Rookie Question: Double clutching??
04-26-2002, 04:27 PM
#31
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I've just started trying this (double-clutching) today after reading this thread yesterday. Here are some comments/questions from someone just learning:
Firstly, it seems to me that double-clutching would only be useful for downshifting. When upshifting, you are going to a gear that will rev the motor at a lower speed. When engaging the clutch, the engine speed drops anyway, and when you engage in the higher gear you should be able to match the revs without the need of double clutching or "blipping" the throttle.
Secondly, I rapidly saw what I think is the benefit of "rev matching" while downshifting, whether I was turning or going in a straight line. When downshifting, you are going into a gear that will rev the motor at a higher speed, which goes contrary to the fact that the engine speed drops when you engage the clutch. I was on the highway driving at 75 mph in fifth gear, and I decided to downshift to third as if I wanted to pass. I first did this the way I have in the past: (1) engage clutch, (2) shift from fifth to third, (3) disengage clutch. I began somewhere around 3000 rpm and by the time I released the clutch the engine was turning about 2000 rpm. The car kind of "bucked" as I released the clutch and the engine tried to get up to around 5000 rpm (or wherever it was). I then tried doing what I assume was what is proper double-clutch technique: (1) engage clutch, (2) shift from fifth to neutral, (3) disengage clutch, (4) blip throttle up to near redline, (5) engage clutch, (6) shift to third, (7) disengage clutch. The car made the transition much more smoothly, as when I disengaged the clutch in third the engine was still turning something near 5000 rpm, which is where it wanted to be in third. The engine didn't have to rapidly transition from 2000 rpm to 5000 rpm as I disengaged the clutch.
Is this the way it is supposed to work? Am I in the right ballpark? I can see why this would work in a turn if you were slowing down, but wanted to shift into a low gear and accelerate hard coming out of the turn. I tried this on my morning commute, but I couldn't safely take any of the turns aggressively enough to where it mattered.
Here's my big question, which it seems has been asked before, but I'm not sure that I've seen a clear answer: Why the need to disengage the clutch prior to blipping the throttle? In my outline of the steps I used for double-clutching, why can't steps #3 and #5 be eliminated? Can't the throttle be blipped and rev's matched while the clutch is fully engaged???
Cheers,
Mark
Firstly, it seems to me that double-clutching would only be useful for downshifting. When upshifting, you are going to a gear that will rev the motor at a lower speed. When engaging the clutch, the engine speed drops anyway, and when you engage in the higher gear you should be able to match the revs without the need of double clutching or "blipping" the throttle.
Secondly, I rapidly saw what I think is the benefit of "rev matching" while downshifting, whether I was turning or going in a straight line. When downshifting, you are going into a gear that will rev the motor at a higher speed, which goes contrary to the fact that the engine speed drops when you engage the clutch. I was on the highway driving at 75 mph in fifth gear, and I decided to downshift to third as if I wanted to pass. I first did this the way I have in the past: (1) engage clutch, (2) shift from fifth to third, (3) disengage clutch. I began somewhere around 3000 rpm and by the time I released the clutch the engine was turning about 2000 rpm. The car kind of "bucked" as I released the clutch and the engine tried to get up to around 5000 rpm (or wherever it was). I then tried doing what I assume was what is proper double-clutch technique: (1) engage clutch, (2) shift from fifth to neutral, (3) disengage clutch, (4) blip throttle up to near redline, (5) engage clutch, (6) shift to third, (7) disengage clutch. The car made the transition much more smoothly, as when I disengaged the clutch in third the engine was still turning something near 5000 rpm, which is where it wanted to be in third. The engine didn't have to rapidly transition from 2000 rpm to 5000 rpm as I disengaged the clutch.
Is this the way it is supposed to work? Am I in the right ballpark? I can see why this would work in a turn if you were slowing down, but wanted to shift into a low gear and accelerate hard coming out of the turn. I tried this on my morning commute, but I couldn't safely take any of the turns aggressively enough to where it mattered.
Here's my big question, which it seems has been asked before, but I'm not sure that I've seen a clear answer: Why the need to disengage the clutch prior to blipping the throttle? In my outline of the steps I used for double-clutching, why can't steps #3 and #5 be eliminated? Can't the throttle be blipped and rev's matched while the clutch is fully engaged???
