brake bias fitting change on 87
08-19-2014, 03:20 PM
#107
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08-19-2014, 04:00 PM
#108
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My 0.02cents:
Nobody writes the truth, specially in such a tricky area. A lot of things get mixed and it is really difficult to translate one experience somewhere / somebody else.
The only real approach to the truth is to log all 4 wheel speeds at 100hz and plot them. It is then easy to check if the slip ratio is even (or too much front or to much rear), if it degrades to the end of the braking, if it is disturbed by downshift, suspension or whatever, first lap to the last.
But there is a very good shortcut: The drivers I work with put more brake to the rear until the rear end gets snappy at the end of the braking, then go a little to the front. If the porsche "limit" valve is actually proportional-proportional (not the usual racing limit valve) then why not install the 50 bucks adjustable unit I suggested and be done with it? ABS will not know.
Best regards,
PS: (not that it is important), it is easy to lock the inner rear with the engine connected, provided the diff does not lock enough (usual). Revs drop very quick to half. You can lock both rears also (not so easy) and the engine stalls. ABS prevents that. You can see the modulation in the rpm, actually.
PS2: old ABS may cause braking distance to duplicate when the optimal slip ratio is high, like, for instance, gravel. That being said, and keep in mind I don't have any experience with racing ABS, to me if the braking with ABS is longer than without it, ABS is probably having issues.
Nobody writes the truth, specially in such a tricky area. A lot of things get mixed and it is really difficult to translate one experience somewhere / somebody else.
The only real approach to the truth is to log all 4 wheel speeds at 100hz and plot them. It is then easy to check if the slip ratio is even (or too much front or to much rear), if it degrades to the end of the braking, if it is disturbed by downshift, suspension or whatever, first lap to the last.
But there is a very good shortcut: The drivers I work with put more brake to the rear until the rear end gets snappy at the end of the braking, then go a little to the front. If the porsche "limit" valve is actually proportional-proportional (not the usual racing limit valve) then why not install the 50 bucks adjustable unit I suggested and be done with it? ABS will not know.
Best regards,
PS: (not that it is important), it is easy to lock the inner rear with the engine connected, provided the diff does not lock enough (usual). Revs drop very quick to half. You can lock both rears also (not so easy) and the engine stalls. ABS prevents that. You can see the modulation in the rpm, actually.
PS2: old ABS may cause braking distance to duplicate when the optimal slip ratio is high, like, for instance, gravel. That being said, and keep in mind I don't have any experience with racing ABS, to me if the braking with ABS is longer than without it, ABS is probably having issues.
08-19-2014, 05:09 PM
#109
Rennlist Member
Thread Starter
My 0.02cents:
Nobody writes the truth, specially in such a tricky area. A lot of things get mixed and it is really difficult to translate one experience somewhere / somebody else.
The only real approach to the truth is to log all 4 wheel speeds at 100hz and plot them. It is then easy to check if the slip ratio is even (or too much front or to much rear), if it degrades to the end of the braking, if it is disturbed by downshift, suspension or whatever, first lap to the last.
But there is a very good shortcut: The drivers I work with put more brake to the rear until the rear end gets snappy at the end of the braking, then go a little to the front. If the porsche "limit" valve is actually proportional-proportional (not the usual racing limit valve) then why not install the 50 bucks adjustable unit I suggested and be done with it? ABS will not know.
Best regards,
PS: (not that it is important), it is easy to lock the inner rear with the engine connected, provided the diff does not lock enough (usual). Revs drop very quick to half. You can lock both rears also (not so easy) and the engine stalls. ABS prevents that. You can see the modulation in the rpm, actually.
PS2: old ABS may cause braking distance to duplicate when the optimal slip ratio is high, like, for instance, gravel. That being said, and keep in mind I don't have any experience with racing ABS, to me if the braking with ABS is longer than without it, ABS is probably having issues.
Nobody writes the truth, specially in such a tricky area. A lot of things get mixed and it is really difficult to translate one experience somewhere / somebody else.
The only real approach to the truth is to log all 4 wheel speeds at 100hz and plot them. It is then easy to check if the slip ratio is even (or too much front or to much rear), if it degrades to the end of the braking, if it is disturbed by downshift, suspension or whatever, first lap to the last.
But there is a very good shortcut: The drivers I work with put more brake to the rear until the rear end gets snappy at the end of the braking, then go a little to the front. If the porsche "limit" valve is actually proportional-proportional (not the usual racing limit valve) then why not install the 50 bucks adjustable unit I suggested and be done with it? ABS will not know.
