# Too Much Compression



## AGETECH (Nov 4, 2009)

Okay- here is what I have, 1972 400 block, 1969 #69 heads, an old HO Specialties cam with .500 lift, (rest of cam specs unknown), Rhodes lifters.
The car runs good, and everybody likes the way it sounds at idle, but I have detonation problems on hard acceleration. Do I need to replace heads? Thicker head gasket? Replace cam? Dished pistons? Can't find gas around here higher than 90 octane. I am running about 30 degrees total timing. Open to any suggestions.


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## 646904GTO (Feb 10, 2008)

Dish pitons will bring it down to about 9.2-9.4 to 1. I did it with my '69. Runs grat no ping stock iron head. I also restricted my advance so I have 34 total degrees. 30 is not really enough for the Pontiac. You just beat up the oil.


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## Rukee (Feb 8, 2007)

Contact any local race track/drag strip/ circle track and ask were they sell the fuel. There are usually 2 or 3 gas stations within 20 miles of the track the drivers can buy the fuel off the tracks site. Or keep reducing the timing till the pinging stops.


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## geeteeohguy (Feb 2, 2008)

Dished pistons are the best solution I've seen. I have a couple of friends with dished pistons and the stock, high compression iron heads (the GOOD heads!), and their cars run hard on pump gas (89 octane) and don't ping with full advance (36 degrees total). I put bigger- chamber heads on my '67, (87cc) and it runs ok, but if I did it again, I'd put dished pistons in it and bolt the GOOD 670 heads back on it. On my 10.75:1 389 in the '65, I run a big cam, stickshift, and octane boost/racegas. You can get away with higher compression if: you have a stickshift car with a shorter gear (3.36--4.33), if you have a lopey cam with a lot of overlap (your Rhoades lifters will eliminate that benefit), and if you can keep the car really cool. To do it once and actually fix the problem and end up with a great running car, dished pistons are the best way to go in my opinion.


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## AGETECH (Nov 4, 2009)

Thanks for the replys guys- I do have a 4 speed with 3.90 gears (and headers) and I only get the pinging when I nail it. Normal driving is no problem, at least I can't hear any ping noise although it may still be doing it. I have 3.55 gears that I will be installing soon, I hope that does not add to the problem. Any short term solutions till I can get into the engine? Are the #62 heads worth keeping? 
Does anyone here have any experience with the TREMEC 5 speed? I want to be able to drive the car on the highway and still have hearing left.


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## Too Many Projects (Nov 15, 2008)

Have you tried the octane boosters in the gas ?


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## 646904GTO (Feb 10, 2008)

#62 are a good head I have a '68 YS and a "69 YS engines and they both are #62 headed engines. The '69 is the one I put the dishes in. I would do it again, it is the best way to go.


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## geeteeohguy (Feb 2, 2008)

Never mind the Tremec. You'll be surprised at the difference between the 3.90's and the 3.55's. Better still, throw some 3.36 gears in there. Those, and a 27 inch tall tire will get you a 70-75 mph cruise at about 3 grand. I put a set of 3.36 gears in the '65 and I can now actually enjoy the car on long trips and on the highway. It cruises at 75, and it's no longer out of breath at that speed. The taller gears WILL make your pinging problem worse. Since it's happening under full load and not just off idle, it tells me it's an octane problem instead of a hot combustion chamber problem. Retarded timing or higher octane is your only fix untill you lower the compression a smidge. good luck!


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## AGETECH (Nov 4, 2009)

I did find a set of 3.42 gears, I have a 12 bolt posi and can't find any 3.36 gears for that rear. 3.36 is what I really wanted, I think the 3.08 would be too high.

I think I'll try the octane booster for now, I haven't had the car long, and I don't think the wife would like me pulling the engine right now.(But I think I could get her to go for an overdrive trans) Would a 17cc dished piston be about right?

Thanks for all the replys

Posted a picture of the 72 in the pictures forum.


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## geeteeohguy (Feb 2, 2008)

I'm not sure about the exact amount of "dish". 17cc sounds excessive to me. That would give you about an 89cc chamber with the standard 72cc heads. Personally, I would rather run about an 82-84cc chamber. I am running 87cc chambers now, and it runs on 87-89 octane. That tells me it's probably less than 9.2-9.3 to one. You want to run about 9.5 to one if you can. You might try Ross Pistons, or Rock & Roll Engineering for the piston specs.


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## plmberkevin (Aug 16, 2009)

Are you sure it is 30* total how are you checking it? maybe the blancer spun ?Also it may just be real lean carb under sized?


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## 646904GTO (Feb 10, 2008)

plmberkevin said:


> Are you sure it is 30* total how are you checking it? maybe the blancer spun ?Also it may just be real lean carb under sized?


Thats a valid point! the Pontiac damper is prone to moving a little. When I bought my '69 it crank longer than I thought it should and when I tried WOT it puked black smoke. I checked the damper and it was off 8 degrees. The previous owner had the timing set at 4 degrees BTDC when it was supposed to be 9. so it was really running at 4 degrees ATDC. I would verify it is correct.


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## AGETECH (Nov 4, 2009)

I checked the timing by measuring 12 degrees on the pointer, then I put a mark at 12, 24, and 36 degrees on the damper. I did not know that the damper could slip. 
The carb is a stock SUMMIT (Holley wanna be). The carb came with the car, I guess it could be lean, the plugs are clean.


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## GTOJer (Oct 20, 2006)

I would look at the TREMEC 6 speed before I yanked the gears. I am sure there must be a conversion bellhousing. Then you have 2 overdrive gears. That would make your 3.90s 2.61s when in 6th.


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## Rukee (Feb 8, 2007)

What kind of distributer does it have?


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## AGETECH (Nov 4, 2009)

I think the distributer is a MSD Street Fire, I havent pulled it out to be sure. It does have an adjustable vacuum advance. 
I put some Octane Boost in the last tank of gas and it did help. What octane boost does everyone run?


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## 646904GTO (Feb 10, 2008)

My chemist son tells me that gasoline is distilled so changing its properties is not really a successful method to modify it. You can add higher octane fuel to lower and make some improvement. Dropping the compression is really your only answer unless you want to buy $7 a gallon fuel.


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## Rukee (Feb 8, 2007)

AGETECH said:


> I think the distributer is a MSD Street Fire, I havent pulled it out to be sure. It does have an adjustable vacuum advance.
> I put some Octane Boost in the last tank of gas and it did help. What octane boost does everyone run?


My car had a Malory in it when I first got the car and I could not set the timing for crap!! Either I had too much total advance and it pinged like a big dog or it would hardly idle if I backed it off till it stopped pinging. I ditched the Mallory and went back to the original distributor and the difference was night and day. It runs like it's supposed to now. The aftermarket one I had was swinging the timing in the wrong direction making tuning impossible. Might wanna consider swapping it out for an original style distributor.


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