Cheers,
Mark
04-26-2002, 04:34 PM
#32
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Oh yeah, one more question: How closely is anyone actually "matching" the rev's? Although this was my first drive trying the technique, I had a bitch of a time being very accurate in exactly how many rpm I got the engine to with my blip. If I wanted, say 6500 rpm, I'd have to really concentrate on what I was doing, watch the tach, and probably get into an accident. Furthermore, once I let off the accelerator and start to disengage the clutch, the rev's drop pretty quickly. Also, how do I know exactly what rev's I want in the first place!?
Do good drivers actually calculate on the fly that they want a particular number of rpm, like within a hundred? Or is it more a matter of getting in the general ballpark and developing a feel over time.
As I practiced today there were times I was clearly way off, like when my throttle blip was too weak and only bumped the tach up to 3000 rpm. Other times the downshift came out really smooth. I think at the latter times I just managed to get close to where I needed to be.
Cheers again,
Mark
Do good drivers actually calculate on the fly that they want a particular number of rpm, like within a hundred? Or is it more a matter of getting in the general ballpark and developing a feel over time.
As I practiced today there were times I was clearly way off, like when my throttle blip was too weak and only bumped the tach up to 3000 rpm. Other times the downshift came out really smooth. I think at the latter times I just managed to get close to where I needed to be.
Cheers again,
Mark
04-26-2002, 04:50 PM
#33
Nordschleife Master
Well I don't calculate it out, I just have kind of got a feel for it over the years. better drivers may do quick figures in thier had, but that seems like too much do in such a short period of time.
04-26-2002, 05:34 PM
#34
Race Director
Sajid:
Regarding the blip of the throttle: it's not a quick stab or jerky motion, rather a smooth rolling of your foot to bring the revs up. You may need to keep your 'side of the foot' on the throttle a little longer than the term blip implies.
Mark:
Regarding double de-clutching:
1. Most people say if you are doing heel and toe, the need for a double de-clutch is not necessary, since our syncros are strong enough to handle the pressure. Bottom line: if you heel and toe, don't worry about double de-clutching. (the purists of course, would disagree!)
2. Why do I double de-clutch on an upshift? Well, the only time I double de-clutch ever, EVER, is in the morning or when it is cold and I'm shifting into second gear (from first). I have a hard time getting the car into second (from a 1-2 upshift) in those conditions. (Most 944's have this problem) I have found that double de-clutching in cold conditions makes it tons easier to get into 2nd gear! If my gear change is smoother, I am not wearing the clutch or tranny as much. Again, that's the only time I double de-clutch!
Some other things to consider:
One should NEVER use downshifting to slow down: you have something else that you can use: namely your brakes! (Last time I checked, brake pads are cheaper than trannies!)
And as stated before, the heel and toe process is meant for smoothness in the transition from braking to turning/accelerating, as well as putting less wear on the tranny. It is not meant to get you through a corner faster only smoother It is not to be used to slow you down either.
Disclaimer: I do not consider myself an expert of heel and toe. What I've stated is just based on what I've read, heard and experienced.
I hope that clarifies things rather than confusing them! I also hope to see you guys out there this weekend trying this stuff out! Good luck!
-Z-man.
Regarding the blip of the throttle: it's not a quick stab or jerky motion, rather a smooth rolling of your foot to bring the revs up. You may need to keep your 'side of the foot' on the throttle a little longer than the term blip implies.
Mark:
Regarding double de-clutching:
1. Most people say if you are doing heel and toe, the need for a double de-clutch is not necessary, since our syncros are strong enough to handle the pressure. Bottom line: if you heel and toe, don't worry about double de-clutching. (the purists of course, would disagree!)
2. Why do I double de-clutch on an upshift? Well, the only time I double de-clutch ever, EVER, is in the morning or when it is cold and I'm shifting into second gear (from first). I have a hard time getting the car into second (from a 1-2 upshift) in those conditions. (Most 944's have this problem) I have found that double de-clutching in cold conditions makes it tons easier to get into 2nd gear! If my gear change is smoother, I am not wearing the clutch or tranny as much. Again, that's the only time I double de-clutch!
Some other things to consider:
One should NEVER use downshifting to slow down: you have something else that you can use: namely your brakes! (Last time I checked, brake pads are cheaper than trannies!)
And as stated before, the heel and toe process is meant for smoothness in the transition from braking to turning/accelerating, as well as putting less wear on the tranny. It is not meant to get you through a corner faster only smoother It is not to be used to slow you down either.
Disclaimer: I do not consider myself an expert of heel and toe. What I've stated is just based on what I've read, heard and experienced.
I hope that clarifies things rather than confusing them! I also hope to see you guys out there this weekend trying this stuff out! Good luck!
