Just a SWAG ( Scientific Wild A** Guess) but the common numbers folks throw around are in the 1.8-2.2 HP per cfm. It depends on *IF* the engine can use the added airflow..meaning cam, exhaust and rpm range are within the heads range.
So a good serious port job that gave you airflow numbers you said could get you 35-40 hp or so. Maybe a little more.
The horsepower that a head can support is CFM x .257 x 8 cylinders. That is assuming no intake loss. The normal intake carb loss is in the 20% range and if over 25% you should look for a new intake.
This is all theory of course.
I have my stock 350 shortblock with stock cam (1.52rr right now though).
Max lift is a paultry 434 on the cam.
Looks like I won't be putting the new heads on until winter, so I'll have to make 'due' with the ones on there now.
The Superram on teh intake and accel lower manifold should be good for
280cfm, which supports a possible 560HP or more. The heads can flow right at the 280cfm range at 500 -600 lift (which I'll get to ones the new cam mayeb next year that goes in). Just trying to see what difference these heads would make now. Sounds good though.. I was counting on about 20-25HP over the Darts on there now at my current lift.
if you're increasing the flow rate, why not get a better cam while your at it, you'll get much better performance, and cams dont cost that much compared to porting.
I was planning to put the new heads on this weekend and see whats the absolute MAX I can get out of the car before the season ends on the stock '85 shortblock/cam, but looks like I will be doing it over the winter. Then I'll probably drop a new cam in. Nothing crazy though, as I enjoy driving the car as a daily driver whenever I want to, and not only race it.
Re: Heads Flowrate vs performance. (norvalwilhelm)
Quote:
The horsepower that a head can support is CFM x .257 x 8 cylinders. That is assuming no intake loss. The normal intake carb loss is in the 20% range and if over 25% you should look for a new intake.
This is all theory of course.
Is that figured at 28 inches.
ANyone have a formula for figuring how much the flow increases for every pound of boost that is applied
Bart, you may want to check in one of the well written books by Corky Bell for the serious math. Here is the crude math: divide the boost psi by 14.7, that will be the potential % increase in flow over the n/a numbers. I have to emphasize "potential % increase" because boost psi as measured in the intake manifold is really a measure of backed up airflow, rather than actual airflow.
Suprisingly, the crude math gets pretty close if the total engine combo is decently matched. My supercharged 468 for example: at 17.1 peak boost psi that would represent a 116% gain over the n/a power. N/A power was in fact ~ 480, the fuel supply limited 951 peak supercharged hp recorded could have been close to 1,000 +/- hp with fuel system changes.
Re: Heads Flowrate vs performance. (The Green Rocket)
My supercharged 468 for example: at 17.1 peak boost psi that would represent a 116% gain over the n/a power.
The Green Rocket what are you running for a supercharger? What is your compression to start with? Is your block oringed?
Do you run a Boost timming retart system.
Re: Heads Flowrate vs performance. (norvalwilhelm)
Norval, I'll only hijack this post a little - if you are building an engine and need technical help just email me with further specific questions.
The blower is a ProCharger D2R centrifugal (rated to be capable of ~ 34 psi of sustained boost). Compression ratio is ~ 8.3:1. Big blocks are good to ~ 20 +/- psi before o-ringing of the cylinder heads is needed. Big blowers run better with locked out advance on the distributor, this engine runs mostly between 28 to 32 degrees depending on boost & fuel.
Re: Heads Flowrate vs performance. (The Green Rocket)
Green Rocket I have been running my present 8-71 blown big block for 4 years. I start with 8 to 1 compression, run a maximum of 12 pounds of boost for an effective compression of close to 15 to 1. I run 43 degrees of timming with a boost master pulling the timming out as the boost builds.
I also O ringed the block to be safe.
I was just curious how it works with 17 psi of boost.
Re: Heads Flowrate vs performance. (norvalwilhelm)
Norval, that is one cool ride you have - many long years ago my dream car was exactly what you have. My engine had a Weiand 8-71 on it before the ProCharger, ran 17 lbs boost with that too.
Re: Heads Flowrate vs performance. (The Green Rocket)
Norval, that is one cool ride you have - many long years ago my dream car was exactly what you have. My engine had a Weiand 8-71 on it before the ProCharger, ran 17 lbs boost with that too.
Thomas
Thanks Green Rocket. Why did you dump the 8-71? I really like the down low grunt of the motor. Boost is at any rpm. I can also run on any gas as long as I don't push it.
Double click to enlarge
Re: Heads Flowrate vs performance. (norvalwilhelm)
I didn't dump the 8-71, there is a crazy story about how I got this engine. I bought the 468 (minus the 8-71 and related gear) from a crazy Chevelle guy who wigged out when a little oil leaked out of the engine. I know the people who installed the engine, it had less than 50 actual street miles at the time and they tried to assure him that it was no problem to reseal the intake. He would have no part of that, he said that he lost faith in the engine. So he had them yank the engine and build a new 540 (he spent ANOTHER $17,000+ on the new engine and trans etc.), onto which he placed the same 8-71 blower (driven slower than when it was on the 468). When the 8-71 was on the 468 running ~ 17 lbs boost I believe it was never dynoed past 5,000 rpm; made like 734 hp and 774 tq, all by 5,000 rpm (more rpms would have seen more boost and more power). At that boost level the instant off idle torque of a roots blower made his '69 Chevelle undriveable - it required eggshell throttle operation. His wife would not get in the car.
I had a few tuning ideas that I wanted to try with a centrifugal, and I wanted less off idle torque to make hooking up an easier task.
Now that they have the new air-loc rotors for the roots blowers I wish I had a good excuse for building another blower engine - but I haven't even installed this engine in a car yet!
Corvette Store
Heads Flowrate vs performance.
