Shift points and dyno graph???
09-14-2010, 11:25 AM
#16
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In real life it should 
If your HP figure right before redline is much lower than the one 1000-2000 rpms lower, then it's time to open the engine. All stock engines have a lot more power at redline, and for racing cars, normally you select a safe redline to keep engine from blowing up and then you tune cams to give max torque as closer up to redline as possible, because that gives you more hp, again with the same result.
It's just a rough explanation, as you don't have infinite gears, but you get the point.
In any case, the shift-delay will render all fine considerations useless excepting in real fast, shifter assisted dog-clutch boxes.
In fact, I have engineered a lot of tests, and I'm pretty sure of what I say. Moreover, rising slightly (200-400rpm) the rev limiter improves acceleration in most stock cars. The only knid of cars requiring more complex calculation are restricted supercharged cars with max power looooong before redline, like WRC (5200 top power, 8500 redline, ideal shifting around 6000).
If your HP figure right before redline is much lower than the one 1000-2000 rpms lower, then it's time to open the engine. All stock engines have a lot more power at redline, and for racing cars, normally you select a safe redline to keep engine from blowing up and then you tune cams to give max torque as closer up to redline as possible, because that gives you more hp, again with the same result.
It's just a rough explanation, as you don't have infinite gears, but you get the point.
In any case, the shift-delay will render all fine considerations useless excepting in real fast, shifter assisted dog-clutch boxes.
In fact, I have engineered a lot of tests, and I'm pretty sure of what I say. Moreover, rising slightly (200-400rpm) the rev limiter improves acceleration in most stock cars. The only knid of cars requiring more complex calculation are restricted supercharged cars with max power looooong before redline, like WRC (5200 top power, 8500 redline, ideal shifting around 6000).
09-14-2010, 01:31 PM
#18
Rennlist Member
no, it doesnt have to be symmetrical, it just has to be such that the post shift rpm is equal in power. at that point, the loss of hp due to shifting is such that it would be better to fall further down in HP, going to a higher rpm. (asuuming you have room up to redline)
holding rpm to redline is usually always the best idea for max acceleration. its rarely not, but in the case of some 84s and 79 928s it can be.
holding rpm to redline is usually always the best idea for max acceleration. its rarely not, but in the case of some 84s and 79 928s it can be.
09-14-2010, 01:34 PM
#19
Rennlist Member
why would you do that, when shifting shifting higher gives you more power at both points. Ive already given you the values for Brian. are you talking for a 5 speed, that has less of a drop?
anyway, you see why using HP is so convenient? you can almost eyeball the answer with the knowing the gear shift rpm spread.
mk
anyway, you see why using HP is so convenient? you can almost eyeball the answer with the knowing the gear shift rpm spread.
mk
09-14-2010, 01:37 PM
#20
Rennlist Member
Yep, right on!
However, a stock 928 of the early years can yeild some pretty plummeting HP curves at 5500rpm, but you can look at them and use the gear ratio RPM % drop to see what shift point is optimal. all about maximizing HP .
your point about shifting is a good one! think about it, a shift not only is a pause, it is a NEGATIVE braking force for .1 to .3 seconds! double whammy! thats why folks spend 1000s on dog boxes and sequential gear boxes.
However, a stock 928 of the early years can yeild some pretty plummeting HP curves at 5500rpm, but you can look at them and use the gear ratio RPM % drop to see what shift point is optimal. all about maximizing HP .
your point about shifting is a good one! think about it, a shift not only is a pause, it is a NEGATIVE braking force for .1 to .3 seconds! double whammy! thats why folks spend 1000s on dog boxes and sequential gear boxes.
In real life it should
If your HP figure right before redline is much lower than the one 1000-2000 rpms lower, then it's time to open the engine. All stock engines have a lot more power at redline, and for racing cars, normally you select a safe redline to keep engine from blowing up and then you tune cams to give max torque as closer up to redline as possible, because that gives you more hp, again with the same result.
It's just a rough explanation, as you don't have infinite gears, but you get the point.
In any case, the shift-delay will render all fine considerations useless excepting in real fast, shifter assisted dog-clutch boxes.
In fact, I have engineered a lot of tests, and I'm pretty sure of what I say. Moreover, rising slightly (200-400rpm) the rev limiter improves acceleration in most stock cars. The only knid of cars requiring more complex calculation are restricted supercharged cars with max power looooong before redline, like WRC (5200 top power, 8500 redline, ideal shifting around 6000).
If your HP figure right before redline is much lower than the one 1000-2000 rpms lower, then it's time to open the engine. All stock engines have a lot more power at redline, and for racing cars, normally you select a safe redline to keep engine from blowing up and then you tune cams to give max torque as closer up to redline as possible, because that gives you more hp, again with the same result.
It's just a rough explanation, as you don't have infinite gears, but you get the point.
In any case, the shift-delay will render all fine considerations useless excepting in real fast, shifter assisted dog-clutch boxes.
In fact, I have engineered a lot of tests, and I'm pretty sure of what I say. Moreover, rising slightly (200-400rpm) the rev limiter improves acceleration in most stock cars. The only knid of cars requiring more complex calculation are restricted supercharged cars with max power looooong before redline, like WRC (5200 top power, 8500 redline, ideal shifting around 6000).
09-14-2010, 01:41 PM
#21
Rennlist Member
generally, skewing your shifts to the right is better, just because of the time to shif. But, if you end up with a post shift power that is greater than your top rpm power, then of course, shift. after the shift power at that same speed is going up, where going past it on the high side, the power is going down. very very simple.
Mark --
I think skewing it to the right is wrong.
For the purposes of computing the average acceleration or the terminal speed, I think you have to take into account the time you spend at each point of the dyno graph. In contrast, for picking the optimal shift points, you don't have to make such adjustments and making them will give the wrong result.
There's even an error inside your error! Even if your weighing/skewing logic would be correct (it's not), it would go the other way around. If you shift at the optimal point (pre-shift and post-shift hp are the same), then the engine (not car) accelerates slower at the low, post-shift rpm. The car spends more time per rpm range at high gear, post-shift, low rpm than at low gear, pre-shift, high rpm. But that is irrelevant for shift points.
Best, tuomo
I think skewing it to the right is wrong.
For the purposes of computing the average acceleration or the terminal speed, I think you have to take into account the time you spend at each point of the dyno graph. In contrast, for picking the optimal shift points, you don't have to make such adjustments and making them will give the wrong result.
There's even an error inside your error! Even if your weighing/skewing logic would be correct (it's not), it would go the other way around. If you shift at the optimal point (pre-shift and post-shift hp are the same), then the engine (not car) accelerates slower at the low, post-shift rpm. The car spends more time per rpm range at high gear, post-shift, low rpm than at low gear, pre-shift, high rpm. But that is irrelevant for shift points.
Best, tuomo
