I'm looking for some opinions...If you were to build a 4" stroke SBC, why should you, or not, think of it like a BBC; regarding port CCs, cam-timing,etc. BBCs' run real big ports(350+CC) stock, and seem to make big TQ. Why couldn't you run bigger ports on a SBC, and still make good numbers. A 4" stroke, is a 4" stroke whatever block it's in. I'm thinking the BBCs ports are longer, but I don't see how they could make that much differance.

 

You are correct in my opinion. You would need to size the heads and cam on an SBC 427 similarly to what you'd do with a BBC 427, within reason and within the abilities of the SBC architectural limits.

Unfortunately, finding SBC heads that flow as well as even small port BBC heads is a challenge and not cheap. I built a 11:1 compression SBC 427 based off a Dart Iron Eagle (Rocket Block) that made 650 hp naturally aspirated on pump gas and used Dart 257cc 18* heads that flowed in excess of 378cfm @.700". At the time, they were some of highest flowing BSC heads available for SBC's that were reasonably streetable. If I could have found bigger heads I would have used those.

I also used a solid roller custom cam with 256/264 duration and about .670" lift.

 

Big block and small block ports are not comparable by volume. In order for the flow of the ports to be similar, the cross sectional area of the ports would have to be close to one another. Because the distance from the intake gasket surface to the valve is so much greater, equal cross section ports will have vastly different volumes due only to the difference in their length. There are other reasons, too. If they come to mind, I'll repost.

RACE ON!!!

 

IMO ci = ci. One of the car rags tried to do a comparison of a 406 ci SBC against a 407 or 408 BBC. Very bogus data. The carb, cam specs, and simular ci was all that was really close on the engine buildups.

To really do the test right you would have to use some heads like Monty's Dart 18 degree so the CFM flow of the heads was the same. You would also have to have simular tuned intake manifolds. Then the headers they had on the BBC were 2 or 2 1/8 and the small block had 1 3/4

I not sure how the additional pounds of the BBC rotating assembly affect the TQ numbers.

What ever the vehicle or motor type, it really comes down to HP per pound.

 

Just look at the new LS7 cylinder heads which are 12 degrees or less and LS6 are 15 degree. These are used on the new 427 C.I. they flow comparable to a big block heads to produce comparable power as you would expect from a big block but in a smaller package. Check out
http://www.gmhightechperformance.com...0601htp_heads/

These are for LS heads but they are still small block heads

Yedi

 

But I'll bet those LS7 heads have NOWHERE near the intake ports ccs of a BB head.

RACE ON!!!

 

The aftermarket LS stuff is very impressive. I just ordered one of the new aluminum Warhawk LS7X blocks and a pair of the aluminum LS7X 12* heads from World Products today for a twin turbo LS7 427 I'm starting.

As just mentioned, the heads are still only 255cc, the Dart 18* heads I used on the first TT 427 were 257cc. It's still physically impossible to get ports much bigger in the SBC/LS architecture. I came across some heads in the mid 280's but the LS7X heads had some other fearures which were attractive to me for my particular project (6 head studs per cylinder vs. 4).

Most BBC heads start out at at least 265 or so cc's. I used some relatively small AFR 315CNC heads on the 700hp 9:1 Dart 540's I built for my boat and they easily flowed 380cfm+ at typical mild hyd roller lift ranges. The 18* heads I had in the Vette maxed out at ~378cfm but that was at .700". More realistic street roller lifts had them peaking around 355-360 cfm.

 

This is what I'm talking about. Good "food for thought". I know you can make 500+ HP with AFR-180CC heads on a bigger small block, and yet BBCs' have 350CC oval-ports and 315CC sq.-ports. From what I've gathered, the Oval heads flow about 264 CFM at .600..sq-port 291CFM at .600. Not all that great by todays standards, but I know 454s make plenty of TQ. All thoughts and input appreciated.

Monty...Those sound like some scarey fast motors to me.

 

Unless you are limited to stock cast iron production heads, you should at least be in the 330+ cfm range for aftermarket BBC ovals (360+ cfm for better oval heads) and 360+ cfm for BBC rec's (425+ cfm for full race).

 

You guys are still ignoring the LENGTH of the various ports. The BB ports are longer and are automatically, going to contain more ccs. Even the 12° and 15°, Gen III and up engines will get by with less ccs that the 23° Gen I heads.

RACE ON!!!

 

I don't think anyone is ignoring the length of the intake runner, we're just skipping that because we are talking about heads with known flow numbers. I don't think anyone disagree's or is ignoring what you said in your first post, but we're not fousing on port length or cross section, merely cfm.


Theoretical power potential can be generalized by what the head/intake will flow in terms of cfm, of course any inefficiencies anywhere else in the system (cam, compression, leak down, exhaust, anything) detracts from that theoretical potential.

While port length is certainly one of the multipliers that determines port volume, port cross section, which varies along the ports length does as well, hence the resultant port volume measured in fluid cc's. In any event, we're just talking power potential, not power at any given point within the rpm range which would be when you'd really start focusing in on port dimensions as it relates to intake charge inertia, port tuning, reversion etc. When you get into tuning or factoring in the port length, you also have to consider the port length of the intake manifold as well, as you can't consider one without the other. A 3" port in the head coupled with a 7" port in the manifold should be theoretically equal to a 4" head port coupled with a 6" manifold port, all else equal.

 

Most of the new Brodix heads for serious race engines start out at 285cc and go up to 330cc. I think that they are using some of the same logic as you guys are to make the 1000+ hp drag motors.