87FORMULAL98

So...I'm ready to squeeze the trigger on a set of AFR 180's (#0919). I call AFR to get the milling specs to yield close to but no more than 10:1 on my stock '87 L98 bottom end. I was expecting the tech to say something like... mill them to 60cc's and use Fel Pro 1003 .041 gasket and live happily ever after. He says he needs more info so I dig through my REALLY old and faded notes and give him what I think are my stock head/piston specs. I get put on hold for a minute or so. The guy comes back and says my numbers don't add up to the factory rated 9.3:1. Here's what I have in my notes. Someone please correct my error.


64cc Combustion Chambers
Deck -.025
Minimum Combustion Chamber 75.47cc @ TDC
Compressed Gasket .021
4" Bore
3.48" Stroke

sofakingdom

iTrader: (1)

Define "stock bottom end".

Direct from GM? "Direct replacement"? "Rebuilt"?

????

That's a CLASSIC case of "measure with micrometer, mark with chalk, cut with axe". If there's ANY ONE THING that shows a lack of understanding of just how much tolerance there is in everything (and by extension, the value of "blueprinting") it would be, quoting ANYTHING about compression ratios, with decimal points in them. Let alone MULTIPLE digits to the right of the point.

Show us photos, part #s, and measurements of your short block, and we'll be able to guess your static CR to maybe within a quarter-point or so. Maybe.

Just for starters, I'll bet MONEY your deck clearance isn't .025". I'll bet it's more than that. I'll put a whole NICKEL on that. (If I can find one?) Additionally, I'll put a whole DIME (one-tenth of a dollar... now that's RICH) that there's MORE THAN .010" difference between the most and the least. What do you suppose THAT will do to your "calculations"?

We on? or are you gonna whup out the mic and actually start MEASURING stuff? (including of course, head chamber volume... any idea how much difference there is between flat valves https://www.summitracing.com/parts/slp-v-1876/overview/ and tulip ones https://www.summitracing.com/parts/m...60-8/overview/? just picked some examples to show the shape, nothing particularly specific to the motor at hand)

Building a motor, as opposed to slapping some stuff together that you order out of a catalog, is HARD. You might be amazed at how much difference there is between "spec" and any one given part, in stroke, rod length, block deck height, piston height, .... for every dimension, there's a tolerance. And in a "stock" engine, they typically run pretty wide.

ULTM8Z

What gasket gives you a compressed thickness of .021? The .041 gaskets from FelPro are .041" compressed.

Also, you need to know what kind of pistons you're running... flat top (w/ valve reliefs)? dished? My guess is they're not domed... Whatever the dish/valve-relief volume is needs to enter into the equation.

Summit has an online calculator by the way... https://www.summitracing.com/experta...ion-calculator

Everything else being equal, a change of .010" in deck height or gasket thickness will move you up or down by ~0.2:1 in compression. So for example if your deck height is say .035 rather than the .025 you think it is, it'll result in a ~0.2:1 drop in your actual static compression. Or if you're banking on .021 gasket thickness, that .041 gasket will decrease your compression by ~0.4:1.

Sofakingdom is correct, you need to know all these items if you want to get the compression ratio you want. If you end up lower than 10:1, you may never know it except you may suspect the power is lacking a little... but if you're shooting for 10:1 and go over that significantly, you could end up with knocking/pinging and/or having to reduce the spark advance significantly enough that it'll cancel out a lot of the benefit of having that high a compression ratio.

87FORMULAL98

The motor is the original L98 that came in my car. It has never been opened up. It has 23,xxx original miles on it. AFR wants me to give them the "stock" specs for my engine and my notes are obviously incorrect.... Probably came out of an old car magazine from back in the 80's . I'd like to give AFR enough accurate information (without tearing my motor down) to get a set heads ordered. Any help would be awesome.

scoflaw

iTrader: (2)

Why would you want to put expensive heads on top of those dished slugs ?
IIRC mine were .040 in the hole Then you want to deck the heads to be compatible with the stock piston ?

Then when you want to upgrade your pistons and tighten up the quench, you already will have a boatload of material missing on those heads.

ASE doc

What I think folks are trying to say is, you need to BUILD this engine with the aid of a good engine machinist. Starting with pistons and deck height that will compliment the AFR heads' chamber. Part of the machining process will be milling the block to set deck height whereby you will establish quench. Deck height, piston crown volume, and the chamber size of your heads, with your machinist there to verify it, will get you to the actual CR you want.

Engine machining isn't as expensive as you might think and is the difference between precision and rolling the dice.

daferris

iTrader: (8)

[QUOTE=87FORMULAL98;6143345]The motor is the original L98 that came in my car. It has never been opened up. It has 23,xxx original miles on it. AFR wants me to give them the "stock" specs for my engine
you can find the detailed spec's of the entire car including what the engine is supposed to be off the line at this site lots of good info straight from GM...
https://www.gmheritagecenter.com/gm-heritage-archive/vehicle-information-kits.html

cardo0

If achieving 10:1 compression accurately is so important to you then you need to pull the head and measure the piston volume at TDC. Ordering new heads should only take a couple weeks so as soon as you get the correct piston volume you can calculate bwhat chambers you need and place your order.

Otherwise just use your published compression and head chamber volume number and do the math to solve for the head chambers that will get you close to 10:1 compression.

Good luck.

87FORMULAL98

Thanks for everyone's input. After spending a few days searching the net I found no solid head/piston data...only bits and pieces. I eventually called Clay at TPIS for a little help. Here's what he had to say.


Stock L98's have flat top pistons with valve reliefs. Pistons are -.025 (.025 below deck). There is always a little variation but -.025 is REALLY close. He recommended milling the AFR 180 (0919) to 60cc's and using a .029 gasket to yield approximately 10.2:1. This is a common combination that TPIS uses. He also said that if 10.2:1 seemed spooky then I could run 62cc chambers with the same .029 gasket for a little less compression. I didn't have him get out his calculator again but he said it would be just under 10:1.


Thanks again to all of you who tried to help me fill in the blanks.

scoflaw

iTrader: (2)

They ain't flat tops, it's a dished cast piston

87FORMULAL98

Yep.... I expected someone to say that. Glad you have the confidence to say so. During my research I found that most folks say they have flat tops with four valve reliefs but every now and then someone will pull their heads and say they have shallow dished pistons. Don't know if GM snuck in both or if our cars are just so old that original pistons are getting rare.

87FORMULAL98

I think you nailed it sir. The missing piece of my notes is piston data. A couple of cc's does indeed make a big difference. Thanks for your input.

NoEmissions84TA

iTrader: (1)

.025" in the hole + .029" gasket gives you a quench distance of .054. That sucks. Shoot for .040".
Why would you want to mill the heads and then lower the CR with a thick gasket?