31 replies to this topic

[diogoteix]

Hello guys,

Due a poor work done on my cylinder head, I'm currently having trouble with a compression ratio of about 12/12.5 (read on a compression meter). My calculations were supposed to lead to a ratio of about 11.0... I'm not sure if the mistake has been done by the people who cut the head or me (my guess, it's them, I'm in Portugal and quality of service is not something very common...).

Anyway, I need to reduce compression ratio to about 11, which means incresing cylinder head volume by about 3 to 4 cc. I see 2 options:

• 3 to 4cc corresponds to the volume of a head gasket. Do you think it's feasible to put 2 head gaskets or will I just end up with a leaking head? I've got a dry deck so water galeries are shut, but oil and air (between cylinders) might be an issue?

• Alternatively, I can have to head volume increased. It will be obviously more costly, and I'm not sure if it's easy to remove 3cc to a head. Despite being an old "SPI stage 3" from Minisport, the head volume seems to be standard (20 to 21 cc). Do the valve seats need to be reground or can the "meat" be taken around without disturbing to much the head chamber eficiency.

Thanks for your suggestions

Diogo

[AndyMiniMad.]

What sort of pistons are you fitting? Could you have the dish size made a bit bigger?

[jaydee]

You can open the chamber a bit and gas-flow them, and you'll have a chamber size increase.
Two gaskets together can be done, but it will end up as an unreliable bodge, but you can buy a decompression plate instead, like the one used in some forced induction conversions.

[cooperrodeo]

Excuse my ignorance but how does a compression meter give you an accurate measure of compression ratio?

[colinu]

As Cooperrodeo said… a compression guage won’t give you an accurate measure of CR (only really an indication of CR problems such as one cylinder being low compared to the others). You really need to do some maths to determine static CR… measure bore, stoke, ring landing, piston dish, piston/deck height, head chamber volume, etc. Once you know the actual (static) CR then you can calc how much CC you need to add to the unswept volume in order to arrive at e.g. your 11:1 CR.
I guess there are several ways to do that amogst which include:

• Remove material from the head chambers
• Fit pistons with a larger dish
• Use a thicker head gasket

I recently had to reduce my CR (from 11.5 to 10:1 - engine rebuild and using a different spec cam). I opted for a thicker gasket to add about 5cc to the unswept volume (a Cometic MLS gasket – they have several thicknesses or can make a custom one to your spec).

[diogoteix]

Thanks a lot for the custom gasket reference. I will have a look at their web site

As for the math, I did it before the work was done, but not after since the engine was rebuild before i could check it. But I really suspect the CR to be too high for 2 reasons: maximum advance i can put without pinging at high load is about 20 degrees, and when i put low grade petrol, then i cannot even think about fully opening throttle below 3000 rpm.... The block and pistons, originaly 11cc omega 73,5, have been planned/cut supposedly by 0.8 mm, to get about 11 in CR, but i suspect that the real cut has been deeper.

What king of relation exists between static CR and the one mesured by a compression meter. I would think that static CR would always be above the mesure taken from a compression gauge, since you must add any leaks and the fact that, despite opening the throttle at full, there is a bit of depression in the intake?

Thanks for your help
Diogo

As Cooperrodeo said… a compression guage won’t give you an accurate measure of CR (only really an indication of CR problems such as one cylinder being low compared to the others). You really need to do some maths to determine static CR… measure bore, stoke, ring landing, piston dish, piston/deck height, head chamber volume, etc. Once you know the actual (static) CR then you can calc how much CC you need to add to the unswept volume in order to arrive at e.g. your 11:1 CR.
I guess there are several ways to do that amogst which include:

• Remove material from the head chambers
• Fit pistons with a larger dish
• Use a thicker head gasket

I recently had to reduce my CR (from 11.5 to 10:1 - engine rebuild and using a different spec cam). I opted for a thicker gasket to add about 5cc to the unswept volume (a Cometic MLS gasket – they have several thicknesses or can make a custom one to your spec).

[diogoteix]

Excuse my ignorance but how does a compression meter give you an accurate measure of compression ratio?

Indeed, I suppose it does not, but should give a minimal value, since the real CR is not affected by leaks or intake depression. Since I read very high values with the compression gauge, between 12 and 12.5, I suppose the real CR should be at these levels or slighly above (the block/pistons are new so the leaks should be reduced to a minimum)

But maybe my reasoning is just totaly wrong?

[bmcecosse]

So - ONLY EVER use top grade fuel, and set the timing back slightly........ DO NOT attempt to use 2 headgaskets....... If you really must - open up the chambers almost to the gasket line.
Edit - You say '12 to 12.5' - is that Bar? If so = 178/185psi which is NOT high, in fact it's not much better than 'average'. Compression ratio is NOT the problem. What mechanical ignition advance is in the dizzy? And is the vacuum advance working as it should??

