CMW heads (big intake valve) anybody using them?

[JohnC]

I'm considering purchasing a set of CMW heads with the larger intake and larger exhaust valve, dual plug too.

The engine would be a 3.3l bumped to 3.6l turbo.

Any experiences with these heads or the larger intake valve?

[Milu]

I remember a favourable write up from sometime back, 911 World I think. But I don't remember any hard facts being quoted.

[JohnC]

Thanks

I'll check to see if I can order back issues.

I've seen a set of race heads (na) that had 52mm intake valves. The valve stretched from about an 1/8" from the sparkplug hole to the edge of the combustion chamber.The 52mm would only work with the 100mm bore. He recommended I use the largest intake valve I could fit.

Interesting stuff.

[jyl]

This is an interesting topic -- alternative heads for the air-cooled 911 -- that doesn't get talked about hardly at all. (In fact, as has been noted on prior threads, the whole subject of flowing, porting, valve sizing, etc gets far less attention in Porsche talk than in, say, small-block Chevy talk).

Other than the CMW big-valve heads, didn't Wayne post something once about picking up some Porsche Flugmotor heads with big valves? And I've seen the photo of an aftermarket four-valve head in the BA book.

If anyone can shed any further light on what these heads are like, the supposed benefits, how much they're actually used in racing or otherwise, and so on, I'd be awfully interested.

[dwightp]

John (s):

I took the plunge with CMW for head work on my 3.2l to 3.6l twin plug project. They did the larger intake, exhaust valves, their valvetrain, port and machine for twin plug. Upon receipt, the quality of the work looks great! I'm several weeks away from firing it up. As soon as I do and get some results, I'll post further info.

Unfortunately, I do not have any flow #'s. They probably do have some before and after numbers.

I have been to their shop and rode in a twin turbo with similar head work. IMPRESSIVE. But as is the case, it had so many other mods, not sure how to differentiate them.

dwightp

[JohnC]

Dwight: I'm going from 3.3 to 3.6L with longer stroke and 100mm cylinders. Sounds like you have a similar project. THe flow number at standard lift (.50") and standard depression(1 atm) I believe is 280cfm. But don't qoute me..Ok? What size intake valve did you choose? And what intake manifold?

JYL: The supposed benefits are that the larger valve, at maximum valve lift, does'nt get you much additional flow. But the low level lift and mid-level lift are where the gains are at, adding to the TOTAL volume of air moved. Makes sense when you think about it. As the valve moves away from the seat it becomes less of a factor in restricting the air.

Porting , I feel, is necessary for the turbo heads due to the small port sizes. You'll notice any time dyno (turbo) charts start popping up, the guys with the 3.2 Carrera heads reach high HP/torque numbers earlier in the rpm range (larger ports) and at less boost. Perfect for a street motor.

Port velocty I think becomes less of an issue with positive manifold pressure. THe newer turbos and twin turbos allow boost way sooner in the rev range. From what I hear there's positive press. shortly above idle.

Chevy talk: I think the only parallels we can draw between the two is that chevy did use the 1.94"= 49mm (power pack heads) intake valve size and then the much feared 2.02". But most hot rodders know the 2.02" intake valve is too large for the street.

My reason for asking for people's experiences is to find out, for turbo applications, is the 52mm or 51mm useful in generating big HP numbers early on in the RPM range(without big boost)....... my intuition says yes but .....who knows?

[dwightp]

Sounds similar, but I'm staying N/A. 100mm bore also. 52mm intake, 43mm exhaust. Didn't go with the d-inserts. Nothing aginst them, I just had to stop somewhere. Extrude Honed Carrera intake. Motec 48 with map sensor.

Yip, I'm getting excited!

dwightp