Best regards,
PS: (not that it is important), it is easy to lock the inner rear with the engine connected, provided the diff does not lock enough (usual). Revs drop very quick to half. You can lock both rears also (not so easy) and the engine stalls. ABS prevents that. You can see the modulation in the rpm, actually.
PS2: old ABS may cause braking distance to duplicate when the optimal slip ratio is high, like, for instance, gravel. That being said, and keep in mind I don't have any experience with racing ABS, to me if the braking with ABS is longer than without it, ABS is probably having issues.
However, most of what im talking about is straight line. The rears already are near max of what physically they can withstand without breaking loose. any additional braking force just shifts the force from the engine to the brakes. no real gain there. in a lighter powered engine with less compression, it could be 50% less force. but still significant slow down forces.
As far as ABS.... I'm going to bet the farm, because of my extensive experience driving the 928 and doing the tests. At same breaking points, I've been able to almost drive off the track due to the ABS just 'letting the car go" when engaged, compared to a skilled foot. Anderson has confirmed this as well. Somehow, Greg thinks just because there is a computer (albeit 30 years old) that it is faster than human processing. yes, it can think faster, but what it is thinking about is deploying a stopping frequency that a skilled foot can match and certainly in this cast, be better than. especially with slicks, as the have a narrow sweet spot for Slip percentage and slip angle, vs a street tire.
08-21-2014, 02:54 PM
#110
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yep, I agree. the inner tire,without ABS is going to lock at some point and then the diff is going to wear out real fast. with ABS, it will spin at some average speed that is what ever percentage the ABS controller wants to see as a differential. Since ours is 3 channel, is it comparing to the front tire speed or each other?? with engine braking, in the turn in point, it goes to the unloaded wheel, (just like if you burn out with one wheel on the snow and the other pavement, the wheel spins on the snow), so not much, if any engine braking under turn in on the rears. this is where the rear bias to the rear can help.
However, most of what im talking about is straight line. The rears already are near max of what physically they can withstand without breaking loose. any additional braking force just shifts the force from the engine to the brakes. no real gain there. in a lighter powered engine with less compression, it could be 50% less force. but still significant slow down forces.
As far as ABS.... I'm going to bet the farm, because of my extensive experience driving the 928 and doing the tests. At same breaking points, I've been able to almost drive off the track due to the ABS just 'letting the car go" when engaged, compared to a skilled foot. Anderson has confirmed this as well. Somehow, Greg thinks just because there is a computer (albeit 30 years old) that it is faster than human processing. yes, it can think faster, but what it is thinking about is deploying a stopping frequency that a skilled foot can match and certainly in this cast, be better than. especially with slicks, as the have a narrow sweet spot for Slip percentage and slip angle, vs a street tire.
However, most of what im talking about is straight line. The rears already are near max of what physically they can withstand without breaking loose. any additional braking force just shifts the force from the engine to the brakes. no real gain there. in a lighter powered engine with less compression, it could be 50% less force. but still significant slow down forces.
As far as ABS.... I'm going to bet the farm, because of my extensive experience driving the 928 and doing the tests. At same breaking points, I've been able to almost drive off the track due to the ABS just 'letting the car go" when engaged, compared to a skilled foot. Anderson has confirmed this as well. Somehow, Greg thinks just because there is a computer (albeit 30 years old) that it is faster than human processing. yes, it can think faster, but what it is thinking about is deploying a stopping frequency that a skilled foot can match and certainly in this cast, be better than. especially with slicks, as the have a narrow sweet spot for Slip percentage and slip angle, vs a street tire.
Take my knowledge with care, I'm no expert. As far as I know:
1. ABS computes vehicle speed trough all 4 wheel speeds (average discarding outliers) and then uses an algorithm to detect locks (based on speed difference compared to vehicle speed and decel rate).
2. If a front wheel is beginning the lock, it goes in a "freeze pressure" mode. In that state, if you increase the brake pressure, the wheel will not see more pressure, just equal. In a ideal world, that would keep the tire in the optimal slip ratio.
3. If the front wheel "recovers" normal speed, ABS goes back to normal mode. If the wheel locks, ABS vents all pressure and cycle back to 1. Then you feel one "bump" on the pedal.
4. If a rear wheel is beginning the lock, it does the same as #2, only that it freezes the pressure at both rears.
So the "frequency" you feel in the pedal is not the ABS blindly pumping the brakes. Ideally, ABS could engage and spend half the braking in state #2 (no vibration on the pedal).
What happens when you vary the relative diameter of front and rear tires? Normally, ABS works but its performance is diminished.