-Z-man.
04-26-2002, 05:42 PM
#35
Three Wheelin'
As for knowing what RPM's to be at, it really depends on how well you know the car. I don't even have a tach, so I just try to get close.
If you're within a few 100 RPM, you should be ok. But you need to work on it to get smooth.
If you're within a few 100 RPM, you should be ok. But you need to work on it to get smooth.
04-26-2002, 06:08 PM
#36
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I completely agree with Z-man's excellent discussion on this matter... especially the part about not doing this during a turn!! Re-read and burn into your mind the line about "ALL braking and downshifting should be done before you enter the turn". If you do it everyday on the street, you won’t have to worry about doing it on the track (where things can get real ugly real fast!!).
As far as the throttle blip, it is a quick jab to get the revs raised. Don't worry about trying to match the rpms exactly to the gear that you are going down into... just give it a good jab!! More often than not, it will be just fine to get the job done. I recently re-read/browsed through my copy of the book Porsche High Performance Driving (by Vic Elford), and in it, Vic states exactly that... to give it a good poke, and not finesse the pedal.
Practice, practice, practice (which is what I definitely need to do with my S2, especially before I go out on the track)!!
I hope that helps.
Hey Z-man... maybe I'll see you someday at LimeRock, or Summit Point, or VIR!! We can talk heel-and-toe some more...
Jim
As far as the throttle blip, it is a quick jab to get the revs raised. Don't worry about trying to match the rpms exactly to the gear that you are going down into... just give it a good jab!! More often than not, it will be just fine to get the job done. I recently re-read/browsed through my copy of the book Porsche High Performance Driving (by Vic Elford), and in it, Vic states exactly that... to give it a good poke, and not finesse the pedal.
Practice, practice, practice (which is what I definitely need to do with my S2, especially before I go out on the track)!!
I hope that helps.
Hey Z-man... maybe I'll see you someday at LimeRock, or Summit Point, or VIR!! We can talk heel-and-toe some more...
Jim
04-26-2002, 09:45 PM
#37
Ok, I will take a stab at the question..."why isnt one of the other methods better?" (I cant remember the actual question.
Double clutching is IMHO a crucial skill for manual transmission driving in general. The only thing on your car holding you on the road is your tires. When weight shifts suddenly, as happens when you downshift without matching revs and surge the engine up when the clutch engages, energy is transfered rapidly in the direction on the inertia at that instant. (I am not sure my terms are 100% correct.)
The adhesion properties of tires are more suited to slow movement. This is what guys like Schumacher excel at, that beautiful smooth line. They never have abrupt movement, so they maximise the adhesion of the tires.
I cannot back this next statement up with math, but I theorize that abrupt motion causes the rubber to crystallize and shear. The molecules cannot stand the violence of the pull between tire and road, and "crack", like taffy will if you hit it hard. But ever slowly stretch taffy? That stuff will get 25' long, if you work really, really slow.
And as far as only being useful on downshifts, I would disagree. Any time you can change your ability to apply power, without upsetting your balance, you will gain in that quality we all want to have in Don King proportions, smoothness.
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
Double clutching is IMHO a crucial skill for manual transmission driving in general. The only thing on your car holding you on the road is your tires. When weight shifts suddenly, as happens when you downshift without matching revs and surge the engine up when the clutch engages, energy is transfered rapidly in the direction on the inertia at that instant. (I am not sure my terms are 100% correct.)
The adhesion properties of tires are more suited to slow movement. This is what guys like Schumacher excel at, that beautiful smooth line. They never have abrupt movement, so they maximise the adhesion of the tires.
I cannot back this next statement up with math, but I theorize that abrupt motion causes the rubber to crystallize and shear. The molecules cannot stand the violence of the pull between tire and road, and "crack", like taffy will if you hit it hard. But ever slowly stretch taffy? That stuff will get 25' long, if you work really, really slow.
And as far as only being useful on downshifts, I would disagree. Any time you can change your ability to apply power, without upsetting your balance, you will gain in that quality we all want to have in Don King proportions, smoothness.
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
04-26-2002, 11:52 PM
#38
Three Wheelin'
First: What Z-man said, including disclaimer.
Double-clutching is kinda useless if your car has non-malfunctioning synchros. Don't bother with it when you upshift, downshift, heel-toe, drag race, stop at a stop sign, whatever. Z-Man mentioned it for cold starts in first to second gear, which I've read other people doing with success. Doesn't seem to help me much.
