Flow testing

[superbike]

I have recently found different results when flow testing the same two heads one stock and the other modified by myself using two different size bore adapters.

My stock head on a 100mm and a 104mm bore adapter was 172 on both ?
My modified head with larger valves was 195 on the 100mm and only 183 on the larger 104mm adapter ?

The gains are easier to see and look much more impressive using the 100mm bore adapter.

Now i wonder what the gains really are ? The engines true bore is also 100mm.

Very odd whats happening ?

[SirYun]

That seems odd. i would expect the opposite.

Have you made shure that test have been conducted with similar temperatures and barometric conditions (accounted for warm due to the fan motors and such) ??

have you tried the effects of shifting the head on the bore adapter?

[djaychela]

At risk of sounding as if a little knowledge is dangerous, could it be that there is shrouding occurring with the smaller adaptor that is helping the flow?

Given that it's the "correct" size, this is a good thing, isn't it?

[Guy Croft]

Doesn't sound right to me at all Chris. However I'm only saying that because I have never seen it work like that..

GC

[superbike]

Before and after testing should all be done on the same size bore right for repeatable results !

It is weird and somone suggested it may be something to do with pressure recovery and that the air is clinging more tightly to the 100mm bore than the 104mm bore. The valves on this head are so big there just short of touching the 100mm bore its self . Feel free to post follow ups

[SirYun]

air is a fairly odd entity it seems.

the air could be sticking to the surface (it seems similar to coanda effect... maybe) thus guiding it along the cylinder wall

thus the shrouding of the valve might actualy lessen the enegry loss by expansion.

this is pure speculation..


but it is very interesting.
I would suggest a plexi glass/windowed bore adapter a strobe light and a fog source (darin morgan suggested dryice in a large softdrink container).

it might give you some idea where the air is actualy going