What horsepower and torque differences typically occur in third gear pulls versus fourth gear pulls? Is there a rough rule of thumb?

 

from what i've been told its best to use a gear that is as close to a 1:1 ratio as possible.... i would think that a 4th gear pull is best on an fd because of this

 

Thanks. I do understand 4th gear is the standard. I'm specifically interested in the difference of values between the two gears.

 

I tried it once in 3rd gear.

From 2k rpms to red line took all of 3-4 seconds. Pretty damn quick.

4th gear gives you a more consistant run.

 

Here read this article:

http://www.enginelogics.com/dynod.html

should clear it all up!

 

Again I appreciate the replies and I do understand the reason 3rd gerar is not used on the dyno, but I am specifically interested in results from 3rd gear compared to 4th for the same car.

 

Reading into this, I take it you had some 3rd gear runs done, and you want to predict how it would have looked in 4th gear?

Dave

 

Personally, I prefer 3rd gear pulls. 4th gear puts a LOT of load on the motor, and unless you're really making serious power 3rd gear gives plenty of time for a good dyno plot.

Typically, 4th gear will actually show *higher* readings on a dyno than 3rd.

I don't really see any reason behind going for a 1:1 gear on a dyno. I've always done dyno pulls in 3rd, never had a problem with it .

Dale

 

What about choice of wheel/tire on a dynojet (roller) type dyno?

FDs are faster on the street with lighter wheels/tires.....any gain by dynoing with stockers on? An odd concept I have often thought of.

 

So what is the gain between 3rd and 4th gear pulls? in %?

 

I have yet to dyno, but I am researching some specifics that interest me before I dyno tune. As importants as 3rd gear is on the track I am hoping to study 3rd and 4th gear plots. Maybe there is nothing for me to learn from it, but I would like to determine that myself.

Dale-

Do you have any plots or stats? And thanks for the valves. I have not plug them in yet.

 

I don't know about the wheels but my friend has a 1000hp Supra and on the dyno he fills his tires to like 50 psi. He made like 20 hp more just from filling the tires.

 

^^^ nice

 

Mine was dyno'ed with 3rd gear too..

 

the reson is because your (rwhp) readings will be incorrect in 3rd gear because
(as stated before) its not your 1:1 gear. meaning in 4th gear you'll have the least amount of drivetrain losses because your trans is giving you the most correct egine rpm-rwhp as possible. if your tuning on your car then there is no need for it to be in 4th because all your looking for is to be consistent.

 

I assume you would see a differnce. I was thinking about testing the difference between my stockers and my 18x12 GT-7s

 

4th gear gives the lowest drivetrain losses because there are two less gears being meshed and losing energy. In 4th your tranny is basically one long shaft.

Dave

 

What he's doing only makes since if he's driving around with 50psi tire pressure, then again what's the point of doing such when a street tire will never allow you to put any of that power down at that psi and with that kind of hp. It is true however that more tire pressure will allow you to trap higher speeds.

 

I made 4 passes the other day.. 2 3rds and 2 4ths. all to 7800 rpms. my 3rds were 369 and 373. my 4ths were 367 and 370.. Go figure?...

 

On what?

 

This seems correct to me, especially if it was on a Dynojet.

The dynoject just counts the number of turns the drum makes and since the weight of the drum is known the horsepower can be calculated.....torque can be derived if the rpm pick up is attached

Outside of drivetrain losses the peak horespower readings should be nearly identical, regardless of what gear you are in. An easy way to confirm this is not to have the rpm hook up attached, this will eliminate any operator errors. Just plot the graph in hp vs. speed

 

thats my piont....
~J

 

thats why dynojets dont give you correct hp readings....the correct way to figure
hp is (hp=torque/rpm@5252) thats why you use an rpm pickup.