Matching revs is independant of double clutching. You should always match revs. That's pretty much what shifting is all about. I don't ever know what RPMs I want to be in or am at in a given quick moment, I just go by feel...I aim high, because I'm usually more accurate that way (seems smoother).
You should never downshift to slow down. Brake or coast until your at your desired speed/stop, then reevaluate what gear you want to be in and shift into it.
Heel-toe is neat. The way I do it is by sticking my toes out to the gas pedal (I tried it today). It works well...until I have to release the brake. It just doesn't seem to work well. I'm no good.
When you want to pass someone on the highway, you engage the clutch, blip the throttle so your revs match the gear lower with the speed of your car (ie, significantly more RPM), then engage the clutch and floor it. Becomes kinda one motion. Downshifting from 5th to 3rd at, say, 75mph without revving your engine higher than it was when you were in 5th is...well, really bad. No offense.
I once went from 5th to 1st in an old GTI (just like Britney in the Ferrari). No amount of rev-matching, heel-toe, or triple-clutching will make that one smooth. No damage. I love those things.
When I say "blip" I mean gas it until it's at the revs you want, then engage the clutch. When you get used to it, it'll be quick...blip like. I've heard of people going blip blip blip while they brake and then blip when they engage the clutch, but this makes no sense to me (unless your car stalls at idle). You should only have to blip once.
Can anyone comment/improve on my habits?
Double-clutching is kinda useless if your car has non-malfunctioning synchros. Don't bother with it when you upshift, downshift, heel-toe, drag race, stop at a stop sign, whatever. Z-Man mentioned it for cold starts in first to second gear, which I've read other people doing with success. Doesn't seem to help me much.
Matching revs is independant of double clutching. You should always match revs. That's pretty much what shifting is all about. I don't ever know what RPMs I want to be in or am at in a given quick moment, I just go by feel...I aim high, because I'm usually more accurate that way (seems smoother).
You should never downshift to slow down. Brake or coast until your at your desired speed/stop, then reevaluate what gear you want to be in and shift into it.
Heel-toe is neat. The way I do it is by sticking my toes out to the gas pedal (I tried it today). It works well...until I have to release the brake. It just doesn't seem to work well. I'm no good.
When you want to pass someone on the highway, you engage the clutch, blip the throttle so your revs match the gear lower with the speed of your car (ie, significantly more RPM), then engage the clutch and floor it. Becomes kinda one motion. Downshifting from 5th to 3rd at, say, 75mph without revving your engine higher than it was when you were in 5th is...well, really bad. No offense.
I once went from 5th to 1st in an old GTI (just like Britney in the Ferrari). No amount of rev-matching, heel-toe, or triple-clutching will make that one smooth. No damage. I love those things.When I say "blip" I mean gas it until it's at the revs you want, then engage the clutch. When you get used to it, it'll be quick...blip like. I've heard of people going blip blip blip while they brake and then blip when they engage the clutch, but this makes no sense to me (unless your car stalls at idle). You should only have to blip once.
Can anyone comment/improve on my habits?
04-29-2002, 02:29 PM
#40
Race Director
[quote]Originally posted by JGriff:
<strong>...Re-read and burn into your mind the line about "ALL braking and downshifting should be done before you enter the turn". If you do it everyday on the street, you won't have to worry about doing it on the track (where things can get real ugly real fast!!).</strong><hr></blockquote>
My scariest experience on track was following a guy into Big Bend at Lime Rock who decided to hit the brakes at the top of the corner! He was lucky he got away without spining or hitting me!
[quote]<strong>
I recently re-read/browsed through my copy of the book Porsche High Performance Driving (by Vic Elford), and in it, Vic states exactly that... to give it a good poke, and not finesse the pedal.</strong><hr></blockquote>
I've got that book: good resource for high performance driving. I would also recommend: Bob Bondurnat's book, as well as Russ Bently's Speed Secrets.
[quote]<strong>Hey Z-man... maybe I'll see you someday at LimeRock, or Summit Point, or VIR!! We can talk heel-and-toe some more...
Jim</strong><hr></blockquote>
That would be cool! My current DE plans are:
Pocono: July 6-7 (maybe), Sept 21-22
The Glen: Aug 16-18, and Oct. 7,8
Lime Rock: Sept. 13-14
VIR is just too far for me, as is Summit Point: Gotta get a trailer!
Gotta check when CVR is going to be at Lime Rock, maybe I'll sign up for their days... (How much $$ do I have left in my track budget again?!?!)
