Thread: Technical Question: Compression Ratios

Sorry if this has been posted before, or if this is in the wrong section, so feel free to move/remove if necessary.

I'm curious to see static C/R rates dependent upon piston type, head gasket thickness, etc.

What I'm thinking here, is that we could get a good sticky regarding static compression ratios, based on piston/rod used, and headgasket usage.

For instance, we all know that DET motors come stock with 8.5:1 compression ratios, with a stock headgasket. When you go with say a 1.1~1.5mm headgasket, how is the compression ratio affected? DE motors have 9.5:1 compression ratios, from factory. Dropping the DET pistons in lowers this CR to 8.5:1, correct? So it's obvious that the piston itself would be dished more on the DET pistons, than on the DE pistons.

Also, I don't know how easy it is to check C/R's with certain cam specs, so if this data is easily acheiveable, then that'd be a great addition. For instance, S4 cams would lower your C/R by .1 (Just an exaggeration).

Anyhow, I hope you see where I'm going with this, and personally I think it would be a great idea to list the entire SR family, not just the DE's and DET's.

Sorry again if this has been posted somewhere on here before. I am a bit lazy today (beautiful weather in Iraq), and didn't do any real IN DEPTH searching. I just did some basic skimming.

33 views and no replies? Is it a pretty basic question that I should be slapped for, for asking?

If so, I'll go educate myself. Just thought it'd be a worthwhile discussion.

i think this is a super technical question, esp if you get into camshafts and aftermarket pistons, then the bores and compression, along with piston volume and all that changes significantly over stock pistons, becuase you can have a much larger piston in diameter, or much larger compression piston, or much less static compression piston.

a thicker headgasket will cause problems with your quench (ability to resist detonation)

you have to measure stuff like:

Bore.
Stroke.
Deck Height.
Compressed Thickness Of Headgasket.
Piston Top Volume.
Combustion Chamber Volume.

i think if you are talking stock to stock compression its not that bad to try and get the true compression ratios just given you are using a aftermarket headgasket,then factor that into stock configuration ie: bore,piston volume,deck height compression headgasket thickness ect, but if you are talking about dropping cams in and aftermarket headgasket with aftermarket pistons then all of the formuls are going to be changed given, the piston isnt stock form ie: all the same measurements.

i know i didnt fully answer anything really, but its really hard to without some numbers to go by... if anyone else wants to take a crack...

Okay, well to simplify the loaded question, lets stick to stock bores on the motors, then. We all know that you can bore/stroke these motors, but lets leave that out of the equation. Throw out the cam part of the equation, too.

Something simple like this:
SR20DE (US) - 9.5:1 CR - x.xx H/G thickness - .xxx cylinder volume - xx.xx head volume
SR20DE (UK) - 10:1 CR - x.xx H/G thickness - .xxx cylinder volume - xx.xx head volume
SR20DET (Non GTI-R) - 8.5:1 CR - x.xx H/G thickness - .xxx cylinder volume - xx.xx head volume

Now, a question I would have relating to the above information provided would be, are the headgaskets the same on all of these motors? If so, then it's literally just the cylinder reliefs that are different, and nothing else?

Does this make sense? A cheat sheet, so to speak, so that figuring out your desired static C/R is a no brainer, based on simple values.



I guess the reason I ask this, is because I'm a real noob when it comes to deck height, etc, and I'm trying to learn as much as I can. I figured that by starting this thread, and others offering input, I'd start to understand it more.

(For instance, what does a 1.5mm head gasket drop the C/R to on a US motor? UK motor? What about the 1.1mm head gasket? What ranges of HG's are available?)

BUMP, I would like to learn this stuff too.

you just need to build up a spreadsheet,
you include the following numbers:
head volume, de=46, ve=41cc etc,
head gasket 1.09mm x87mm bore = 6.42cc
piston in the hole 0.45 to 0.55mm FSM figures, = 2.90 +-.14cc
piston dome dish = y cc
swept volume =86x86... = 499.6cc

now you put all those numbers together and get a "Static CR" of whatever
Then you use some trig and the final closing angle of the inlet valve and you get the "dynamic CR" then you look at the VE and dynamic CR and you get the CR in use at a given revs.

That will give you plenty to google for. Some web pages serve up the calculations as javascript, so if you save the complete web page you will be able to play and do what if's on your PC.

Mike

accually tekkie. i have heard the stock DET's have 8.3:1 because of a slightly differant head design. however DET pistons in a DE block is 8.5:1


but seeing as how there are quite a few motor builds poping up (all in the midd east coast area BTW ) and it seems like high CR N/A builds are the new hotness this is a very good idea tek

sr20det= 8:5:1 gtir=8:3:1 differences in the combustion chambers between the 2 heads which gives the compression differences

well as there are so many ways to achieve the desired CR, a spreadsheet is the only sensible way to do it.

Mike

^^ i fully agree. have no idea to set one up tho.