I use a CFM rule. Now I don't proclaim that it's the "right" way or anything like that... just my way of going about it.

I size a head based on airflow and an HP goal. For street engines (max effort daily driver) I use 1.9-2.0hp per cfm. Real radical street builds (more track than street) with solid rollers might be able to bump that to 2.1 or so. This is just empirically derived and based on a database of really exceptional small block builds I've come across these past few years. I encourage everyone to draw their own conclusions.

I also look at flow near actual "net lift" and not peak flow bench numbers. If the head flows 320cfm at .700 and you're only lifting the valve .600, then that .700 number doesn't mean a whole lot.... we'll never see it.
Based on the 1.9-2.0 cfm per hp rule, a head flowing 250cfm at or near peak valve lift has the potential of making 475-500hp at the crank. The build we mentioned a couple of pages back. He was putting down 430-440rwhp? When you consider a loss of 12-15% (M6 right??), that seems about right to me. The velocity through the port is probably high and his torque curve is likely strong in reflection of that.

The heads on my engine flow ~325cfm @ net valve lift. 325 x 2.0 = 650hp. That's not far away from where I'm at.

The Joe Sherman build up from the 2002 EMC. He was using an AFR 215RR with ~315cfm at .600. Joe was netting .565 lift and I'm gonna guesstimate 300cfm of flow at that lift. 300 x 2.0 = 600hp.... Joe made 604 and a gob of torque along the way.

I can list some other builds if you guys want to see more examples. Just trying to let you know where I'm coming from on port sizing.

This in no way is a measure of port efficiency other than to say, most really good ports I've seen (23º) flow ~1.4cfm per cc of volume. Check out some of WeldTech's cnc heads and you'll see what I'm talking about. The 187cc Smokey Yunick head for instance... 263cfm. Then there are the really exceptional ones that are closer to ~1.45 but most of those are raised runner types. There are better methods of measuring overall port flow.. this isn't meant to be one. I'd suggest hunting down Mr. Horsepower's posts on that stuff.

I'm not basing this on old combustion chambers either. You can really see the power differences in say a Vortec vs an old 041 or something similar. No comparison there, even if there are similar flow rates. Nor would I consider botched ports formed by some amateur porter.

So... if I were looking to make 500fwhp (~435rwhp), I would be shopping for a head that flows in the 250cfm neighborhood (net valve lift) at the smallest port volume necessary. On these high hp stroker LT1 builds, I think you're fighting against the intake too. That's where something like Bret's BRE Ram intake would really start to shine. Either that, or do some extensive porting (possibly welding) on the stocker. Either way, I think the intake plays a bigger part in these stroker builds. That and I don't doubt that minimum cross section areas could limit things when the revs get high.

Which brings up that "sizing for giving displacement & RPM" thing. The smallest port cross section has to be large enough so that critical port speeds don't become a limiting factor. Too small and the engine will fall on it's face at an rpm where the air speed is too high. Cam it all you want... it won't make much more power.
David Vizard talks about this in his book on small block chevy heads. Using an overly simplified formula....

CA = (.00353 x rpm x stroke x bore²) ÷ 690

CA = cross section area (in²) and 690 is the critical air speed in feet per second.

I'm working on a few programs right now. Plan is to build a mathematical model that looks at this in more detail utilizing piston speed and diameter in relation to crank position/cam timing. Not too difficult and I think it'd be a better representation of particle speeds through the smallest area.

So that's some of my theory... agree? disagree?? or just want to bat it out of the park???

Mr Meaux, Mr Riddeck, head porter guys... any opinions. Feel free to share some knowledge.