Quick math: If your car weighs 1000kg (we wish) and you have 1.3g decel, you are averaging circa 640 braking horses from 200 to 60kph. 70%F 30%R (which will require pretty high cofg) would mean average 191hp rear brake power. How much engine braking power you got? I don't think more than 40-50. And that is average. At higher speed, the braking power is much more (900 braking horses in the first second). Before doing good math, I don't think engine braking is close in importance to rear brake braking. Also, keep in mind the effective engine braking torque is the one you see when you force it to slow down at a given rate. Meaning that if the engine slows down (rpm/min per second) at the same rate with the clutch on and off (which is quite true in short gears under heavy braking), engine braking is zero.
To sum up: I wouldn't be so sure you have the rears in the limit of adhesion just because you have a big engine with high compression on the front. You can do two things:
a) do the real math, using your estimation of cofg and decel rate, you know how much braking force you would use in the rear in ideal bias. Then calculate the braking force at the wheels due to engine braking (braking torque times gbox ratio times diff ratio divided by tire radius) in top gear and add it to you brake dyno results. Then, see where you are.
b) Buy a variable valve and do some testing.
08-21-2014, 05:11 PM
#111
Rennlist Member
Thread Starter
Mark,
Take my knowledge with care, I'm no expert. As far as I know:
1. ABS computes vehicle speed trough all 4 wheel speeds (average discarding outliers) and then uses an algorithm to detect locks (based on speed difference compared to vehicle speed and decel rate).
2. If a front wheel is beginning the lock, it goes in a "freeze pressure" mode. In that state, if you increase the brake pressure, the wheel will not see more pressure, just equal. In a ideal world, that would keep the tire in the optimal slip ratio.
3. If the front wheel "recovers" normal speed, ABS goes back to normal mode. If the wheel locks, ABS vents all pressure and cycle back to 1. Then you feel one "bump" on the pedal.
4. If a rear wheel is beginning the lock, it does the same as #2, only that it freezes the pressure at both rears.
So the "frequency" you feel in the pedal is not the ABS blindly pumping the brakes. Ideally, ABS could engage and spend half the braking in state #2 (no vibration on the pedal).
What happens when you vary the relative diameter of front and rear tires? Normally, ABS works but its performance is diminished.
Quick math: If your car weighs 1000kg (we wish) and you have 1.3g decel, you are averaging circa 640 braking horses from 200 to 60kph. 70%F 30%R (which will require pretty high cofg) would mean average 191hp rear brake power. How much engine braking power you got? I don't think more than 40-50. And that is average. At higher speed, the braking power is much more (900 braking horses in the first second). Before doing good math, I don't think engine braking is close in importance to rear brake braking. Also, keep in mind the effective engine braking torque is the one you see when you force it to slow down at a given rate. Meaning that if the engine slows down (rpm/min per second) at the same rate with the clutch on and off (which is quite true in short gears under heavy braking), engine braking is zero.
To sum up: I wouldn't be so sure you have the rears in the limit of adhesion just because you have a big engine with high compression on the front. You can do two things:
a) do the real math, using your estimation of cofg and decel rate, you know how much braking force you would use in the rear in ideal bias. Then calculate the braking force at the wheels due to engine braking (braking torque times gbox ratio times diff ratio divided by tire radius) in top gear and add it to you brake dyno results. Then, see where you are.
b) Buy a variable valve and do some testing.
Take my knowledge with care, I'm no expert. As far as I know:
1. ABS computes vehicle speed trough all 4 wheel speeds (average discarding outliers) and then uses an algorithm to detect locks (based on speed difference compared to vehicle speed and decel rate).
2. If a front wheel is beginning the lock, it goes in a "freeze pressure" mode. In that state, if you increase the brake pressure, the wheel will not see more pressure, just equal. In a ideal world, that would keep the tire in the optimal slip ratio.
3. If the front wheel "recovers" normal speed, ABS goes back to normal mode. If the wheel locks, ABS vents all pressure and cycle back to 1. Then you feel one "bump" on the pedal.
4. If a rear wheel is beginning the lock, it does the same as #2, only that it freezes the pressure at both rears.
So the "frequency" you feel in the pedal is not the ABS blindly pumping the brakes. Ideally, ABS could engage and spend half the braking in state #2 (no vibration on the pedal).
What happens when you vary the relative diameter of front and rear tires? Normally, ABS works but its performance is diminished.
