SBC point of diminishing return on cubic inches??

[73Nova]

Ive allways heard talk about big inch small block chevy's and that there is a point where you cant put "enough" cylinder head on it if you stay with a 23* design. So with todays modern "top shelf" CNC ported cylinder heads like the M2 227cc Track 1's and the 227cc AFR's, at what cubic inch would this happen or start to take place? Is there a certian point of diminishing return where you simply have cubic inches for the sake of cubic inches??

[Freiburger]

Re: SBC point of diminishing return on cubic inches??

What rpm and application are you talking about?

Generically, you can build about a 450ci small-block without too much trickery. A typical 227 CNC small-block head in an NA application probably won't be too happy above 6,000 rpm. Imagine a 454 BBC with peanut ports; they have about the same crossection (guessing there).

[JeffMcKC]

Re: SBC point of diminishing return on cubic inches??

I would not go bigger than a 420 with those heads, 4.155 bore X 3.875 stroke with a 5.85 rod. The 4 inch and bigger strokes need some very large heads and camshafts, the piston travels down the cylinder so fast it makes it hard to fill.

[MadmanMark]

Re: SBC point of diminishing return on cubic inches??

There are some pretty stout sbc heads on the market. There was a recent PHR article that used AllPro heads on a SAM-built 400 and hit 574ftlbs at 4800 and 639hp at 6500. I'd bet tq & hp would pick up if they used a 4" instead of 3.75" crank. If you call AFR about a 434, they'll more likely steer you to their 210s instead of the 227s unless it's for a full-race application, so I'm not convinced sbc size is entirely head limited.

A lot depends on the intended rpm. If you're going to be driving your car in the 6000 range and shifting near or above 7k, big strokes seem trickier to work with. If you can live with a low-to-mid 6000 top end, I think the more cubes you can get the better if you're without power adder. If you want to run some spray, I don't think you'd want a max bore. I've heard (although 2nd hand) about people having problems even with 150 shots with 450+ ci.

The biggest downside I saw going with 4" vs 3.875" is there's not a lot of room for the rings with the 4". It makes me a little uncomfortable having a street motor with the pin intruding into the oil ring (1" CH). A taller deck block could fix this, but would require some custom headers.

[GibTG]

Re: SBC point of diminishing return on cubic inches??

I would guess that the port length would be limited to around 5.5-5.6 inches on a standard valve location 23° head. This would make the cross sectional area of a 225-230cc head only about 2.5 square inches. We all know this is highly theoretical but...

This would only power a 434 to 6500 or so RPM's.

People underestimate how much power can be made from 420+ cubic inches when the peak horsepower is REALLY above 6000 rpm though.

[Eric68]

Re: SBC point of diminishing return on cubic inches??

I would not go bigger than a 420 with those heads, 4.155 bore X 3.875 stroke with a 5.85 rod. The 4 inch and bigger strokes need some very large heads and camshafts, the piston travels down the cylinder so fast it makes it hard to fill.

I'm getting ready to find out. AFR 227 comp port heads on my workbench and a Callies 4.125" bore x 4" stroke rotating assembly in on its way. 12.8:1 compression pistons should work with my E85 tune. Goal is max power in the 5000-7000 RPM band.

I personally think 427" and 7000 RPM is about it for the heads you mentioned. If you only want to pull a dump truck a 450" SBC with those heads would work (below 6k like DF said). If you want to turn 8000 RPM I'm thinking a 406-410" motor is as big as you would want to go with a good 230 cc 23* head.

Now if you want to pony up for some 18* (or better yet 12* or SB2.2) stuff then I think the sky is the limit with the right ports. A 500" SBC will run like a raped ape with some big cross-section 450 cfm ports . . .

Jeff and I just had a long phone conversation about this ;D Thanks for the call by the way Jeff ;)

[Eric68]

Re: SBC point of diminishing return on cubic inches??

I would guess that the port length would be limited to around 5.5-5.6 inches on a standard valve location 23° head. This would make the cross sectional area of a 225-230cc head only about 2.5 square inches. We all know this is highly theoretical but...

This would only power a 434 to 6500 or so RPM's.

People underestimate how much power can be made from 420+ cubic inches when the peak horsepower is REALLY above 6000 rpm though.

According to my measurements, my AFR 227s have a minimum port cross-section of right about 2.45 sq.in at the pushrod pinch. That takes into account the height, width, and radius of each corner of the runner at the smallest spot.

Now, if I plug this info into a David Vizard formula that assumes a limiting port velocity of 690 fps, I can predict that a 427" engine will hit limiting port velocity (690 fps) at 7000 RPM. Of course this is approximate and not necessarily a "stone wall" but it gives a little math to support what I am saying about LVP and 23* heads on an SBC.

Keep in mind that from what I have read, the further away from the valve a choke point occurs the less it hurts power output, so maybe you could get a little more RPM out of a head like this.

[MadmanMark]

Re: SBC point of diminishing return on cubic inches??

Eric - In your call with Jeff, did he give in that 420 isn't the limit for the 227s? Also, are you running a 6" rod and stock deck?

I got a serious case of DW-envy. Of all the events out there, DW pulls me in the most. I'm doing a serious push on my bro to cage his car to do it next year. If I don't succeed, I'll eventually cage mine, but I think I'd wait a couple years for my 5 yr old to appreciate it a bit more. I wouldn't care if we finished last as long as we finish (I might be happy if we could survive 4 of 7 track days).

