Flow
Comparison

Flow comparison 4.030" vs 4.125" bores &
various back cut valves on a Cleveland head

Over the years I've seen a lot of heresay stories on forums etc. regarding flowing Cleveland heads using various bore sizes & valve types & back cuts etc. So, I figured I would do a half a dozen flow tests to give some indications as to what you may expect. Over the years I have found that bore sizes don't always make a difference, sometimes I've seen zero change. With Chev heads, I've never seen any noticeable change, however most Cleveland heads I generally see 10 or more cfm. increase. As for back cutting, I always see a good flow increase, when using Ferrea or Rev valves. Manley stainless valve heads are too flat & have seats that are too narrow, which prevents back cutting, hence they just won't flow air, at least that's what I've found.

Here's a quick flow test on a very well ported Pro Comp Cleveland head. This is NOT a test about the PC head, but about flow comparisons when changing from 4.030" to 4.125" bore size & using stock / 32 deg. / triple back cut valves. Both bores have valve scallops out to near the gasket line.

Test 1. Ferrea 6000 no back cut 4.030" bore

Test 2. Ferrea 6000 no back cut. 4.125" bore

Test 3. Ferrea 6000 valve 32deg back cut 4.030" bore

Test 4. Ferrea 6000 valve 32deg back cut 4.125" bore

Test 5. Ferrea 6000 valve 3 angle back cuts 4.030" bore

Test 6. Ferrea 6000 valve 3 angle back cuts 4.125" bore