How to work out compression ratio

[Driftit 1944]

Hey guys, thought I would ask here to see if anybody knew how to work out compression ratios on your car.

Now this is for a SR20DET. Not my BMW motor.

The motor has been rebuilt and not run yet. But I am worried the compression will be too high.

Remember its a 4 banger.

It has 9:1 pistons installed. They are 0.5mm bigger than normal (not sure if this is already taken into consideration) e.g. are they 9:1 + 0.5mm or is the 0.5mm already included in the 9:1 figure.

Not I also had to skim the block by 1mm. This is where the 9:1 must increase. I used the factory head gasket.

Can somebody figure out my gibberish?

Cheers

Dan

[Gus 5]

the 0.5 mm refers the the bore size....should not affect compression

skimming the block will most likely have raised the compression...i am not sure how to tell if it would be too much however. are the pistons sticking out 1mm?

[*Glenn* 854]

The formular is :

Swept volume + unswept volume divided by unswept volume

Go figure ???......lol

Can really only be done properly with head off...unless you already know the cc rating of the combustion chamber

Combustion chamber=unswept volume

You must also allow for the headgasket too because that creates more unswept volume

The swept volume = Bore area X stroke

Domed or dished piston tops also + or - the unswept volume

You must measure the combustion chamber with fluid, using the spark plugs you will run in the car.

Spark plugs also have volume around the electrode. When balancing an engine all this is taken into account, measuring and equalising the combustion chamber volumes so all cylinders are equal.

All this must be measured to be 100% accurate

Confused now ??

Cheers

Glenn

Edited December 12, 2007 by botanymotorworx

[CamB 48]

The formular is :

Swept volume + unswept volume divided by unswept volume

Hehe, I made a spreadsheet for my M10, but since I relied on internet measurements it could be way out.

For drift it

For an SR20 you could try the spreadsheet linked on this page (thanks google):

http://www.rs-enthalpy.com/info.htm

While it might not be accurate exactly for your engine (ie if the US spec engine is different to JDM, or whatever), the CHANGE in c/r should be close enough for your purposes. Instead, lets assume the US engine is the same as JDM:

- if I change the bore to 86.5mm, the c/r only goes to 8.6:1. If the pistons are 9:1 then the dish is less, so

- I guessed at -12cc piston volume - gives about 9:1

- now, skimming the block reduces the deck clearance from -0.05 to +0.95, increasing c/r by 0.75:1 to 9.75:1

... which suggests you should look at a thicker head gasket, and/or find someone (not me!) who really knows what they're talking about, since it seems you may have pretty high c/r.

[Driftit 1944]

Hey thanks guys. The motor is already all together so I cant get any measurements that way. The pistons are dished typed pistons that also had a bit skimmed off the top.

My Dish depth is apparently -0.115inches which converts to -2.921mm's

Im not 100% sure on deck clearance though.

But I have a feeling just from some quick work outs that the ration is far to high. It will be high 9's and maybe even low 10's.

So I am pretty much screwed. Wish I had been more patient and purchased a thicker head gasket. But I have been desperate to get the car running again.

[*Glenn* 854]

You can always do things twice

I have had too many times with modifications....you cant always source every bit of info you need

[Driftit 1944]

Yeah I am not happy about pulling it to bits again. But it looks like I have no choice.

[*Glenn* 854]

Yeah I am not happy about pulling it to bits again. But it looks like I have no choice.

Very true

You'll end up screwing it, if the compression is to high

When I turbo'd a ford motor years ago I ran 8.5:1 comp and 20lbs boost

Flared occasionaly to 22lbs

We balanced the engine & cc corrected the combustion chambers

We also did the calculations correctly and got the right comp ratio

All good things take time...just be patient and get it right

Cheers

Glenn

[Driftit 1944]

Well I decided to pull the motor apart again. Haha. Just need to track down a 1.9 or 2.0mm head gasket now. Not very common. But they do make em.

Cheers Guys.

Been a good leason and great to know how to work out your compression ratio.

Dan

[tibbs.james 1]

Better a new head gasket than some toasted pistons and having to really start all over again.

