[FOUROHOH]

I have a 1970 400 HO in my GTO running #13 heads. Block is .030 over and pistons are forged TRW (mid-1980's vintage). Piston reliefs have 6.7 cc and deck clearance is ~.025. With a 0.41 compressed head gasket I calculate a static in the neighborhood of 9.8. Enigine is running the 068 "S" camshaft. I get pre-ignition with 93 octane and need to mix in race gas to run it right. I want to bring the static down so I can run pump gas with this cam/head combo. What is the static I should shoot for and what is the best way to do it....keeping the #13 heads. I can have the pistons dished or I can go with a thicker Cometic gasket. Any suggestions?

Thanks,
FOUROHOH

[PunchT37]

Go with a dish piston. The thick head gasket will kill your quench.

I`d prolly set it to around 9.3 or so.

[getmygoat]

Dynamic compression is what matters - depending on when your intake valve closes in the cycle will make a big impact on compression - 9.3 static is pretty high for a street car unless you have a cam with a very late intake closing, and the 068 is fairly modest in this regard - I'd shoot for more like 9.0:1 or even 8.8. For a street car, you don't want to rely on a perfect tune to avoid detonation, and detonation will ruin your engine very quickly. You don't give up that much power by bringing the compression down modestly. You can make it up with head porting or cam selection.

[geeteeohguy]

I agree with getmygoat. I'm running about 9.3 compression in my '67 400, and it's too high for the 91 pump gas in 100+ degree weather. I have the same pistons you do (1988 rebuild) and the same cam. Dished pistons for a max CR of about 9:1 would be my choice.

[paul s.]

combustion space configuration is EVERYTHING. With open chamber heads there isn't much quench/squish. I agree with the suggestions of utilizing a dished piston and I will add that you should strive for a zero deck, or close to it to try to attain any mixture motion.

PunchT37 is dead on.

[70 bird]

Try water/meth injection before you tear your motor apart.

[chiphead]

There are several shelf forgings available with a 10-14CC dish that would work perfect. Zero deck the shortblock.

The .025 deck height and the 068 cam is what is getting you. If you're running freeway gears, hot sparkplug heat range and a lot of timing, that makes the problem worse.

[Brent Flynn]

Originally Posted by geeteeohguy

I agree with getmygoat. I'm running about 9.3 compression in my '67 400, and it's too high for the 91 pump gas in 100+ degree weather. I have the same pistons you do (1988 rebuild) and the same cam. Dished pistons for a max CR of about 9:1 would be my choice.

I know that .025 down is a problem, but, the cam is a big problem with that set up...Id go with a split pattern cam in the 230/238* range... i bet that would help a bunch ,if you are having detonation probs... Either that, or get a set of #64's and change them out. I ran a Comp .525/305*advertised in a 400, with #48's and dual 625's and never had an issue... TRW flat tops, block wasnt decked... I would run 91-93 in it...
The duration i recommended may be a little extreme, but you get the idea... maybe a 112 LSA... Im sure there will be alot of opinions on this subject...lookin forward to hearing them, as well....Im sure you could get a custom grind lunati(bullet) cam, around the .460 lift range, 285/295Adv duration with 112-113LSA that would allow you to run your deal...especially if you zero decked, and put hardened seats in your heads , and oil consumption was down to a minimum... Its doable...good luck!

[PunchT37]

I`m running 9.7 compress. down here in Louisiana heat. Cast iron heads. TRW piston. 93 octane pump piss. No problems in 10 yrs. or so.
It can be done. Especially at 9 to 9.3 or so. Like stated before, keep a good quench, a good tune, and maybe a cam swap as the 068 was made back in the day for the higher compress options. Course, the 455HO ran that cam at 8.4 or so, as the book says. But, we`re talking a 400 vs. a low compress 455.

[74Grandville]

you could also unshroud the intake valve removing the eyebrow. won't give you a lot but may be worth it.

[FOUROHOH]

Thanks to all for the critique of my set-up and the recommendations. I will report back with results. Lots of work to get done.....

[72LuxuryLeMansLa.]

Originally Posted by FOUROHOH

Thanks to all for the critique of my set-up and the recommendations. I will report back with results. Lots of work to get done.....

Check these guys out. http://www.racetecpistons.com/rt_athome.htm

custom pistons, compression height, dish, all at shelf prices.

Karl

[archerytony]

Originally Posted by FOUROHOH

Thanks to all for the critique of my set-up and the recommendations. I will report back with results. Lots of work to get done.....

If its in your area check out e-85 its 108 to 110 its not hard to convert over bigger fuel line 130 gph pump and a converted carb read up on raceone-85.com

[mechanic17]

I had the same setup in the '80s and eventually punched holes in a few pistons. I agree with others who say change the cam--why get into the bottom end and sideline the car all year?

If you have a 3-series gear and a 4-spd or a bit of a TC, try the 041 cam with or without Rhoads lifters. If not, a wide LSA aftermarket. PT Engine Analyzer will tell you if you've got the right one. Hell, even the Summit 2801 retarded 4 degrees might do it.

Funny how those old TRWs degraded the quench by going down to .025, trying to do a good thing, when the factory short blocks I've measured have all been between .010 and .015".

[HWYSTR455]

I must be an exception to the rule, because I have successfully run a thicker head gasket (Cometic) on that exact config and had positive results. It did in fact lower SCR enough to allow for a good tune/curve and prevent det. And the Pontiac open chamber heads, it's true that there's not much quench area to begin with, so the effect is limited in regards to zero decking. IMO, too much emphisis is applied to zero decking, or at least with Pontiacs.

In that same respect, when it comes to zero decking, many who build boosted engines intentionally go below deck to stablize the pistons under high loads. Some go as much as .040 or more.

Usually, additional duration and LSA will help when SCR is at the border. More LSA has a positive effect on reducing det, though if it is a 'true' 068 S cam, it would be 116 LSA. I seriously doubt it is, many 'claim' it's an 068 just for comparison purposes. As an example (LSA), newer engines in performance apps are in the 11+ SCR range, calculate the comparison to iron heads, drop a point, and you're still over 10:1. Look at the LSAs being run, and you will see they are in the 122-126 range. And for a reason. Late intake closing events also plays a part as previously mentioned.

To address the original poster, how many miles are on the shortblock? Is it something that can use a refresh anyway? If not, I believe it would take a combo of actions to 'correct' the current config. Like head work and cam change specifically, would be one recommendation. You can clean up the chamber some to achieve a lower SCR and change the cam. If you need to go through the shortblock, then you can band aide it with head gaskets and tune, until you get another engine built.

Do you want it to remain original, original appearing, or it doesn't matter?

.