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Post by magoconnor on Mar 20, 2018 16:14:23 GMT -5
Hello.
Has anybody tried running a 72cc cylinder (46mm piston) with a 47mm Bvh?
Any experience shared, will be much appreciated.
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Post by magoconnor on Mar 20, 2018 18:58:59 GMT -5
Any first hand experience?
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Post by 90GTVert on Mar 20, 2018 20:01:56 GMT -5
You need to look for BVHs that have an equal or smaller bore. A lot are 50mm+ and that makes a sort of mushroom shape up there. Not good and compression will be very low. Even some ppl with 50mm kits complain of low compression with BVHs.
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Post by magoconnor on Mar 20, 2018 20:53:02 GMT -5
The head i've got is a 47 mm BVh.
Have you tried running a 72cc cylinder with a 47mm Bvh ?
Do you think the compression will be to low?
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Post by 90GTVert on Mar 20, 2018 21:03:50 GMT -5
I had a Scrappy BVH that I think was 47mm and used it with a 47mm bore and 50mm bore. I had other issues with it on the 47mm bore so I can't say for sure how it was then.
Each setup can be a little different. Can't really know without some measurements.
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Post by magoconnor on Mar 21, 2018 11:19:36 GMT -5
What do you think will work best (performance wise)
72cc cylinder - stock head VS 72cc cylinder - 47mm BVH ?
I understand there is a couple of variables there comes in to play.
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Post by 90GTVert on Mar 21, 2018 12:01:10 GMT -5
That's a tough guess. Could go either way. Basic question there is does the engine respond better to more compression or larger valves and ports. Ideally, you'd cc all the components and try to figure out how to get the BB/BV head setup with good compression and squish specs and then it should win. Effort is free, and that will help you get the most from any setup.
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Post by GrumpyUnk on Mar 21, 2018 12:27:01 GMT -5
Ayup. I will take a SWAG that the improved airflow will beat the increased compression ratio. An engine is an 'air pump', and you'll flow more air with larger valves. IF. If the current or 47mm head is restricting flow, then I'd bet the larger valves would help. If the original can flow the necessary lpm/cfm, then having larger valves would not help. Math may be required. You are hereby warned.
If you do not regularly run at WOT, or in other words, you actually cruise a bit, having the higher CR with the original heads may likely give better response(useful response instead of at the high end). OTOH, if you want top speed, or run at or near redline all the time, you'll get better airflow with the BVH.
Do you feel the engine is restricted due to limited ability to intake enough air? Does it 'struggle' a bit at higher rpms?
tom
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Post by fugaziiv on Mar 21, 2018 13:18:35 GMT -5
We find that bigger ports beat bigger valves all day long in performance heads. Basically, make sure you've got the ports to match those big valves, or it's not worth it.
Matt
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Post by magoconnor on Mar 21, 2018 17:12:06 GMT -5
47mm Bvh = 21,5mm intake / 19mm exhaust. The ports was already matched with the valves.
I will be runing it with a 21mm carb.
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