Just tore apart my motor to inspect the parts and see what I REALLY had bought last year. It is a 350 4-bolt, bored 60 over with #492 angle plug camel back heads and the pistons are TRW 11.78:1. The head gaskets are .039 thick. Should I install a little thicker head gaskets to drop my compression down so I can run pump gas? I have talked to a few people and they all have a different answer so I thought I would ask you guys. I learn more on this website by accident. Thanks for any responses!!!

It's the dynamic compression ration that you are after. Go here to learn more. http://cochise.uia.net/pkelley2/DynamicCR.html

Basically, I can tell you and not knowing your cam specs, you have way toooooooo much static compression for iron heads. I would keep it around 7.9 DCR for pump gas with the proper quench and timing put in. It's a combination of all these.

just my "un educated" resonse based on what i have also learned here thru accidental osmosis:

ditch the pistons in favor of some true flat tops with 2 valve reliefs, and get the thinnest head gasket you can find. this will lower your compression a couple points and tighten up the quench, which is reportedly good for everything from detonation resistance to power output across the board. what you end up with is an engine that makes as much power on lower octane (cheaper) fuel and is just flat out more fun to drive.

So is installing thicker head gaskets just "cheating"? We are trying to figure out what the cam is. Found a number on it when we pulled the cam plug off but we can't decode it yet...And I forgot to say that the heads are 64cc. if that matters. Thanks again to all!!!

From all ive gathered on this topic, by doubleing your head gaskets and increasing your quench to .080 youde be more prone to detonate than if you left it alone

Originally posted by Novaguy73:
From all ive gathered on this topic, by doubleing your head gaskets and increasing your quench to .080 youde be more prone to detonate than if you left it alone Exactly . . .

Combustion chamber shape and quench height plays a very important role in detonation resistance. With an ideal quench the piston will miss the flat part of the cylinder head by about .035-.040". This really helps prevent pinging.

However, this only works to a point so if you truly have 11.78:1 compression I look at ditching those dome pistons in favor of some flat tops as the others suggested. Or go for aftermarket maybe aluminum heads that flow better and have much larger combustion chambers (like 76cc.

You need to come down a tad more than a full point of compression to run iron heads. I doubt you'll achieve that with head gaskets. I second the advice on pistons and quench. Keep the quench area close to 0.040".

This motor ran when I bought it. The previous owner had it in a drag car for awhile. He used it for a few years. He says it worked "mint". I just wanted to pull the heads off to see what I had actually bought and to inventory my motor. Didn't older LT1's and 302's have 11:1 compression or more and didn't they run O.K. on pump gas? Could I just put it all back together the way it was and throw some octane booster in it and hit the streets? Or would I have to buy 55 gallon drums of race fuel and keep them at home? Thanks again for all replys!!!

LT1s and 302s ran good but you could buy 103 octane leaded gas not 91 unleaded crap.If you plan to run it on the street with 11.78 comp you will be buying race gas in 55 gal. drums.The best octane booster will only give you .5 to 1 point more octane 91 to 92.

One more thing. The motor is balanced so if I install new pistons I will have to have it re-balanced, right? Is there a way to mill part of the domes off of each piston the same so I could re-use the pistons and still have a balanced motor? As you may of guessed, I don't build motors monday thru friday!!! Can you tell???

If you change pistons or mill the dome off the motor would need to be rebalanced. Milling the dome would lighten the piston.It depends on the brand of piston some have solid domes others are hollow as to whether you could mill the dome down.You could probably get the milling and balance done for $250 to 300. If you run it at the current comp. ratio and bust a piston you could lose th whole engine.

All things being considerd,maybe it would be easier for you to get some other heads with larger comb.chambers.The trend is Flat tops and smaller chambers for a faster,better burn.

if you can weigh the pistons before you buy some, maybe you will get lucky and find a set of flat tops that are really close. in the end, tho, you're always better off with lighter pistons, anyways, so ditching the domes will also help in that respect. if you have floating pins that don't need a press to take apart, perhaps you could sell the pistons to someone who's gotta have domes for whatever reason and offset part of the cost of the new pistons and re-balance?

I had the same problem. 302 with 12.5 domes. Had to buy racing gas. I finally got tired of it and built a new motor with 9.5 and aluminum heads. Hellow pump gas. Back in early 70,s i had a 69 Z28 and i ran it on 102 pump gas. 91 octane is tops here in calif at the pump now. So lower comp. is the only way to go.

Well, I might just run high octane gas for awhile. I did order thicker copper head gaskets, at the request of my engine builder, for it. I also am swapping the victor jr. for a air-gap. I'll find out tomorrow what my cam is. He is measuring it this week. I think from what I have read that buying larger cc. heads would be about the easiest fix. Thanks to all!!!

