MILD THUNDER

Was thinking about this the other day. I remember a few guys on here being big fans of notching the bores on a 454 block, for airflow gains. Some even mentioned they did it and picked up HP by doing it.

i was curious, was this test done using a cylinder head with stock 26* valve angles, like a GM or Merlin head? Or a head that has shallower valve angles, like a dart, afr, brodix, etc?

Camalot

Tinkerer

I believe it was done with Dart heads. The gain was at least 25 HP.
I don't think the valve angle matters at all.
The reason for valve notching the cylinders is because the bowl of the head because of the valve size is wider than the cylinder at the valve pockets.
A large part of the air/fuel mixture coming out of the intake port runs right into the top.of the block. This is almost 1/4 inch of blockage on a 454.
The exhaust valve pocket is blocked in the same way by the right angle turn caused by the top of the block.
when you grind the cylinder away from just above the top ring mark to just inside the PROPER head gasket you allow the mixture and exhaust a cleaner path.
The bigger the bore the less gain.
I didn't bore notch my 598.
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getrdunn

20-30 hp depending on combo. Especially smaller bore engines with larger valves. 454-496. Both intake and exhaust.

MILD THUNDER

Valve angles, can absolutely change the relationship between the valve, and cylinder walls.

I'd like to think that there are several variables that come into play here. Valve diameter, valve angles, valve lift, chamber shape, and so on. At least to generalize a claimed power increase on all smaller bore setups.

some bbc heads chambers are larger than others, resulting in more or less chamber overhang on the deck. Take an afr head with a cnc chamber for example. That head needs a 4.540 bore gasket. If you put a standard 4.280 bore felpro on that head, the fire ring will protrude into the chamber, yet it doesnt on most if not all factory heads. So, id like to think, that bore notching would net different results when using different heads.

ive seen here guys say 20-30hp, and seen on other forums where guys say they saw 3-5hp gains.

getrdunn

Baja Rooster

Tinkerer

When I said valve angle didn't matter I was saying that it didn't matter what the valve angle was because You would would still gain from notching with any valve angle.
If the head overhangs the cylinder it will smooth the flow out instead of running into the top of the cylinder.
The more that you unshroud the valves the more the head overhangs the cylinder.
I didn't consider stock heads because who is going to valve notch the cylinders without unshrouding the valves.

MILD THUNDER

I agree. Plus, a 2.30 or bigger valve , would exacerbate the issue, requiring more unshrouding. Prob Not much of an issue when using smaller valves, like truck head.

i know gm put eyebrows in the old hi perf mark iv blocks at the factory, i dont think the gen 6 models had them? Although, i believe the reasoning for them on the old blocks was back when small, closed chamber heads were used

Rookie

Valako did mine and I do run 2.300" in my 454's. This is a 4.320" (+0.70" over bore)


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