Flow Bench Fixture Bore Size Differences

[skinny z]

There was a single hit when using "Speed Talk Fixture Bore" as search terms so I'll ask again.
Subject: RHS Pro Torker 12410 SBC Heads.
I've assembled a spread sheet using data from Stan Weiss' website. I've also added these heads after they came back from the porting bench.

Flow.xlsx

Flow.jpg

I've read that anywhere from 6-8 CFM could be attributable to varying fixture bore sizes to as little as nothing.
What's the real world experience?

What I'd like to know is how does what I have now stack up against what I had then. As in out of the box vs ported. For the record, the porting was described as minor short side radius work, open up exhaust port (described as "small and restrictive"), open up and blend bowls, 5 angle valve job (although it may be 3 angles). Pictures available.
Thanks in advance.

[skinny z]

I came across these two threads here at ST but it doesn't really answer the question.
Mathematically, I'd think there would be a formula to allow for meaningful comparisons.


viewtopic.php?f=1&t=8676


viewtopic.php?f=1&t=7665

[SpeierRacingHeads]

I just did this a few weeks back.

The head is a dedicated small bore, 4.060 wide. Flowed on a 4.030 and 4.155 fixture.

[skinny z]

Thanks for that Mr Speier.

Flow 2.jpg

It sure doesn't amount to much. Especially at the .500" lift level that concerns me the most.
There might be a case made for another 1% or so if the bore diameter goes up to 4.200" which is the as tested diameter provided by RHS.

[3V Performance]

If your using the Brzezinski head slider the bore sleeve will have very little affect because the slider acts as the bore against the head.

[skinny z]

With the exception of the last entry of that list (Flow) those values were gleaned from Weiss' website. There was a mention of Brzezinski however I can't say in what context the name is used.

[skinny z]

So, it would seem that there's not a lot to be gained from the larger fixture bore.

What I was trying to do is reconcile what RHS posts on their website regarding these particular heads and their use of a 4.20" bore. I can see how the flow figures would be enhanced somewhat but with what Speier Racing Heads posted, there's not a lot to be gained from a comparison standpoint. About 1% in the area where my heads will have peak lift (.575"). There may be more of a percentage gain if the step goes from the 4.03 they were flowed at (in my case) to a 4.20 rather than the 4.125 that Speier's experiment (4.03 vs 4.125) was tested at.

It'd be nice to know if the money spent had any real gains.

[3V Performance]

https://www.castheads.com/equipment-too ... ing-stand/

The slider bore size controls the bore diameter for the first 1" off the cylinder head deck. Below that the cylinder size selected has very little affect on shrouding.