Hi all,

Just wondering why SBC heads are siamesed in the middle 2 exhuast ports? Also, I see that the sb2.2 heads are not siamesed on the intake side but still have the siamesed 2 middle ports on the exhuast side. Is there any advantage or disadvantage to theese heads being designed that way? I hear that the head gasket is prone to failure between the 2 middle cylinders due to the exhuast ports being siamesed. Is this true or is it only a factor on nitrous and/or turbo cars?

Are there any affordable heads for a sbc that are not siamesed?

I have been looking at head designs of sbc heads for a few months now and it dawns on me that many performance non-sbc engines do not have the siamesed exhaust ports or intake ports for that matter. Are the bbc heads/ford heads/LS1 heads a better design?

Sorry for all the questions. I am just looking for answers before I decide on which heads to use on a turbo sbc street/strip setup.

Thanks,

 

The temps are a little higher because of the exhaust port pairing. More so in a standard port vs spread.

If you're routing coolant then you can take care of the heat without much trouble. If you search the forum for "reverse cooling" you'll find more info on this. Brodix also covers the routing in their catalog.

Affordable for who?

Most of the non-siamesed heads are splayed & canted valve types. Dart/Buick, Brodix BD2000, Splayed GM, etc., and while their initial casting price isn't bad... getting them machined and fully loaded is an expensive endeavor.

Depends on what you mean by "better". Do they have a better exhaust port layout? Maybe.
But which port does all the breathing? And, does the exhaust port layout really effect the scavenging efficiency of the motor?

You can make more than 1500hp from a set of "standard config" exhaust ports... so I don't think they're truely a problem per se.

If I were looking to build a 1000+ hp turbo setup... I wouldn't think twice about using an sb2.2 or GM 18/15º head FWIW.

-Mindgame

 

Don't make me make up something stupid here to argue about so you can explain this one.....

Bret

 

Why are sbc exhaust ports siamesed in the middle ports?

In the original SBC design done in the early 50's COST was a major driving factor. Siamese exhaust ports allow an exhaust manifold with only three branches, which means a simpler core for the casting. It also uses fewer attaching bolts, and has fewer pads to machine on the exhaust flange of the head. Overall it's also lighter, and therefore cheaper to make. Remember the first SBC was about 160 gross hp at the flywheel.

Stamped, stud mounted rocker arms were also lots cheaper to make than the cast or forged shaft mounts everyone else used at the time. The stamped stud-mounted rocker was actually invented by a Pontiac engineer (Clayton Leach). Because patents belong to the Corporation, not the individual or a certain car division, Chevy "borrowed" the idea. Both Chevy and Pontiac hit the market with their V8s in 1955 model year with the very similar rocker design as well as siamesed exhaust ports.

The other posts discuss the merits/deficiencies of the layout concerning power quite well.

 

Thanks for the replies! Very interesting stuff. Olds, I like the history of that...cost always is a main driving force in manufacturing. I should have thought about that in the first place..lol.

mindgame, et al, Do you know of anywhere I can find a 15/18 degree/sb2.2 used top end that flows in the upper 300's intake and 260-280 exhaust with a 60cc or bigger combustion chamber? All of the stuff I have been looking at online has 46-50cc combustion chambers and to get the compression I need with a dished piston I need at least a 64cc chamber.

Thanks and kind regards,

 

I agree. Nice to hear the manufacturing perspective behind things. It's all about them darn bean counters.

Is this something you'll be building or something you're having built?

The sb2.2 stuff can be tricky, where as the 18/15º stuff is a little more straight forward to build.

sb2.2's can have chambers in the 56cc range and with a 355 build that'd get you right around 9.0:1 with a 26-28cc reverse dome piston. The heads would like a larger bore but there's not much you can do about that IF you stay with a gen2 block.

On the other hand... 18/15º heads can be had in chamber sizes ~64cc. You can do some calcs from there.

Now, before we get too far gone here (and maybe we already are)... I wouldn't even consider this unless I just had to have a 1000+ hp force induction build that could run on pump gas. If you'd be happier with less, then stay with a 23º casting.

-Mindgame

 

Mindgame, This will be a 1000+hp turbo setup. I would liek to make at least 700rwhp through a th400 on pump gas and around 1100rwhp on race gas. I am going to be switching to a gen 1 406ci w/dart block for this so everything will be gen 1. It seems that 18/15 degree top ends for $4000-4500 are the way to go as getting a 23 degree top end porter with all the parts new would run into the same proce range as the 18 degree heads. I will be building this combo as soon as I can find a top end. Any info on where I can find a set of heads or advice will be appreciated.

thanks and kind regards,

 

GMPP 18 degree heads are good for 750-830hp with no boost easy. They are a good proven casting to base a build off of.

 

Most of the 18/15º heads I see (used but rebuilt) seem to go for $1500-2000 on ebay. That usually includes a set of titanium valves, retainers and springs. Some of that can be reused and some of it probably can't, but you could always sell the stuff you don't need. Rocker shaft setups usually go for ~$250 used but in good shape... sometimes I see them included in that $1500-2000 package price I mentioned earlier.

Some of these heads come out of shops like Hendricks, RCR etc., and if you take them to a good porter he can probably find another 10-20 cfm from them.
If you buy bare, with no seats or guides... expect to pay ~$750 to have the seats and guides installed with the seat work. So be careful what you get into and ask a reputable cylinder head guy how much it will take to get them street worthy.

FWIW, the sb2.2 stuff is just about as low $ wise just because most people don't want to mess with them. Probably not the best choice for your build but deals aren't too hard to find.

-Mindgame