New Race engine build

[rgh0]

it isn't 12 bar - his gauge would (ef) indicate a ratio of 10:1 (ca) if the reading would've shown something between 9 and 10 on his instrument, so at way over 12 (where the needle stops) it could even be 14:1!!! plus, as i said, mine have a dome height of 5 to 6mm and schaffner "uses" 3,8mm for a 11:1 compression - i'm also just thinking about shaving off 2 (or so) mm with a mini mill (magnetical) as included (pistons centered at the very top and everything well covered to prevent shavings getting anywhere Sandy - .......... just maybe a crazy idea

Sandy I have never seen a compression gauge that reads in compression ratio. Everyone i have seen reads in PSI and bar and I think yours reads in bar, can you show a photo of the dial that shows the gunge calibration and the makers name. Regardless you need to calculate the actual geometric compression measured ratio as you cannot simply determine it by measuring compression pressure even if the gauge is marked that way. Once you have done that correctly and provided information on the details of the calculation and also provided details on the cam duration and timing you have, then it will be possible to advise how best to achieve the required compression ratio if a reduction is required.

There is no specific intruder height requirement for a specific compression ratio as there are to many other variables. You need to also allow for e.g valve cutout diameter and depth , intruder cone diameters, gasket thickness and bore, engine bore and stroke, head thickness and combustion chamber volume and then what compression ratio is acceptable depends on the cams and the fuel you will be using and how its then best achieved depends on all the preceding data. Not a simple exercise.

If you don;t believe me then talk to your engine specialist Schaffner and if he does not believe me then get him to contact me for some discussion as i am always happy to discuss the best way to build a good twink and sometimes I learn something new which is always good.

I guess you could use a magnetic mill to reduce the piston height in situ if needed and if that is the best way to reduce compression but you would need to firmly lock the pistons with wedges in the top of the bore as they can wobble which would give a mill a real problem. I have been involved with a lot of in situ machining on large industrial equipment and it is never an easy exercise and need real expertise to do well.

cheers
Rohan

[el-saturn]

i should've copied the height of the last size which was working beautifully, rather than listening to my motor man. my last were 83,5 and the next size is 83,65 - according to omega/accralite these two are identical with only the dia. difference!! - the height is 3,00mm - schaffner was saying his have 3,8mm and i could go to 4,00. the ones i'm using have 6,00; i think!? do those 2mm male that much of a difference? sandy

[2cams70]

the height is 3,00mm - schaffner was saying his have 3,8mm and i could go to 4,00. the ones i'm using have 6,00; i think!? do those 2mm male that much of a difference? sandy

Measure what you actually have now in compression ratio!!! Total waste of time trying to guess what to do next without this information.

[nmauduit]

As Rohan pointed out, compression ratio is a theoretical build characteristic and canno't be measured directly on most modern engines, including our beloved LTC, as the inlet valves will still be opened for a little while after BDC when the piston starts rising for the compression cycle.

Also, in order to infer theoretical compression ratio from a measured pressure by a gauge (such as that of a compression tester) one would need to know the temperature difference between uncompressed and compressed air inside the engine (PV=nRT). Even at low starter speeds like 100-200 rpm, since compression heats up the air, the actual readout in pressure is higher than what it would be if done quasi statically for a compression ratio measurement (would need to take in account camshaft profile, too, for exact intake closing time after BDC, and that with zero air loss)

I use compression tests as an indicator of engine health, for variations in time or across cylinders : e.g. on my street engine last I measured was (in psi) :
215 210+ 220 210
that is with Q420 camshafts timed as per the manufacturer (In Ex 100?/106?), standard big valve pistons (no race intruder, see photo taken when I installed a bottom breather)

piston top and cutout

[2cams70]

As Rohan pointed out, compression ratio is a theoretical build characteristic and canno't be measured directly on most modern engines

Static compression ratio is a build characteristic that exists and which can be easily measured as described earlier!! Whilst the dynamic compression ratio under running conditions is something different as it considers actual cylinder fill including the effects of valve timing etc. the static compression ratio is still a good yardstick to estimate what is appropriate for a particular state of engine tune and fuel octane rating.

[nmauduit]

As Rohan pointed out, compression ratio is a theoretical build characteristic and canno't be measured directly on most modern engines

Static compression ratio is a build characteristic that exists and which can be easily measured as described earlier!! Whilst the dynamic compression ratio under running conditions is something different as it considers actual cylinder fill including the effects of valve timing etc. the static compression ratio is still a good yardstick to estimate what is appropriate for a particular state of engine tune and fuel octane rating.

well, if you have a device that would fill up two cylinders of a measured quantity of oil and compute the ratio, then you would have a compression ratio direct measure of sorts... plus a mess to clean up when you're done, not mentionning the camshastfs to refit not exactly what I would call easy either, but I guess I'm lazy...

[2cams70]

well, if you have a device that would fill up two cylinders of a measured quantity of oil and compute the ratio, then you would have a compression ratio direct measure of sorts... plus a mess to clean up when you're done, not mentionning the camshastfs to refit not exactly what I would call easy either, but I guess I'm lazy...

If you already have to remove the head to replace the head gasket it's not that hard. You don't have to remove the camshafts either. Just back off the cam caps enough so the valves close. A couple of graduated syringes for filling the cylinders are easy to find and cheap. Furthermore if you can divide one number over the other you can do the compression ratio calculation.

Quite easy I would have thought. Maybe even Sandy could do it in one of his more sober moments!!

[Davidb]

Sandy: Did you replace the con rods during your last rebuild? Some of the rods supplied have the longer "Cosworth" length and will bump the compression ration by at least one point.
Ask me how I know!