Compression Ratio...please Read

[richardm6994]

Hi everyone, this may not work out how I hope, but you'll never know if you don't try....

 

Basically I'm trying to collect as much "real life" data as possible regards your engine's compression ratios.

 

If you know the compression ratio of your engine (e.g 10.5:1)...and you know what that ratio gives you on a compression test (e.g in psi or bar) can you please reply with your engine's readings.

 

Compression test readings need to be "warm engine" & wide open throttle.

 

 

Hopefully with enough responses...a pattern between PSI compression test readings and compression ratio's will start to emerge? Then with this data, I hope to produce a rough scale to indicate what PSI readings equate to what compression ratio....

 

The more people who respond...the better chance this stands of working, and I think it may end up actually being useful to people in the future.

[sj-bradley]

I have found from the Net the following:

 

http://en.wikipedia.org/wiki/Compression_ratio

 

About half way down under Fault Finding and diagnosis:

"There is, however, no formula to calculate compression ratio based on cylinder pressure."

 

So, there is no easy way to do this. All you can get from psi measurements is a state of health of the engine. I.e balanced psi between cylinders to indicate good head gasket etc. Also if the rings are worn which would indicate a lowered psi.

 

All I think you can get is an upper limit to psi which the engine cannot cope with unless forged pistons and stronger con rods are used etc.

 

Simon.

[sj-bradley]

Added:

 

Also this gives a "ball park figure" - so they say.

 

http://www.hotrodder...atio-66664.html

 

But please don't take this as gospel. It's likely to be highly inaccurate.

 

On second thoughts - after reading further on - ignore those readings ! You'll see when you read that site

 

Simon.

[knights_templar]

not sure why you would want to go to all that trouble

 

agree with simon, to many variables.

 

regards

[sj-bradley]

That wikipedia site I mentioned earlier is quite good. Engine compressions of between 150 and 200 psi are considered normal.

 

It also mentions the factors that would result in poor psi readings and the best way to diagnose the problems.

 

Interesting reading

 

Simon.

[sj-bradley]

Also I've read the simplistic formula given here (see picture) "might" give you an initial idea of the compression ratio.

- But again - Too many other factors will influence the true figure.

 

Simon.

[Bob Tucker]

Hi,

My PSI readings are generally 215-220 psi, taken regularly over the past 4 years. (highly tuned XE) which has 11.5:1 (forged rods & pistons)

At the end of last summer the performance started to drop off, no power below 5500, then a real kick up to 7800.

I checked comp readings again, & they had all started dropping, ending up around 175PSI. I expected head gasket damage, but I also ran a leak down test (courtesy of my brother's love of gadgets) which showed no change at all from previous readings. I used an endoscope (my brother's again!) to examine bores & pistons, & everything looked fine. Eventually, I found that the inlet valve vernier had come loose & cam timing was out by just 5 degrees. Correcting it put compression back to 220 & the kick now comes in at 4k. just like before.

It surprised me how much difference it made, 220 to 175psi, from an identical mechanical setup & comp ratio, just down to wrong valve overlap.

So no real link between comp ratio & cylinder pressures at all.

 

HTH Bob

[Snapperpaul]

Simon, Ta, now I have some maths to look at.

I have been asking the relationship between static compression ratio, dynamic compression ratio and actual psi seen at the guage but no one came up with an answer.

I have static and dynamic compression ratio calculators on Excel spread sheets and will now construct one from the formula you have provided.

Perhaps, just perhaps it will make the relationships a littl clearer.

 

It won't however tell me what the pressures are once the cam starts working

[Guest Getting There]

Hi

 

Unfortunately for what you are trying to do you can only get ball park figures very roughly. I work in an engine testing place at Huddersfield and one of the Phd students tried to do a very similar thing to what you are trying to do. In order to get repeatbale values that are real life the engine has to be running, not just on the starter motor and it requires a load applied. When the work was completed the only way that the CR could be determined was when the engine was running, and an in cylidner Kistler transducer was used, which maps the cylinder pressure during a cycle above 1500rpm when the pistons are moving fast enough to get a correct reading.

 

The other problem we have found is that the meters that are sold in shops can vary as much as 15psi on the same cylinder and operating conditions meaning that unless the figures use the same meter then the inaccuracy is too great.

 

Thanks

 

Chris

[richardm6994]

Simon.....great stuff...v.helpful

 

Chris, that's very interesting about the PHd student.

 

I understand that there are loads of variables to consider.....which I why I was going to ignore the variables and just collect data from everyone. By doing it this way, you are not looking for a mathematical formula, but simply a pattern......and then extrapolate from that.

 

For example...lets say for instance that 10 people confirm they are running 10.7:1 ratio's....and there compression readings are between 195 - 210 psi. You can then take a good guess when measuring an "unknown" engine that if it's compression reading is between 195-210psi, it is also running around 10.7:1.

 

Collate as much info as possible from as many engines as possible, and it will soon become apparent what PSI reading equate to what ratios. The you don't need a formula, because you can just average these these figures and plot them on a graph?

 

The reason for doing this is 1) simple curiosity and 2) I come across no end of "tuned" pinto engines which people ask me work on...and half the time I do not believe the CR I'm being told.

