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.

[DeuceCoupe]

Originally Posted by oldskool

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.

Agree on the gear comments - that's what I see in the Gonkulator. A "rule" often used is "gear it so the engine hits peak HP thru the traps" - but even then, less gear might be better if you're just frying your street tires.

Maybe 4.33 was a compromise - barely driveable on the freeway if you limp along at 55-60mph (we used to run our Model A Ford on the freeway at 50-55mph, MAN that got hectic even back then!)
But also a decent gear for a big block with drag tires.

On the factoring: That's consistent with what I have down that Paul C once said here:
RamAir1 GTO: 360hp
RamAir1 Firebird: factored to 340hp

I often wondered why they weren't BOTH factored to 360hp - with headers and a carb fully opening, arent the GTO and Firebird identical engines then?

[oldskool]

"...arent the GTO and Firebird identical engines then?"

Yes. There are several '68 & '69 400 D-port engines that are basically the same, whether in a Bird or a Goat.

The cams may be SLIGHTLY different. But with the unlimited duration & valve spring pressures, those slight cam differences won't really make a difference.

Here's the '68 330hp 400ci Bird engine specs. Current NHRA factor is 333hp. Came down from 338, the season before.

http://www.classracerinfo.com/Engine...9&MAKE=Pontiac

1968 350hp 400ci GTO engine specs. NHRA = 325hp. This was set in 2001. Don't know when it came down.

http://www.classracerinfo.com/Engine...5&MAKE=Pontiac

'69 330hp 400ci Bird. NHRA = 345. Was raised to this in '14. The specs look just like the '68 engines, to me. So, why is the '69 NHRA factor so high ? Guessing it's because the one quick '69 Bird Stocker was running so quick. That car belongs to long time Pontiac racer John Schloe, in div 7.

http://www.classracerinfo.com/Engine...9&MAKE=Pontiac

'69 350hp 400 GTO. NHRA = 325hp. The last change is dated 1/1/27. So I don't know when the change was made, nor when it was 1st reduced from the 350hp rating it had back in '75. One thing is for sure. At the current NHRA factors, a '69 GTO with the base 400 engine should be more competitive than a '69 Bird with the base 400 engine.

http://www.classracerinfo.com/Engine...4&MAKE=Pontiac

'70 330hp 400 Bird. NHRA = 350hp. Specs look real similar to the '68 & '69 engines. Don't look to have been any hp factor changes in recent years.

http://www.classracerinfo.com/Engine...5&MAKE=Pontiac

'70 350hp 400 GTO. NHRA = 350hp. Can't see any reason for the '70 engines to be rated higher than the '68 or '69 engines.

http://www.classracerinfo.com/Engine...8&MAKE=Pontiac

I CAN see that maybe some allowance should be made for a body which is more aerodynamic. For example: A 1st gen Bird body should offer less wind resistance than a '68-'70 GTO body. But, using the '69's for an example, I don't think the Bird should get a 20hp penalty. Maybe 5hp, 10 max. That would put the '69 Bird more in line with the '68 Bird.

Under NHRA rules, cars are penalized for going too quick. Lots of guys would probably go slower, if they had it all to do over again. Many have made their combo much less competitive, because of going too quick. Sandbagging has become a major aspect of Stock/SS racing. It's just part of the game, if you wanna protect your combo, in order to avoid a hit.

As it stands now, looks like the '68 & '69 Goats, with the base D-port 400, would be just as competitive as a 330hp '68 Bird. But, I don't know of a single one still active, unless someone is racing the former Adam Strang '68 Goat. I assume Tim Gillespie still has his '68, but don't race it anymore.

[Cstockbird]

Not sure if it’s the same for 68 RAII but it was for 69 RAIV that the power to weight ratio dictated the HP be less for firebird which was taken care of mainly with the different throttle linkage bracket.

This thread needs pictures something awful.

[Stan Weiss]

Originally Posted by Cstockbird

Not sure if it’s the same for 68 RAII but it was for 69 RAIV that the power to weight ratio dictated the HP be less for firebird which was taken care of mainly with the different throttle linkage bracket.

This thread needs pictures something awful.

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Actually it needs more tech.


Stan

[oldskool]

Originally Posted by Cstockbird

Not sure if it’s the same for 68 RAII but it was for 69 RAIV that the power to weight ratio dictated the HP be less for firebird which was taken care of mainly with the different throttle linkage bracket.

This thread needs pictures something awful.

"... with the different throttle linkage bracket..."

Yeah, those Q-jets were rigged up so they would not open all the way. But, that was easily fixed by hot rodders & racers.

On a Pontiac forum, I've been scolded for not posting pics to go along with what I was saying. I suppose that's one reason why I have collected so many Pontiac pics on my computer.

I think MOST guys like pics. At least I usually see several guys thanking the guys who post pics from races.

I don't care about MOST of the pics. But, I always like to see pics of the few Pontiac powered cars that are still active.

I assume that Adam Strang & Tim Gillespie have driven the quickest '68 GTO Stockers, ever. Tim Made the field at Indy a few times. He was #83 in 2006, @ .981 under. Adam was #105 on that list, but had already switched over to his '68 Bird. Tim also later switched over to a '68 Bird. Julie Biermann has been driving it for the last few years. It now runs high 10's.

https://www.dragracecentral.com/DRCS...r2006#indextop

Don't know the best time for either of the '68 Goats. But here's a 2004 race where Adam ran more than a sec under. He ran it mostly as a stick car. Won class at some races.

https://www.dragracecentral.com/DRCS...r2004#indextop