More compression or flat tops

[ClassAct]

...Not all of us desire a stroke so long it could be a diesel and get compression with a small dome.

That's the way a shop I know does it, to make compression with the huge chambers of a Hemi. He doesn't normally build anything under 572" and most are 636", 678", etc.

Well yeah!!! A hemiroid had a chamber the size of Texas and you need stroke to keep the piston weight down on that. I was speaking of wedge stuff. LOL.

[ptuomov]

If you have a given cam with a 106 L.S.A and the valves are lets say .150" off their seats at T.D.C. on the exhaust stroke (installed straight-up, sometimes called) , and you have a second cam identical except you give it a 110 L.S.A. you will now have less lift at T.D.C. and more P/V clearance . Mark H.--------- Camking if this is not correct please speak up.

Yes. The two biggest factors in Piston to valve clearance are Duration and Lobe Centelines(I don't like to say LSA, because LSA doesn't tell you where the cam is installed). The lobe lift has a much smaller effect, because occurs so far from TDC

One question that I have is that how many cam lobes in various applications have longer than desired duration to get enough lift and area with acceptable mechanical reliability?

[Steve.k]

Thanks guys. We have custom pistons already built with popup dome. However switch to different head has little different configuration than previous. So we thought rather than modifying combustion chambers a bit we go flat tops and peel heads down. However with 780 lift and zero deck leaves us with big relief so not gaining much. We will modify heads a smidge get domes to fit and see how she looks.

[MadBill]

...
One question that I have is that how many cam lobes in various applications have longer than desired duration to get enough lift and area with acceptable mechanical reliability?

A friend of mine has a number of BBC-based Batten four valve engines. The later ones have cams with at least 15° more duration than Dynomation thinks is necessary. Apparently with direct-acting buckets the 'too-long' duration was the only way Batten could get the desired lift without the huge contact stresses from cam profiles resembling dunce caps...

Someone here speculated on the possibility of killing low lift flow to band-aid such a situation

[houser45]

With the difference in compression only taking a very small dome? (Guessing 5-9cc. ) i would take the dome with the higher compression option. The torque number would probably be 25-30 ft pounds higher. If you make your dome profile with nice round 1/4” or larger radius it wont lose very much flame travel to the flat top. Prep your pistons. It will pay off

[ptuomov]

...
One question that I have is that how many cam lobes in various applications have longer than desired duration to get enough lift and area with acceptable mechanical reliability?

A friend of mine has a number of BBC-based Batten four valve engines. The later ones have cams with at least 15° more duration than Dynomation thinks is necessary. Apparently with direct-acting buckets the 'too-long' duration was the only way Batten could get the desired lift without the huge contact stresses from cam profiles resembling dunce caps...

Someone here speculated on the possibility of killing low lift flow to band-aid such a situation

Direct acting bucket lifters limit the maximum valve velocity and the limit depends on the lifter diameter.

If you could reliably kill very low lift flow, that wouldn’t be just a band-aid, it would in my opinion be a solution.

[rustbucket79]

No mention of the heads used, why?

[CamKing]

One question that I have is that how many cam lobes in various applications have longer than desired duration to get enough lift and area with acceptable mechanical reliability?

Out of all the cams I sell, none.
That's one of the advantages of being able to machine profiles with inverse radius's.
As for flat follower profiles, you're limited to a maximum velocity, but you're not limited to how quickly you can reach that max velocity, or how long you can stay at it.

[Stan Weiss]

One question that I have is that how many cam lobes in various applications have longer than desired duration to get enough lift and area with acceptable mechanical reliability?

Out of all the cams I sell, none.
That's one of the advantages of being able to machine profiles with inverse radius's.
As for flat follower profiles, you're limited to a maximum velocity, but you're not limited to how quickly you can reach that max velocity, or how long you can stay at it.

Mike,
Since that would produce zero acceleration. Would you happen to have a picture of the acceleration graph for one of those cams? I have never seen one like this and it would be interesting to see one.

Stan

[Steve.k]

IMG_3608.JPG

No mention of the heads used, why?

The heads are blue thunder 3.6. The original pistons were designed for a3. The spark plug location is in a different location that requires modification to get dome to clear. The pistons i have pretty much mirror cleveland combustion chamber. We thought rather than machine combustion chamber to fit we figured maybe a zero deck flat top get us to our goal. The wallace says 13.5 with flatty. Mid 14 with popup.The pistons are coated didn't want to machine them.

[CamKing]

Mike,
Since that would produce zero acceleration. Would you happen to have a picture of the acceleration graph for one of those cams? I have never seen one like this and it would be interesting to see one.

Stan

Ignore the blip in the accel curve, after the dwell. That was a measuring error.

[Steve.k]

IMG_3613.PNG

The area above spark plug where it comes in has to be relieved to alow part of dome. No big deal i guess it will end up similar to a cleveland combustion chamber

[Stan Weiss]

Mike,
Thanks! Does that have a constant velocity lash ramp?

Stan

[CamKing]

Mike,
Thanks! Does that have a constant velocity lash ramp?

Stan

Yes. That accel spike you see before the constant Velocity lash ramp, takes place in the first .001" off the base circle. The spike in acceleration you see at the end of the lash ramp is deceiving, because that's the lifter rise with zero lash. In reality, the valve acceleration would not be effected by the constant velocity ramp, because it happens before the lash point.

[Walter R. Malik]

Yes i can mill these heads to 55cc easily without a drastic flow change. They are 64 now. So yes thats the answers im looking for. Even with a deep relief and mid 13 compression we maybe further ahead than custom dome to get is up in 14+ area.

Personally, as long as the dome does not hinder flame propagation or cross-flow within the chamber, the dome size shouldn't be an issue.
The problem is that most big domes, do create some kind of hindrance.