# ‘65 GTO Compression Ratio ?’s



## Roqetman (Aug 31, 2015)

Greeting GTO Fans, Looks like I’m going to have to pull the engine in my ‘65 to repair rear main seal leak. What’s the recommendations for a seal that will not leak? Also, because it has 10.75:1 compression, I have to add 2.5 gallons of 112 octane race gas per fill-up to prevent spark knock. That’s an extra $20 bucks per tank… on top of the 93 octane cost. I have not found an octane booster that is as effective or worth the money. While it is out of the car I’m considering having 9.0 or 9.5:1 compression pistons installed. Has anyone done this? If so, how much did it effect the engine’s performance? And… how much will it devalue the car? Thanks for your advice on this!


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## Banjos (May 10, 2021)

I recommend the Viton Rear Main Seals. You can find them on the Butler Performance Website.


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## Lemans guy (Oct 14, 2014)

I have 9.5 compression runs great on 93 octane pump gas. Plenty of power. No detonation.

If you are only pulling to fix the main seal leak I would first recommend trying the ME Wagner dual flow PCV valve. You can install it yourself, and it stops many main seal leaks and also valve cover leaks, breathers dripping oil, etc.

Now it won’t fix a bad main seal, but many are leaking because of too much crankcase pressure. Go to the ME Wagner website and read up on their valves. It may help you.


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## bootleggerjim (Apr 12, 2019)

My 65 GTO has a 67 GTO 400 with a Ram Air cam and Tri Power....#13 heads and 10.5 cr.....34 total timing and pump high test all day no problems.....my center carb was jetted rich w/64 jets and I just ordered some 55 jets.....it might get messy with the leaner mixture......I can let you know when my parts get here......Pontiac's love compression and if you drive you car like I drive mine, I punish the hell out of mine, you will miss that compression........


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## BearGFR (Aug 25, 2008)

I doubt very seriously that your car actually has 10.75:1 static compression. Yes, that's the factory spec (well, actually the factory spec for the 389 in a GTO was 10.5, it was the 421 that was spec'ed at 10:75 with the same heads) but "what they don't tell you" is those figures are for an engine that has been machined to "factory blueprint" specs, which none of the cars that ever rolled off the assembly line and were sold to the public had. This was a little game that all the manufacturers played back in the day. This allowed them to build engines in production cars that were reliable on the street, but could be "blueprinted" (machined to the actual factory blueprint measurements) by racers and made competitive in the NHRA Stock classes and still be 'within the rules'.

It's very hard to find accurate chamber volume specs for earlier Pontiac heads, plus even from the factory the volumes were known to vary "some", but the 1967 670 heads were a popular performance closed chamber head and the specs I've been able to find for that head claim it had a nominal chamber volume of 70 cc's. So if we "do the math", using the standard 389 bore (4.0625), stroke (3.750), "usual" deck clearance (0.015), factory spec compressed head gasket thickness (0.042), bore size (4.200), valve pocket volume (6 cc) combined with that 70cc figure, that works out to an actual static compression ratio of 10.06:1 --- quite a bit under 10.75. To get to 10.75:1 the chambers would have to be cut down to 64 cc's.

The only way to know for sure what your actual static compression ratio is, is to pull the heads and measure "everything", including cc-ing the heads.

10.06:1 is still "a lot" for today's pump gas but not nearly as risky as trying it with 10.75.

Bear


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