Quick math: If your car weighs 1000kg (we wish) and you have 1.3g decel, you are averaging circa 640 braking horses from 200 to 60kph. 70%F 30%R (which will require pretty high cofg) would mean average 191hp rear brake power. How much engine braking power you got? I don't think more than 40-50. And that is average. At higher speed, the braking power is much more (900 braking horses in the first second). Before doing good math, I don't think engine braking is close in importance to rear brake braking. Also, keep in mind the effective engine braking torque is the one you see when you force it to slow down at a given rate. Meaning that if the engine slows down (rpm/min per second) at the same rate with the clutch on and off (which is quite true in short gears under heavy braking), engine braking is zero.
To sum up: I wouldn't be so sure you have the rears in the limit of adhesion just because you have a big engine with high compression on the front. You can do two things:
a) do the real math, using your estimation of cofg and decel rate, you know how much braking force you would use in the rear in ideal bias. Then calculate the braking force at the wheels due to engine braking (braking torque times gbox ratio times diff ratio divided by tire radius) in top gear and add it to you brake dyno results. Then, see where you are.
b) Buy a variable valve and do some testing.
welll, i can tell you engine braking is MUCH more thatn 50hp. I have the dyno results.
we dont look at top gear here we look at the gears we are in ,while decelling at max threshold braking .... starting out at 130mph and going down to 45mph. sure the total KE is much higher but the braking force is near constant. this is why ill have a diffrent LB force in the lower gears vs higher for engine braking. we are talking downshift from 4th to 3rd and then 3rd to 2nd. thats 5:1 and 7:1 (roughtly).
the engine braking is well over 150ft-lbs and 150HP. , or 75hp/75Hp per wheel. if you have 300lbs on each wheel and we are downshifting into 3rd at near 5:1, thats 375lbs per wheel of engine braking force. well over the 300lbs sitting on the wheel and very close to "at the limit" decel forces on a Cf tire that is DOT or a slick. this would show you need NO rear brakes while iin gear. but the reality is also that due to using the fronts to sllow the chassis down, the rate of decel is such that the engine braking is less. maybe 1/2 or 1/3rd. even still, engine braking is a substantial part of the decel, and is significant.... this is why , especially with a front engine'd racer, folks get into trouble with rear bias issues and lock up. 911s, have 2x the capacity to put more rear brake bias down, plus engine braking is a lot less.. (all other things being equal)
now, dont confuse engine braking with RPM drops with the clutch out. if you can match this rate, you are right, but generally, you cant. you rev your engine to 6000rpm and see how long it takes to get to 3000rpm. so, i dont believe it is "quite true under heavy braking" It's .5 seconds or so, we are talking a decel rate of near 2 seconds per gear , so yes, that diminishes the engine braking, but i would say by 1/2 or a 1/3rd as i mention earlier. stilll, engine braking is a serious force and one that saved my bacon by knowing and being familiar with it. blowing a brake line at laguna at 120mph and downshifting twice , allowed me to stay on track without slamming into tires. I would imagine a weak bias to the rear and engine braking, gets you to the limit of the tires on decel.
EDIT: one interesting point, is that you're right. 120 to 80mph vs 80 to 40mph is almost twice the KE, even with a constant rate of decel do to a relatively cosntant braking force. With the constant decel power, that means, braking force goes up proportionally with speed loss..
(remember, acceleration is proportional to power and goes down inversely with speed. decel.. just the opposite.
so, if you bias for max deceleration at top speed, you might find that you have too much rear bias due to engine braking near the last second or two of the slow down sequence.
certainly, i dont think there is much time to be found by getting greater bias in the rear, only potential lock up issues. certainly , it wont save the fronts from overheating from their huge KE task in 4 seconds from 130mph to 45mph. however , larger rotors will and so will even higher temp pads.
so, you just did the power math above and made my point engine braking is almost all of the decel force in a high performance race car on slicks. any more just adds to it, and could make stability worse by inducing rear lock up and high slip ratios.
Last edited by mark kibort; 08-21-2014 at 09:28 PM.
08-21-2014, 07:37 PM
#112
Rennlist Member
Just to add more of my weird experiences...
The 90GT I had engine braked so much so that I could not drive the car in icey or snowy conditions unless on tires specifically for it like Blizzaks. Otherwise, any lift off the throttle at all would send the car into a spin.
As annoying as that feature was on slick roads, it made it a joy to drive on long trips doing excessive speeds on a four lane highway. To slow down from triple digits just merely lift the throttle, then press down to resume. Rarely did you ever have to move your foot from the accelerator to the brake. Doesn't sound like much, but really does make a big difference. On the GTS lifting is more like pressing the clutch.
