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Discussion Starter · #1 ·
I’m constantly learning, tinkering and reading (book number 5 now) about performance engines. Specifically the SBC but also carburetors.

Just can’t help but want to swap the cylinder heads on a 385 I built. About 10.5:1 compression with Promaxx maxx series 200 heads and a crower .236/.240 .555/.559 hydraulic roller.

The truck only made like 340 to the wheels. I have since learned a lot about tuning a carb and it feels stronger but I haven’t put it on the dyno since. I have also “upgraded” the trans to an AR5 unit for a Colorado and it definitely feels like much less of a “truck” transmission over the NV3500. So I think drivetrain loss is down some as well.

The Promaxx 200 heads have a flow rating of 263/195 at .500
The AFR 195 flow at 293/224 at .500 and jumps to 304/229 at .550 (right at the cam specs).
The AFR 210 flow at 296/224 at .500 and jumps to 310/229 at .550

I mean that’s a big cfm jump over the Promaxx heads. Anyone have a guess or experience with how cfm relates to power gains? Even more so what it would improve my current engine?

210’s or 195’s?
 

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first big problem is "hydraulic" roller cam. Search Chad Spieir 379 cube sbc making 538 hp with 205 cc Pro filers heads and solid roller cam. Thats a monster improvment over what you have.

For the math on a "perfectly" matched V-8 with 11:1 cr you can get 2.2 hp/cfm air flow on the intake side with about 80% exhaust numbers in ideal situation.
2hp/cfm is a real well built combo.
you could do over 500 hp with those heads if you took it to 7k rpm with all the good matching parts
 

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Take that baby up to Temple Academy Dragway, and see how she does. Its really not far away from you at all, and you can make full 1/4 mile runs.

Your cam is really close to what Straub recommends for those heads. https://straubtechnologies.com/sbc-3-75-stroke-with-edelbrock-profiler-promaxx-camshaft

Promaxx is claiming they flow 280CFM and can achieve 550 HP on a 383. So you might have something on the table somewhere. Timing maybe? How does it perform on the street?

My build is very similar with this cam
and the (budget) Pro Header/EBay casting. My pickup weighs 3980 with me in it, and goes 12.6s at about the same altitude. Only difference is I have a 406, so just a little more bore but same stroke as you.
I gave up on HP numbers a long time ago, and just shoot for a good ET. According to wallace my combo is only making 393 at the crank. But everything else says the 406 should be closer to 500...
On the same hand, people say 12.6 in that big ass truck is good so thats what I go with. LOL!

Have fun, dont overthink it. Also I got tired of playing games, and am building a Dart Block 427 SBC this time because I didnt want to put more head on the 406 to get only minimal results.

I'm sure others will jump on this like a dog on a bone, so stay tuned for some good info.
 

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Power at the wheels is very problematic. The purveyors of chassis dynos like to talk about installation losses of 15 to 20 percent. The reality is these losses can easily hit 50 percent, it takes damn few sins in driveline component choices, part alignments and exhaust components and pipe lengths to end up with severe power losses. The only real test is to have both an engine dyno test and an chassis dyno test. From there you have an idea of what needs working on.

We have huge test resources to draw upon to relate engine component and configurational information so it is pretty easy to put an engine together to arrive at a pretty sound comparable crankshaft number. But as soon as you step into chassis dynos the details of the installation get lost.

More air flow begets more power as long as the valve train is up to the stresses of more duration, more lift and more RPM.

A thing to focus on is how the head flows beyond .5 inch. Many if not most flop around .5 inch. By that I mean they either greatly reduce, or stop, or even reverse flow gains past that point. This signals loss of flow control on the short side turn. This is somewhat like stalling an airplane wing where the flow detaches from the surface, in this case the short side turn of the port floor. It then goes turbulent and obstructs flow. So looking at head’s you want those that keep up reasonable gains with valve opening beyond your anticipated maximum lift.

You need to consider that unless your engine is using direct cylinder injection you are flowing an air fuel mixture against the fact the port is tested with air only. So now a large portion of the flow inside the operating engine contains much heavier than air molecules which are unable to take as sharp a turn of direction as the air thusly their separation occurs sooner and this begins to cause disruption to the flow before the port gets to the test point in the specs where you see the floor side flow go wonky.

Bogie
 

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bogie explain this wing stall for me? I thought the wing stalls from too low of air speed? Or in some cases extreme angle of attack Vs directional speed? Kinda opposite of port flow separation?
 

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Discussion Starter · #7 ·
I do need to take it up to temple and just run it down the track. If nothing else, it will be fun!

After that I may end up just pulling these heads and doing a little hand porting on them. Installing a better spring package and maybe swapping to some 1.6 rockers. I’m apparently lacking something here as everyone thinks I should be making more power!

If I get real antsy I may just pull the motor. Do the head work and then engine dyno it. That will leave me with a real answer on drivetrain loss.

That will be a whole different adventure!! The truck has new bearings, driveshaft, u joint, brakes, everything spins/rolls like you would expect. The new trans/clutch feels a lot less “draggy” than the previous one. I wouldn’t even know where to start!
 

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bogie explain this wing stall for me? I thought the wing stalls from too low of air speed? Or in some cases extreme angle of attack Vs directional speed? Kinda opposite of port flow separation?
Wing stalls happen at too low and too high of an airspeed. In the high airspeed this is an angle of incidence problem where the angle of the wing is too steep an angle relative to the direction of airflow. These are high speed stalls where the airflow cannot stay attached to the wing or tail surfaces, goes turbulent and takes a different path rather than following along the surface. Probably the classic of this is the Bill Odom crash at the Cleveland National Air Races in 1949 where a high speed course recovery maneuver stalled the wing; the plane, a modded P51, flipped and hit an occupied house with expected death and injury. This was the last time the National Air Races were held in Cleveland or any other populated area.

Back to ports, what you are seeing where the flow increases suddenly reduce, stop or even reverse with increasing lift is the short turn flow has detached from the port floor and has gone extremely turbulent keep in mind that minor boundary layer turbulence being normal and desired in a port. But the extreme turbulence in this case essentially is acting like a Tesla Valve and shutting down the flow. The whole point of canted valves whether they are in a hemi, pent, or BBC or Ford Cleveland or 385 BBF canted wedge is to ease this turn angle and keep the heavy breathing going.

A trick the SBC L31, Vortec employed was to widen the port floor as it approaches the short side turn. This slows the local flow allowing it to navigate the turn yet keep the overall port small which keeps velocity up which is good for ram filling the cylinder thus supporting good low RPM torque and in used to wash the pooled fuel off the backside of the intake that the sequential EFI puts there between intake cycles.

Now if you’re a follower of brother David Vizard, he, especially in his older writings, follows the old hot rod axiom of enlarging the upper port corner to make room for the majority flow in the upper roof and port wall on the bore side of the valve. I’d say that I was and mostly still am more in the camp best described by Larry Widmer of the twisted trapezoidal port. This works especially well on Oldsmobile and Ford Windsor ports, but holds pretty well for anybody’s wedge chamber porting. Then of course there is the simply big theory (Head Bytes) which works well on big displacement engines and smaller high RPM engines but is something of a pain if you drive a smaller engine on the street with humongous ports.

A take away about porting is your in an arena of gross flow measures where local flows are adjusted to to change the gross flow. As with everything else little or local changes have large effects on the total performance. If I’ve learned anything it is you just can’t let the flows meander, that in my book is vote against simply big holes as ports.

So there ya go, a bit about how little I know about porting.

Bogie
 
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