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Port sizes - how much is too much?
https://www.ausmini.com/forums/viewtopic.php?f=5&t=60634		 Page 1 of 1
Author:  Asphalt [ Wed Jan 27, 2010 1:50 am ]
Post subject:  Port sizes - how much is too much?
Hi!

I was wondering, what the average port size for a road going 1275-1310 motor is? Resp. whats reasonable for it not to loose too much 'grunt' in the medium rev range (for me: 2500/3000 to 5000/5500)? Let's assume cams in the MD266-SPVP3MN range (rev limit = 7800).

I know that's a gross generalisation - but maybe I could extract a good impression to 'get the idea'?

Or maybe I should pose the question the other war 'round: are ports with a 36mm diameter already too large for a torquey 1275cc road motor (w. SPVP3MN cam)??

Fire away! :)

Cheers,
Jan
Author:  mickmini [ Wed Jan 27, 2010 4:02 am ]
Post subject: 
Mine has 36mm diameter inlet, 1410cc, RE282, 10.9:1. 127hp @6500rpm, 113ftlb @5000rpm. Also note that the Weber has 36mm chokes, and 36mm inlet manifold. The gas does not have to slow down and speed up due to inlet diameter changes, but siamese inlet port pulses still have an effect. Rev limit is/was 7000rpm due to crank material.

I think a bit smaller capacity but higher rpm -ie 7500 will have similar gas speeds, but you will lose a little bit of torque down low. But of course with the different cam and capacity, different gas speeds will also result.

What is the specs on the cam?

cheers
michael
Author:  Asphalt [ Wed Jan 27, 2010 4:14 am ]
Post subject: 
Hi Micheal!

That's lots more than I'm expecting :) 113lb/ft is imense! :shock: I like the idea about a continuous diameter from start to end.

What I got from APTfast about the cam:

"This cam is a hot street cam, ideal for 1275cc and bigger. Intended for use with 1.5 ratio rockers, but could be used with standard or "tweaked" rockers to make it a little more docile with better torque below 4000 RPM. Lobe center angle 102.5/105.5, duration@ 0.020" In./Ex. 268-272°, cam lobe lift 0.312-0.319, lash clearance (hot - max) 16 In. /18 Ex. RPM range 2200-7200, Max. RPM 7600, idle Characteristics slighty lumpy, suggested comp. ratio 9.0-10.4. Cam duration @0.050 lift 234/242"

Also it's a scatter pattern profile :)

Cheers,
Jan
Author:  drmini in aust [ Wed Jan 27, 2010 5:53 am ]
Post subject: 
My 1412 stroker has 35.5mm inlets, it also made 113lb/ft torque at 5000 rpm, with a single HS6 carb on a RE 7" manifold. Different cam to mickmini's though, it runs an RE13 with Toyota Corolla 1.46:1 rockers.
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