Was expecting the same with the 94 GTS, but nope. Almost no rear braking when lifting off the throttle.
In racing terms, the throttle lift rear braking is so slight balance is not moved forward enough for the front tires to grip more and tighten the 4 wheel drift thru the apex.
In fact, there is such a difference I wonder if there is some way to increase engine braking. And/or what made the difference in the amount of engine braking.
The 90GT I had engine braked so much so that I could not drive the car in icey or snowy conditions unless on tires specifically for it like Blizzaks. Otherwise, any lift off the throttle at all would send the car into a spin.
As annoying as that feature was on slick roads, it made it a joy to drive on long trips doing excessive speeds on a four lane highway. To slow down from triple digits just merely lift the throttle, then press down to resume. Rarely did you ever have to move your foot from the accelerator to the brake. Doesn't sound like much, but really does make a big difference. On the GTS lifting is more like pressing the clutch.
Was expecting the same with the 94 GTS, but nope. Almost no rear braking when lifting off the throttle.
In racing terms, the throttle lift rear braking is so slight balance is not moved forward enough for the front tires to grip more and tighten the 4 wheel drift thru the apex.
In fact, there is such a difference I wonder if there is some way to increase engine braking. And/or what made the difference in the amount of engine braking.
08-21-2014, 09:20 PM
#113
Rennlist Member
Thread Starter
Just to add more of my weird experiences...
The 90GT I had engine braked so much so that I could not drive the car in icey or snowy conditions unless on tires specifically for it like Blizzaks. Otherwise, any lift off the throttle at all would send the car into a spin.
As annoying as that feature was on slick roads, it made it a joy to drive on long trips doing excessive speeds on a four lane highway. To slow down from triple digits just merely lift the throttle, then press down to resume. Rarely did you ever have to move your foot from the accelerator to the brake. Doesn't sound like much, but really does make a big difference. On the GTS lifting is more like pressing the clutch.
Was expecting the same with the 94 GTS, but nope. Almost no rear braking when lifting off the throttle.
In racing terms, the throttle lift rear braking is so slight balance is not moved forward enough for the front tires to grip more and tighten the 4 wheel drift thru the apex.
In fact, there is such a difference I wonder if there is some way to increase engine braking. And/or what made the difference in the amount of engine braking.
The 90GT I had engine braked so much so that I could not drive the car in icey or snowy conditions unless on tires specifically for it like Blizzaks. Otherwise, any lift off the throttle at all would send the car into a spin.
As annoying as that feature was on slick roads, it made it a joy to drive on long trips doing excessive speeds on a four lane highway. To slow down from triple digits just merely lift the throttle, then press down to resume. Rarely did you ever have to move your foot from the accelerator to the brake. Doesn't sound like much, but really does make a big difference. On the GTS lifting is more like pressing the clutch.
Was expecting the same with the 94 GTS, but nope. Almost no rear braking when lifting off the throttle.
In racing terms, the throttle lift rear braking is so slight balance is not moved forward enough for the front tires to grip more and tighten the 4 wheel drift thru the apex.
In fact, there is such a difference I wonder if there is some way to increase engine braking. And/or what made the difference in the amount of engine braking.
with around a 8:1 84 928, i was at near 150hp for decel engine braking.
easy to find out.... just go to the dyno and at redline, just lift and leave it in gear and dont touch the brakes.... the dyno will record it all.
there could be one difference, and thats the cams.
08-22-2014, 06:55 AM
#114
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yep, I agree. the inner tire,without ABS is going to lock at some point and then the diff is going to wear out real fast. with ABS, it will spin at some average speed that is what ever percentage the ABS controller wants to see as a differential. Since ours is 3 channel, is it comparing to the front tire speed or each other?? with engine braking, in the turn in point, it goes to the unloaded wheel, (just like if you burn out with one wheel on the snow and the other pavement, the wheel spins on the snow), so not much, if any engine braking under turn in on the rears. this is where the rear bias to the rear can help.
However, most of what im talking about is straight line. The rears already are near max of what physically they can withstand without breaking loose. any additional braking force just shifts the force from the engine to the brakes. no real gain there. in a lighter powered engine with less compression, it could be 50% less force. but still significant slow down forces.
As far as ABS.... I'm going to bet the farm, because of my extensive experience driving the 928 and doing the tests. At same breaking points, I've been able to almost drive off the track due to the ABS just 'letting the car go" when engaged, compared to a skilled foot. Anderson has confirmed this as well. Somehow, Greg thinks just because there is a computer (albeit 30 years old) that it is faster than human processing. yes, it can think faster, but what it is thinking about is deploying a stopping frequency that a skilled foot can match and certainly in this cast, be better than. especially with slicks, as the have a narrow sweet spot for Slip percentage and slip angle, vs a street tire.
