Don Terrill’s Speed-Talk

I am considering a DOHC motorcycle and maybe this question will apply to other applications. Will someone explain the difference between the profile of a camshaft intake lobe and an exhaust lobe of same lift and duration? And, why there is a difference? I understand that the intake has a different "job" than the exhaust so there must be a subtle difference in the ramps. I haven't done any direct comparisons, but according to some manufacturers they are not interchangeable.

dhidaka wrote: ↑Fri Dec 29, 2017 8:20 am I am considering a DOHC motorcycle and maybe this question will apply to other applications. Will someone explain the difference between the profile of a camshaft intake lobe and an exhaust lobe of same lift and duration? And, why there is a difference? I understand that the intake has a different "job" than the exhaust so there must be a subtle difference in the ramps. I haven't done any direct comparisons, but according to some manufacturers they are not interchangeable.

A lot has to do with getting maximum cylinder pressure to ignite yet not allowing excess combustion pressure, (torque), to go out the exhaust pipe, while still evacuating the cylinder for the next intake charge.

Compression ratio and how it affects combustion will have a bunch to do with intake and exhaust camshaft requirement differences.

People have been running single pattern cams since the beginning of the four cycle engine so I don't really understand your question. Is a single pattern cam the "ideal" cam? I really don't know. I've had plenty of engines that ran very, very well with single pattern cams and I've had super "trick" cams with exotic intake and exhaust lobes that didn't work worth squat so I'll say that the answer is less than perfectly clear. If you flip thru some cam catalogs you'll see a lot of shelf cams that are single pattern, or the exhaust lobe is from the same lobe family but one size larger.

10 pages on it....
https://www.speed-talk.com/forum/viewtop ... &start=135

I think it was Ed Isky that once said that the CORRECT single pattern cam will run best if the engine combination is right.

Please check me here because its been years ago.

pdq67

Exhaust valves usually run larger valve clearances, like 20 thou vs 16 on the intakes. In order to keep valve seating velocity the same IN vs EX, the grinds would have to differ ever so slightly at the low lift area where the valve is closing/opening.

If the same exact gring were used at EX as in IN, the EX cam/valves, as installed would hmeasure slightly less valve lift and slightly less duration, due to the larger valve clearance used.

#1 reason why intake and exhaust lobes match: it's the cheapest way to make the cam.
The intake and exhaust ports have completely different functions, and what happens in each has almost no similarities (except gas volume).

difference between the profile of a camshaft intake lobe and an exhaust lobe of same lift and duration? And, why there is a difference?

I have not seen this done but my experience is limited on this but i'am long time rider and tinker. Do you have an example? Considering the RPMs some of these bike engines turn valvetrain stability control would a priority. The engine designer could alter the phase of the intake and exhaust cams on DOHC if it was needed for whatever reason... emissions being one of them.

I do know of at least one company who would turn the master around with flat tappet cams and run the asymmetrical master lobe backwards on the exhaust side but, still the same pattern - reversed.
With the lighter valve the exhaust side could close faster.

So the question is about the valve lift profile normalized to gave the same lift and duration? Or talking about an engine combination that happens to want the same lift and duration intake and exhaust cams, and then asking if there are any remaining differences?

In a four valve DOHC engine with solid bucket lifters, I’d expect a lot more thermal expansion and movement on the exhaust side, which might impact the required lash and thereby the required lash ramp. This was mentioned earlier.

In a forced induction engine with any flex between the exhaust live and the exhaust valve head, the worst example being the pushrod cam in block setup, the opening of the exhaust valve against the combustion pressure loads up the whole valvetrain like a spring. I’ve read somewhere that the cam exhaust lobe and springs need to be designed to dampen the resulting vibration before the valve closes again, for if the valve close impact happens to time that vibration right (or wrong) the valve really bounces at EVC. This just from me reading stuff.

panic wrote: ↑Mon Sep 14, 2020 2:13 pm #1 reason why intake and exhaust lobes match: it's the cheapest way to make the cam.
The intake and exhaust ports have completely different functions, and what happens in each has almost no similarities (except gas volume).

You raise an important point that should be considered. The intake and exhaust have completely different functions; they operate at different pressures, densities, velocities, and temperatures. But the volumes will always be the same, equal to displacement, as will the gas masses.

The time frames in which they operate are quite different as are their effective durations.
An intake lobe must deal with slow moving, cool, dense air, over the period of piston downward motion.
An exhaust lobe must deal with high speed, very hot, low density air over a short period called blowdown.

For an intake lobe, the designer must also consider induction pipe characteristics; for an exhaust, the properties and functions
of the exhaust system must be considered.

dhidaka wrote: ↑Fri Dec 29, 2017 8:20 am I am considering a DOHC motorcycle and maybe this question will apply to other applications. Will someone explain the difference between the profile of a camshaft intake lobe and an exhaust lobe of same lift and duration? And, why there is a difference? I understand that the intake has a different "job" than the exhaust so there must be a subtle difference in the ramps. I haven't done any direct comparisons, but according to some manufacturers they are not interchangeable.

Are you are talking about an engine with a "finger follower" type valve train design. If that's the case, the cam profiles will need to rotate in the proper direction even if the intake and exhaust profiles are the same. The "finger follower" creates a variable ratio through its movement. The cam profile design matches this variable ratio to create a smooth valve motion. The intake and exhaust cam profiles will be the mirror of each other.