Just ordered a set of PRC 225 As Cast heads with 62cc Chambers for my SBE LS1. Also ordered the Cometic 0.40 head gaskets. Do you think I’m ok with this combo as far as PTV clearance?
Also within the next month I will be adding a ECS Supercharger.
231 / 234 631" / 615" 112.5 lsa is the cam I currently have in the car.

 

When I contacted Texas Speed they recommended that I go with 0.045" gaskets because they don't like the clearance with 0.040" gaskets, a bit too tight. A lot of people run them like that, so you'll probably be ok, I'm just relaying what they told me.

That cam is a lot like the Torquer V2 and V4, I would think you'd have plenty of PTV clearance. With my cam (232/234 112 lsa 110 icl) they told me I could mill my PRC 225s down 0.018" to 59cc and still have plenty of clearance without needing to flycut.

 

Well look like that 62cc head is not gonna play well with my supercharger. Everyone is saying not to run over 10:5 compression o supercharged engines. Will be around 11:1 with that head

 

Is 11:1 with .040 gasket or not. You could run thicker than stock gaskets to lower compression...

 

If you just ordered the head, I'd cancel the order or change it to the appropriate chamber size. You really want to run the thinnest head gasket you can get away with, for optimum quench. Quench will help with detonation.

 

Agreed. I don’t like to use the headgasket to dictate compression. Shoot for the most efficient quench you can safely run. Adjust compression with chamber volume.

 

I a having TSP switch the 62cc heads to the same ones with the 68cc chambers. Should work much better

 

What I've been led to believe-
First figure how much desired (required?) quench in thousandths and also, know how much piston is out of the hole,
Gasket thickness is quench, PLUS out-of-the-hole measurement.
Figure compression at that point. THEN figure how much to mill to arrive at desired final static compression.
Sound right?

 

In a nutshell, yes. There’s a lot of mock-up to do when getting this right. One will have to build the engine a few times before final assembly. Parts selection makes a huge difference on how tight you can go. An Oem crank and rods will have too much variance in tolerances to go super tight. The Oem stuff also has too much flex and stretch to really push things.
Measuring piston out of the hole numbers is easy with the right tools. A deck bridge with dual mics is how I do it and always have. Finding that number on all 8 holes is mandatory. It’s an art because oil pressure centers the rod on the journal. Without oil pressure present, there’s a good chance that as you turn the assembly by hand, the piston top won’t be where it’s going to land while the engines running, with oil pressure keeping everything in check. Doesn’t really matter if your shooting for .035”, but think about shooting for .027” or .028”…that’s where .001” matters. I’ve seen much lower numbers than that in fact. We would spend 8 hours measuring conn rod center to center distance and setting up cranks in fixtures to find the most accurate ones. I’m talking about measuring out .00001”. We would start with 10 sets of rods and 4 cranks ($35k right there) and sort through it all to find 8 matched rods, that would always still require machining of the pin end to get enough oil clearance. I’ve seen many a piston set up in a milling fixture as well to get out of hole numbers exact as well…There’s a process called compression balancing that most have never heard of, that requires all of the same procedures here as well as combustion chamber work on all 8. It’s crazy tedious also….It all has to be balanced along the way after final mock-up also. 99% of folks here won’t ever have to go through the process I just talked about, but I do know that Tony Mamo set an engine up at .028” once and it didn’t last because he had one conn rod that was a scoche longer than the other 7, and he had measured everything out pretty well…so it does happen to even pro builders. Of course your north of 7500k rpm when you find the error. Ouchy.
Gasket thickness is generally fairly accurate, so that’s usually not a concern in figuring quench. Just always find the compressed thickness, not the packaged thickness. Man did I just ramble on here…

 

Not rambling at all! And thank you for the VERY thorough explanation, as far as a new engine build goes.
I was thinking more along the lines of an existing engine being refreshed, as I have alluded to in the past.
When I ever get to pull the heads off my trusty 5.3, I would measure deck clearance (out of the hole), then figure the right gasket for good quench, which ideally should be .035-.045, or so I'm told. A .045 gasket would likely work here, being that many LS engines have .005-.007 out of the hole. That should put quench around .038-.040 or so.
Then I would mill the heads to arrive at 10:1 compression.
This might be a while, but I WILL be ready... LOL
Thanks again Scott!

 

Yup. Same principal as a new build. Make sure you are dead nuts on your out of the hole measurements. Measure all four corner cylinders at minimum. .045 gasket works 99.9% of the time with stock bottom end. You don’t want to be the .1%er.