Matching chambers to the bores

[steve316]

Notched top of stock block sbc for years. Even after market blocks the chambers always seem to be larger than the bore.

[MadBill]

Good to hear; surprising it's not more widespread.

[David Vizard]

Matching the combustion chamber to the bore and beveling / chamfering the bore to match the chamber are two entirely different things and have much differing consequences for every different application.

Trying to paint ALL combustion chambers and bores with this same wide brush is absolutely stupid.

Walter,
Thanks for emphasizing the point here regarding BBS 's. I did say that this was explicitly a BBC deal but is is amazing how often people don't read exactly what is said or extrapolate the data to far along from the original tests.

I think it's time to reveal what happened with the SB Ford and Chevy tests I ran and if your experiences can add (and I feel inclined to believe they they could well do so) anything more please share with us.

DV

[David Vizard]

Here are some results of tests I did some time back. This applies to SBF and Chevy. Also I should say that it is not a universal deal although, for most of the V8 heads I have worked on, the air flow more often than not drops on the exhaust side when the chamber is bore matched. The reason is that the predominant flow is from the direction of the center of the cylinder. When the cylinder wall side is relieved to the bore diameter more air attempts to flow on that side and it has an effect of shutting down the flow from the center of the cylinder more than the flow on the cylinder wall side contributes toward increasing it.
But on the dyno it is a different deal and is mostly an issue of supersonic flow. It varies from engine to engine but for the most part for a typical V8 sized cylinder and ex. valve the flow is supersonic at and around the ex. valve seat. This sort of velocity persists for about the first 100 to 200 thousandths lift. If the bore wall is shrouding the ex valve it has the effect of reducing the area for the S/S flow to occur. What I have typically done here is to un-shroud the ex. valve to a 0.200 diameter ball and limit that deshrouding only when it is constrained by the cylinder wall. Using a ball for this ‘pull through’ from beneath the valve to above (as described by my ‘Flow Balls’ technique described a couple of months back) I have been able to see a general trend toward more HP.
I am sure someone is going to ask ‘how much more HP?’ I can tell you now it is not huge by any means but the gain seen always beats even the smallest loss. On my SB builds in the 600 hp plus range I have tweaked CNC heads after I have done the out of the box tests. The results is anything between about 4 to 10 hp and this occurs in the last 1000 to 1200 rpm of the rev range.
How often have I run this test? Well I cannot remember for sure but certainly no more than 7 -8 times. In my book these test do not constitute a definitive result. So use the data as a gjuide.
For sure I would like to see other folks findings/experiences here.
Chad – I am sure you have something to say on this so let’s hear it.
DV