Hopefully we can get together sometime and like you said, talk and experience heel and toe face to face!
Thanks guys: I've learned much from this post!
-Z.
<strong>...Re-read and burn into your mind the line about "ALL braking and downshifting should be done before you enter the turn". If you do it everyday on the street, you won't have to worry about doing it on the track (where things can get real ugly real fast!!).</strong><hr></blockquote>
My scariest experience on track was following a guy into Big Bend at Lime Rock who decided to hit the brakes at the top of the corner! He was lucky he got away without spining or hitting me!
[quote]<strong>
I recently re-read/browsed through my copy of the book Porsche High Performance Driving (by Vic Elford), and in it, Vic states exactly that... to give it a good poke, and not finesse the pedal.</strong><hr></blockquote>
I've got that book: good resource for high performance driving. I would also recommend: Bob Bondurnat's book, as well as Russ Bently's Speed Secrets.
[quote]<strong>Hey Z-man... maybe I'll see you someday at LimeRock, or Summit Point, or VIR!! We can talk heel-and-toe some more...
Jim</strong><hr></blockquote>
That would be cool! My current DE plans are:
Pocono: July 6-7 (maybe), Sept 21-22
The Glen: Aug 16-18, and Oct. 7,8
Lime Rock: Sept. 13-14
VIR is just too far for me, as is Summit Point: Gotta get a trailer!
Gotta check when CVR is going to be at Lime Rock, maybe I'll sign up for their days... (How much $$ do I have left in my track budget again?!?!)
Hopefully we can get together sometime and like you said, talk and experience heel and toe face to face!
Thanks guys: I've learned much from this post!
-Z.
04-29-2002, 05:29 PM
#41
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a few people have stated so far in this thread that one should never downshift to decelerate, but no one has backed this up with an actual reason... anyone care to take a stab at that?
i regularly downshift to decrease my speed (in street driving only) because my brake pads are getting kinda thin and i'd rather use gas than that last couple of millimeters of brake pad. I can't think of any reason why this is bad for anything other than fuel milage. as long as you rev match (which I do) and then let off the throttle completely, this seems like a pretty harmless way to slow down. this is especially useful on long downhills where you would have to stand on the brake for a considerable time just to maintain a constant speed.
i have heard of this referred to as "engine braking" (no, not breaking!)... probably because the engine absorbs more energy during the compression, intake and exhaust strokes than it generates during the power stroke. i think some big rigs even have a second throttle on the exhaust side so they can make the engine even more inefficient and absorb that much more energy without sacrificing brake pads. this is what is referred to as "jake brakes", right?
i regularly downshift to decrease my speed (in street driving only) because my brake pads are getting kinda thin and i'd rather use gas than that last couple of millimeters of brake pad. I can't think of any reason why this is bad for anything other than fuel milage. as long as you rev match (which I do) and then let off the throttle completely, this seems like a pretty harmless way to slow down. this is especially useful on long downhills where you would have to stand on the brake for a considerable time just to maintain a constant speed.
i have heard of this referred to as "engine braking" (no, not breaking!)... probably because the engine absorbs more energy during the compression, intake and exhaust strokes than it generates during the power stroke. i think some big rigs even have a second throttle on the exhaust side so they can make the engine even more inefficient and absorb that much more energy without sacrificing brake pads. this is what is referred to as "jake brakes", right?
04-29-2002, 05:45 PM
#42
Race Director
[quote]Originally posted by bs:
<strong>...i regularly downshift to decrease my speed (in street driving only) because my brake pads are getting kinda thin and i'd rather use gas than that last couple of millimeters of brake pad. I can't think of any reason why this is bad for anything other than fuel milage. as long as you rev match (which I do) and then let off the throttle completely, this seems like a pretty harmless way to slow down. this is especially useful on long downhills where you would have to stand on the brake for a considerable time just to maintain a constant speed...
</strong><hr></blockquote>
Well, using the engine to slow down is NOT a harmless way to slow down. Even if you match rev's, you're still putting extra wear and tear on your car: namely the clutch and tranny. So, by downshifting, you're saving maybe $100 (cost of new pads), but you're also wearing something that costs $1000-2000 to repair. (New pads vs. new tranny/clutch) You do the math: it does not compute. Bottom line: Why put unnecessary wear & tear on expensive components of your car, when your CHEAP brake pads can do the work for far less money?
