Technical Flatty six dual pattern cam?

Talking to a local cam grinder the other day. He has a dual pattern grind for the M47 Chriscraft dodge chrysler flat six. It is .404 intake and .378 exhaust.

Curious weather it would be worth getting both the intake and exhaust profiles reground to the .404 or leave it as a dual pattern favoring the intake. Going to be running dual carbs and tube headers so not sure this is ideal for a street engine in a light car.

Thoughts?

 

Considering that the intake side depends on atmospheric pressure for induction, that accounts for both a larger valve in the intake tract and, perhaps, greater lift/duration to insure as much volume enters the cylinder as possible. On the other hand, the exhaust is forced out of the cylinder by the piston and can pass the volume required more easily. I would think, though, at some point, exhaust restrictions would come into play and create more resistance to piston movement on the exhaust stroke.

Maximum efficiency is only really required at increased rpms as the actual time the valves are open decreases, regardless of the profile. The boat engines usually run at mid to upper range rpm. So, if it works for that (assuming it does), why not for a street engine? Just some thoughts, others may differ.

Ray

 

My general thought on cams is that you have two valves doing two different things so my way of thinking is you need different lift and duration on intake and exhaust.

 

Generally, what the others have said is true.
But to be the best informed, you need to know how well the intake and exhaust ports flow. THAT in itself would be the main teller. Now knowing that it's pretty difficult and expensive to flow ports on a "block" (rather than a cyl. head).
I'd give it a try. Understanding that you don't mention the duration or the overlap...!!

Mike

 

226 and 110* is what he said if I recall

 

I would check with the cam grinder as to the power band that was ground for.
Most marine engines are designed to run at a specific RPM range.
Automobile engines are a more broad range of RPM's.
Just a thought, KK

 

As I understand it the rpm range or lack of it has to do with the separation angle more than anything.

The cam in my power wagon 251 flatty was 102* or something. It had a real narrow range.

 

I would tend to agree that a marine cam typically is designed to run at mid to upper RPMs. Depending on what you are doing it could be OK, but not what I would want for a driver/cruiser. I wonder if they kept the exhaust more closed because of some reversion issues in a marine exhaust. Idk.

 

Dual pattern is a duration difference not a lift difference with most cam grinders .Lift is somewhat restricted by the head clearance in a flathead, duration and lobe centre will give you more of an idea how a cam will work..Have another chat with your cam grinder so he can explain his grind .

 

It is a lift difference with this cam.
I could easily go to .450 lift without head interference.

The separation angle will be basicly stock, he can only do much with that without building up the cam blank.

 

what are the valve sizes ?

 

Would have to measure them. The intake is much larger than the exhaust. As it should be.

 

Intake valve is 1.525 exhaust is 1.400

 

Nobody knows?

 

I think you need to talk to Edgerton. http://www.edgyspeedshop.com/index2.html

 

I have.

 

I'd run the .404 on both sides, however, you are dealing with a very small difference and all of this is in a flathead 6.
Marine engines may have different requirements than a pass car engine but they still spend part of the time at idle as well as steady state cruise rpm, just like a car.
Typically, at least in my little world, we would put more lift/duration on the exhaust if we were keeping a stock-ish manifold or extra quiet mufflers.
If you have an hour to kill then call George Asche 814-354-2621. If there is a Mopar flathead expert he is probably that guy...

.

 

I for got about him. I will give him a shout.

I am going for as much power out of this little motor as possible. Our highway speed limits are 120KM/H which is over 70M/H flopping along at 55 is not my style.

 

Digging this one up!
In doing more research it seems Harley used almost the exact same cam in the KR and KH series race bikes. The cam specs I mentioned and second or third stage over stock and were used in addition to their head milling and port work which I plan to copy as best I can to the milled down factory iron head.