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Old 01-12-2011, 10:40 PM
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Intake port flow required for 320 hp?

Any idea for the optimum intake port flow for a mild cam
(206 @ 050 & .435 lift) to support about 320 hp
at 4500 rpm on a 9-1 cr 350 inch engine?

I'm trying to determine which of 2 heads will not leave any
power on the table at 4500/5000 rpm, but without getting
too big of a port - capable of too much airflow - so the flow
slows down at lower rpm so it won't fill the cylinder properly.
Torque & throttle response at 1600 to 1800 rpm will be very important
for this vehicle's gearing on the highway.

I've looked and can't find any calculators or formulas for that -
of course there are quite a number of variables.

I'm looking at a couple options that are good quality heads
and within my budget -

One head flows about 210 and the other flows about 185 at .435 lift.

Would 185 restrict it at 4500/5000, or would 210 be too much
for 1600-1800? Which would be better and why - if you have
any more of an idea than I do.

Appreciate any thoughtful, informed SWAG estimates! :-)
Of course with suporting info / theory / what have you.

Even some actual experience, maybe even on a similar engine,
would be just super.

Thanks!

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Last edited by macx; 01-12-2011 at 10:52 PM.
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Old 01-13-2011, 12:08 AM
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what is the intake runner cc on each head? since you are running a .435 lift cam what does each head flow at .385 lift?
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Old 01-13-2011, 07:25 AM
How fast is fast enough?
 
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"camel hump" heads or Vortecs will work well in your application. Don't worry too much about being too big, the 6.2L truck engines coming in the new silverados have HUGE csa.
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Old 01-13-2011, 09:05 AM
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The lower one will be a better fit IMO. A 210 for a mild 350 is a bit much and low end torque will suffer.

Generally on a decent engine you can get about 2HP per CFM. This is a very "Ball Park" rule. So a 185CFM port would support 370Hp +/-
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Old 01-13-2011, 09:12 AM
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ever try getting 2hp/cfm with low compression and a small cam? 210 is adequate, more would be better. Poor flow is poor flow, it doesn't make a head magically better at low RPM, neither head's flow is great at all.

Assuming a 1.94" valve you're going to want about a 170cc intake runner and ideally your flow should be about 260cfm. The further you vary from that the worse things will get.

Its not about flow, its about velocity. You need a runner that fits your valve and gets the best velocity out of that combo. 1.94" valves are best suited to about a 170-185cc runner on a typical sbc head.
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Old 01-13-2011, 09:20 AM
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I agree... I chose 402 cfm heads for a 750 hp build and 240 cfm for a 425 hp build. The 425 hp build was a Vortec, so I just had the heads mildly bowl-ported and I knew the smallish ports wouldn't hurt torque. The 750 hp build was in a 529 cube caddy, so low end torque wasn't an issue

I like to find the CFM I need and then get the head that provides that flow with the least port volume
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Old 01-15-2011, 11:13 PM
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I agree totally.

I'm focused on 170 intake runner heads, if I can't find some good used ones
such as base level Performers I'm looking at Vortec style heads, probly
aftermarket ones, with the 1.94 intakes. There are some aftermarket
ones that have some improvements over oem Vortecs (better exhaust flow
and exhaust valve seat cracking fixed) that are in the same price range.
I really like the RHS and Dart iron Vortec style heads, but they're about
$400 higher and do flow more which for my needs isn't necessarily a good
thing. I've got a 670 cfm TB efi system that will support a max of about
325 hp so don't need big flowing heads for hp.

Vortecs of course have good velocity flow at low rpm yet support a decent
hp level at moderate rpm ranges.

My engine will be running 1600 at ~58 mph and 2200 at ~ 73 mph so you
can see my need for good low rpm torque. Being an older (81) shortblock
that's in good serviceable condition, and my budget doesn't want me to
upgrade to a newer Vortec roller block or even change pistons and freshen
the rings & bearings etc, I don't want to overstress it, esp with any
higher rpm. Strictly a cruiser for good gas mileage but with some added
throttle response over stock. I'm going from the oem 2.87 to 3.31 gears
and swapping out the 3 spd TH 350 for a mildly built 200-4R & 2200 stall
with a lower 2.84 first gear and a .67 OD for the mileage.

My cam will be in the 206 to not over 212 @ 050 range with 435 to not
over 450 lift, and the heads I'm currently looking at, which I discovered
after the above post, have 148 flow at .300 lift and 195 at .400 which
should work well with that cam in the lower rpm ranges and,
being Vortecs, should have good velocity too.

Last edited by macx; 01-15-2011 at 11:27 PM.
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Old 01-16-2011, 12:03 AM
How fast is fast enough?
 
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Given equal volumes and valve size the head that flows the most at .300" will more than likely be the best choice. Look in the 170cc range with 1.94" valves.
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Old 01-16-2011, 10:13 PM
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Recently My 305 got vortec heads and a Performer cam. Fed by a Qjet and performer intake, the mixture velocity and manifold vacuum are awesome. The 906 vortec head has to be the best head for a road fairing vehicle. Intense manners, more hot than expected. 3 changes so far induction, cam , and 2.73-3.73 gear. Heads made the biggest change. Gear a close second. Cam so mild , added a little power everywhere no big E.

Heads were $275 with LS6 beehive springs and retainers. Clean up and re seal.
Cam and lifters $90.
3.73 Gears $150 + 250 labor.

Start with the heads, even if with a stock cam. The flow and compression bump will blow you away, or not.
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