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Old 05-03-2013, 02:06 PM
Big Block Notch Big Block Notch is offline
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Default P51 heads: How much more airflow with bigger valves?

I've been running the P51s in my Mustang for several years now and really like them. With some moderate porting, the intake ports are flowing 415-420 cfm right now with the standard 2.250-inch valves. How much airflow can I realistically pick up by stepping up to larger valves? I need to switch over to titanium intake valves anyway, so now would be the time to do it. The new combo will have a 4.600 bore, so I'll have some more real estate in there. Thanks!

Last edited by Big Block Notch; 05-03-2013 at 02:17 PM.
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Old 05-09-2013, 09:12 AM
c.evans c.evans is offline
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I've done this modification about 6 or 7 times I believe. About half of them were titanium valves and about half were stainless steel valves.

You should gain anywhere from 10 to 15 cfm on the intakes at peak flow.

Hope this helps,
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Old 05-18-2013, 01:12 AM
Big Block Notch Big Block Notch is offline
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Originally Posted by c.evans View Post
I've done this modification about 6 or 7 times I believe. About half of them were titanium valves and about half were stainless steel valves.

You should gain anywhere from 10 to 15 cfm on the intakes at peak flow.

Hope this helps,
Precisely the information I was looking for. Thanks, Charlie!

Were those gains with 2.300 valves or something larger?
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Old 05-18-2013, 09:58 AM
c.evans c.evans is offline
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2.300". Technically we're too big in diameter at that number. Measure the o.d. of the seat and you'll see what I mean.

Charlie
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Old 05-18-2013, 10:05 AM
turbo2256b turbo2256b is offline
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You could use this for a compairison
http://users.erols.com/srweiss/calcafhp.htm

Real issue is the complete intake system. Will your intake actually flow more air, the carb.
An intake that flows 212 CFM bolted to a head that flows 235 CFM the intake will still flow 212 CFM bolted to a head that flows 360 CFM.
One shuld if possible flow the completer intake system at least with the intake
bolted to the head on the flow bench.
Flow testing an intake with a common plenum all ports but the one being flowed should be taped closed at the head gasket flange or the reading will be off (much higher).

Most setups I have dealt with the intake manifold is the bottle neck.
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Old 05-18-2013, 06:26 PM
Big Block Notch Big Block Notch is offline
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Originally Posted by turbo2256b View Post
You could use this for a compairison
http://users.erols.com/srweiss/calcafhp.htm

Real issue is the complete intake system. Will your intake actually flow more air, the carb.
An intake that flows 212 CFM bolted to a head that flows 235 CFM the intake will still flow 212 CFM bolted to a head that flows 360 CFM.
One shuld if possible flow the completer intake system at least with the intake
bolted to the head on the flow bench.
Flow testing an intake with a common plenum all ports but the one being flowed should be taped closed at the head gasket flange or the reading will be off (much higher).

Most setups I have dealt with the intake manifold is the bottle neck.
Good point, although my plans to put twin 88mm turbos on this combo probably throws a monkey wrench into the mix. Since I have to upgrade from stainless to titanium intake valves, I figured I might as well put a slightly larger diameter valve in it if it's worth some airflow. Realistically, all the additional airflow will be doing with a turbo combo is make the exact same power as it would with a smaller valve but at slightly lower (ie negligible) boost pressure.

Last edited by Big Block Notch; 05-18-2013 at 06:47 PM.
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Old 05-20-2013, 12:38 PM
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Quote:
Originally Posted by Big Block Notch View Post
Good point, although my plans to put twin 88mm turbos on this combo probably throws a monkey wrench into the mix. Since I have to upgrade from stainless to titanium intake valves, I figured I might as well put a slightly larger diameter valve in it if it's worth some airflow. Realistically, all the additional airflow will be doing with a turbo combo is make the exact same power as it would with a smaller valve but at slightly lower (ie negligible) boost pressure.
out of curiosity what kind of number are you trying to make? I am also in the process of building a twin 88 BBF combo and am yet to identify what heads to run. Do you think the p-51 provide enough flow to make the big number?
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Old 05-21-2013, 09:56 AM
Big Block Notch Big Block Notch is offline
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out of curiosity what kind of number are you trying to make? I am also in the process of building a twin 88 BBF combo and am yet to identify what heads to run. Do you think the p-51 provide enough flow to make the big number?
Twin 88s are typically good for anywhere from 2,500 to 2,800 hp. Not sure I ever want to max the turbos out, but I'd like to run 7s on a low-boost (15 psi), pump gas tune.

IMHO, I think the P51s are very much up to the task. From what people much smarter than I have told me, a max effort P51 head will flow in the 430 cfm range vs. 490 cfm for a max effort A460 head. On a naturally aspirated motor, you'd be crazy to sacrifice those 60 cfm (granted you can afford them), as they'd be worth an extra 120-150 hp on a high compression race motor. However, that same logic doesn't necessarily apply to a forced induction combo.

Boost and airflow are two very different things. In essence, boost is merely a measure of restriction inside the intake manifold. Let's say a 532ci combo with twin 88s and A460 heads makes 2,500 hp at "X" boost pressure. If you took that same motor and put P51 heads on it, the motor would make the exact same hp but at a slightly higher boost pressure. Sure the P51s would be more restrictive, but that restriction isn't nearly enough to reduce the total airflow capability of twin 88mm turbos.

An extreme example would be the Titan Motorsports ADRL Scion. With a single 91mm turbo on a 190ci inline-six, it ran 4.30s in the 1/8th at 50 psi of boost. There's no way six intake ports on a Supra head flow nearly as much as eight P51 intake ports, but by cranking up the boost that little six-banger makes enough power (with no weight break) to run with the twin-turbo big-blocks. That same 91mm turbo would make less than half as much boost on the big-block as it does on the Supra motor, but not any more power. Actually, it might make less power since you'd have to spin the snot out of it, take it out of it's ideal efficiency range, and seriously heat up the intake charge.

