Some Math about piston to valve clearances

[Curiousyellow71]

TN mopar is the fly cutting you mentioned what just came with the pistons or are theycut for bigger valves. If I remember right I think 2355 is the forged six pack piston for 2.08 valves... future head upgrades may make your clearances unsuitable if it has bigger valves. 2355 piston valve reliefs are not too deep. My guess from experience things are going to get tight at .550 lift if you have had any head surfaces machined you may not be able to run that much. Not much more to add then what had already been said you have had sound advice. But when you push clearances at .080 on the intake and .1 on the exhaust the larger duration cam will make your valve clearance go away fast. One option is to use a cam ground with a centerline that moves the valves away from the piston as the valve is shutting and run more duration.

 

[PRHeads]

General question.......(for those who think the lift matters with regards to valve-to-piston clearance).......

Where is the piston when the cam is at maximum lift?

Along those same lines........ two .630 lift roller cams, ground on the same lsa and installed on the same intake C/L.
Do they have the same valve-to-piston clearance?

 

[451Mopar]

General question.......(for those who think the lift matters with regards to valve-to-piston clearance).......

Where is the piston when the cam is at maximum lift?

Along those same lines........ two .630 lift roller cams, ground on the same lsa and installed on the same intake C/L.
Do they have the same valve-to-piston clearance?

Piston position at max lift is the Intake Centerline, around 108 degrees, so the piston is around 1/2 way down the bore.
The two cam with same max lift question depends on the cams duration. Really duration is just measuring the tappet lift at pre-defined lift points.
The P-V issue is at exhaust/intake overlap where the exhaust closes after TDC and the Intake opens before TDC.
I found another piston position link that accounts for piston pin offset.
http://homepages.cae.wisc.edu/~hessel/faqs/kiva4faqs/zpistnCalculationWAndWOWristpinOffset.html

When looking at valve lift and piston to valve clearance, the valve is also at an angle to the piston travel, to that would need to be accounted for also.

 

[PRHeads]

I think the first clue, that the piston is about 1/2 way down the bore, should dispel any ideas about peak valve lift playing any part of valve-to-piston clearance.

The answer to the second question was really, “not enough information”.
Since the important stuff wasn’t listed, there would be no way to know.

For example, looking at the specs two actual .420 lift roller lobes(.630 with 1.5 RR).......
Cam A has .108” tappet lift with the cam installed at 106c/l, with the piston at TDC.
Cam B has .180” tappet lift with the cam installed at 106c/l, with the piston at TDC.

If the motor had .100” valve-to-piston clearance using cam A with 1.5 rockers...... it has plenty of clearance.

If you slid cam B into the motor, it would have less than zero clearance.
The difference in lobe lift at TDC is .072”, times the 1.5 rocker ratio is .108”....... so you went from plenty of clearance to less than zero clearance without increasing the total valve lift at all.

 

[Curiousyellow71]

I agree with the last couple posts. Their are so many different cam profiles out there. Even more challenging cam companies often are not consistent on the the way they rate there cams. Advertised durations don't mean much from one cam company to another. Two cam with the same .05 duration and same lift can have drastically different dynamic compressions. One will bleed compression off the other will build it up. One will have gobs of valve clearance the other will hit. Some companies have lots of info on their profiles, others like mopar performance you have to physically measure it if you want to know what you are actually running. Some cam companies like MP consider there cam profiles trade secrets and are counting on us to just drop there cam in and not measure it. Other companies are great about sharing the profiles. When I order a cam I want to know exactly what I am putting in. That's why I haven't run a mopar performance cams in 20 years. They are great cams...but they don't share enough info for me. But I am a engineer and like that stuff...figuring it all out is part of the fun. Especially when it runs hard after you get it together.