Edited by bmcecosse, 04 October 2011 - 11:01 PM.

[diogoteix]

What sort of pistons are you fitting? Could you have the dish size made a bit bigger?

Pistons are 11cc omegas, put supposedly cut by 0,8 mm to achieve the "right" CR. As a result their dish is now only about 8cc. I dont know if another 0.8 mm can be dug into this king of piston without risking getting a hole?

[diogoteix]

So - ONLY EVER use top grade fuel, and set the timing back slightly........ DO NOT attempt to use 2 headgaskets....... If you really must - open up the chambers almost to the gasket line.
Edit - You say '12 to 12.5' - is that Bar? If so = 178/185psi which is NOT high, in fact it's not much better than 'average'. Compression ratio is NOT the problem. What mechanical ignition advance is in the dizzy? And is the vacuum advance working as it should??

Yes indeed it is in Bar units. Since the intake pression at full throttle is at or close to 1 bar, atmospheric pressure, I supposed a 12.5 reading can imply a 12.5 CR. But again, this is my best guess based on fragment of basic college physics! I m no expert in static versus dynamic compression readings on Mini engines...

Ignition is mapped in my ECU (emerald K3) so there is no issue related to vaccum or dizzy adjustments. The only margin of error could come from the point 0 reading, from the alignment of the crack sensor. On a 36 tooth sensor wheel, the distance between tooths is 10 degrees, so margin of error is 5! Meaning that when i say 20 degrees of advance it can be in fact 25...

[diogoteix]

By the way how much did you pay for such custom gasket? They dont give any prices on their website since most items are indeed custom specs...

As Cooperrodeo said… a compression guage won’t give you an accurate measure of CR (only really an indication of CR problems such as one cylinder being low compared to the others). You really need to do some maths to determine static CR… measure bore, stoke, ring landing, piston dish, piston/deck height, head chamber volume, etc. Once you know the actual (static) CR then you can calc how much CC you need to add to the unswept volume in order to arrive at e.g. your 11:1 CR.
I guess there are several ways to do that amogst which include:

• Remove material from the head chambers
• Fit pistons with a larger dish
• Use a thicker head gasket

I recently had to reduce my CR (from 11.5 to 10:1 - engine rebuild and using a different spec cam). I opted for a thicker gasket to add about 5cc to the unswept volume (a Cometic MLS gasket – they have several thicknesses or can make a custom one to your spec).

[diogoteix]

So - ONLY EVER use top grade fuel, and set the timing back slightly........ DO NOT attempt to use 2 headgaskets....... If you really must - open up the chambers almost to the gasket line.
Edit - You say '12 to 12.5' - is that Bar? If so = 178/185psi which is NOT high, in fact it's not much better than 'average'. Compression ratio is NOT the problem. What mechanical ignition advance is in the dizzy? And is the vacuum advance working as it should??

If anyone could check its compression reading on a compression meter and compare it to its static CR, it could help us see what relation we have between these 2 measures. Even better if we have a small set of readings since engine wear will have some impact...

[diogoteix]

I ve got a nice link for those who would like to explore compression ratio versus engine pressure relationship:
http://mgaguru.com/m...power/pp105.htm

In brief: engine pressure is usualy somewhat lower to compression ratio since valve timing, usually overlapping in the case of our favorite tuned specs, imply that part of the air/fuel intake goes back to the manifold.

Thanks a lot BMCECOSSE for pointing me in the right direction

In a "perfect engine", and with a totaly accurate gauge, readings should be the same even if you need some calculation:
In my case 12.5 bars=12.5x14.7=182psi

CR=(182/14.7)^0.71=5.96 well quite different from my static CR whatever it is...

Cam timing have a great impact on engine pressure, the wilder the cam the lower the pressure for a given CR. In my case i m running a sw5-7.

Wikipedia also gives some nice ranges: for a "perfect engine" CR of 10:1 would read in a compression of 25 bars. In real world engines, readings go usualy from 150 to 200 psi (abou 10 to 13 bars).

So bottom line:
.The readings i get are not anormal
. Its impossible to estimate CR from compression readings
. My detonation problems must come from another situation. I have a few guesses, like for exemple some

[diogoteix]

... Continued

Some issue with a much too hot intake air (my ecu air temp sensor reads about 45 deg these days, climbing even to 50 in Lisbon trafic...) .

There can be also some margin of error in my advance setup since the reference point might not be that accurate. Meaning that I may be having in fact 25 deg advance when my ECU displays 20.

And i will stick to higher grade fuel...

[colinu]

Sounds like your problems are not CR based, but to answer your earlier question of where did I get my Cometic head gasket from and at what cost...

Ebay USA, this item (0.075"), US$126 + delivery (ouch!)
http://cgi.ebay.com/...=item35af7375ca

Not cheap but the only option I had - my head is already heavily worked and taking any more material out of the chambers could have trashed it