However, most of what im talking about is straight line. The rears already are near max of what physically they can withstand without breaking loose. any additional braking force just shifts the force from the engine to the brakes. no real gain there. in a lighter powered engine with less compression, it could be 50% less force. but still significant slow down forces.
As far as ABS.... I'm going to bet the farm, because of my extensive experience driving the 928 and doing the tests. At same breaking points, I've been able to almost drive off the track due to the ABS just 'letting the car go" when engaged, compared to a skilled foot. Anderson has confirmed this as well. Somehow, Greg thinks just because there is a computer (albeit 30 years old) that it is faster than human processing. yes, it can think faster, but what it is thinking about is deploying a stopping frequency that a skilled foot can match and certainly in this cast, be better than. especially with slicks, as the have a narrow sweet spot for Slip percentage and slip angle, vs a street tire.
welll, i can tell you engine braking is MUCH more thatn 50hp. I have the dyno results.
we dont look at top gear here we look at the gears we are in ,while decelling at max threshold braking .... starting out at 130mph and going down to 45mph. sure the total KE is much higher but the braking force is near constant. this is why ill have a diffrent LB force in the lower gears vs higher for engine braking. we are talking downshift from 4th to 3rd and then 3rd to 2nd. thats 5:1 and 7:1 (roughtly).
the engine braking is well over 150ft-lbs and 150HP. , or 75hp/75Hp per wheel. if you have 300lbs on each wheel and we are downshifting into 3rd at near 5:1, thats 375lbs per wheel of engine braking force. well over the 300lbs sitting on the wheel and very close to "at the limit" decel forces on a Cf tire that is DOT or a slick. this would show you need NO rear brakes while iin gear. but the reality is also that due to using the fronts to sllow the chassis down, the rate of decel is such that the engine braking is less. maybe 1/2 or 1/3rd. even still, engine braking is a substantial part of the decel, and is significant.... this is why , especially with a front engine'd racer, folks get into trouble with rear bias issues and lock up. 911s, have 2x the capacity to put more rear brake bias down, plus engine braking is a lot less.. (all other things being equal)
now, dont confuse engine braking with RPM drops with the clutch out. if you can match this rate, you are right, but generally, you cant. you rev your engine to 6000rpm and see how long it takes to get to 3000rpm. so, i dont believe it is "quite true under heavy braking" It's .5 seconds or so, we are talking a decel rate of near 2 seconds per gear , so yes, that diminishes the engine braking, but i would say by 1/2 or a 1/3rd as i mention earlier. stilll, engine braking is a serious force and one that saved my bacon by knowing and being familiar with it. blowing a brake line at laguna at 120mph and downshifting twice , allowed me to stay on track without slamming into tires. I would imagine a weak bias to the rear and engine braking, gets you to the limit of the tires on decel.
EDIT: one interesting point, is that you're right. 120 to 80mph vs 80 to 40mph is almost twice the KE, even with a constant rate of decel do to a relatively cosntant braking force. With the constant decel power, that means, braking force goes up proportionally with speed loss..
(remember, acceleration is proportional to power and goes down inversely with speed. decel.. just the opposite.
so, if you bias for max deceleration at top speed, you might find that you have too much rear bias due to engine braking near the last second or two of the slow down sequence.
certainly, i dont think there is much time to be found by getting greater bias in the rear, only potential lock up issues. certainly , it wont save the fronts from overheating from their huge KE task in 4 seconds from 130mph to 45mph. however , larger rotors will and so will even higher temp pads.
so, you just did the power math above and made my point engine braking is almost all of the decel force in a high performance race car on slicks. any more just adds to it, and could make stability worse by inducing rear lock up and high slip ratios.
we dont look at top gear here we look at the gears we are in ,while decelling at max threshold braking .... starting out at 130mph and going down to 45mph. sure the total KE is much higher but the braking force is near constant. this is why ill have a diffrent LB force in the lower gears vs higher for engine braking. we are talking downshift from 4th to 3rd and then 3rd to 2nd. thats 5:1 and 7:1 (roughtly).
