dyno of 351

quote:

Originally posted by René #4406:
And if I see that there is not enough clearance, what do I do? I deposit the head, and I dig valves relief ..... if there is enough margin with the top ring.

But I agree, the weak spring is more practical than the clay.

How are you going to change the weak spring to a triple spring without taking off the head?
You are going to use air pressure to hold them on their seats?
Good luck with that.

Only one piston is mounted to check the valves clearance, so the head must be removed and then the final assembly done.
To temporarily mount a cylinder head and to deposit it, it's fast, the difficulty is to modify the pistons if the control isn't good, that's why it's better to know the valves lift curves before ordering the pistons.

All you need to know is the clearance at maximum lift and the piston at top dead center.

The valve is no closer to the piston at any other point. You don't need to know the clearances at any other point so what is the point of graphing the lift "curve"?

I don't get it?


Unless you are using piston blanks from another engine or machining them down out of a solid piece of aluminum billet, ALL pistons are going to have the valve reliefs cut into them.

The manufacturer can tell you the maximum lift of the cam that you can use with them before you need to flycut the valve reliefs.

Even so, that usually is because you are going to a much larger valve outside diameter then the enging ever was built for.

I do'nt agree, the minimum clearance may occur a bit before or a little after the top dead center, I checked it on other engines where I was using a camshaft with a lot of overlap and very fast openings and closures.

Anyway, I never asked how to check that, I know how to do it, I just said that having the lift curve facilitates this check.


My subject was above all the supply of lift curves by cam grinders in order to really compare camshafts with each other.

I have never seen a graph of a cam lift.

The closest clearance is at maximum lift at the highest point the piston is at, unless you have invented a new form of physics?

I suppose in this era of alternative facts anything is possible? I must live in an alternate reality?

quote:

Originally posted by PanteraDoug:
I have never seen a graph of a cam lift.

The closest clearance is at maximum lift at the highest point the piston is at, unless you have invented a new form of physics?

I suppose in this era of alternative facts anything is possible? I must live in an alternate reality?

No need to invent new laws of physics, it is enough to well understand those that are known and particularly those of kinematics.

To determine the clearance between TWO moving parts we must take into account the position of BOTH parts and therefore not only that of the piston but also that of the valves!

Therefore, the minimum clearance is not necessarily at the top dead center simply because the valves, during the few degrees before and after the top dead center, can open or close faster than the piston goes down or up.

I do not live in an alternative world but it is not the first engine that I rebuild by modifying it and I never affirm anything without being absolutely sure of it. I also avoid making fun of people when I know badly what I'm talking about.

If you've never seen a lift curve, it's that you still have a lot to learn about engines.

It simply doesn't matter. The closest possible place the piston can be to the valve is at maximum lift and the pistons highest point.

You need to maintain .100" clearance on the exhaust. .080 on the intake.

If the piston is on it's up stroke still .250 down in the bore and you happen to have so much duration on the cam that you are at maximum lift there, great, you have .250 more clearance there, which again, doesn't matter.


Listen. I'm the original poster. If you guys want to debate this crap, go start another thread and get it out of here.

You want to talk about derailing a thread, this is a perfect example.

WTF, does this have to do with the dyno chart that I posted here?


While you are at it, go debate how many angels can sit on the head of a pin? Oh and look out for the pin. It's a little sharp.

you have a nice day Doug

If I may comment.

The dwell period occurs from about 30° BTDC to 30° ATDC. There is very little piston motion during the dwell period, the pistons are as high in their bores as they can be during the dwell period. Unlike the reciprocating motion of the pistons, the cam rotates at a consistent speed, thus the valves are in motion during the dwell period even though the pistons' motions have ceased. Since the valves are in motion during the dwell period, the clearance between the valves and the pistons is in a continual state of change during the dwell period.

The exhaust valve clearance should be checked as the piston finishes rising in the bore from about 30° BTDC to about 10° BTDC. The intake valve clearance should be checked as the piston prepares to descend into the bore from about 10° ATDC to about 30° ATDC. That's all the clearance that ever needs to be checked, from the mildest street engine to the hottest race engine.

Piston to valve contact normally occurs when the "closing" exhaust valves float, because the pistons chase the exhaust valves as they close, so this clearance is the more critical of the two to check ... and this is why we normally expect more clearance (0.100 inch) at the exhaust valve. The intake valve clearance is not so critical because opening intake valves don't float, and because the pistons are "fleeing from" from the opening intake valves.

Doug I would have commented about hijacking the thread but you've been right in the middle of the comments being made regarding piston to valve clearance. So long as you were commenting I assumed the hijacking met with your approval.
.

YOU derailed the post:

quote:

Originally posted by PanteraDoug:

The procedure for determining piston to valve clearances is a standard/known procedure.

You don't calculate it, you measure it with clay.

For safety a "standard" number is used. I think it is .080" on the intake, .100" on the exhaust.

US pistons are all made with those considerations and you don't get close until you are around .650" lift.

So that really puts the "Pro Stock" and NASCAR type engines in danger with .700+ lifts.

And I tried to "refocus":

quote:

Originally posted by René #4406:
I do'nt agree, the minimum clearance may occur a bit before or a little after the top dead center, I checked it on other engines where I was using a camshaft with a lot of overlap and very fast openings and closures.

Anyway, I never asked how to check that, I know how to do it, I just said that having the lift curve facilitates this check.

My subject was above all the supply of lift curves by cam grinders in order to really compare camshafts with each other.

But George agrees with me, we have to check a bit before and a little after the top dead center, not just at the top dead center.


George, do you know why cam grinders usually refuse to communicate the lift curves?