Thread: need some piston help

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ok ive got some dome pistons and some almost flat top pistons for my 350. with the domes the motor would have around a 10.5 :1 CR with the others it would be a little over stock i think a 9.2:1. this motor is gonna be in a bracket car. it has ported 193 heads with the stock 1.94 valves and probaly roller rockers. the bottom end will be stock and this is going to be mated up to a 3 speed. ive been told the 10.5:1 CR isnt real streetable which could be an issue for me. if i use the flat tops and buy a good cam accordingly what kind of power can i expect as compared to the domes?

and assuming the domes have a -.5cc dome will i have problems with the valves hitting the pistons?

Even though the domes will hinder flame propogation to a point, they'll allow you to run more cam and thus make more power.

Clean up all sharp edges in the chamber and on the piston crown, match your deck height and gasket thickness to yield a squish of around 0.040" and you'll be able to operate on pump gas.

Here are examples of the type cam I might choose with your motor. I'm not saying run right out and buy either one, I'm just offering you some parameters to look for. The final choice for a cam should always be made by one of the techs at your favorite cam grinder.....
http://www.cranecams.com/?show=brows...tType=camshaft

http://www.cranecams.com/?show=brows...tType=camshaft

You'll want to use a dual-pattern cam with production heads. They need help on the exhaust side. (more lift and duration on the exhaust side than on the intake side).

I wouldn't run the domes, you can go plenty fast for bracket on 9.2:1 compression and it will save you money in the long run on fuel and engine wear.

You can run any were between 9:1 to 12:1 on pump gas and it is streetable all of the ls1 to ls7 that came in the Camaros and GTO's, Corvettes have 11:1from the factory right now I am in the process of build a 12:1 motor and its streetable if it was me I would use the 10.5:1 CR pistons it will increase your power and it will have no problem being on the sreet well besides gas mileage... but its a sacrifice all of us street rodders pay right.

Originally Posted by Speed Freak

You can run any were between 9:1 to 12:1 on pump gas and it is streetable.

I'm gonna start buying my gas where Speed Freak does.

Art

I may be old, but something or other!

hmm can anyone give me a power estimate with the flat tops which worked out to be around a 9.3:1 CR
and thanks guys

im also worried about piston to valve clearance. the domes of the pistons stick out of the block at TDC and my heads have 64cc chambers. at what size will domes interfere with the valves??

Originally Posted by muteboy49

im also worried about piston to valve clearance. the domes of the pistons stick out of the block at TDC and my heads have 64cc chambers. at what size will domes interfere with the valves??

Sorry, there is no standard answer, too many variables. You'll have to measure everything before it is assembled. Now you know why engine builders get the big money.

Speed Freak is talking aluminum heads and EFI together with some close squish and good spark management.

yeah i knew 10.5:1 CR is kinda high to run on pump gas. im thinking of getting the heads milled or finding better ones and running the flat tops strictly because i really dont feel like buying race gas (well all the time anyways)
i dont need a killer motor. just something to run decent numbers and to have something for the kids around here in their probes and civics and all those other little sh*tboxes

Unless you are running a power adder that 10.5:1 is fine on pump gas I would use premium gas but now I can see were you might run in to piston to valve clearnce and I could not tell if you are or not like Tech said to many variables if in doubt always check!

yeah ill have to check it. however i seriously believe they will make contact

Originally Posted by Speed Freak

You can run any were between 9:1 to 12:1 on pump gas and it is streetable all of the ls1 to ls7 that came in the Camaros and GTO's, Corvettes have 11:1from the factory right now I am in the process of build a 12:1 motor and its streetable if it was me I would use the 10.5:1 CR pistons it will increase your power and it will have no problem being on the sreet well besides gas mileage... but its a sacrifice all of us street rodders pay right.

Your Gen II and Gen III motors also have reverse flow cooling systems which keep the combustion chambers nice and cool to prevent detonation. They also have fast burn combustion chambers which reduce the pockets of unburnt fuel and air in the far corners of the combustion chamber that cause detonation. You can't really compair the motors.


And muteboy, with carful parts selection, you can make over 400 hp on 9.3:1 compression. When I say careful, you have to be very careful not to overcam the motor. 9.3:1 is pretty low and won't support a lot of duration. When you buy your cam, try to select the one with the most lift for the duration you select. This will help to maximize cylinder pressure which is what you need to do to make power on a low compression motor. Your static compression ratio may be low, but careful selection can bring your dynamic compression ratio into the same arena as the higher compression motor.

thanks 76gmc1500.i really appreciate your help

ok i just mocked up the engine to see what the pistons look like at TDC. the flat tops sit a little bit in the block still and the domes poke out a little. so idk still.
and what do you guys think of ordering a custom grind cam? worth it? lunati customs are only like 180