the engine braking is well over 150ft-lbs and 150HP. , or 75hp/75Hp per wheel. if you have 300lbs on each wheel and we are downshifting into 3rd at near 5:1, thats 375lbs per wheel of engine braking force. well over the 300lbs sitting on the wheel and very close to "at the limit" decel forces on a Cf tire that is DOT or a slick. this would show you need NO rear brakes while iin gear. but the reality is also that due to using the fronts to sllow the chassis down, the rate of decel is such that the engine braking is less. maybe 1/2 or 1/3rd. even still, engine braking is a substantial part of the decel, and is significant.... this is why , especially with a front engine'd racer, folks get into trouble with rear bias issues and lock up. 911s, have 2x the capacity to put more rear brake bias down, plus engine braking is a lot less.. (all other things being equal)
now, dont confuse engine braking with RPM drops with the clutch out. if you can match this rate, you are right, but generally, you cant. you rev your engine to 6000rpm and see how long it takes to get to 3000rpm. so, i dont believe it is "quite true under heavy braking" It's .5 seconds or so, we are talking a decel rate of near 2 seconds per gear , so yes, that diminishes the engine braking, but i would say by 1/2 or a 1/3rd as i mention earlier. stilll, engine braking is a serious force and one that saved my bacon by knowing and being familiar with it. blowing a brake line at laguna at 120mph and downshifting twice , allowed me to stay on track without slamming into tires. I would imagine a weak bias to the rear and engine braking, gets you to the limit of the tires on decel.
EDIT: one interesting point, is that you're right. 120 to 80mph vs 80 to 40mph is almost twice the KE, even with a constant rate of decel do to a relatively cosntant braking force. With the constant decel power, that means, braking force goes up proportionally with speed loss..
(remember, acceleration is proportional to power and goes down inversely with speed. decel.. just the opposite.
so, if you bias for max deceleration at top speed, you might find that you have too much rear bias due to engine braking near the last second or two of the slow down sequence.
certainly, i dont think there is much time to be found by getting greater bias in the rear, only potential lock up issues. certainly , it wont save the fronts from overheating from their huge KE task in 4 seconds from 130mph to 45mph. however , larger rotors will and so will even higher temp pads.
so, you just did the power math above and made my point engine braking is almost all of the decel force in a high performance race car on slicks. any more just adds to it, and could make stability worse by inducing rear lock up and high slip ratios.
Sure the slower you go the greater the engine brake at the tires. Engine brake torque is the same, but gear is progressively shorter. That's why most big racing engines are tuned to brake more the longer the gear. Specially critical in MotoGP.
That's why engine braking is not very easy to use in racing. Tell current F1 teams, with 120KW of extra engine braking this year. Most are still struggling to fine tune the thing, even with electronic brake control, after a bunch of weekends. They would love to move the MGUK past the gearbox, so braking through the downshift would be much easier.
And that's why I think I would consider to reduce engine braking (open idle, remove idle cutoff...) and move a bit of braking force to the rear.
Keep us posted when you try the different valve!
08-22-2014, 07:04 AM
#115
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Can you share with me (maybe PM) the engine braking curve? I'm curious. I've never seen one in a standard throttled car (not throttle by wire or direct injection in which you pretty much program it the way you like).
You considered the drivetrain losses add in the braking part, right? (unlike the power part, in which they substract). Or you dynoed the engine outside the car?
Sorry if I offended you with the question, sometimes I tend to forget easy stuff and I wrongly assume everybody does.
I know drivetrain losses are there in real life, but it's action is not related to gearbox ratio and, therefore are less important the lower the gear.
You considered the drivetrain losses add in the braking part, right? (unlike the power part, in which they substract). Or you dynoed the engine outside the car?
Sorry if I offended you with the question, sometimes I tend to forget easy stuff and I wrongly assume everybody does.
I know drivetrain losses are there in real life, but it's action is not related to gearbox ratio and, therefore are less important the lower the gear.
08-22-2014, 12:33 PM
#117
Inventor
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There is a bit in the LH2.3 (S4-up) constants which enables engine braking, IE. injector cutoff on throttle lift. (It is selectable via the Sharktuner.) Even disabled, depending on the RPM, it will go off-scale lean on a log.
On the LH2.2 (S2/S3), there is an (unused) hard wired input which flags for injector cutoff on decel. This coding input is used for automatics on Volvo/SAAB.
Automatics feel sportier with no decel cutoff as engine rpms don't immediately drop to idle on liftoff, keeping the torque converter spooled up for better throttle-back-on response.
On the LH2.2 (S2/S3), there is an (unused) hard wired input which flags for injector cutoff on decel. This coding input is used for automatics on Volvo/SAAB.
Automatics feel sportier with no decel cutoff as engine rpms don't immediately drop to idle on liftoff, keeping the torque converter spooled up for better throttle-back-on response.
