I have eagle 3.875" crank, eagle 6" rods, SRP pistons (SRP-144995) which all should get me around the area of 11.4:1 compression. I was going to use this to go NA but now changed my mind and want to go turbo. They make low compression SRP pistons and all I would need to change are my pistons to (SRP-148988) and rods to 5.85". I can get 9.2 compression on those but I want to go down to 8.5:1. What can I do to get 8.5:1 CR? I would love to keep my 3.875" crank and 6" rods and just get new pistons but are there any companies out there that can make me custom pistons to my application? Is there a way I can say get shorter rods to get lower CR? Sorry if it sounds like I don't know ****, cause that would be the truth

 

Sure, BME, JE, CP, Ross... any of those guys can build a custom piston. Looks like you need about a 28cc dish. Plenty of companies make those... just not any that I know of that'll have the right comp. height with the 3.875 stroke. So... keep searching and you might find one, or get a set made.
You want to keep your quench height, so you don't want to lower the piston in the block any.
How do you know you're at 11.4:1 now? I only ask because it seems a lot of people miscalculate their static compression.

-Mindgame

 

Let's talk about quench height and gasket thickness.

When you're running a low compression piston,
does the quench height change, or is it proper
to dish the piston and/or add 'eyebrows'/'valve
relief' to drop the compression?

What is the risk of running a thick compress
gasket?

I am currently running a very low deck height,
and using a thin compress steel gasket. My pistons are weak and I am able to drive 235 PSI
of static compression with a Dart Sportsman II head with 72 c.c. chambers.

Would LT1RX7 be able to use a thicker gasket to
drop the compression half a point and stick him
down in range?

 

I think you're a little confused about the terminology here. The quench height is the distance between the piston top and the deck of the head. The relation of the piston to head is a matter of compression height in the piston (the distance from the wrist pin centerline to the piston crown) + the rod length + stroke/2. So you have a 5.7 rod a 3.48 stroke and a 1.575 compression height piston. We'll assume the deck is 9.025, so 9.025 - 5.7 - 3.48/2 - 1.575 = .010.... so the piston is .010 down in the bore at TDC. Now there's the gasket thickness (compressed thickness that is), let's say it's .039... so you have a quench height of .049 (.039 + .010).
Now you should be able to answer your own question. The 'low compression' pistons have a reverse dome... the larger that dome volume, the smaller the compression ratio. Trying to lower comp by using a piston further down in the hole is the wrong way to think on this, which brings us to the next question...........

The risk is that you'll lower the quench effect, the combustion mixture will be less turbulent and the engine will have a greater propensity to knock. Tighter is better to the extent that the piston doesn't hit the head. I've read about some engines that actually do... a 'pinched head design' but that's a little beyond my meager knowledge of engines.

Read the above.

-Mindgame

 

Thanks for that clarification. I know the terms, just not sure
of the methods.

In summary, the best method is to build for maximum quench
effect. You want the combustion to occur in the chamber, not
down in the bore.

That's not good news for LT1RX7, it's going to be a pricey rebuild!

Thanks to the formula you posted, I'll be able to get a pretty
decent deck height measurement this coming Spring. The
machine shop decked my block, but didn't take the measurement!

I'll use a depth gauge to the top of my piston from the deck as
a reference and work backwards.