Going for 3 dyno pulls next weekend. What gear should I be in?

Strong set-up Chevy 350. Supposed to max hp at 6800rpm and >350rwhp.

I was thinking 3rd because 4th would be a long pull (time wise). Any suggestions?

Thank you

 

Dyno pulls are always done in high gear or whatever is closest to 1:1 if you have a tranny that doesn't actually have 1:1

Any other gear ratio throws off the dyno results because the dyno is measuring torque and the other gears change the torque multiplication.

 

Doesn't really matter; the dyno is measuring both engine RPM and wheel RPM, and makes the necessary calculations. That's why it's accurate no matter what your rear gear is, as well. Think about it....

The way they work is, the operator sets the rate at which the engine is allowed to accelerate, usually around 200 - 250 RPM / second for a street car, maybe 350 RPM / second for a circle-track race car, and sometimes as much as 600 RPM / second for a strip car. The dyno applies enough resistance to limit the engine's acceleration to this value, and it calculates the wheel torque and HP on this basis, then multiplies that by the ratio of engine RPM to wheel RPM. Which is also why a pull from 3000 to 5500 on a 140 RWHP LG4, lasts exactly the same length of time as on a 550 RWHP big block over the same RPM range. It's ..... 250 RPM / second, or whatever.

Note also, that this is why an aluminum drive shaft and light weight wheels, will make a typical chassis dyno read higher than a steel drive shaft and heavy wheels. The energy required to accelerate the wheels is less with the lighter stuff, therefore the resistance the dyno has to apply to limit the engine's RPM/sec rate to 250 or whatever, will be higher. The engine's HP isn't actually higher, but the acceleration on the dyno will replicate the car's behavior at the strip, so that's not entirely a bad thing. Just something to be aware of in interpreting dyno #s and trying to compare apples to oranges.

Dyno rollers have an RPM limit, same as anything else. They'll want to do your pulls in a gear such that the "road" speed is less than about 75 or 80, tops. As Alky said a 1:1 gear is preferable, just because there's no extra losses; but most of the time, especially in a car with street rear gears like 3.73 or lower #, they'll have to do it in 2nd for an auto or 3rd for a stick.

Prepare to be humbled. Most people's expectations get a rude shock at the dyno; much like, first time at the strip. If your engine is "supposed to" do 350 RWHP @ 6800 RPM, don't be too surprised when you see like 285 RWHP @ 6200. Won't be the first, nor the last time, that's happened. An HONEST 350 RWHP (not, people are paying me money to run on my dyno and want to see good numbers, so to make them happy I set some of the correction factors a little bit "extra friendly", like setting it at 4000' altitude when I'm really only at 250') is ALOT LOT of power for a 350.

 

The OEM 700-R4 will not go 100% throttle without downshifting into 3rd gear. I've always done 3rd gear pulls.

 

Thanks guys that really helps.

I'll probably go for second or third. On the street I'd probably reach about 80 in second gear at top rpm, maybe more. Ive never found a road long and straight enough to bury the rpm needle in even 3rd gear.

Im definitely prepared for that humbling experience haha. But I do expect higher than 300. I saw the same set-up have a dyno reading at 420+ at the crank. Obviously it will go substantially lower than that for the wheels but the car feels great. Regardless, it should be fun and nice to know. Thanks

 

I went 4th gear (out of five) made 348 at the wheels which I was told should be about 420 at the crank

 

id say third gear just cause it seems optimal and my car seems to pull harder if i keep it in third..

 

Good to know guys, thanks.

I'll probably go 3rd, 3rd, and then make a decision on the last pull. I don't even know what the resistance will feel like from the dyno. Ive got a really good feeling on the way the engine hits and revs, and its mostly top end power.

Thanks again and I'll try to remember to post the results and what my feelings were with the gear selection.

 

The dyno operator will probably tell you what gear to be in anyway.