Piston to valve clearance calculator for DOHC motors?

[yoshimitsuspeed]

I am starting to do more piston design and I would like an easy way to calculate minimum safe valve relief depth for a given cam.
Does anyone know of a program, or calculator designed for this purpose?

 

[yoshimitsuspeed]

It looks like engine analyzer pro will but that's out of my budget for the time being.
I have heard good things about pipemax for other things but It doesn't look like it calculates valve to piston. Can anyone confirm or deny this?

 

[pontiacjack]

Preface: "calculation" implies, to me, analog variables. Admittedly digital iterative solutions for non-uniform functions might also be labelled "calculation".

For a "calculator" to be possible would require the cam profile to be a 'regular' function which can be described algebraically. Even if some cam grinder were to employ some regular function for some small range of cam rotation, he would never disclose it. Yes, the piston position can be described algebraically (given stroke, rod length, and cylinder/crank offset). So you're left with merely plotting valve lift (cam profile times any rocker/follower ration) on top of piston position.

 

[pontiacjack]

Oops! Along with multiplying cam lift by rocker/follower ratio, the component of lift parallel to cylinder axis must be obtained by multiplying by the cosine of the valve inclination angle.

 

[yoshimitsuspeed]

Although you would need a complete profile for perfect calculation you can use given numbers such a lift at .050" and 1mm along with max lift and valve lash to come up with a fairly accurate guess. You will want to add some margin for error anyway so if you know your calculation will be accurate within .5mm you just make your pocket 1mm or 2mm or deeper than your calculation or whatever you feel is necessary to give you enough margin for error for your calculations as well as the possibility of the customer changing headgasket thicknesses or shaving the head and or deck and so on.
I feel like with the information provided here along with a couple other things like valve angle and valve to deck clearance you could map out the profile plenty accurately enough to get very close on valve to piston clearance.

http://www.tomei-p.co.jp/cam_spec/4ag4/4AG_16VALVE-264-8.15E.pdf

 

[BrianPetersen]

Be careful if you try cutting it too close. Slight differences in cam timing setup, including due to wear in chain and guides over time, will make a large difference.

Ruff rule of thumb. The halfway-open time will be roughly (not necessarily!) halfway between the nominal opening timing and the lobe center timing. If nominal IVO is at 30 degrees BTC and the lobe center is at 105 degrees ATC, the halfway-open point will be somewhere near 15 degrees ATC and that's probably pretty close to closest approach of piston and valve. For an engine having a stroke length in the automotive range, that 15 degrees of crank rotation is going to be in the 1 - 2 mm range of piston movement, which is in the vicinity of the minimum piston to valve clearance that you want anyhow. So if you have enough clearance running through TDC for the valve halfway open, it ought to be enough. If your cam timing radically differs then put in your own numbers to estimate whether this is going to be out to lunch or not, in your particular case. If you have a cam profile that snaps the valve open and leaves it at max lift for an extended dwell period, this is not going to work but nor will your quick and dirty calculation.

This ruff rule of thumb won't work for diesel engines that have nominal intake opening after TDC (after the piston has already left). VW diesels have piston-to-valve clearance comparable to the squish clearance. The clearance factor-of-safety is in the cam timing, not in the amount of clearance provided.

There is NO substitute for doing a plasticine-blob check when test-assembling the engine, then pull it apart again and inspect.

In one case where I wasn't sure where a certain camshaft was going to work, I did my test-assembly without even bolting the head to the block. When the head started lifting clear of the block when I started running the cam cap bolts down, I knew it wasn't going to work.

 

[tbuelna]

It can be quite difficult to accurately predict valve-to-piston clearance. This is due to the huge number of variables/tolerances involved. If you were to do a dimensional tolerance/thermal stack-up of every component between the valve and piston at worst case conditions, you'd likely come to the conclusion that the "safe" depth of the valve relief required was unacceptable.

 

[yoshimitsuspeed]

Thanks for the input. Not nearly as definitive as I was hoping but definitely helpful.

I send the designs out to be made by companies like Wiseco and JE so I'm sure they have their own methods of trying to confirm it will be safe. I just want to do as much as possible and be in control of as many variables as possible.
All the pistons I have designed so far are non interference so that's easy enough but I expect to start doing some very high compression big cam designs soon and want to minimize relief while still being safe.

I'm trying to upgrade to a decent CAD package right now so I have been playing with the idea of making an assembly where I could link the crank cams and valves and then play with valve timing and possibly even figure out how to cut the relief out of the piston at the closest point. This way I could play with cam timing and other variables to make sure I left enough margin for error.