2270 Deck Height and CR, Pistons sitting proud of the deck, how many shims is too many shims? Options

[vjb206]

Hey folks! I have the bottom-end of my 2270 assembled and spinning freely. Thanks to all who helped me get to this point!

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To quickly recap: I bought the 2257 kit from AA Pistons (78mm stroke, 5.158" rods, etc.). And when I threw a P&C on for a test-fit last weekend, I immediately noticed the piston sitting proud of the deck height at TDC (negative deck) by about 2mm.

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Before I overreact, or buy the wrong stuff, I thought I'd ping the community...

1. Is this normal for a stroker set-up?
2. Can you stack multiple shims when getting up into the .06 >> .09 range?
3. (If no-stack) What's the best place to have custom shims cut?
4. I recall Jake saying using large shims requires lots of advance, anyone have experience here? What am I looking to adjust for upon start-up?

Once I figure out whether or not this is normal (I.E. verifying that I don't have to backtrack), I'll get into the CR questions (just waiting on clear plastic from Amazon so that I can measure my CC volume this weekend) (IMG:style_emoticons/default/biggrin.gif)

Thanks!

[Jack Standz]

Proper quench distance is far more important than staying under a specific compression ratio number.

You take an engine with no quench that requires 93 and INCREASE the compression ration by decking the block (adjusting shim size on a T4) to achieve optimal quench and the engine can now run regular without detonation.
The really wild thing is the higher compression engine will also require 5-7 degrees LESS timing to achieve MBT.

MBT stands for Mean Best Torque and is how a steady state dyno tunes.
You continue to add timing (at a specific rpm) until either the engine detonates (detonated limited timing) or you see no further increases in torque for additional timing.

Also, your compression ratio is low enough already that a decrease of 1-2cc in compressed volume is only worth .1-.2 increase in compression. IE you end up with a 9.0 or 9.1 engine instead of an 8.9...

Do some reading involving "quench".
IMO it's the MOST important clearance to get right.
Getting this wrong can be worth 10% less power and a requirement for higher octane fuel.

For a customer job on a sub 4" bore with decent rods (h-beams) and forged pistons I want to hit .045.

On my personal stuff I might go a tiny bit tighter. I'd run as tight as .041-.042.

This clearance must be verified for all cylinders and the tightest used as the reference point for shim selection.

Just noticed the baby syringe in the video. A real burette is far more accurate!
I use a cow syringe in a pinch. They are really handy for chambers at or under 60cc as the cattle syringe is a 60cc unit.
The cow syringe is also not as accurate as a burette but getting the measurement in a single syringe load helps accuracy and if you use the same syringe for each cylinder measurement you will be fairly balanced between cylinders when you are finished.



What cam will you be using?

Induction?

Exhaust?

+1

We use a 100 ml burette with a three finger clamp for accuracy and repeatability. BTW you can fly cut the heads to reduce chamber volume and increase compression ratio among other ways.