[Guest old_timbo]

The snag is you will need to know the cam being used too. A standard 2.0L Pinto has a 9.2:1 compression ratio, and a good psi reading is about 175 psi with the standard cam IIRC. I put a Piper 285 cam (much more overlap than standard and a bit more lift) in my otherwise standard engine and got a psi reading of 147psi on all cylinders. This is why you need to skim the head to get the compression ratio back up to get the benefit out of a hairy cam!

I hadn't realised the cam made so much difference so did a leak down test to see where all my pressure had gone - everything was fine. I've now got a Piper 134 injection cam which has less overlap and more lift than the 285. One of these days I'll do another compression check and see how the reading has changed.

[Snapperpaul]

This is why the Americans use Dynamic Compression ratio a lot more than we do as it uses rod length and cam duration in the calculations.

There are cams out there for oval racing where the engine is mostly standard, a Lightning Rod cam is designed to give more time for the cylinders to fill but gaining back some compression by clever closing of the inlet valve ( lobe ramp) and early exhaust closing.

Longer inlet duration, shorter exhaust duration and less overlap

[richardm6994]

I know people are getting sick of me talking about compression ratios, but in a last ditched effort, I'm hoping someone can cast some light onto my concerns;

 

My Engine; 2.1 pinto (93mm bore from memory and 94.5mm x 70tho thick cometic gasket).

Pistons are approx 10tho above the deck height.

Standard crank stroke of 76.95mm

Combustion chambers were 48cc (std I think), but the head has been skimmed 15tho and I didn't measure them after skimming. For the sake of arguing, I'm going to assume they are 46cc (kind of worse case senario)

Cam is Kent FR32

 

Okay, now for the maths (for ease assume 40tho is 1mm);

 

1) Because the piston is 10tho higher than the deck, it only travels 76.7 (0.25mm less) in the bore than standard. As such, Piston - Cyl displacement = 532.28cc

2) The 10tho that the piston is above deck high, it is in the 94.5mm bore of the gasket.

Piston - gasket displacement = 1.75cc

3) The gasket is 70tho thick, therefore 60tho is left empty when the piston is at top stroke.

Gasket remaining volume = 10.52cc

4) Assume combustion chamber are 46cc.

 

Total volume = 590.55cc

Total compressed volume = (10.52cc+46cc) 56.52cc

Therefore CR = 10.45 : 1 suggesting that cast pistons will be okay.

 

In practice, I'm getting around 220psi on compression test (it has always been this high). After talking to JEM engines (who supplied my bottom end and head), they have said that this pressure is too high as a safe figure is no higher than 210psi. I am also aware Burtons suggest no higher 190psi

 

Jem engines are also advising me that if I am getting these readings, to make sure my ignition timing does not exceed 33degrees to prevent predet. I current run a nice 3d map on my megajolt, but it has figures around 38degrees in it (worrying).

I'm really struggling what to think. Either leave it as it is and let the rolling road guys set it up (but make them aware of the compression readings), or take the head off and fit a 10tho ticker gasket (this is the last thing I want to do!!)

 

....other than that, I'm borrowing another compression tester tonight to double check the compression readings I've been getting against my £15 Draper tester.

Is there anything you can see that I've overlooked...miss calculated etc...?

 

Also if any nearby feels like popping in to have a look over the engine....it would be greatly appreciated and the kettle will be on....or beers in the fridge!

 

In short.......I had hoped that after this winters hard work, I would have a sound engine which I could simply enjoy.......however all this CR talk (and my own compression readings) has got me in a spin!

 

HELP!!!!!!!

[Guest mower man]

Hi I have a Snapon c/tester if you would like to try, it lets org a time and day ,me to you ,you to me it doesn't matter my 2.1 regs 175 psi cold ,pstons are flush to deck 1 mm gasket [comp]head has had a clean up lick ,no more than 10 thou ,std chambers [?] get in touch --mick

[richardm6994]

I've had a good long chat this afternoon with the guys at the rolling-road place I'm going to on the 30th April. They got quite a lot of experience with Pinto's and are specialist in Megasquirts. With what they have told me, they sound like they know what they are talking about and have put my mind at ease somewhat.....

 

When the car goes for the RR session, they will MAP the megajolt accordingly and make sure I dont get any pinking.

 

Also, they seem to think that my "mathematical" compression ratio as detailed above (10.45 : 1) is probably a closure representation to my actual CR is, and that the compression test reading (220 psi) I'm getting doesn't give any real indication to the CR......it only shows that I'm getting a good piston and valve seals.

 

They have told me that at cranking speed, two different engines both at 10.5 : 1 CR can give very different compression readings (they can range from 180-225 psi), however when these engines are both at running speed, their combusion pressures will be very similar to each other becasue their CR's are both at 10.5 (assuming same cam).

 

So the long and short of it is that the mathematical CR is what I need to work to, and although it's close to the limit for cast pistons....I should be okay (he says very nervously).

 

Thank you for your offer Mick, but I think I've just made the biggest mountain out of a mole-hill I'm going to leave it now and let the boys at the RR set things up and tell me if I've got any problems.

 

And to everyone else......I realise that I have just gone around in one big circle back to what you was all saying at the start of this thread (and the other thread I hi-jacked). Sorry! I'm 30 this year so not many marbles left!!!