IMHO, DON'T increase the quench dimension by running thicker headgaskets to lower your CR. B/C it's been proven that you may detonate worse!!

Try to stay between .038" and .045" to make quench work good for you..

pdq67

Okay! Okay! You guys won!!! Any part numbers on a good set of pistons for me? I'll probably keep the camel back heads on it for now. It will be a intake, cam, and piston swap.Thanks Again!!!

At what point does one see the law of diminishing returns come into play with respect to quench? How tight can you go? Or is it a physical limitation set forth by the clearance between the piston and the head.

Come Again!!!

Originally posted by psalm69:
At what point does one see the law of diminishing returns come into play with respect to quench? How tight can you go? Or is it a physical limitation set forth by the clearance between the piston and the head. The law of diminishing returns for quench height takes effect when your piston splatters after hitting the head :D

For an engine with tight fitting forged pistons, tight bearings, and steel rods about .035" is generally considered a safe minimum. There are some motors that have run well for a long time a little tighter than that, but its not worth the risk IMO.

I run .034-.036 and when I took it apart this fall I could see where carbon build up on the piston had just barely kissed the head.

I Hope your all not tired of my dumb questions about this compression problem yet, BUT, I have one more... Can I leave my pistons alone and change my cam to get my motor to run on pump gas? Does a different cam change the compression ratio?I thought I read somewhere once that changing to a bigger cam does change compression somehow. I think I read too many articles!!! Thanks once again!!!

Cam selection can be used to manipulate the dynamic compression ratio by adjusting the closing of the intake valve relative to piston position. IOW yes. In short, getting a stick with more duration closes the intake valve later thereby not building as much pressure on the compression stroke allowing you to stay with the set of pistons you have. But the everyday drivability of the big cam prolly won't be worth the effort.

I would normally agree with psalm69 but in this case we are talking about a static compression ratio of 11.78:1 and old style iron combustion chambers.

You would need a HUGE cam to bleed off that much cylinder pressure which would make the car completely undriveable on the street.

IOW, no.

Eric68, What compression do you have in your motor? Do you have any other specs from your car? That is a pretty impressive 1/4 mile time.

Thanks, my compression ratio is 11.3:1. With a tight quench, modern combustion chambers, and a pretty healthy cam I am at about the max for pump premium gas.

Here's a brief rundown of my combo:

Ported TFS 23* aluminum heads
SRP flat tops
Eagle 3.75" cast crank
Eagle SIR 5.7" rods
Comp Magnum 294s cam with 1.6:1 roller rockers
Victor Jr intake with 1" spacer
Speed Demon 750 carb
Hedman 1-3/4" headers
Flowmaster 2-1/2" pipes, Dynomax mufflers
3000-3500 Coan stall, home built TH350 trans
Eaton posi, 3.55 rear gears

Yeah I was just trying to explain the principle of DCR is all. Drivability with that combination at that altitude would be rough for sure. But as you go higher up the more static compression and, therefore, DCR can go up as a result. Up here at around ~5000 ft an increase of approximately 1 full point of static compression with iron heads can be used, which translates into another approximate 1 full point of DCR. Not that any of this has any relevance to KCS' original question, but I just thought I would share. Who wants a hug :D

Hey, good point about altitude . . . you are right on. Thinner air means less cylinder pressure and you can get away with a bit more when up high. ;)

kcs just put your motor together get a 55 gal drum of race gas and mix it with premium pump gas! At least have some fun with the motor the way it is and when it starts to get tired then go through the motor and build it the way you want it.

Start off by mixing half and half, and decrease it until the motor doesn't like it! Once you've found what the motor likes your on your way! The only downer to this is carrying race gas with you on long trips! The up side though is that awesome race gas smell that makes the crowds at cruise-ins go into tears. LOL

Mike

Mikey007, You have a great point. I've been thinking about what you said. My buddy races cars and has drums of race gas in his shop. He's all for letting me use his shop as a "gas station"!!!

Use a case of this kemco (http://www.kemcooil.com/product_info.php?pId=61) . It would get you through a lot of gas without having a drum of racing fuel around. This stuff seems to be the real deal for true octane boost. Of course it is for off road only. ;)

phel69, Sold!!! That is the answer. I'm going to have to order a case of that. Thanks for the link to their site. Has anybody used their product and if so, is it a good product? Their website info sounds like it is the answer for allot of people needing a boost!

[ 02-23-2004, 11:35 AM: Message edited by: kcs ]