-Mindgame

 

I like the initial formula for getting a "ball park" estimate. I've nearly all the data on my current engine and it's fairly close to that formula:
Stock LT1 bottomend, ported LT1 castings, 242/252 .594/.612 hyd cam.
RWHP was 450 through a 4L60E, 4400 Yank locked up, carbon fiber driveshaft, 3.73 gears, & 26" tall BFG drag radials at 36psi. Now I'm figuring the drive train loss to be at just 17% with the converter locked up.
Using 17% loss we get 526.5 at the flywheel. The average peak flow for the heads (@.550-.600) was 247cfm. This results in a factor of 2.13 for the initial formula to arrive at the estimated 526hp. Not too far off in my book. Also the intake runner volume was 177cc. This nets 1.39 cfm/cc... makes me happy, LOL. Anyway we'll have a chance to test this further in the near future as I'll be dropping on another set of LT1 castings that are pushing 1.45 cfm/cc, but I may have engine dyno figures this time before the chassis dyno... great more variables for us to play with!
I'll be finished up flowing/measuring 95Bird's heads tomorrow. Then we can see how close the solid roller figures are.
Steve...

 

Based on your cfm rule, i am down on power. My heads flow (270 @ 500-600) WIth the 847 cam in there...i have dynoed 380rwhp and that is it..mind you i make 380 @6000 all the way to 6600....M6 car..

I can't fig out where the missign 20-30rwhp is and the low Tq 350ftlbs
any ideas?

 

mindgame...man...sometimes I just love this site. Thanks for explaining everything. The preliminary results of my old set of head on the motor show a pretty good head. With lots of room for improvement

I do agree that our intake is a huge restriction on high hp/high rpm motors. I've seen what our stock intake can do a head on the flow bench and how much improvements help that.

 

How about what the heads REALLY flow, that plays into MG's formula as well.

I don't get into who's heads do what, I pay people I am comfortable with to do the work I expect. (BTW if a guy has a conservative bench and you see that yourself, that's a dam good thing in my opinion)

I commonly get "oh yeah they are 300cfm heads" you wanna know what. 300cfm is something I would like to see out of a sub 230cc 23 head on my bench. Best heads I have seen yet are a 288cfm LT4 with stock valves and a 295cfm Weld Tech Track 1.

I would say that there are some very honest places which are close to what they say they flow. Some places that claim "290cfm out of a LT1 casting is easy", I wonder why because I've never seen one of their heads go over 260cfm on any bench anywhere else.

Then there is the flow curve.....

I've seen heads that claim 275cfm out of a LT1 castings do it, but the rest of the curve was not as fat as the 270cfm heads I have had on it. As of right now 275cfm @ .500 is the best I have seen out of a LT1 casting. Then again that casting was fatter at .200-.400 then most AFR and LT4 castings I have seen on the bench.

If you really get into some crazy stuff, like stock castings for race cars with stock valves.... Think closer to 2.3 hp per CFM, but then again that's a port job that is about 3.5-5 times more than you want to pay for your street heads.

My goal is still to get a stroker LT1, as a street car with a Hyd Roller cam in the 500rwhp range with standard 23deg LT1 heads (not converted SBC heads)

My opinion of EXCELLENT flow charts for a street small block would be around this.... no shaft mounts, no off set lifters etc...

.200 147
.300 213
.400 258
.500 288
.600 290
.700 295

or

.200 152
.300 219
.400 266
.500 288
.600 308
.700 312

Those heads are 215-220cc and they move that kind of wind. By MG's scale that's 580-620hp at the crank or 500-540rwhp.

Now when you go to SR's then there are some really exceptional heads, but I would want them on a 400cube motor. Then with lots of work you should be able to get 600rwhp, but as MG can attest that is not very easy.

Bret

 

I think that the 1.9 - 2.1 X cfm rule is a good guidline for figuring out minimum requirements from a head, but if you're building a 400 hp motor and you install a hypothetical set of 300 cfm heads, then you can install a milder camshaft and make the engine more streetable.

Also, having 270 cfm heads doesn't mean that you'll make 540 hp, it just means you CAN make 540 hp with just the right cam, intake, compression, exhaust, etc. . .

Mike

 

I agree with this 100%. I am friends with two local engine shop owners and they experience the same thing. 300 cfm out of a 23 deg standard port location head is very rare, but everyone thinks they have a set. Competition ported AFR 220's come pretty close.

Mike

 

what about when you apply boost into the equation? doesn't that throw the critical velocity up in the scale? I mean there is more density and there is more volume being flowed, doesnt that mean more velocity beyond what is considered critical?

