3VZ-FE Head Skimming & Compression Ratios

[cdwood2010]

Good input!

Also Pat, if you really want to get all technical, get a small Perspex disc just bigger than the cylinder bore, drill a hole in the middle and stick it to the top of the block with the piston at TDC / the face of the head etc with a smear of grease to seal it.

Get a syringe and pump oil in through the hole. This will give you a near exact CC measure of how much fluid can occupy the available space.

Factor that against the total displacement area of the piston, and you can pretty accurately calculate the compression ratio from an empty cylinder to a full cylinder.

You can then start working out how many thou off the head will get you what ratio and so on, working in the thickness of a gasket etc etc.

I've not seen any info on what a difference the compression ratio will make on a 3VZ, but going by old school engine stuff, the results can be quite impressive.

C.

[adamh]

the above formula is missing the combustion chamber volume :icon_rolleyes: . dont forget inject that also.

2959/6 =493cc per cyl/9.6:1= 51.4 cc for comby chamber + gasket thickness+deck height.
bore is 8.9cm in gasket (guess) therefore stock 1.2mm gasket thickness vol = 7.46cm^3 vol .
change stock gasket thickness by 0.1mm = 0.11cm(pi8.9^2/4) = 6.83cc new gasket vol.
(51.4-7.46)+6.83 = 50.77 (new assumed number of comby chamber. 493/50.77= 9.71:1 therefore 0.1mm=0.1:1 increase.
comby chamber walls are not perpendicular but it aint going to be far off.
it would be nice to have that confirmed using syringe method , the maths for me adds up, but absoloute values are always better (measured)

the main power comes from the explosion pressure and not specifically the cylinder air compression, although substitute the petrol for plain air the cylinder Psi would increase if ratio increased, therefore the force in psi pushing on the crank would increase proportionally. works for air / torque, force x D etc. not sure / know nothing about hp

[cdwood2010]

No it isn't. :) I put block face / head face. But point well made. Pat is hopefully bright enough to figure out that the combustion area should be included in the maths.

It's only a very rough guide, and I've used it in the past to ensure combustion areas on heads that have been reworked remained as close as possible.

Your equations look like they might add a more accurate approach to my thumb in the air method.

I would be interested to see what sort of results could be achieved from this, so expect some more questions flying your way Adam!

:)

C.

[wood_patrick]

Adam, I think that's wrong, I get it to 7.454289cm^3 ;-)
(Haven't got a clue really, kinda feel like a schoolboy watching Countdown)

When I get the engine in bits, I will get the measurements for re-assurance as always good as a reference point.

From a more simplistic perspective however, we know that the standard compression ratio is 9.6:1 so the question Im wondering is how high can the compression ratio be increased (using Adams method for calculations if 0.1:1) before integrity of the internals become compromised or maximised. Establishing the exact internal dimensions will help to be more precise but the ultimate factor is how much the stock internals will cope with... Which is a debate in itself really. :-/

Soooo, if compression is increased from say 9.6 to 10.1 how much will that increase the actual combustion by? 5%/10%/15%?

I would've thought/guessed that a 15% increase would be acceptable but how can I translate that back to a compression ratio? Would I need to take off 0.1mm or 1.0mm to achieve that?

Or am I just trying to make this too simple? :-)

[wood_patrick]

And I know that increasing the compression ratio isn't relative to the actual combustion, simply because the physics are different and in effect I'm talking about compressing compressed air which means the compression ratio to combustion will increase relatively.

So, 0.1 skim could (for example) increase combustion by say 1% but a 0.2 skim could increase 4% etc etc

[cdwood2010]

If you really want to get it exact, do a fill test with the same method with the piston at BDC. Then you will have all the values.

I have big syringes, bought them off of the 'bay.

Also, don't forget that a compression test at cranking will give you some more info.

And let's just do one and see what happens.

:)

[wood_patrick]

3VZ-FE
Stroke 82mm
Bore 87.5mm
Compression Ration 9.6:1


To convert compression ration to PSI x 14.7 for combustion
So, 9.6:1 compression ratio - 9.6 x 14.7 = 141.12 psi (10.6 Compression = 155.82 psi)

Converting psi to Newton/Square meter
1psi = 6894.744825 n/m2
So, 141.12psi = 972986.389774 n/m2

Full bore area in M2 = 0.006013984375

So, in summary, I think the following is fairly close... (Not perfect as I only used pi to 3 decimal places)

Compression Ratio Piston Force Increase as %
9.6:1 - - - - - - - - - - -5,852 - - - - -N/A
9.7:1 - - - - - - - - - - -5,912 - - - - -1.02%
9.8:1 - - - - - - - - - - -5,973 - - - - -2.06%
9.9:1 - - - - - - - - - - -6,034 - - - - -3.11%
10:1 - - - - - - - - - - - 6,095 - - - - -4.15%
10.1:1 - - - - - - - - - - 6,156 - - - - -5.19%
10.2:1 - - - - - - - - - - 6,217 - - - - -6.23%
10.3:1 - - - - - - - - - - 6,278 - - - - -7.28%
10.4:1 - - - - - - - - - - 6,339 - - - - -8.32%
10.5:1 - - - - - - - - - - 6,400 - - - - -9.36%
10.6:1 - - - - - - - - - - 6,461 - - - - -10.41%


Used a few different websites for the formulas, and this is the force of the piston not the combustion.
The force of the piston is in Newtons (N) and if anyone interested the formulas for calculating it are here
http://www.ehow.com/how_7476402_calc...ton-force.html

PSI to N/M2 calculator - http://www.endmemo.com/sconvert/n_m2psi.php

Really dont know just accurate it is, but it gives an indication...

Anyone who is a little more keyed up on the physics side feel free to check through and correct me. :-)



So, I'm currently thinking that I need to check the clearance between the head and the valves then go from there, but at the moment I'm pretty much thinking "Just take a full millimetre off...." At worst I need to rebuild it, and throw a set of heads.

[adamh]

Sorry chris you are correct, i missed that part, i will cane myself later :)

[cdwood2010]

Sorry chris you are correct, i missed that part, i will cane myself later :)

No worries Adam, it was a bit vague, but worth stressing the point. As you can see, Pat loves getting stuck into stuff like this.

I'm actually quite excited about this. Last time i played with compression ratios was on an A Series....

:)

Edit - I have changed the name of the thread slightly to reflect the who discussion, and stickied it.

[wood_patrick]

Well will have a block and heads on the bench within next couple of days and will measure clearance and do the syringe test and update. :-)