Does gear ratio affect RWHP #'s???
Originally Posted by atljar
The dyno will figure a final drive ratio, and compute torque from that. How it does it????
Originally Posted by DynoDR
The dyno knows the roll rpm, roll force and engine rpm. No magic, just a few inputs and a little simple math.
Ahhh ok so by knowing the roll rpm and the engine rpm it determines the ratio right there then between the 2
Originally Posted by atljar
What about stuff that doesnt require a engine RPM input?
Example: Dyno target RPM set to 3000. You bring vehicle up to 3000 rpm in the testing gear and select "CALIBRATE", "OKAY" or whatever and the dyno know that vehicle engine rpm is 3000 at whatever roll speed it had at that point.
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An "inertia style" DynoJet (or any brand that is an inertia style dyno), will read lower power figures to the tires with the addition of steeper rear end gears (higher numerically). A move from 342's to 3.90's hurt my dyno figures by roughly 8 RWHP (I say roughly because I didn't back to back the two on the same day but it was the same chassis dyno). I have seen guys on this board report in the neighborhood of a 10 RWHP loss with the steeper 410's which makes sense considering the eight or so loss I witnessed with the 3.90's.
There has been a few threads on this topic if you want to spend some time with the search feature that should back the figures mentioned above.
Hope this helps...
Tony M.
There has been a few threads on this topic if you want to spend some time with the search feature that should back the figures mentioned above.
Hope this helps...
Tony M.
Last edited by Tony Mamo @ AFR; Feb 28, 2006 at 02:44 PM.
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From: Omaha, NE
Yes, especially on an inertia dyno, you will see a HP/TQ drop with steeper gears mainly due to the engine/drivetrain's acceleration rate increase, it revs quicker. This increases the parisitic inertia of these componants which will subtract from the dyno's readings. Now if you perform the same comparison on a load dyno that holds both tests to an engine acceleration rate of 300 RPM/second, which keeps the engine/drivetrain's inertia constant, you will not see much of a difference.
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From: Omaha, NE
I do not have the time and space here to explain this fully and correctly but it has to do with the TQ multiplication to the driving wheel's axle. Even though you lost engine HP due to its increased accelleration rate, at any given MPH in the usable MPH range of the new gears (we lost some) we overcame this because we can more quickly put the engine at it's peak HP RPM and keep it there easier and longer. It's important to understand that the axle's peak TQ occurs at the engine's peak HP RPM due to transmission and rearend gear TQ multiplication. It helps to think of these physics in relation to the vehicle's MPH. Remember we "robbed Peter to pay Paul" by stacking the useable MPH in a more narrow range. Clear as mud, right?
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P.S. If this were a perfect world all chassis dynos would measure "force" instead of HP/TQ. Force, measured in Lbs, is the actual thrust that the drive tire's patch is applying to the dyno's rollers. As we make engine and/or drivetrain changes we would monitor these numbers at given MPH increments and then we would have a real world indication as to how our combos are actually performing. We often make the day for the guys that have new high stall speed converters by overlaying thier before/after "force" numbers after they see their HP/TQ numbers go south after their install. As an example, a stock '97' manual transmission Vette put down 1218 Lbs of force at 75 MPH in 3rd gear. If we keep going to a higher numeric rear gear, we would get more and more force (get quicker) at that speed until the engine's HP fell below the point where the increased gear's TQ multipication to the axle decreased. This force figure is what it's is all about in the performance world (18 wheelers too).
Originally Posted by dynocar
Yes, that's one of many reasons that we went with a load control/measuring chassis dyno.
Dan
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IMHO, the best way to tune an engine is by simulating as close as possible the actual load and accelleration sweep rates that an engine will actually utilize. The highest transmission gear that will be used to race should be selected and then tuned using it. As an example, if a manual transmission car is tuned for a qtr mile race and it clears the traps in third gear and that is the gear where the engine spends most of its time, that is the gear it should be tuned in. This is assuming that you can simulate that gear's actual load with a load control type of dyno. As an example, tuning with not enough load that would allow the engine's accelleration sweep rate to increase quicker then normal would cause the ignition timing to be overly advanced. To much load, such as no accelleration, would cause a very conservative timing advance. Different loads (engine accelleration sweep rates) also cause different fuel requirements. This is why some serious drag racers retard their timing at higher MPHs.
I agree and this is what I liked about the dynopack (for tuning). You can tune for any load in any gear for any RPM. When doing this, you can also determine the amount of timing it likes as well. It's good to tune for all loads just in case, whether you actually see it or not, your covered. Fact is, very few people tune like this, mostly because you can't do it on a plane jane dynojet, plus you really need to know your stuff. I'm definately no expert, just learned a bit of new info hanging around while getting my car tuned
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