Just want to understand a little engine "101" on this issue.......

Does increasing bore diameter also increase head velocity in a normal engine rebuild? Is head flow affected at all?

Thanks, Charles

Normally, increasing bore size helps unshroud the valve. Assuming something like say a 350 with a 4.00" bore, do the math of a 2.02" intake and a 1.60 ex. valve. Add them together and that leaves a total of .380" of an inch to divide 3 ways between the outside of the intake, the space between the two valves and the outside of the exhaust. Not too bad....but add in some 2.050's and a 1.625 exhaust and now you're down to .325".

As the valve opens it can get real close to the cylinder wall and the flow just doesn't want to come around that little space, so flow decreases. That's part of the reason why big block Chevy's and Fords, Cleveland Fords, Hemi's and many more all have the valves canted at angles so that as they open they move away from the cylinder wall as much as possible. Same thing the Twisted Wedge heads by TFS do. Make sense?

Think of a big block with 2.25 intakes and a 1.88 ex. With a 4.250 bore breathing would really be terrible with only .120 divided the same 3 ways if the valve didn't move away from the wall as it opened. The poor 'ole 396 had to have notches at top of cylinder bore to allow decent breathing with the 4.094 bore.

There are times when engine builders will use a smaller intake valve than they wanted because of a limited bore size. The smaller valve *unshrouded* will flow better than the larger one that is *shrouded*. Lots of chamber work is done to help this area too.

So, increasing bore size just about always increases head flow on the motor, as well as allowing more chamber work to help too. Big bore engines will just about always make better HP as rpm climbs.

Some builders have been known to intentionally bore the cylinders *offcenter* to scoot cylinder a little away from intake valve. Also, look at the offest cylinder head dowels that move head .030/.050 away from intake side of cylinder wall. Doesn't work on small blocks due to the way intake/ex valves are arranged in order in head.

Hope all this helps,


JIM

540Hotrod, that really helps clarify an area that isn't discussed too often on this forum.

I believe that an argument can be made about keeping smaller valves instead of opting for bigger valves in many cases, especially when it comes to a pure street performer. You make sense about cylinder boring actually helping to unshroud an intake valve and aiding flow as well.

Thanks, Charles

I had a extremely lengthy discussion about this very topic with a high end engine builder in my area. Eventually we ended up deciding to stay with the 2.07/1.72 valves in my Vortec L29 heads because of shrouding issues with the small chambers. I don't have any hard numbers yet on power, but my butt dyno definately likes the low end grunt.

Well now I understand what they mean by "unshrouding" the valves. Great post!

It's been proven that on a set of new Merlin ovals, just cleaning the stock, 2.19"/1.88" valve ports up performed slightly better then adding 2.25"/1.90" bigger valves until you do the same thing to the bigger valves ports!!

The same thing applies to the 2.06"/1.72" smaller valves when adding the 2.19"/1.88" valves..

And it is said that GM's optional 2.300" intake valve was too big, even for the big 495"/510" Can-AM motors!! Or whatever size Bruce McLaren used to run??

Interesting reading and dig the power they made at speed!!

http://www.vintagerpm.com/can-am_history.htm

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