Race engine port sizes
 

I have a rather generic question:
Recently I have run across a couple of sets of heads that have 42mm intakes and 40mm exhaust ports. They are twin plugged. This seems like a lot of size for engines in the 2.2-2.4 range? My guess is that would need lots of carb, and cam, and only produce in the high end?
I would love to hear, from the Pelican Experts.

Your guess is a pretty good one.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/519112-early-911-cylinder-head-cross-reference.html


Those ports are larger than the ports of a 3.2 Carrera head. Your ports are much larger than most hot 3.2 short stroke motor builds.

I can not imagine those heads ever being on a streetable motor in that displacement range.

What's the Year marked on the heads and do you know the valve sizes?

Race engine port size
 

heads 11-72. Valves 46 and 40.
talking strictly as a RACE engine. Zero Street.

Bruce Anderson's book has a list of all the motors and port sizes. You would have to be able to rev to 8K rpm to use those heads.

Porsche's 2.8 and 3.0 RSR motors had 43/43 ports, to go along with 49/41.5 valves, and peak HP was listed at 8,000 rpm. Since you shift above peak HP to get the most out of your torque curve, you are looking at maybe 8,500 or so at a guess - you'd need a torque curve and gear chart to calculate actual optimum upshift points. The two flavors of 2.5 race motors had 41mm ports and 8,000 peak HP listed.

So can be great for racing if you can invest in the lightened parts and other race bits to go along with that, and probably more frequent rebuild intervals.

However, in building a short stroke 2.8 race motor using 3.2 heads I was advised by a respected source to leave the ports alone at 41/41. I think that the balance between port size and port velocity can be a tricky thing, at least absent DFI.

My understanding is that the theory of race porting has changed over the years since the days of the early rsr motors in favor of smaller ports that have more velocity.

Generally, valve size is a limitation so that ports that exceed 85% of the valve size is probably a waste anyways since it won't flow any better. For instance, probably the largest port that would flow with a 46 intake valve is probably 39.

According to Wayne's book, in 1971 there was a 911/70 (Factory race engine) with 86.7X70.4 (2.5L) with 46/40 valves and 41/41 ports and MFI. According to his chart, the largest ports up to that date.
Next up is a 1972 911/72 with 92X70.4 (2.8L), 49X41.5 valves and 43/43 ports and MFI. Factory RSR race engine.
Both of these engines violate the above 85% rule. (But I'll bet they were fun to drive).
Bob B

Race engine port sizes
 

thanks for all the response. Good confirmation of my thoughts.

you are so right!

regards