20 replies to this topic

[Java_Green]

Nope you have to measure the actual position at actual valve closure. It is significantly different to using the manufactures cam figures.

Assumption is the biggest killer of engines........

Ac

I can not see why the effective CR wouldn't be possible to calculate. Cam timings/crank stroke/rocker arm ratio should be according to specification (or?). To that the valve clearance need to be added. The rest is the same figures as for calculating static CR. The main uncertainty (as I see it) is the slack in the timing chain. Guide me. What is it that I am missing?

[timmy850]

There are so many things that go into timing the valve opening - cam timing, cam profile, rockers, rocker clearance, etc so the best way to calculate the effective CR is to rotate the engine around and measure it

[Spider]

 

Nope you have to measure the actual position at actual valve closure. It is significantly different to using the manufactures cam figures.

Assumption is the biggest killer of engines........

Ac

I can not see why the effective CR wouldn't be possible to calculate. Cam timings/crank stroke/rocker arm ratio should be according to specification (or?). To that the valve clearance need to be added. The rest is the same figures as for calculating static CR. The main uncertainty (as I see it) is the slack in the timing chain. Guide me. What is it that I am missing?

 

 

Nothing.

I do a DCR Calculation for an engine build, then do a further calculation during the dry build when I can measure the point of the stock where the Inlet Valve closes. There's always some variation between the initial calculation and the measurements in the build, but they are only usually small.

[ACDodd]

The point is the parts arn’t to specification usually and therefore measurement is the most accurate way to obtain this information.

Ac

[Java_Green]

The point is the parts arn’t to specification usually and therefore measurement is the most accurate way to obtain this information.

Ac

That is exactly what I thought you would answer... And I am 100% with you that measuring doing dry build is as close you can get and at the same time hard to do before you have decided on bore size, choice of pistons and the verdict on how much you need to skim.

Where should you expect the biggest deviations to be? I assume your cam specifications are fairly accurate, the same goes for CNC machined rocker arms and I hope so as well when it comes to an A+ crankshaft.

[ACDodd]

The cam is much more accurate than the clearance in the bearings can hold, follower fit affects the readings as does follower face radius, push rod flex etc. Every engine is different in these respects. The camshaft also flexes when loaded.

Ac