How to determine engine compression
 

Hello,

the previous discussion brought me to the question of how to determine compression on the engine I am building. It is a former 1.8L engine, with stock crankshaft, and I was planning on putting on it the larger 96mm cylinders to bring it to 1911cc. (Heads are stock.) If I understand correctly I can adjust the final compression ratio by putting shims at the bases of the cylinders, to leave more or less space between the piston top dead center and the top of the cylinder. This is called deck height here:

Van's Porsche 914 Engine Build

Two questions: how do I calculate the compression ratio for a given deck height? In particular, if I don't shim at all, what compression ratio can I expect? And what would be a good compromise target compression ratio? The car will never be used for racing, mostly as a sunny day driver, but I'd like to get some decent power. And if I can get away with using only 87 octane gas, that would be very nice.

Thanks,
Andrei.

You have to measure the combustion chamber volume, with the valves and spark plugs installed. Then you add the static volume contributed by the deck height (PI * r^2 * h). That's the "fixed volume". Then you add the "swept volume" (that's (bore/2)^2 * PI * stroke), and divide that total by your fixed volume. That's the static compression ratio.

This is relatively basic engine building stuff. You might want to read through a few primers on how to spec and build and engine if you're going to be looking at questions like these.

--DD

Dave, thanks for your answer. That was not my question. (Background -- I have a Ph.D. in math; I can compute the volume of a cylinder.) I was wondering where I can find the combustion chamber volume for a 1.8L head; and also what do people recommend for a reasonable compression ratio given the uses I plan for the motor.

Thanks again for your patience with my noob questions.

The chamber volume must be measured directly. Small changes in it can have pretty large changes in the ratio. This is usually done using fluid of some kind, most often by cutting a flat piece of plexiglass to fit the recess in the head, with a couple of holes (to fill through and to let air out).

Here's a random pic of someone doing this:

http://www.wpvw.com.au/images/Alfie%...%20%282%29.JPG


The compression ratio you want varies depending on a lot of factors. Relatively fixed things like the combustion chamber shape, and things you can change like the cam grind. In general, more aggressive cams need higher compression, and less aggressive cams need lower compression. If you go too high in compression, you can experience pinging which can actually destroy your engine if you let it go on long enough. You can mitigate that by using a higher-octane fuel, so that also has an effect.

The stock cam in our cars is very mild. The stock compression ratio for a 1.8 was 7.6:1, I believe, and that allowed the car to run on 87-octane fuel. (Some say that the factory was overly conservative with their figures, though.) You should be able to go up to about 8.5:1 if you're willing to run 91-octane or better all the time with the stock cam--but I wouldn't go any higher than that. I would probably stick with 8:1 to give myself some extra safety margin, because blowing holes in your pistons can ruin your whole day.

If you have a more aggressive cam, you want more compression. And that is where we get outside of my comfort zone. 8:1 would be very safe with an aggressive cam, and 8.5:1 might be fine as well, but I'm really not sure.

Good luck!

--DD

Compression Ratio
 

This my help. Compression Ratio Calculator.

Engine Compression Ratio (CR) Calculator

I finally got around to measuring the combustion chamber volume and to compute the desired deck height for getting the compression to where I want it (8:1). I just would like to run these numbers by you guys to see if they make sense, primarily to make sure I didn't do something stupid.

My measured head chamber volume was between 53 and 55cc. Is this typical for a stock 1.8L head?

I put this into my calculator, and it appears that with a 2mm (0.08in = 5/64") deck height I should end up having a compression ratio between 7.9 and 8.1 (due to variation in chamber volume). Again, is this deck height typical? I am aiming for a CR of about 8:1.

I am waiting to get my new oversized bearings from the shop that align bored the case, so I can measure the actual deck height on my engine, without spacer rings, and to determine if I need extra spacer rings. The cylinders are 96mm bore, stroke is 66mm (stock from a 1.8L crankshaft). The engine is completely stock, with the exception that the P&C are the slightly larger bore (96mm versus 94mm).

If someone could just look over these measurements and calculations and tell me if they sound ballpark-right, it'd be helpful.

Thanks.

They do sound ballpark-right. But you should do what you can to make sure that the head volumes on all four cylinders measure the same. Uneven compression ratios mean uneven amounts of power mean uneven wear, which will lead to things getting out of tolerance sooner.

--DD