Rebuilding ls1. Going into my 85 iroc with a t56. Im trying to build a high compression motor. No boost. Need advice or point in right direction for piston choice. Using stock crank. Forged rods. Texas speed 227cc/62cc chambers. What style of piston do i use. How can i get compression to 12-1 range?

i was told by local shop the use forged icon pistons. Anybody run those?

 

Cut those 62's down to 56-57cc's , cut the decks so the pistons are out of the bores by .003-5", use .040" head gaskets and you might get there with 3cc flat tops. Otherwise look for light pistons with slight dome, JE makes some. Cam events will have a lot to say about how much 12:1 scr will help you.

 

thanks for the advice. I was looking at some +4cc domed pistons. In terms of cam i was going with 235/240 .629/.615. What exaclty do you mean in regards to the cam will have a lot say how much 12:1 will help me? Should i stay away from high compression? Or need to choose my cam carefully?

 

There are two numbers to consider when talking about compression. Static compression, and dynamic compression. In this thread, you are referring to static compression. Once a camshaft is introduced into the equation, its valve timing events, most notably overlap, will bleed off some cylinder pressure. This effect is generally more pronounced with larger cams, and can be calculated. This figure is known as dynamic compression.

 

Fixed.

 

Try this Calculator to determine what dome you will need.

 

Hi Grainy, I too like an Intake Lobe as you chose.
I too like an Exhaust Lobe of similar duration or just slightly longer, mine would have LOWER lift (.567") due to the smaller exhaust valve size, a "lazy" lobe acceleration rate.
I would like you to STATE your chosen L/C ?
I would use a 116* C/L for your "guessed" requirement.

The amount of Valve Overlap has a GREAT effect on engine operation and if too much for the RPM Range/Throttle Opening will ADD pressure to the cylinder mostly from HOT exhaust gasses.

The report from KCS is correct with a "wider" C/L helpful for the LATER intake valve closing point, keeping cylinder pressure LOWER at low RPM.
This is VERY good for the engine ALLOWING a higher static C.R. for everyday driving.

The report by "64" is ALSO my method.
1. My RacTec FT piston @ 99mm +.004" OS bore with a "deck" of .007" "proud".
2. Pistons to be used with your stated forged rod, a .927" pin, clips, NPR steel rings, at your door for $650.00.
3. I would use a FelPro 1041 O-Ring head gasket a .041" thick.

THUS for your case, no bore, no deck, no head machining costs, the cost of a CORRECT piston set !

Lance

 

after using the calculator with deck of .007, same heads with 62cc chambers, .041 gaskets with 4.135 bore, slight hone to 3.903 bore /3.62 stroke, i got 12.3 with 10cc domed pistons and 12.02 with 8cc domed pistons.

my next question is if i need to choose a different cam, and what pistons and rods to use. I looked at the race tek but it looked like all bore sizes were over 4. Also im running pump gas 91. Is 12.3:1 to high?

 

So... those heads you're looking at. I'd run them at 58cc with a Wiseco -2.8cc reverse dome piston. Most LS pistons come out of the hole around .010" and with a .040" gasket, you're going to be really close with such a tight quench for things to touch (nothing to do with PtV - just really close piston to head). That's 11.8:1 CR and you won't notice a difference between that and 12:1 with those heads and a hydraulic cam. Even a really big one.

Do you want to just build a high compression motor to build it? Or do you have a specific goal? Street-driven? Pump gas? Race fuel? Run 9s all motor? That will help determine the cam. And then once you know the cam, you build the motor with the proper CR for the valve events to maximize power, drivability, etc.

Assuming this car will be a hot street/strip machine, I'd aim for the 11.8:1 with a hydraulic cam or a Cam Motion LLR solid setup. That's about as much as I'd run on any kind of stock cube NA LS1 motor. Going 12.5 or more doesn't net you much NA. It's really diminishing returns. It's not like going from 9:1 to 10:1 or even from 10:1 to 11:1.

You have to get to 13.5:1 or 14:1 or something like that to make it worth it. And if you do that, I'd def go solid roller, a carb-style intake and just rev it to the moon with C16 in there. But then for the price, I'd just ditch the LS1 block, get a 6L block (or 6.2L LS3), PRC 247s, LS3 255s, or small bore LS7s and build a 408/416 as it's easier to build high compression with a bigger motor using off-the-shelf pistons and heads. Price would be pretty close... only difference would be the cost of the block.

 

thanks for the info. Very helpful. So with the cam i stated above. Go with some -2cc flat tops for a high 11ish:1 ratio for pump gas. It wont see race gas its a street car.

 

Ah yes, thanks for the correction. My brain wasn't functioning quite right at that time of night.

 

Yeah with the 235 cam... I would match it with 11.8:1. That'd be a perfect match with the PRC 227s and that compression.