08-22-2014, 12:50 PM
#118
Chronic Tool Dropper
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Last edited by dr bob; 08-25-2014 at 07:06 PM. Reason: [Done]
08-22-2014, 12:51 PM
#119
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Thread Starter
so, i removed it. also , had is switchable for the S4, because its more computer involved with the S4 vs the '84s. (more things effected, so there were sometimes i wanted it , like start up. but never on the track, AND it only effected low RPM fuel cut off, not high speed downshifts, where the fuel was cut off either way. that might not be happening due to he shark tuner changes.
Can you share with me (maybe PM) the engine braking curve? I'm curious. I've never seen one in a standard throttled car (not throttle by wire or direct injection in which you pretty much program it the way you like).
You considered the drivetrain losses add in the braking part, right? (unlike the power part, in which they substract). Or you dynoed the engine outside the car?
Sorry if I offended you with the question, sometimes I tend to forget easy stuff and I wrongly assume everybody does.
I know drivetrain losses are there in real life, but it's action is not related to gearbox ratio and, therefore are less important the lower the gear.
You considered the drivetrain losses add in the braking part, right? (unlike the power part, in which they substract). Or you dynoed the engine outside the car?
Sorry if I offended you with the question, sometimes I tend to forget easy stuff and I wrongly assume everybody does.
I know drivetrain losses are there in real life, but it's action is not related to gearbox ratio and, therefore are less important the lower the gear.
I stand corrected, then. As I said before, each situation is a little bit different. I didn't speak lightly, when I said it is easy to have same rpm drop rate in neutral than in 2nd braking hard. But not in your car, ok I believe you 100%.
Sure the slower you go the greater the engine brake at the tires. Engine brake torque is the same, but gear is progressively shorter. That's why most big racing engines are tuned to brake more the longer the gear. Specially critical in MotoGP.
That's why engine braking is not very easy to use in racing. Tell current F1 teams, with 120KW of extra engine braking this year. Most are still struggling to fine tune the thing, even with electronic brake control, after a bunch of weekends. They would love to move the MGUK past the gearbox, so braking through the downshift would be much easier.
And that's why I think I would consider to reduce engine braking (open idle, remove idle cutoff...) and move a bit of braking force to the rear.
Keep us posted when you try the different valve!
Sure the slower you go the greater the engine brake at the tires. Engine brake torque is the same, but gear is progressively shorter. That's why most big racing engines are tuned to brake more the longer the gear. Specially critical in MotoGP.
That's why engine braking is not very easy to use in racing. Tell current F1 teams, with 120KW of extra engine braking this year. Most are still struggling to fine tune the thing, even with electronic brake control, after a bunch of weekends. They would love to move the MGUK past the gearbox, so braking through the downshift would be much easier.
And that's why I think I would consider to reduce engine braking (open idle, remove idle cutoff...) and move a bit of braking force to the rear.
Keep us posted when you try the different valve!
now as far as the RPM drop in the lower gears, yes, you are right. each gear has about 30% faster drop for a constant decel. (which we can safely assume, due to near constant braking force), SO, it also has 30% more braking force, so the engine braking force should be constant through the gears. thats the big variable i cant really accurately predict, right? on a dyno, the force is all engine under decel in gear. on the track, if you could decel with front and rear brakes, at the same rate as the engine RPM falls, then there would be "0" force on the driveline.
somewhere earlier i took all this into account for an approximation.
200hp decel engine braking forces would slow the car down from 120 to 40 in 8 seconds.... so, i figured if it happens in 4seconds under braking, maybe its effects are then only 1/2. take out the aero we can easily approximate, and the rolling friction and that value changes a little but it stays relatively constant as you slow, even in the lower gears with shortened RPM drop time.
Ill pm you the graph.
08-22-2014, 12:58 PM
#120
Rennlist Member
Have GT cams in GTS.
Before gong to ShartTuner Mechanic assured me injector cutoff on throttle lift was working and enabled.
After hooking up SharkTuner on or off did not make any difference.
Looked at and tried everything because I really like the throttle lift decal on road trips. Slows down quickly when coming up on a group of slow moving traffic without flashing brake lights to alert any officials behind.
Before gong to ShartTuner Mechanic assured me injector cutoff on throttle lift was working and enabled.
After hooking up SharkTuner on or off did not make any difference.
Looked at and tried everything because I really like the throttle lift decal on road trips. Slows down quickly when coming up on a group of slow moving traffic without flashing brake lights to alert any officials behind.