Where to rev too

Anonymous · Jul 9, 2003

Execute.
This is probably a good question for you.
If I was going to extract maximum power from a 250 and say for example the maximum allowable revs were 4500. If I then bought a cam that brought in its peak hp at 4500 would this be advisable if the next gear change was to cause a drop of 1000 rpm to 3500. abviously peak power would be lost and then only gained and held again for a fraction of a second before changing again and causing another loss.
It may sound like spliting straws but wouldnt it be better that if 4500 were to be the maximum intended revs to have a cam that made its peak power at 4000 or even lower (somewhere in between the 1000 rpm drop when a gear change is made)
All things being equal what would be the better way to go get maximum power matched with the gearing and the rpm. Or have I completely lost it.

StrangeRanger · Jul 9, 2003

Let me jump in with my .02.
You're right that there's no point in revving much above the engine's HP peak, you're just wasting gas and losing performance. As long as the RPM you drop to after the shift is at or above your torque peak, you'll be fine. What you don't want, especially in the upper gears, is to have to climb back onto the fat part of the engine's power curve.

Anonymous · Jul 9, 2003

so if you`r engin makes its peak power at 4,000 then you would shift around 5,100 ?

Anonymous · Jul 9, 2003

If I was going to take a guess at it I would not go more than 500 rpm past to aviod risking too much loss of power. I suspect there a many out there who reving there engines too much and think for all that noise they are "go-en when they are just slow-en".

StrangeRanger · Jul 9, 2003

If the engine makes it max power at 4000, shift at about 4200 or so UNLESS that drops you below your torque peak in the next gear.

xctasy · Jul 9, 2003

Cutting to the chase, over nine months of testing him out, StrangRanger is right to these questions.

I'll post some optimum shift point stuff in a moment to show you why!

The actual rule is, any engine will stand between 5 and 10% over -reving from the point of peak power, unless its rev limited to 7500 rpm or so like a AVESCO or NASCAR racer.

In the contracting industry, my workmates have a saying. "We are not here to f*** spiders". (Meaning were not here to do stupid tasks or talk at length about inconsequential things that someone else with a degree can do).

To be a "spider stuffer" about it, you do need a curve for the likely power or torque figures, gear ratios and other stuff. I'll put in some default values from an EFI XF Falcon 4.1, as a start to the Virtual Reality Racing I've been talking about.

This is the HardCore Session...I'm here to practice taxidermatery on spiders!

Anonymous · Jul 10, 2003

I like the way you get to the point (that bit about the spiders). I think your dead right (and so is the spider).
See ya
Cheers Tim.

xctasy · Jul 10, 2003

Tim, here is why a shift at 4200 rpm is best.

Take an 84 XF Falcon EFI S-pack. First schedule or table is the EFI 2.77:1 diff, and the later wide ratio single rail BW 3.47, 2:1, 1.43:1, and 1:1 ratio gear box. The second is with an XB GT 351 close ratio single rail BW gear box

That gives us shift points to change at. As SR said, as long as its still pulling towards the peak power level from the torque peak side of the curve, then you'll gain in acceleration. If you have wrung its neck in second, and then fallen right on to the peak power level with an upchange to third, the engine will be over its peak power, and won't give its all. Since torque peek to power peak is often 1.25 to 1.4 different in revs, the gear ratios are set at intervals of 1.4:1 or more.

The torque curve is the only thing used, but I've included Hp here as well. I scaled these off the metric DIN 70020 (net flywheel figures, corrected to 760 mm Hg at 20 degrees) sheet Ford supplies for ADR compliance. Care of Wheels, Page 45, November 1984. All were looking at is the potential accelleration due to the twisting force at the out put shaft of the gear box. Later on, we'll look at Potential Acceleration.

The figures are:-
1984 XF EFI (SCALED, NOT exactly THE SAME AS ADVERTISED)

|0750 rpm|210 lb-ft|030 bhp|
|1000 rpm|215 lb-ft|041 bhp|
|1250 rpm|224 lb-ft|053 bhp|
|1500 rpm|228 lb-ft|065 bhp|
|1750 rpm|234 lb-ft|078 bhp|
|2000 rpm|239 lb-ft|091 bhp|
|2250 rpm|244 lb-ft|105 bhp|
|2500 rpm|245 lb-ft|117 bhp|
|2750 rpm|245 lb-ft|128 bhp|
|3000 rpm|246 lb-ft|140 bhp|
|3250 rpm|244 lb-ft|151 bhp|
|3500 rpm|242 lb-ft|161 bhp|
|3750 rpm|229 lb-ft|163 bhp|
|4000 rpm|199 lb-ft|152 bhp|
|4250 rpm|186 lb-ft|150 bhp|
|4500 rpm|169 lb-ft|145 bhp|

TABLE ONE:

Example. There are seven columns. First is engine rpm before gear change, second is engine torque before gear change, third is shaft torque before gear change, fourth is new engine rpm after gear change up to new gear, fifth is the torque at that new rpm, sixth is the shaft torque in this new gear, seventh is the amount of torque lost or gained. The idea is to minimise the loss or gain. Ideal change up rpm is the one that yields a nice fat zero! The best selection is marked with a red S for shift, and the rpm before shifting is made red too. Rocket scientists can interpolate upshift rpm to the nearest rpm.

addo · Jul 12, 2003

OK, then (maybe should have started another thread for this?)...

What of the ignition kill on the clutch? Result is that when you shift, the engine momentarily falters, but you don't have to back off on the accelerator to avoid overrevving. The proponents of this idea (like jmac) say that this keeps the intake flow more consistent and the mix even unlike a closing of the throttle plates and subsequent accelerator pump squirt.

Of course, you have a bypass switch for non-competitive situations.

Adam.

Anonymous · Jul 12, 2003

At that point why not just an appropriately set rev limiter? Shift it like you got a pair and it ain't tappin that limiter for long...

-=Whittey=-

xctasy · Jul 13, 2003

The savage nature of a rev limiter (that's one that doesn't have a soft limiter to short out odd or even cylinders first, and then go for a full ignition shut -off) is going to hurt things only if that happens to occur at the optimum gear shifting point.

If you decide to control ignition limiting by the clutch, then thats just how the Japanese have done it with the Asian Warner 30/40 transmissions on the Lexus and post 1988 Cressida models. A good idea, I think.

David Vizard had some truely weird ideas on controling a turbo engine just on the ignition, sort of like a deisel. He fitted the Pinto 2.0liter engine with a turbocharger , and a fully movable baseplate on the distributor to create massive variations of advance and even retard. Then the control of startline lauching with a C4 transmission was done with a wide open throttle, and the engine speed with the distributor!

Fun, huh?

Anyway, I've been fllowing the cumulative total of all the Wheels magazines I have, and am charting the Potential Acceleration figures form known torque curves...many of which have been tested on the Bennets dyno in Sydney for 1978 to 1981 vehicles. I've also got some SuperCars of the Seventies data which I've worked through. The trends are fairly rigid.

I've come up with similar results to what something like a Quarter Pro would do, and its pretty good stuff. What I've found is acceleration is a fixed function of:-

1. the gross thrust at the wheels, [which is just the potential accelration (torqueing force through each gear, at each revolution of the engine, based on the tire rollout, weight acting on the rear wheels for our RWD cars here)],

2. minus the air,
3. drivetrain,
4. body,
and 5. rolling resistance,

The rest of it is just simple math!

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