compression ratio of my engine

[disco-v8]

ok ive done alot of work to my rover V8 and alot of money has been spent on it.... first off its a 3.9 injection engine with the 9.35 heads, so should have a compression of 9.35:1 ........... but ive had the heads skimmed more than enough and have fitted the thinner tin head gaskets, to help increase the CR.... plus crower cam shaft and rhoads lifters along with alot of other things................

......... when i first did a compression test on the engine when i first had her i was getting an average of about 9.5 BAR, which i thought was ok, then i rebuilt her and then got an average of 10.5, which i thought was great.... but after doing abit more head work and fitting alot of mods i now get 15BAR SUPER i thought..........


unless i take off the heads i cant realy messure the smaller combustion chambre so i cant work out mathmaticaly what the CR is



NOW MY MAIN QUESTION.... is there a way of working out the CR of my engine with knowing that the compression pressure is 15BAR????

[kiwi303]

well, 1 BAR is 1 atmosphere, so 15 BAR means your engine is compressing the air in the cylinder to 15 times the resting static pressure, you're therefore prsumably close to 15:1-ish CR... the question is how much the head work means the airflow velocity through the ports is causing the air to cram in and compress the air already in the cylinder as the piston reaches BDC and the air's inertia keeps it moving despite the piston stopping.

13 to 15 : 1 CR depending on port flow efficiency is my guess

[CastleMGBV8]

Disco?

Firstly static compression ratio is determined by calculating the swept volume of the cylinder which on a 3.9 is 494cc to which you add the volume of the head chamber 36cc (unskimmed) the gasket thickness 3.5cc(tin) and the deck height which can be anything between 20-60thou say 3.5cc-10cc. and lastly the volume of the piston bowl which is i believe with h/comp pistons 9.35/1 = 14cc.

So for a standard engine cyl vol. 494cc + 36cc. + 3.5cc. + 14cc. + say + 6cc. for deck height = Total of 553.5cc. divided by 59.5cc = 9.3/1 which is about as close as you can get without accrate measurement of every component.

Just to clarify it is the piston bowl size that determines the compression ratio on a standard engine not the cylinder heads.

The question is just how much did you have machined off the heads because the figure of 15 Bar just does not look realistic for a relatively unmodified engine with standard pistons and would suggest a compression ration in the region of 11.7/1 !

One other point is that if you have had excessive amounts machined from the heads is that your lifter pre load will be well in excess of the maxcimum recommended 60 thou, 20 thou is the recommended optimum pre-load.


Kevin.

[disco-v8]

well, 1 BAR is 1 atmosphere, so 15 BAR means your engine is compressing the air in the cylinder to 15 times the resting static pressure, you're therefore prsumably close to 15:1-ish CR... the question is how much the head work means the airflow velocity through the ports is causing the air to cram in and compress the air already in the cylinder as the piston reaches BDC and the air's inertia keeps it moving despite the piston stopping.

13 to 15 : 1 CR depending on port flow efficiency is my guess

i used to think this at one point but, 15bar doesnt mean 15:1 compression ratio....... as the psiton compresses the air inside the cylinder it creates HEAT and that extra heat increases the pressure....... so mathmaticaly it my be only 11:1 but just like i used to think, its 15 BAR so 15 atmospheres just like you said......WRONG!!!

Disco?

Firstly static compression ratio is determined by calculating the swept volume of the cylinder which on a 3.9 is 494cc to which you add the volume of the head chamber 36cc (unskimmed) the gasket thickness 3.5cc(tin) and the deck height which can be anything between 20-60thou say 3.5cc-10cc. and lastly the volume of the piston bowl which is i believe with h/comp pistons 9.35/1 = 14cc.

So for a standard engine cyl vol. 494cc + 36cc. + 3.5cc. + 14cc. + say + 6cc. for deck height = Total of 553.5cc. divided by 59.5cc = 9.3/1 which is about as close as you can get without accrate measurement of every component.

Just to clarify it is the piston bowl size that determines the compression ratio on a standard engine not the cylinder heads.

The question is just how much did you have machined off the heads because the figure of 15 Bar just does not look realistic for a relatively unmodified engine with standard pistons and would suggest a compression ration in the region of 11.7/1 !

One other point is that if you have had excessive amounts machined from the heads is that your lifter pre load will be well in excess of the maxcimum recommended 60 thou, 20 thou is the recommended optimum pre-load.


Kevin.

im sure the high compression head like in the 3.9/4.0/4.6 have a volume of 28cc not 38cc, fair enuff the piston size and shape hasnt changed but the combustion chambre in the head has..... it must of changed by alot as when trying to put the inlet manifold back on i had to reinlarge the bolt holes just to get the damb thing to fit..............

the preload is set at 20tho as some of the other work ive done was buying adjustable pushroads,so its set up alot better than most RV8's out there.......... ive also inlarged and ported the inlet and exhaust ports so this will help with the engines breathing

[CastleMGBV8]

All 3.5 and 3.9 heads have 36cc. chambers the 4.0 and 4.6 engines have 28cc chambers but use thicker composite gaskets and have different pistons to adjust the compression ratio.

