frayed

Just curious on a point. . .I did an archive search and saw dyno pulls hitting in the 230rwhp/220tq range, and SC pulled published a 244 number.

Has anyone been able to duplicate that rwhp number? It sounds high given typical drivetrain loss numbers used on other rwd vehicles.

krinkov

244 is exactly 15% drivetrain losses, thats not disproportionatly low at all, especially with the carbon/composite driveshaft that helps to reduce drivetrain losses, I would actually not expect it to be to much higher than 15%. Even old muscle cars with automatics and heavy weight shafts and rear ends keep it around 20%

frayed

Hmm, maybe you are right. 15% loss is pretty low. Coming from an S2000 and an E36 M3, the numbers used to represent drivetrain loss are a bit higher.

With the S2000, about 17% is seen, as most dyno right around the 200 rwph mark. 17% is also generally used by tuners in the bimmer community, except that some cars stock cars dyno up to 215 rwhp (most blame this on underrating from the factory).

A lightened driveshaft will have little to no effect on peak hp, especially when dynoing in higher gears to minimize the skewing effects of rotational inertia of rotating components (eg, a car dyno'd in lower gears with a ltw fly will actually 'show' a hp gain, but not in higher gears) This effect has more to do with the type of dyno being used (eg, a dynojet inertial-type dyno) rather than a hp gain do to reduced rotational weight. And further, a driveshaft is a pretty low inertial component, as it is of small diameter (varies as a square function of the distribution of weight from the center of the rotating mass).

In any event, it does not appear that folks have been able to duplicate SC's dyno performance.

krinkov

hmm, your right frayed, the inertia effect of the lightened driveshaft wouldnt be detectable at high rpm. Maybe SCC got a ringer from Nissan?

frayed

JDZ,

The SAE correction formula used on the newest Dynojet software is pretty good, but yes, it doesn't seem to work well with extreme temps and/or humidity. On a very humid day, I got some funky hp readings that I did not expect. But, if the correction factors were perfect, it wouldn't matter.

But, I agree. The young cars will likely dyno low, and pick up hp as you clock up around 10k miles. . . due to the motor and drivetrain components breaking in. I believe this is what some S2k guys have seen.

Krinkov,

That's sort of what I was wondering. It's not uncommon for a manufacturer to stack the deck. I know Honda did this with an NSX that was undergoing some testing by the rags out in Cali. I know one of the techs that was involved with the 'rigging' so to speak.

Of course, it could have just been a fully broken-in car. If so, 244 is pretty strong.

Jeff

Michael-Dallas

Regarding gearing, gears simply act as a tq multiplier. So by dynoing at a gear other than the 1:1 gear, you're just shifting (and changing) the tq curve. And that tq curve will certainly affect the hp curve.

However, even if you dyno at the 1:1 gear, the final drive ratio more than likely isn't 1:1. Therefore, dyno charts are not intended to be read as the Bible, instead, they should be looked as benchmark indicators.

Michael.

frayed

That's not wholly accurate. The torque curve generated from a dyno pull does not shift from gear to gear. Rather, the tq curve is normalized so you can compare gear to gear dyno runs.

OTOH, actual rear wheel tq is decreased as you go up through the gears as your mechanical advantage is lowered.

1:1 has conventionally been considered the most accurate gearing b/c of few frictional losses due to the direct engagement in the tranny, and b/c you get a longer duration pull (that is, more data). And sure, 1:1 is not an overall 1:1 gearing from the crankshaft to the rear wheels. There a big differential in the way which gears things down considerably. By dynoing at 1:1, you are simply optimizing the configuration in gearbox to lower frictional losses.

At least, that's how I understand it.

pacco

Off topic question but you guy's sound knowledegable...so here's goes.

I drive up short steap hills daily. What rpm is easiest on the engine when climbing?

frayed

Paco, one of the worst things you can do is lug it at low rpms. Try never to be caught at a low rpm (say 2kish) going up a hill; you want to be at an rpm where the car is still responsive and has lots of available tq for accelerating.

I can't remember the specifics, but I recall reading a section in one of Paul Frere's books about the mechanics of lugging as well as sustained high rpm running. Both can wear out a motor, but for different reasons.

If I can find the book around among mile piles, I'll post a better answer.

Elistan

I'm not an automotive engineer, but I thought the issue with lugging is that you're injecting a lot of fuel and air into the engine, like you would at high-rpm WOT operation, but when when the fixture is ignitet the pistons are moving relatively slowly so the force and heat of the explosion are essentially contained in the cylinder, rather than quickly exiting out the exhaust ports like normal. This increased stress is what contributes to the accelerated wear.

Michael-Dallas

Yes, you are correct and I stand corrected.

Michael.

frayed

Elistan,

You have it right; I have an engineering background but I'm no automotive engineer either nor can I play one on TV.

I looked up the passage I was thinking of. To paraphrase, it states lugging produces high cylinder pressures, where high temperature gases (PV = nRT and all that) move through the valves that open and close compartively slowly. Result is scorched valves.

I don't want to be an alarmist as modern motors are quite robust, but it still stands that lugging should be avoided to keep your valvetrain happy.

Jeff