Vintage Dirt Bike Q & A

The transfer timing was altered and a piston fed boost port was added. A Sudco intake manifold for a 36mm Mikuni was matched to the cylinder. (Yes Ray it's thin at the gasket mating surface, but it works).

Coll,


Hey, that's all that matters. Check out the gasket surface surrounding the top two bolts in the CR250 head in my other post.

How much did you open up the intake port? The engmod2t software says it flows about 32.2mm equivalent I.D. in stock form, and I've removed .5mm from each vertical edge (33.2mm I.D.) in case the software's a little off. My other motor also had the port lowered 2mm, for 34.5mm I.D. I'm using a 30mm Keihin.

I modeled 10mm and 12mm conventional passage-fed boost ports - the software won't do piston-fed - and got about +1hp starting at 8,000 rpm. I'm using a 9,000 rpm redline, though; if you rev higher, the gain is greater. I believe the piston-fed port works better than the conventional type in some ways, since 1) a large quantity of otherwise-wasted charge under the piston crown is utilized, 2) refreshing this charge every cycle keeps the piston cooler, and 3) very little flow / velocity is 'stolen' from the main transfers, since the under-piston volume is somewhat separated from the main crankcase volume.

At some point I'll upload port diagrams to my Photobucket account and paste links, as the diagram text looks horrible when resized.

Ray

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'74 CR125M (175cc), '75 MR175, '82 RM250Z, '08 YZ250F, '14 Zero FX electric, '14 Zero MX electric, '18 Alta MXR electric