May i ask why 1mm was skimmed off the block, i would ave though a light surface would have done it ?

[Driftit 1944]

It was actually 8 thou that was taken off. Stupid Imperial measurements.

It was taken off as the melted piston had done a good job killing that bore and warped the top.

But the motor is all in now. 2mm Trust/Greedy Headgasket. Should bring it down to 9.1 - 9.2 which is very workable. I will just have a response instead of massive boost. But hey thats cool with me. Dont like having to rely on the hairdrier.

Goes to get tuned 2moro. Will post up the dyno chart. I also have one to compaire it too. Mate did the same mods but without any increases in comp.

Very glad I asked you guys. Thanks once again.

Dan

[c.robertson00 11]

And why are you getting twitchy about 10:1 compression this is lower than stock on a lot of cars and my Alfa ran more than that 30 years ago.

Turbocharged cars require a lower compresion ratio because of the amont of air and fuel going into the cylinder.

[Driftit 1944]

Much much lower. Stock compression ratio for my SR20DET motor is 8.5:1 the NA version the SR20DE is 9.5:1

When running in the range of 200rwkw the cars are running about 1 - 1.2 bar boost on the factory turbo. At 10:1 I would need to be running race gas. And that might not work ether.

[*Glenn* 854]

If you want to run high boost you will need to be within the 8.5:1 ratio

Simple really...its been done so many times before.... just work it out with the formula I gave you before

The higher the compression ratio from this figure means you can only run lower boost settings

On a ford that I turbo'd a few years ago, we ran 8.5:1 comp & 1.5 bar boost pressure ( 1bar = 14lbs )

Reliability will all boil down to how well the motor is built and the internal/external parts that are used

Remember this..... detonation = "bang"........eventually

Checking compression tells you frig all...except that you have good compession. It will not tell you the ratio you have.

Hope this helps

Cheers

Glenn

[*Glenn* 854]

Just an add on to this thread

Everything in "Automotive Engineering" is controlled or determined by ratio's

Exceed the engineering limits.......AND IT "WILL NOT WORK"

Dont....hope....pray.....or think it might work... it will have been tried before you were born

If its beyond engineering limits " IT WILL FAIL"

Always research & do the maths... it will save alot of heartache, $$$ & hard work in the end of the day

[Driftit 1944]

Fired the car up this morn. After spending all night stressing something was wrong. Turned out I had been sent the wron Apexi Power FC. A D-Jetro instead of a L-Jetro. So I ether go and by $500 worth of required sensors. Or wait to after new years to get the correct one. So I am waiting. No huge gains having the airflowless version when I am only making around 220rwkw.

But she runs nice and smooth. But the stock ecu doesnt like the bigger injectors much. Not driving it at all till I have the new ecu and a tune.

[*Glenn* 854]

In my previous thread I said "Compression pressure tells you frig all" I was relating to the original question...." how do you work out compression ratio"

Pressure will tell you alot about engine condition (and yes I do know how to use a compression guage) but it cannot be used to work out compression ratio

A high compression engine will have higher pressure, but you cant use that pressure figure to determine the ratio.

The ratio can only be determined by measurements & maths to be accurate.

and yes you can turbo and supercharge any engine with any compression ratio...but not effectively

You wouldnt go to the trouble of spending thousands on fitting a turbo or supercharger on an engine and only being able to use .25 bar boost...... Ooops.....some people might

The limits are controlled by boost over compression ratio. You cant boost a petrol engine at 1.5 bar using 10 or 11:1 compression ratio .....detonation will occur...and it will eventually go "BANG"

Detonation is the killer

Detonation will occur if the boost pressure is too high ( for the comp ratio ) and if the fuel mixture or igition timing is wrong. Fuel/air/boost ratio and ignition timing must be set up after building the engine.

Detonation = The fuel / air mixture igniting under compression before it is ignited by the ignition system

The knock or pinking ( detonation ) occurs while the piston is still trying to come up the bore ( before reaching TDC ). This damages the pistons. This will also occur in a NA engine if the ignition timing is too far advanced.