 

[TN Mopar]

TN mopar is the fly cutting you mentioned what just came with the pistons or are theycut for bigger valves. If I remember right I think 2355 is the forged six pack piston for 2.08 valves... future head upgrades may make your clearances unsuitable if it has bigger valves. 2355 piston valve reliefs are not too deep. My guess from experience things are going to get tight at .550 lift if you have had any head surfaces machined you may not be able to run that much. Not much more to add then what had already been said you have had sound advice. But when you push clearances at .080 on the intake and .1 on the exhaust the larger duration cam will make your valve clearance go away fast. One option is to use a cam ground with a centerline that moves the valves away from the piston as the valve is shutting and run more duration.

This is a great point
And one I had not considered even
I do have 2.14/1.81 valves in my 906 heads
Something else I will consider in my next engine

 

[TN Mopar]

That's why I haven't run a mopar performance cams in 20 years. They are great cams...but they don't share enough info for me. But I am a engineer and like that stuff...figuring it all out is part of the fun. Especially when it runs hard after you get it together.

I agree with you, I thoroughly enjoy measuring and checking everything as I build the engine. And that's where my question came from. I knew there wasn't much room left for a bigger cam and I wanted to learn more. I looked at the profiles iof the MP mechanical cams and short of buying one to test fit I figured I could ask here

 

[BSB67]

I think the first clue, that the piston is about 1/2 way down the bore, should dispel any ideas about peak valve lift playing any part of valve-to-piston clearance.

The answer to the second question was really, “not enough information”.
Since the important stuff wasn’t listed, there would be no way to know.

For example, looking at the specs two actual .420 lift roller lobes(.630 with 1.5 RR).......
Cam A has .108” tappet lift with the cam installed at 106c/l, with the piston at TDC.
Cam B has .180” tappet lift with the cam installed at 106c/l, with the piston at TDC.

If the motor had .100” valve-to-piston clearance using cam A with 1.5 rockers...... it has plenty of clearance.

If you slid cam B into the motor, it would have less than zero clearance.
The difference in lobe lift at TDC is .072”, times the 1.5 rocker ratio is .108”....... so you went from plenty of clearance to less than zero clearance without increasing the total valve lift at all.

Cam (lobe) A, I resemble that.

In my example with the two solid FT cams with 0.480" and 0.580" lift, they have real similar 106 cl numbers as your example cam A and cam B numbers.

 

[Curiousyellow71]

This is a great point
And one I had not considered even
I do have 2.14/1.81 valves in my 906 heads
Something else I will consider in my next engine[/QUOTE

I agree with you, I thoroughly enjoy measuring and checking everything as I build the engine. And that's where my question came from. I knew there wasn't much room left for a bigger cam and I wanted to learn more. I looked at the profiles iof the MP mechanical cams and short of buying one to test fit I figured I could ask here

So are your 2355 pistons stock or are they enlarged for the 2.14. Those are some massive pistons...you could flycut a deep deep relief in there. Are you wanting to stay with a hydraulic cam.

 

[TN Mopar]

So are your 2355 pistons stock or are they enlarged for the 2.14. Those are some massive pistons...you could flycut a deep deep relief in there.

They are stock
and I imagine if I had true zero deck I might have discovered some issues

 

[Curiousyellow71]

You pretty much have to check the clearance with clay too see what the valve clearance is around the perimeter of the valve. It may just barely be clearing w the bigger intake valve...it all depends on what diameter the eyebrows are cut for. My guess is they are cut for 2.08 +.08(intake clearance) and they used a cutter of 2.16 or larger.... hopefully they are larger...because if the 2355 piston eyebrows are cut with a 2.16. You only have .01 around the outside of the valve....and that wouldn't show up turning the engine over measuring valve clearance w/out clay. But I don't want you to get 2 worried...maybe the eyebrows on 2355 piston are cut with a 2.22 and your fine. Need a piston to measure. I used to have one, but sold the engine a decade ago... Surely someone knows or has one.

 

[PRHeads]

Sure, when 2355’s were about all there was to pick from for 2.06” CH flat tops...... they were a logical choice.

Now, there are piles of better choices for HP builds.
Based on a number of perspectives......
-weight
-ring pack
-valve pockets
-pin retention
-pin oiling
-did I mention weight?