Since then, that car has run 3.99 in the 1/8th, but I'm not sure how big the turbo is or how much boost it's running.
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Old 05-21-2013, 11:01 AM
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I'm also not really trying to push the set up to its maximum potential. I'd just like to be able to make close to the 2000 whp range if desired but intend on a more conservative tune up for regular use. I have always had modular motors where the 4v heads from the facotry with minor adjustments is all you need to make 4 digit power. This is my first big block and I just don't want to shoot myself in the foot by picking the wrong heads. Of course you need to know what heads you will run when choosing pistons for final compression ratios etc. That's where I am at with my build right now.
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Old 05-21-2013, 02:12 PM
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I'm also not really trying to push the set up to its maximum potential. I'd just like to be able to make close to the 2000 whp range if desired but intend on a more conservative tune up for regular use. I have always had modular motors where the 4v heads from the facotry with minor adjustments is all you need to make 4 digit power. This is my first big block and I just don't want to shoot myself in the foot by picking the wrong heads. Of course you need to know what heads you will run when choosing pistons for final compression ratios etc. That's where I am at with my build right now.
If you're starting from scratch and can swing the extra dough, by all means go with the better heads (A460 in this case). Not only do they flow more air, but they have 18 head bolts for better sealing. That's what I'd do, but I already have a set of P51s.
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Old 05-21-2013, 04:00 PM
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Quote:
Originally Posted by Big Block Notch View Post
If you're starting from scratch and can swing the extra dough, by all means go with the better heads (A460 in this case). Not only do they flow more air, but they have 18 head bolts for better sealing. That's what I'd do, but I already have a set of P51s.
A460 is exactly what the builder just said I should do when we just talked. Now I have to make sure that my a460 block has provisions for 18 bolts. I guess some do and some don't depending on the generation.
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Old 05-21-2013, 04:53 PM
Big Block Notch Big Block Notch is offline
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A460 is exactly what the builder just said I should do when we just talked. Now I have to make sure that my a460 block has provisions for 18 bolts. I guess some do and some don't depending on the generation.
If it can't take the 18 bolts, upgrade to an Eliminator block and sell me your A460 block. LOL.

The A460 heads have better intake manifold options as well, provided there's adequate hood clearance.
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Old 05-21-2013, 05:51 PM
c.evans c.evans is offline
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The original question was in regards to the additional flow with bigger intake valves in P-51 heads. I answered that question and I do feel it is worth it,,,,,IF a guy is seeking additional power in a N/A P-51 headed engine.

However, now that the rest of the story has been revealed, I would not worry about adding additional intake flow, because that is going to be taken care of by the turbos and forced induction. If you are going with P-51 heads, then just stay with the 2.250" intake valves. The real bottleneck in the system will be with the exhaust ports flowing 215- 250 cfm, depending on whether they are ported or not. With any OEM style BB Ford head and turbos, the exhaust flow will be the problem area.

If other heads are available as an option, then by all means I would step up to any of the 18 bolt heads with turbos. You will have better head gasket clamping force with the 18 bolt heads. Those head options include the TFS A-460-18 bolt, the C-460, the Pro-Filer 205 and the Thor heads. Furthermore, your exhaust flow will be much improved with the TFS A-460-18 heads flowing about 325-330 cfm ported, and the C-460 and Pro-Filer flowing about 360 cfm ported, and the Thors flowing about 370 cfm ported.

The additonal exhaust flow will reduce your boost pressure,(all other factors being equal) because "the system" becomes less restrictive to flow.

Hope this helps,
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Old 05-21-2013, 06:18 PM
turbo2256b turbo2256b is offline
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+1

If under boost anything done to reduce flow restriction will reduce boost and would have to be adjusted for.
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Old 05-22-2013, 09:46 AM
Big Block Notch Big Block Notch is offline
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Quote:
Originally Posted by c.evans View Post
The original question was in regards to the additional flow with bigger intake valves in P-51 heads. I answered that question and I do feel it is worth it,,,,,IF a guy is seeking additional power in a N/A P-51 headed engine.

However, now that the rest of the story has been revealed, I would not worry about adding additional intake flow, because that is going to be taken care of by the turbos and forced induction. If you are going with P-51 heads, then just stay with the 2.250" intake valves. The real bottleneck in the system will be with the exhaust ports flowing 215- 250 cfm, depending on whether they are ported or not. With any OEM style BB Ford head and turbos, the exhaust flow will be the problem area.

If other heads are available as an option, then by all means I would step up to any of the 18 bolt heads with turbos. You will have better head gasket clamping force with the 18 bolt heads. Those head options include the TFS A-460-18 bolt, the C-460, the Pro-Filer 205 and the Thor heads. Furthermore, your exhaust flow will be much improved with the TFS A-460-18 heads flowing about 325-330 cfm ported, and the C-460 and Pro-Filer flowing about 360 cfm ported, and the Thors flowing about 370 cfm ported.

The additonal exhaust flow will reduce your boost pressure,(all other factors being equal) because "the system" becomes less restrictive to flow.

Hope this helps,
Thanks for the great advice as always, Charlie. I was considering the Trick Flow's anyway, and didn't realize how much of an advantage in exhaust flow they have over the P51s.

My turbo headers have already been fabbed up, so I wonder if there's any hope of salvaging what I've got. I'm hoping I can re-flange them for the BBC style exhaust ports, but that's probably wishful thinking.
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