Are you saying you don't know how much was removed from the heads!

How many bolts do your heads have?

Kevin.

[kiwi303]

i used to think this at one point but, 15bar doesnt mean 15:1 compression ratio....... as the psiton compresses the air inside the cylinder it creates HEAT and that extra heat increases the pressure....... so mathmaticaly it my be only 11:1 but just like i used to think, its 15 BAR so 15 atmospheres just like you said......WRONG!!!

Ah, heat from compression is something I overlooked.

[disco-v8]

All 3.5 and 3.9 heads have 36cc. chambers the 4.0 and 4.6 engines have 28cc chambers but use thicker composite gaskets and have different pistons to adjust the compression ratio.

Are you saying you don't know how much was removed from the heads!

How many bolts do your heads have?

Kevin.

its a 10 bolt head....

nope i aint got a clue to how much was taken off as it was done by the person i bought the heads off. and they where taken from a 4.6 V8

[CastleMGBV8]

Well you've something in excess of 11.25/1 far too high for anything but the track and using race fuel!

If you still have the 3.9 heads I'd put those back on.

Kevin.

[sidecar]

In answer to your question about CR, IMHO the only real way to work theoretical CR is by buretting the heads and the pistons at TDC. The head gasket can be calculated. The CR can then be worked out using a bit of maths.

A few months ago you posted about this engine not running at high revs, did you ever get that sorted out? I suggested that the problem might be due to detonation, did you ever get to the bottom of the issue?

The max PSI is affected by too many other factors to be a reliable method of working at the CR.

15 bar is about 217 PSI, from what I've read I think that is too high, especially considering your cam will have a longer than the standard duration so it should be lowering the PSI at low (i.e cranking) speed.

Like I said....AJMHO!

Pete

[disco-v8]

In answer to your question about CR, IMHO the only real way to work theoretical CR is by buretting the heads and the pistons at TDC. The head gasket can be calculated. The CR can then be worked out using a bit of maths.

A few months ago you posted about this engine not running at high revs, did you ever get that sorted out? I suggested that the problem might be due to detonation, did you ever get to the bottom of the issue?

The max PSI is affected by too many other factors to be a reliable method of working at the CR.

15 bar is about 217 PSI, from what I've read I think that is too high, especially considering your cam will have a longer than the standard duration so it should be lowering the PSI at low (i.e cranking) speed.

Like I said....AJMHO!

Pete

yeah got it all sorted.... the ignition amp was breaking down.... didnt find out untill it wouldnt start one day, and found i wasnt getting a spark...... changed itt for a complete MSD ignition... made hell of a lot of difference

it happened again not so long ago, but that turned out to be a knackered idle control valve... it completely came up the ghost, i also recon it way have been on its way out while the amp was failing, so had to problems....


when you said my cam has a longer duration, in what way do you mean???? if you mean overlap then you are wrong, my inlet opens 7 BTDC and exhaust closes 11 ATDC so ive only got 18 degrees overlap which i think is hardly any....... i chose this cam to get as much torque as possible and by having hardly any overlap, to help with compression

[sidecar]

yeah got it all sorted.... the ignition amp was breaking down.... didnt find out untill it wouldnt start one day, and found i wasnt getting a spark...... changed itt for a complete MSD ignition... made hell of a lot of difference

it happened again not so long ago, but that turned out to be a knackered idle control valve... it completely came up the ghost, i also recon it way have been on its way out while the amp was failing, so had to problems....


when you said my cam has a longer duration, in what way do you mean???? if you mean overlap then you are wrong, my inlet opens 7 BTDC and exhaust closes 11 ATDC so ive only got 18 degrees overlap which i think is hardly any....... i chose this cam to get as much torque as possible and by having hardly any overlap, to help with compression

I just assumed that if you had a "performance" cam then it would be one which was longer in duration than the standard one.

[disco-v8]

yeah got it all sorted.... the ignition amp was breaking down.... didnt find out untill it wouldnt start one day, and found i wasnt getting a spark...... changed itt for a complete MSD ignition... made hell of a lot of difference

it happened again not so long ago, but that turned out to be a knackered idle control valve... it completely came up the ghost, i also recon it way have been on its way out while the amp was failing, so had to problems....


when you said my cam has a longer duration, in what way do you mean???? if you mean overlap then you are wrong, my inlet opens 7 BTDC and exhaust closes 11 ATDC so ive only got 18 degrees overlap which i think is hardly any....... i chose this cam to get as much torque as possible and by having hardly any overlap, to help with compression

I just assumed that if you had a "performance" cam then it would be one which was longer in duration than the standard one.

nope everything ive done to this engine is to get asmuch torque out of it as possible, and i think its working well