Cylinder Shim...

[yoh!!!]

Hi,

Finished the crank case tonight and temporarily installed a piston and cylinder to measure the deck height …
The excel calculation that someone on the board put together worked very well. For a 8.0:1 CR I calculated that I would need a 0.047 shim (which of course is not available).
Here I could use some help…
1. If I would use a 0.05 shim I would establish a CR of 7.94:1 and a deck height of 0.062 inch. The problem for this configuration is, that I can’t find a 0.05 shim .
2. By using a 0.04 shim (this seems to be available) I would end up with A CR of 8.12:1 and a deck height of 0.052.
3. By using a 0.06 shim (this one is also available) I would end up with A CR of 7.77:1 and a deck height of 0.072 which is probably to much.

I guess what I’m looking for is some advice regarding the CR and the deck height. I also would like to know if combining shims (0.04 + 0.01 …) might be an alternative that does not result in oil leaks. Maybe someone on this board knows about a place that sells 0.05 shims.

Thanks for your help,

[rice_killer]

use the 0.04 and 8.12:1 r.
thats not much above your default.
and ya might need to run a little higher test gas, OR alternativly lower the timing a few degrees.
shouldnt be much though.

what it all really boils down to is doing a compression test in when the engine been broke in. all the diagrams and excel sheets in the world wont tell you EXACTLY what your compression will be. ya dont know if the deck was shaved or heads modified etc.

an old trick i used once on my dirtbike was, i took the head and lapped it on a piece of glass with 800 grit wet/ dry sandpaper, this raised the compression to the point of pinging on regular. then i kept doing it till it pinged on mid grade.
now it runs on high test and doesnt ping, and has alot more power. point is, what if previous owner of the engine parts did this to your engine? that would not show up in the excel spreadsheet.

btw, where did you get this spreadsheet? i would like to look at it.

[seventythree]

I just went through this with US spec pistons. It sounds like you are measuring .012 deck height without a base shim and head gasket. That is what I measured as well. You say that a .047 shim will give you .060 deck height! Are you not planning on using the head gasket? The head gasket measures .029 uncrushed, and should add at least .025 to the deck once installed. So, the .010 shim supplied with the gasket kit will set the deck height to .047. Add or subtract from there as desired. My heads are torqued, but it is not too late for me to go back and make corrections in case my understanding is flawed. Anybody?

[yoh!!!]

Yes, I will not use a headgasket since I remember that people on this board discussed that it causes difficulties. I think this is what might have caused the head problems on my engine when I bought it...
... Head work was done. The sealing surface was mildly cut, probably for the first time. I meassured 58ccm volume in the combustion chamber and another 2ccm in the piston (K&S Euro Spec)
Deck height... I think under 0.05 might not be a good idea. And I would not know where to get better gas than the 93 Octan...

[Dave at Pelican Parts]

Ricekiller, there is a real difference between compression ratio and compression pressure. A compression tester will tell you the pressure in the cylinders that is caused by compression. That is affected by a whole lot of things, including the outside air pressure, the speed the starter cranks, and so on.

The compression ratio is simply the ratio of (fixed volume + swept volume) to fixed volume in the cylinder and combustion chamber. This will not change unless you change the pistons, heads, bore, stroke, etc.

The spreadsheet does give you correct results for compression ratios--if you feed it the correct numbers for your engine. That means you have to measure a whole bunch of things pretty carefully, like the combustion chamber volume, the piston dish (or dome), the deck height, etc. Note that the spreadsheet doesn't say squat about PSI readings--because that is a different (though not completely unrelated) issue.

You can find the spreadsheet (there are actually several available on the Web) here on Pelican's "Tech Specs" page: http://www.pelicanparts.com/914/technical_specs/914_compression_ratio.htm .

--DD

[yoh!!!]

I know this was not addressed to me...
... but I did the required measurements in order to use the spread sheet correctly.
My deck height is 0.012... I messured 58 ccm of combustion chamber volume (it is lower because of a mild fly cut) and the small piston dish of the 8.0:1 K&S have a volume of 2 ccm (at least in my case).

I need some advice regarding the calculated shim...
... and for making a decission in context with the final CR.

Thanks,

Patrick

[rice_killer]

dave, i didnt say compression pressure and compression ratio are the same thing.
what i did say is that he should chek his compression after the engine is broke in.

and, compression pressure and ratio are almost interchangable meanings. because one directly effects the other. technicly speaking....


things like atmosphere, starter speed etc should not really effect compression pressure unless your engine is bad.
i get the same 185PSI on all 4 cylinders weither i turn it with the starter, or turn it by hand with a socket and wrench on a test stand. turning it by hand is pretty slow too. and as far as atmosphere, most people dont live in the andies or anything and even if they did it would only lower the comp a couple psi at most.
just stuff i learned over the years from personal experence...

[Pritchard]

Did I read correctly? 185 PSI...

My 2.0L is running with 107 psi (this is part of the reason for the P/C replacement I am going to do). I have always heard that 150'ish PSI was about normal.

What kind of "seat of the pants" increase in power would one feel with an increase in PSI from 107 to 185? Would I have to dry my seat afterward?

Just curious.

Pritchard

[yoh!!!]

O.K. ordered my shims at FAT Performance Parts today... They sold me 0.045 wich is almost exactly what I needed according to the calculation. According to FAT the shims should arrive in Austin TX before this weekend, which means that I most likely will finish my engine this weekend... Yeh!!!
I should end up with a CR of 8.05:1 and a deck height of something like 0.056. Will see how this works...
... and we will see what my actual compression reading will be.
185 psi seems a lot, even 107 is not all that horrible if it is 107 on all four cylinders. I also remember that 150 PSI is typically what a new or well rebuild engine should have afer it is broken in.

Thanks for all your help...

Patrick

[Dave at Pelican Parts]

use the 0.04 and 8.12:1 r.
[snip!]
what it all really boils down to is doing a compression test in when the engine been broke in. all the diagrams and excel sheets in the world wont tell you EXACTLY what your compression will be.

I dunno, ricekiller. I may be mis-reading the above (and if so I apologize), but it looks to me like you are saying that a compression test will tell you your compression, as in compresion ratio.

I do not see how you are getting the same readings (and 185 PSI is very high!!) cranking with the starter and cranking by hand. I'm not saying you can't be, just that I find it tough to see how. Perhaps a limitation on my part.

Other people have observed different compression pressure readings depending on if all the plugs are out or not (if they are the starter cranks quicker), the health of the battery (poor battery or connections == slower starter), if the throttle is wide open or not (WOT means less resistance to air getting into the motor) and so on. And there is a rule of thumb for compression readings and altitude--I think it's something like 5 PSI lower per every 2000 feet? I don't remember exactly, but there are folks driving 914s around the Denver area (5000+ feet) as well as the SF Bay area (~20 fteet).

The compression pressure is related to the compression ratio, but the relation is indirect. There are other things that affect the pressure, including how well the rings and valves are sealing. But the compression ratio of a given engine is a pretty decent indicator of what grade of fuel you want to run in that engine. It also varies with a lot of factors internal to the engine (combustion chamber shape, number and location of spark plugs, number and location and orientation of valves, how efficiently the cooling system removes heat from the cylinders and chambers, etc.) but for a given engine the CR gives you a decent idea of the kind of fuel you can use.

Getting the CR even across all cylinders is also important to help the engine have a long and happy lifespan. Compression pressures will change as the components wear-in, so you those numbers are tougher to use when initially setting up an engine anyway.

Compression pressures are a good indicator of the current state of the top end of the motor. But compression ratios are a more useful quantity to know about when you are building the motor, IMHO.

--DD