JAKE

Let me see if anyone knows this:

Advancing or retarding a camshaft changes the valve to piston clearance; that's a given. I've always believed that the maximum clearance change occurs at 10 degrees BTDC/ATDC.

Does anyone know a rule of thumb of how much the clearance changes per degree of advance/retard of the cam?

Say, for example, with the cam installed and valve to piston clearance checks at .100 In/.150 Ex. Now the cam is advanced 4 degrees causing the intake valve to piston clearance to decrease and the exhaust valve to piston clearance to increase.

Yea, but by how much? Any rule of thumb on that? Something like, for each degree of advance/retard of the cam the piston to valve clearance changes by .005, or .003 or whatever, rule of thumb.

Thanks,

Jake

JAKE

I'm surprised that no one know this. This isn't too High-Tech is it? Maybe I'll just have to keep bumping it until someone sees it and chimes in.

Oh, well.

Jake

redrose

too important to thumb/guess at, and so easy to actually check...put a mag-base dial indicator on the head, picking up the valve retainer, rotate the crank near TDC in 10* increments, use a pry bar to push the valve down (gently) till you feel the valve touch the piston or until desired clearance shows on the indicator, whichever comes first.

JAKE

But that won't tell me how much piston to valve changes when the cam is advanced or retarded.

I always use the clay method when screwing one together, but once the engine is together, in the car and running I'm wondering what rule of thund can be used . . .

I'd hate to have to go all the trouble of tearing off the front of the engine, moving the cam, only to find piston to valve is too close. So if I had an idea, up front (before turning any wrenches) I'd know if it would be an exercise in futility.

Of course, after the cam move has been made, the P/V clearance would be verified in the method you outlined.

Thanks for keeping this question alive.

Jake

REDC4CORVETTE

That is like asking how do you set your valves and what clearence did you have as an end result?
Some will say I bring the piston to top dead center and adjust the intake and the exaust at the same time .The other person will say I take the intake rocker to the top and adjust the valve just as the exaust starts to come off the bottom.Both can be adjusted eather way but the end result will not be the same.This is if you are adjusting a solid camshaft.If you clayed the piston at the start then you already set the valve at it's highest point on the cam shaft and advancing and retarding will only move the adjustment to eather side of the cam lobe .
You are not increasing the lift of the valve just changing when the the valve opens.Now I think I am confused.

rodj

"You are not increasing the lift of the valve just changing when the the valve opens."

But Jake is referring to how that opening point change relates to piston/ valve clearance as the cam is advanced or retarded

JAKE

Right.

I know there a half dozen things (maybe more) that effect piston to valve clearance, and re-phasing the cam is one of them. I just want a ballpark figure on how much it'll change if I, for example, advance the cam 4 more degrees.

What peaked my interest in this was an article I recently read in the Engine Masters mag. They tested several different timing chain sets to see if one of them showed a power increase. They then played with the phasing; 101, 103, 105 and 107 logging the power each setting made.

BTW, they all made virtually the same power, BUT changing the centerline showed as much as 10 HP difference between the worse and best readings.

Jake

thirdtimevetteowner

(BTW, they all made virtually the same power, BUT changing the centerline showed as much as 10 HP difference between the worse and best readings)
Jake,
What exactly do you mean by changing the centerline? Are you referring to different cams w/different centerlines? Is it not true that you can not change the centerline because it is ground into the cam?
You can degree it in at various amounts of advance or retard, but the centerline is fixed, correct?

Rich K

rodj

No he is referring to cam centerline to piston relationship.
In a certain cam position (re TDC) the valve has X clearance to the piston.If the cam is advanced or retarded then it's relationship with the piston (through the valve ) has changed because the piston has moved closer or further away from the valve.

LTxDave

I really don't see how there could be a rule of thumb for this. Different cam types and grinds have different rates of lift (profiles), so generating a rule of thumb would be difficult if not impossible. As expensive as it could be if this was screwed up, I would just put some checking springs on, a dial indicator, and slowly rotate the engine checking clearance by pressing the valve down every few degrees until you find the tightest point. But I prefer the clay method personally.

CFI-EFI

There is no easy answer or rule of thumb. There are too many variables. The length of the stroke and even the rod length, both affect the amount of the piston movement per degree of crank rotation. Different cams accelerate the lifters (the amount of lift per degree of rotation) at different rates. You could check it out, for your engine, while it's still on the engine stand, but reliance on a "rule" could kill an engine.

RACE ON!!!

JAKE

Thanks for all the input on this.

If I had known I'd be walking this road, I would have checked the clearances by setting the cam at different centerlines while the engine was on the stand and I was degreeing it in.

Now that it's in the car - has been for a few months now - there's a lot more work involved to see what kind of clearances I'd have if I move the cam 4 degrees.

I've just got to decide if it's worth my time and effort to tear into the engine, move the cam and see what clearance I'll have. Now that I've gotten older, I find I don't look forward to all that work the way I use to.

Jake

thirdtimevetteowner

Ain't that the truth!! I am in the same boat, pushing 60 (Mar. 21st), and it is no longer "fun" working on cars anymore. The only fun part is if you tuff it out and get to see dramatic results. I am in the process of gathering up speed parts, and am in need of cylinder heads, a cam to match my setup, and finally a torque converter upgrade. So far I have a mini-ram, CC full rockers, EM headers due any day now, and a spanking new Dana 44 w/3.54's. Looks like I will be busy this winter, since it is kinda late to tear into it now. Just my $.02 worth.

Rich K

RacerX70CC

CFI beat me to it... too many variables to have a "rule of thumb".


That's why we call them "custom engines"