For what it's worth (probably not much), my bro's 427 pulled 630hp at 6200 with the AFR 210s and a hydraulic roller on an engine dyno. I have a 6800rpm chip in my 4000+lbs chevelle with almost the same engine and can hit the limiter fairly hard in drive (about 160mph). Since destroying a new set of 255 60 15 bfg ta radials (on the front), I've given up the high speed runs, but the thing felt like it was pulling the whole time. What I meant to imply is that sbc heads are probably not a limiter until you're really pushing the envelope.

[Eric68]

Re: SBC point of diminishing return on cubic inches??

That's sort of a loaded question Mark ;) You know Jeff enough to know that he don't give in LOL . . . but I think we agree that RPM is a pretty important factor. If you want to spin a 427 SBC motor to 8k like Jeff's then we agree a 23* 227cc head probably ain't enough. At 7k I think it probably is . . . like I said before, I'm getting ready to find out.

But then there is a whole can of worms about what sort of bore stroke combo you should use to get 427 inches and which one is better for high RPM power.

Then there is that can of worms about how much cam you need to make power with a head limited big inch small block . . .

And the can or worms about how much compression you should have with that cam -- even on pump gas.

;D

[MadmanMark]

Re: SBC point of diminishing return on cubic inches??

The worm cans you mentioned can't be any wormier than the e85 can.

[GibTG]

Re: SBC point of diminishing return on cubic inches??

From my understanding the farther away a choke point is in the induction tract from the valve, the more it hurts power. Larry Meaux describes it as affecting the wave tuning possible in the induction tract - and the apparent length of the port would be changed more by a choke point at the pushrod pinch than a choke point at the short-side radius.

...from working with old 23° SBC heads with pushrod choke, If it's too "choked" at the pushrod area, thats about 4.000" inches before the Curtain Area... the Pushrod Choke problem seems to loose more horsepower than the Short Turn FPS speed problem... so from just those experiments... I'd say it appears if the choke location is located too far from the Curtain Area..that it must also be influencing Wave Tuning.

In other words, you tune your intake length from entry to seat angle or curtain area....but if you put too much choke midway between them... Ithink it disrupts or changes wave tuning enough to hurt torque, by changing its pressure wave "timing."

You can successfully run a lot more short turn apex FPS than you can get away with at the pushrod area.

~Larry Meaux

By the way Eric, thanks for the numbers on your AFR heads - it puts my estimates in the ballpark.

[min301]

Re: SBC point of diminishing return on cubic inches??

I agree, on readily available heads, 420ci, or close is likely it.

[Eric68]

Re: SBC point of diminishing return on cubic inches??

From my understanding the farther away a choke point is in the induction tract from the valve, the more it hurts power. Larry Meaux describes it as affecting the wave tuning possible in the induction tract - and the apparent length of the port would be changed more by a choke point at the pushrod pinch than a choke point at the short-side radius.

...from working with old 23° SBC heads with pushrod choke, If it's too "choked" at the pushrod area, thats about 4.000" inches before the Curtain Area... the Pushrod Choke problem seems to loose more horsepower than the Short Turn FPS speed problem... so from just those experiments... I'd say it appears if the choke location is located too far from the Curtain Area..that it must also be influencing Wave Tuning.

In other words, you tune your intake length from entry to seat angle or curtain area....but if you put too much choke midway between them... Ithink it disrupts or changes wave tuning enough to hurt torque, by changing its pressure wave "timing."

You can successfully run a lot more short turn apex FPS than you can get away with at the pushrod area.

~Larry Meaux

By the way Eric, thanks for the numbers on your AFR heads - it puts my estimates in the ballpark.

Thanks for that -- I was thinking about something Larry posted too, maybe I got it backwards?

[JeffMcKC]

Re: SBC point of diminishing return on cubic inches??

The Diminishing return is the key words for me. The Idea of winding up a 4 inch stroke causes a lot of problems, in the cylinder fill area. Its not that they won’t run and they will turn up but the returns are already diminished by then. A 420 is what I would call a shelf motor you can stock block a 4.155 bore 3.875 stroke or you could go to a after market block and do a 427 with a 4.185 bore and a 3.875 crank this I think you will find to be a better 427 than a 41.25 with a 4 inch stroke.

Now this may be being picky, but there is no doubt in my mind that it will make more power without a 4 inch crank, 427 against 427. The bigger bore allows for a bigger intake valve un-shrouding is better the fact that the piston speed is down for both cylinder fill (valve events) and speed for pulling the rod apart would also be better.

Its not that the Bigger motors wont make they will but HP Per Cube starts going down. A wound up, correct headed and cammed and intaked 406-410 may, make more power than a wound up 434 under cammed and under headed and under intaked

Eric and I talked a lot about this, and His will work pretty good he is not going to try to spin it up. I think E85 takes up more room yet in the port with fuel, worrying about pushrod pinch, fuel shear, cam base circles, If your spraying it Ring packages, comp height, a 5.85 rod looks better too for piston stability.

If you ask me 420 is about it for what I try to do. Eric is right, we will see what my twisted sisters do next year, they flow 450 plus, and good air speed, with small ports but they are a high end head.


These are my thoughts as best as I can type them there is more to it but,,,, hope it helps explain my thinking and your questions

[Freiburger]

Re: SBC point of diminishing return on cubic inches??

I think E85 takes up more room yet in the port with fuel, worrying about pushrod pinch, fuel shear, cam base circles,

How does the volume of fuel in suspension relate to cam base circle?