Dynos?

[rdane]

So tell me, if cars weigh the same, and have HP numbers/curves and torque numbers/curves that are the same, at the same RPM, how close will the cars be for drivability everything else being equal....except displacement?

[dd74]

I posted this on the 3.6 or rebuild thread:

Dane, (in your case) I think the 3.4 should be very streetable considering the extra bottom-end torque the additional displacement will yield.

Of course most of that will depend on the cams you use in conjunction with whatever induction you utilize.

With the 3.6, there's no question it's streetable.

As far as comparisons, I'd honestly have to give the edge to the 3.6 in overall useability - again because the 3.6 has larger displacement.

I would guess, though, that the comparisons might increasingly blur as the 3.4 and 3.6 (both with equal to comparable induction/cam setups - and also in equal cars) rise into the higher RPMs.

[speeder]

Originally posted by rdane
So tell me, if cars weigh the same, and have HP numbers/curves and torque numbers/curves that are the same, at the same RPM, how close will the cars be for drivability everything else being equal....except displacement?

That's a lot of "ifs", and you can forget about HP #s if you are talking about streetability, but if the two motors have the same torque @ the same RPMs, drivability would be identical. As would be performance.

[Bill Verburg]

The run of the mill dynos that we mortals have access to are coarse measuring tools. The will give some idea of the full throttle capabilities of a chassis. As long as absolute faith in the absolute accuracy/repeatability/comparibility of the numbers is avoided they can be useful tools for comparing mods on a given car on a given dyno on a given day.

They fall flat on their face when predicting streetability because the normal test is just a full throttle run from roughly 2k to 7k, no transients, no acceleration/cornering/braking G -forces, no on/off throttle etc.

The true trick is to have a powerful engine that accepts all of these transient conditions gracefully. Factories literlly spend millions to achieve such results.

[Wil Ferch]

Bill:
I think the original post talks about same hp and torque CURVES...not identical PEAK VALUES....so although what you say is correct, that inertial dynos' don't measure drivability well because only full throttle is measured...the point of the question is a bit different.
However, the original point doesn't really make any sense in that ALL functional parameters are assumed identical, but one engine is smaller. If so, some other variable is LIKELY to be different too. Either peak hp or peak torque or the shape of the curve...or something. It would be a real neat trick to have peak hp, peak torque, be the same AS WELL AS the shape and location of the torque and power curves...all with two different size engines.
--Wil Ferch

[rdane]

Thanks guys. I know it is all hypothetical. Obviously I am comparing a couple of known engines. If you saw the other thread my comparison engines are a stock, early model, 3.6 from a 964 and a purpose built, street, 3.4.

I lay no stock in what a dyno tells me on how a car drives. But if I have the same curves and peaks on the dyno graph lines I would expect similar rides.

I understand that no similar engine is going to be the same on the curves if the displacement is different. But I thought same/same, hypothetically.

If the two motors have the same torque @ the same RPMs, drivability would be identical. As would be performance

Thanks for the reality check. Just wanted to make sure I had my thinking cap on correctly