Compression ratios

[bdwoody]

It may well have been asked a trillion time before but here goes. the comp ratio i think is worked out by swept volumn divided by combustion chamber but do you include the head gasket and spark plug and the gap around the piston and valve pockets or is it a bit simpler ?...woody

[Al Weyman]

Yes you do!

[bdwoody]

Hi al, if this is the case 1998 cc / 4 = 499.5cc / 55cc combustion chamber,head gasket ,piston crown ,ring lands and pockets all measured with water in a 10 cc syringe = 9.0818 to 1 comp ratio which is less than standard. how can this be ?

[Al Weyman]

Thicker head gasket than standard maybe a composite competion one where as (like a Chevy) the standard is stamped steel? Over enthusasim with the grinding stones in the combustion chambers? Deeper pockets in the pistons for valve clearance on a high lift camshaft? Wrong compression height on the pistons? Crank ground offset etc etc etc!!!!

[bdwoody]

Nope apart from the cams the motor appears to be stock. i was wondering to up the comp rather than skimming the head which would bring the valves into the pistons ,weld up some of the chamber to decrease the 44cc that is there. the head gasket is a stock payen one 1.23mm thick .

[GORDON STREETER]

You have made the common mistake as a lot of people do and not included the complete volume of the cylinder that includes the combustion chamber and head gasket etc for a start .you have over 10.1 cr

[Al Weyman]

Gordon is right of course, see here http://www.circletrack.com/techartic...e_compression/

[Adam43]

GORDON, are you saying the actual calculation for the example above is (449.5+55)/55 ?

Which makes sense, it is the ratio between volume with no compression and compressed volume.

[GORDON STREETER]

Yes it really is simple 499.5 +55 = 554.5 divide by 55 = 10.08 to 1
You should really use a burrett rather than a syringe as it will be more accurate. also use diesel and not water.

[bdwoody]

that now makes sense now guys , and to up comp ratio it now looks like there aint no cheap way to do it as i had thought ,got to get some more cash from someplace for higher comp pistons it seems .its one thing after another this motorsport malarky init.... doh.

[bdwoody]

Gordon why diesel i thought water had a lower specific thingy ?

[GORDON STREETER]

bdwoody This is static cr and I note that you probably don't have std cams.
You should really set the cr to the cams as the cr will be worse dynamically.
I have to run over 12.1 static with the cam I use.
Water tends to hold bubbles easier.

[tristancliffe]

And water has a higher surface tension, meaning that the meniscus (spelling? Anyway, the 'skin' on the top) is less of a problem on accuracy.

I measure the volume of the combustion chamber, the volume of the piston crown (by putting the piston a known amount down the bore, and subtracting the burette volume from the geometrical ((pi*D^2)/4) volume, the volume of the headgasket (when compressed, NOT when new), and calculate the volume of the swept cylinder (Bore Area * Stroke).

Cr = (Vhead + Vgasket + Vswept - Vcrown) / (Vhead + Vgasket - Vcrown)

[zefarelly]

is red diesel ok Gordon

I have 384cc + 30+5 = 419
____ = 11.97:1
35

I also have a thin warped head and 2 blown head gaskets after trying to push 12.5 :1 !!!

[GORDON STREETER]

I'm not suprised that you blew head gaskets, fancy running it on red diesel.
A 400deg cam will sort your problems !

[dtype38]

Originally Posted by zefarelly

is red diesel ok Gordon

If you're using that, does it mean you classify your car as an agricultural vehicle?

[zefarelly]

400 degree is supermarket sweep territory, no wonder 320 is classed as a shopping car cam

I think 12:1 is a sensible limit CR wise, especially if you only want to build your engine once a year!

[Notso Swift]

Why stop at 400, go the whole hog with 720.

[johnny yuma]

Al Weyman's post has a good site,but don't neglect measuring EVERY head chamber volume as described, if they're different you'll have maybe 4 different C/Rs and it will be impossible to set the timing correctly for all pistons,unless remedial work is done.

[Notso Swift]

Originally Posted by johnny yuma

Al Weyman's post has a good site,but don't neglect measuring EVERY head chamber volume as described, if they're different you'll have maybe 4 different C/Rs and it will be impossible to set the timing correctly for all pistons,unless remedial work is done.

Providing you don't want different compression on different cylinders, that is
not always common, but with some engines various cylinders run hotter than others so you may lower the compression to prevent detonation. Of course the current generation of engine management systems allow you to trim the advance on each cylinder as well, but it is not always the case

Personally I have never bothered, but I know builders who do.

[JohnD]

Thoroughly disagree that water is a suitable volume measuring agent.
It will not wet oily surfaces or fill all the inevitably slightly oily chamber without bubbles. Use liquid paraffin, which is equally hydrophobic/hydrophilic. It will wet oily surfaces AND wash off with water. Ordinary paraffin is just too smelly.

Buy liquid paraffin from a chemist(pharmacist) but be ready for rumours around town about your severe constipation.

John

[SidewaysFeltham]

Originally Posted by Notso Swift

Providing you don't want different compression on different cylinders, that is
not always common, but with some engines various cylinders run hotter than others so you may lower the compression to prevent detonation. Of course the current generation of engine management systems allow you to trim the advance on each cylinder as well, but it is not always the case

Personally I have never bothered, but I know builders who do.

Which is why on the dyno, you measure each exhaust temp.

Or as we used to do, back in the old agricultural days, compare the colour of each pipe!

Or as Freddy Dixon did with his Rileys, test run at night and compare the flame colour! But then Freedy was an ex bike racer and that's why he used four Amal carbs.

Of course, if you are really building your engines properly, then you dry build and carefully compare the throw of each crank pinion and thus the swept volume of each cylinder to try and arrive at the same value.

Thereafter, if each cylinder space (provided you actually have any! Some Torroidal designs have very little and the early Ford crossflow 1100 was a totally flat head!), is the same, you have balanced the BMEP/cylinder and you have greater smoothness.

As an example of this, most production BMC A Series cranks were out degrees in timing (i.e. not fractions!) throw by throw and quite alarming amounts in terms of stroke!

[bdwoody]

the measuring of exhaust temp is interesting any more info on header temps when running if all is correct?

[SidewaysFeltham]

Not in specifics, I'm afraid. And nothing for modern "Lean Burn", as they run much, much hotter!

On the dyno, we used to look for a nice cherry red colour on the exhaust pipes just immediately after the port. Any sign of a whiter colour meant over-heating and first step was to enrichen that cylinder.

Most light airplanes have exhaust manifold temp. gauges. This is a check on over-lean mixture once the airplane has reached operational ceiling and speed and the prop pitch has been wound back.

An engine manufacturer should be able to provide exhaust temp data as the exhaust valve will be designed to handle that.

Of course, once you start deviating from manufacturer's specs and increasing power, the run temp will naturally be higher, as you are seeking to achieve a greater volumetric efficiency and therefore burn more charge weight per cycle; ergo = greater combustion temp.

[tristancliffe]

The most important thing is that they are the same. The overall mixture is set looking at emissions values, and each cylinder is tuned to acheive a balance.

However, some cars do have hotter front cylinders than rear (or vice versa), and attempting to balance EGT on those will actually be making it worse. Talk to people who 'know' that particular engine well.