So why do people downshift? Back in the old days, people used the engine to slow down (ie engine braking). Back then, the brake technology wasn't up to the task of stopping an automobile well. (Remember: they had drum brakes at all four corners, and even the early disk brakes were inferior by today's standards!) So they used the engine to assist in the stopping power. Nowadays, that is no longer necessary: our brakes are far superior and have adequate stopping power that is necessary to haul our cars down from high speeds. (Side note: There's nothing like getting up to 115 mph on the mainstraight at Lime Rock and going deep, braking at the 2-3 mark and being able to slow the car down for Big Bend! What fun!)
So like double de-clutching, downshifting to slow a car down is a technique that was needed in the past, but is really senseless today.
That said, it is ok to grab a lower gear for long downhills: just do it before you're going too fast.
Brakes are for braking. Engines are for going!
-Zoltan.
<strong>...i regularly downshift to decrease my speed (in street driving only) because my brake pads are getting kinda thin and i'd rather use gas than that last couple of millimeters of brake pad. I can't think of any reason why this is bad for anything other than fuel milage. as long as you rev match (which I do) and then let off the throttle completely, this seems like a pretty harmless way to slow down. this is especially useful on long downhills where you would have to stand on the brake for a considerable time just to maintain a constant speed...
</strong><hr></blockquote>
Well, using the engine to slow down is NOT a harmless way to slow down. Even if you match rev's, you're still putting extra wear and tear on your car: namely the clutch and tranny. So, by downshifting, you're saving maybe $100 (cost of new pads), but you're also wearing something that costs $1000-2000 to repair. (New pads vs. new tranny/clutch) You do the math: it does not compute. Bottom line: Why put unnecessary wear & tear on expensive components of your car, when your CHEAP brake pads can do the work for far less money?
So why do people downshift? Back in the old days, people used the engine to slow down (ie engine braking). Back then, the brake technology wasn't up to the task of stopping an automobile well. (Remember: they had drum brakes at all four corners, and even the early disk brakes were inferior by today's standards!) So they used the engine to assist in the stopping power. Nowadays, that is no longer necessary: our brakes are far superior and have adequate stopping power that is necessary to haul our cars down from high speeds. (Side note: There's nothing like getting up to 115 mph on the mainstraight at Lime Rock and going deep, braking at the 2-3 mark and being able to slow the car down for Big Bend! What fun!)
So like double de-clutching, downshifting to slow a car down is a technique that was needed in the past, but is really senseless today.
That said, it is ok to grab a lower gear for long downhills: just do it before you're going too fast.
Brakes are for braking. Engines are for going!
-Zoltan.
04-29-2002, 07:48 PM
#43
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so are you saying that running my engine at 4500 rpm with the throttle closed to slow down is harder on it than running it at 4500 rpm at WOT to speed up? or just that it's better to let the engine idle and wear out the brakes because revving an engine is bad for it?
04-29-2002, 08:18 PM
#44
No, I believe the point is this...
While big diesel truck are frequently designed to absorb power via engine braking (you take every inch you can with 80,000 pounds of weight behind you), your Porsche engine is designed to apply power, not to absorb it.
By engine braking, you effectively reverse a great deal of the torque loads back into your engine. This causes loads and stresses that the engine is not designed to accept.
Think about this...
Now instead of the spark firing the fuel, and the resultant forces shoving the piston away to make the engine rotate...the piston is now shoving the exploding fuel back into a smaller space. It seems to me that engine braking would/could significantly increase compression levels during fuel burnng, possible damaging engine components or seals, maybe even breaking the wave front of the fuel burn and causing detonation?
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
While big diesel truck are frequently designed to absorb power via engine braking (you take every inch you can with 80,000 pounds of weight behind you), your Porsche engine is designed to apply power, not to absorb it.
By engine braking, you effectively reverse a great deal of the torque loads back into your engine. This causes loads and stresses that the engine is not designed to accept.
Think about this...
Now instead of the spark firing the fuel, and the resultant forces shoving the piston away to make the engine rotate...the piston is now shoving the exploding fuel back into a smaller space. It seems to me that engine braking would/could significantly increase compression levels during fuel burnng, possible damaging engine components or seals, maybe even breaking the wave front of the fuel burn and causing detonation?
<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />
04-29-2002, 08:41 PM
#45
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damn... that sounds kind of convincing... and i was just starting to get good at real smooth rev matching double clutching downshifts... oh well, i guess it's time to dig into the archives about what kind of brake pads i should get. and i think that when the rotors have a big "MIN THICKNESS 1.024" stamped on them and they are 0.905 thick that i might need more than pads.