 

Bret, I flowed some nice new AFR heads yesterday that had some great # for the runner volume... too bad their an LS1 casting though, LOL.
The new "Mongoose 205" has a port volume of 200cc on the nose. Peak flow was 293 @ .560 lift and had some nice bottom #'s as well, the flow had broken the 200cfm mark @ .300 lift. Looks like a nice little 2.02 head the LS1 crowd will have their hands on.
FWIW the average port volume I've see on some big dollar LS1 heads to get these king of mid flow #'s is in the 230 to 238 range... looks like once again AFR has done their homework.
Steve...

 

Absolutely!

Just a measure of potential, not a holy grail.
Ties into Schurter's question. Looking at a dyno sheet isn't going to tell you, myself or anyone where the power went. It could be a mis-match of components, a cylinder head port that wasn't good in the first place, tuning, machining tolerances, poor ring seal, wrong camshaft, valvesprings, a combination of things.... hard to say. Just means you have more power to be found. So get to looking.

Not easy at all.
If you build a stroker 383/396 it's going to take a great cylinder head and alot more rpm to make anything close to those numbers. There are 23º heads out there flowing over 300cfm but they're usually ~220cc+ and those are gonna require a conversion for the LTx block. Not to mention, by the time you buy the shafts and all the other parts to make them work, you could have bought a 14/15/18º head.
After all that, you still have an LT1 block when you could have built a 400+ ci small block on a better foundation in the first place. Sure there's some work involved with accessory brackets and other little crap but who said aiming for this kinda power was for the faint of heart.

So, if I had it all to do over again, I would have built a 427 (4.125 bore 4.0" stroke). No conversion for the heads and even these large race heads are not so big for an engine of this caliber. Look at the 454 BBC.... a good head for a strong street BBC is gonna be 280cc and larger. And here we're talking about a small block that's just 27 ci smaller! So bring on those 250-280cc race heads and get ready to make 650-700hp before you ever see 7k rpm. With those options... I think it'd be foolish for anyone to try and replicate my setup. It just gets too expensive for most people.

You could look at Chuck's combination for inspiration. 434ci engine, sb2 heads, custom intake and I think ~640hp at the rear wheels. It's an ultra high dollar build for anyone who can't build the stuff Chuck does. But you could do something real similar using what's available and a lot less money. Amazing thing about this ride is its street manners! I just got a ride... no driving, but I was in shock and awe.

So pick your poison boys and girls. Just make sure you have your mind made up before you start buying the parts.

And thanks for the info on your combination Steve. I'll add that one to my "database".

-Mindgame

 

Mindgame: so if you had it to do again, you would go totally GEN 1?

I suppose we're getting a little off topic. Is the general consensus (for the $$$) to go with the AFR 210's for a ~350ci LT1 due to the better exhaust side over a AFR 195? And the general consensus also seems to be, if you want "real power," go GEN 1 and scrap the LT1 block. ???

 

Absolutely. Either that, or an aluminum BBC.

I bought a BMR k-member anyway (for weight reduction and header clearance). If they had come up with their gen1 swap k-member before I was too deep into this project, I would have went that route.

I already have an aluminum tall deck Dart block that would go 434ci. From there I'd buy a set of heads (or use the set of sb2.2's I sold), figure out a way to bracket the accessories and have an intake built with a front mount throttle body. That would be the easiest way to avoid hood and cowl clearance issues. The rest of the stuff is just a matter of sorting.

-Mindgame

 

Elysian,

Please redefine a few things... are we talking about a head for your goal of 500rwhp or one for this build you're currently putting together?

I'd say the ported heads you have now are more than enough to make power with the 355, for whatever that's worth.

No, you can make alot of power on the LT1 block. Look at these supercharged guys.... 700rwhp, 800rwhp. Then there's George Baxter's engine that was making more than 1,000hp.

The potential is there. My route in using the big small-block build is in building a street engine that doesn't need a lot of revs to make big power. Anytime you want to do that normally aspirated, you need lots of cubes. Revs and reliability..... directly proportional to one another.

-Mindgame

 

Or... inversely proportional, LOL

 

What kind of compression are you guys running to get over 450rwhp N/A ?