1968 Pontiac GTO

[oldskool]

Originally Posted by Stan Weiss

I have never done a 3.90:1 to 4.33:1 gear change. But IMO I would be very surprised if you saw anymore than a .1 of second improvement.

Stan

Assuming max traction, I suppose the ET difference caused by a gear change would strictly depend on the torque & power range of the engine.

If the engine is already well passed it's peak power range BEFORE the finish line, then obviously, if you add more gear, the engine will pass it's peak power range even farther from the finish line & slow the ET.

On the other hand, if you are way under-geared, & you cross the finish line a 1000 rpm below your engines peak power range, then adding more gear will obviously utilize more of the engines potential, & lower ET.

If the car is just barely under-geared, & you changed the ratio enuff to make it slightly over-geared, then the ET change would likely be very small or may even go slower.

So, the amount of ET change would depend on several factors.

Back in the old days, the Chevy street guys who had sbc power would say they had a "three quarter cam & 4.11 gears". It seemed that was the hot "Street/Strip" set up.

Since 4.11 gears seemed to be so popular, I've wondered why Pontiac seemed to skip that ratio & go from 3.90 to 4.33 gears. Had to be some reason they chose 4.33, assuming that many of those who ordered that ratio would be drag racing the car.

I have to assume that they did a lot of testing & had determined that 4.33 was the best ratio for drag racing a 400 Pontiac A-body, with the factory RA cams. Otherwise, they would have offered 4.11 or 4.56. But hey, what do I know ? They may have stopped at 4.33 because that's all they thought the cast rods could stand, crossing the finish line.

In '75, I had a 400 built to 350hp Stocker specs. Changed to 4.56 gears & ran 12.90's right off the bat. Put that engine in my girlfriend's '68 E/SA Bird, with a 330hp Lunati Stocker cam & 4.56 gears, & it turned 12.80's. The Bird was a MUCH more competitive combo. NHRA rated basically the same engine 20hp LESS, in the Bird. And the Bird was 200lbs lighter. It was a perfect fit for E/SA. 3300 lbs/330hp = 10 lbs/hp = E/SA.

The '68 Birds have been a popular & competitive combo right up to the present time. I suppose Julie Biermann drives one of the quickest, today. She runs a little more than 2 secs quicker than we did. The round port '68 Birds have also been very competitive, thru the years.

Anyhow, I never changed JUST the gear ratio ONLY, to see the ET difference it made. So, any numbers I mention would be strictly a guess, or estimate.