Arrg?

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Obviously I need an education here. What your saying makes sense cos that's what people tend to do- big overlap cam shaved & polished head.

But......

My understanding was that the big cam, extractors and tuned inlet were designed for good scavenging to allow 110-120% of the swept volume into the cylinder, giving you more power when you get enough revs up for it all to start working. So you're only lowering your dynamic compression at low revs, surley its significantly raised when you are "on the cam"?

Got a good website or something that explains all this?

thanks.

you're right, but what changes is actually the dynamic compression ratio which is more a way of thinking about how much squishing is actually happening before the bang.

The static CR is just the ratio of swept + unswept / unswept volumes. That's just plain old maths and isn't affected by cam timing or extractors or voodoo magic.

When you get a new cam with more overlap, they'll usually tell you to raise the CR, because your big cam is actually lowering your dynamic compression.

Fair call. should have thought a bit harder about it.

like I said, I failed year 9 maths. Just thinking it might be useful if you want to work out your comp ratio without taking it apart. Running with the realities of cam timing, I spose comp ratio varies quite a bit over the rev range depending on how good your extractors & inlet setup are

as for the compression test idea...
although it is a nice idea, it falls down because the inlet valve is still open when the piston is well on it's way up the bore. So by the time the pressure in the cylinder starts to increase the volume is already much less than the swept volume. Maybe if you knew the exact cam timing of the valves opening and closing (not at .050" like cam specs are usually quoted at) you could work out where the piston is when compression starts, and then the volume at that point.
Then you can use simultaneous equations to solve the unswept volume bit.

Brain hurting yet? Basically it's a whole lot of maths and a little bit of physics to work out something that probably just isn't worth it... I would just rip the head off....

wow there are some pretty dubious bits of advice flying around amongst the good advice there..

like 1200 coupe says, perspex on gasket face. I used kerosene, and had two small holes, one to let the air out as the kero goes in. if it leaks much out the valves you might need a valve grind!

you can do the same with the block if you seal the rings to bore with grease too. at TDC of course.

and don't forget the head gasket volume!! measure your old gasket for the thickness.

if you're not too sure on the maths side, google search compression ratio calculator or something like that and you should find a good one, print it out and just work through the calculation as they have it. too easy!

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Michael - the best way to cc your chambers is to cut a piece of perspex or lexan sheet (read hard clear plastic) and drill a small hole in it to suit the diameter of the syringe, use some grease or something similar to seal the perspex around the chamber wall (on the head surface) then fill the chamber with fluid until the volume is full. That way you dont have to estimate for any minuscus (surface tension) in the fluid. Most accurate and easy way to do it. All of this stuff should be available at your work.

Heh I work in the physics dept at a university so I could also get my hands on a graduated syringe I guess. I'll look aroudn for one tomorrow.
Thanks,
michael

Michael, I havent actually done it, but I reckon the best way to cc a combustion chanber at home would be to grab a 5 or 10mL syringe from the chemist and measure it out using that. I have found that they are pretty accurate (the bigger ones are a bit rough) A graduated pipette or burette (or a set of precision scales) would be better, but that's only going to happen if you have access to a chem lab (I do)

Hey grunterhunter - I'm still workign on the rebuild. Got the block back this weekend and I'm waiting on a gasket set to start putting it back together. I basically assumed the original configuration was 8.5:1 as stated in the specs and made a few changes to raise it up to around 8.8 (shaved pistons and decked block which needed it anyway). I'll do a comp test when it's back together and we'll see how the calculations compare. Acutally I shoudl probably cc my combustion chamber before i put thehead back on just to have that bit of info. This is an informative thread- thanks guys.
michael

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That gear is important if you are going the traditional (engine apart) route but if you work it out from a compression test then all that is included automatically. What I dont know, is if the calculation actually works. The theory is all good but I need some data.

Mperdue: you said you just did a rebuild- have you done a compression test? what numbers did you get & what were your engine specs........ Also what temperature was it (approximately) when you did the comp. test.

cheers.

EDIT- Always look at the first post before belting on about physics..... His engine is apart so compression test method is no use anyway

Hopefully it will be useful to someone else though!!!!