A TRW 2355 plus pin weighs about 1100 grams.
An Icon IC968 plus pin weighs about 875 grams.
225 grams difference....... which is very very close to a 1/2 pound lighter........ each!!

 

[TN Mopar]

You pretty much have to check the clearance with clay too see what the valve clearance is around the perimeter of the valve. It may just barely be clearing w the bigger intake valve...it all depends on what diameter the eyebrows are cut for. My guess is they are cut for 2.08 +.08(intake clearance) and they used a cutter of 2.16 or larger.... hopefully they are larger...because if the 2355 piston eyebrows are cut with a 2.16. You only have .01 around the outside of the valve....and that wouldn't show up turning the engine over measuring valve clearance w/out clay. But I don't want you to get 2 worried...maybe the eyebrows on 2355 piston are cut with a 2.22 and your fine. Need a piston to measure. I used to have one, but sold the engine a decade ago... Surely someone knows or has one.

I checked it with clay

 

[Curiousyellow71]

Glad you used clay....

For your reference we last used those 2355 pistons with 906s and a mp .557 solid. But we had 2.08 intakes and oversized ex 1.81. Below the deck .02. I can't remember how much valve clearance she had.

Maybe consider a comp cams extreme mopar hydraulic...the biggest is 5966 lobe lobe you could run .564 lift. Steeper ramps more area under the curve then your .509 Mp cam and I think it would fit fine. Hughes has aggressive profiles as well. I assume you are running 1.5 rockers. 1.6s would be a option.

 

[TN Mopar]

Glad you used clay... Did you notice how much clearance there was around the perimeter of the valve....I have some concerns w a 2355 and 2.14s...hoping your ok. I think .04 is ok around outside of valve..someone else can chime in there.

not particularly however I placed the clay then cut it to measure at the thinnest area giving me the .120 without gaskets
I used the metal shim gaskets for total of .140 but the felpro would give you an additional .020 clearance

 

[Curiousyellow71]

TN mopar...Sorry I didn't notice you were asking for biggest solid cam in your very first post..that comp cams I mentioned above is a hydraulic. Comp has some aggressive mechanicals that i think you could run .6 lift with your current set up. My experience with those pistons in a solid...ends with the purple .557. If I was trying to put in the biggest solid in your combo I would try a comp cams MMsolid. 287 [email protected]. .604 lift. I would suggest you get the clay back out going that big....

 

[PRHeads]

Russ, Bob, 451Mopar........ I think our job is done here(and I don’t think we were entirely successful).

 

[TN Mopar]

TN mopar...Sorry I didn't notice you were asking for biggest solid cam in your very first post..that comp cams I mentioned above is a hydraulic. Comp has some aggressive mechanicals that i think you could run .6 lift with your current set up. My experience with those pistons in a solid...ends with the purple .557. If I was trying to put in the biggest solid in your combo I would try a comp cams MMsolid. 287 [email protected]. .604 lift. I would suggest you get the clay back out going that big....

Well at this point the engine is built
My next one is down the road a ways
I built one with the 484 cam and loved it so I went with the 509 this time but I was wondering what it would take to go to a mechanical
Perhaps I could have asked about better but I did get some very good insight and can file that away for future use
My next build will be completely different anyway as I will probably go with a stroker kit

 

[Curiousyellow71]

Build the stoker...you won't regret it. Those old 6 pack pistons make a good economical build with a lot of stock parts...there is nothing wrong with that!

 

[66Satellite47]

In my experience, you need to MEASURE the piston to valve with particular cam you have(or something close in design). The piston to valve must be measured by rotating the cam through it's travel on both intake & exhaust by about 5* at a time. The tightest clearance is NOT at max lobe lift. Piston chasing intake valve, exhaust valve chasing piston, if my thinking is correct. Be sure the valve diameter is adequate. Bigger is better in case you increase valve diameter. Measure don't guess.