356 compression ratio

[badsix]

help, i'm having problems calculating the static compression of my engine. it's a 58 1600 normal the numbers are as below
bore - 86mm old npr big bore kit
stroke - 74mm
head volume - 60 cc
cylinder volume - 429.85 as per the secrets book
dome volume - 15.02 as per secrets book
they also list a clearance volume of -9.21, i have no idea what this is for so i'm stuck !

i have tried some of the on line calculators but i'm getting some conflicting numbers. what i would like to end up with is 8.0-1 to 8.50-1
i know i will need to use cylinder spacers but just don't know how thick they need to be
thanks Jay D

[JTR70]

Ooh, technical motor stuff is not my strong suit; no questions about rust? If Craig or Jack doesn't field this one I'll forward this on to Tony. Justin

[Craig Richter]

Jay,
It's the volume of the space created by Deck Height, which you set with the .25mm copper cylinder base shims. There must be around .040(1mm)so the pistons don't hit the heads when you rev it up. It's a critical measurement when the heads have been flycut, or you're using big dome racing pistons. NPR's, 60mm chambers, 9:1 CR should add up to no problem. But you should do at least a one-cylinder mock up with some solder on the piston top to check.

[Jack Staggs]

We need more info. What is the chamber angle of the pistons and of the heads? They need to match.

[Craig Richter]

Jay, I was just looking over Jack's shoulder when he made his above post. He picked up on your early 22* heads matched with NPR 30* pistons. You had said they were used pistons, so I assumed (and you know "assume" makes an ass out of you and me) you had run them before, and already taken care of this mis-match. If not, you must!
Putting your combination together with no cylinder base shims will give (generalizing here with round numbers) a 10:1 CR, and you will probably need a .040" cylinder shim to get 9:1. See why people write books on this stuff?

[badsix]

i'm sorry I meant early NPR kit, their new still in the box. everything on this engine is new or rebuilt and yes I know it needs cylinder base shims as the piston will usually hit the head on these I believe. I just need to know how thick.
thanks Jay D

[Jack Staggs]

AFIK, ALL NPR p/c were 30* as delivered. You said "everything on this engine is new or rebuilt and yes I know it needs cylinder base shims as the piston will usually hit the head on these I believe." ugh. New or rebuilt has no meaning whatsoever as to whether or not parts are compatible. Just try to bolt a 5 lug Ford wheel on a Chevy. Might work, probably not. FWIW, 86mm NPR can be installed on certain engines with no, zero, nada, shims with no issues at all. In fact I sometimes wish that I could bump the compression even more. To think that an abitrary thickness could be proper for every engine just doesn't work. Not to be difficult here, but it's really better to make parts work together, not just make the darn thing run. You want expert advice? Many of us long term guys with decades of experience are ready and willing, but we will need all the info that we are asking for. One of my mentors liked to say "If it ws easy, girls would do it".

[Craig Richter]

Yeah, but girls smell good...

[Jack Staggs]

[badsix]

SORRY, I will look for the information elsewhere
thanks Jay D

[Craig Richter]

Sorry back Jay. I don't think we said anything that was offensive. After all, free advice is worth what you pay for it...

The point Jack was trying to making is that the answer to your question of "how many shims" cannot be answered by anyone on a forum. You start at 0, and work your way up in .010" (.25mm) steps until you have enough clearance. The answer becomes obvious during pre-assembley measuring of the various parts that have never been an assembly before. Professional mechanics do this all day long, because we have 50 year-old parts, new parts, machine shop parts, your brother-in-law's parts - takes a lot of time and measuring to make sure everything works!

[badsix]

there is a formula to figure the static compression with the information in my first post other than deck height. and that is controlled by the thickness of the shims to get various compression ratios. when I have that then I can assemble one cyl. and check clearances and go from there. this is all I asked for, I don't care if a ford wheel will fit a chevy I already know that. or how girls smell, I know that also but be warned not all girls smell good.
Jay D.

[Craig Richter]

Ohh, smelly girls...we don't want to go there...

I work a little backwards for what you're asking. First we have to get enough clearance to make sure things aren't hitting each other. Then you will know the "clearance volume" to plug into the CR formula. Then, if you don't like the number you get, add shims to get it where you want. If the number is already too low, you'll have to trim the cylinder bases a little, and start over.
Another detail I just remembered about those NPR pistons: the exhaust valve cut-out isn't very deep. If you are using a re-ground cam, you may need to re-cut that to gain valve/piston clearance. Good luck, Jay.

[356JAGER]

This may help:

http://www.precisionmatters.biz/pdf/compression-ratio.pdf

[Tony G]

The difficulty with figuring out CR on 356/912 engines is that the pistons are domed and the cc displacement has to be known. NPR pistons I have used sometimes provided some information but one really has to do their homework. I typically install the cylinders with pistons to check the case deck as well as the deck height. I also cc the pistons and cylinder chambers (all four). I then do the math and check to see how much deck height I need to arrive at the CR I am aiming for as well as the proper clearance to prevent piston from comming in contact with the heads/valves; I use Claydo for this. Careful, some pistons are designed for certain head chambers. By the way, what information do you have regarding your piston set, cylinders, and heads? Do a line by line thread showing what you have as "verified information" and we can help/guide you to secure the rest of the information you need to plug into the formula; I have my own which even takes into account other minute cc values that are generally not taken into consideration (such as volumetric quantities between the piston diameter, cylinder diameter, and first ring or the value of head spacers, if used, based on the thickness and inner diameter of the shim, etc..). Be careful and do it right, can you mock up the engine to measure and verify certain critical variables?