434 sbc heads and cam

[Marketmerlin]

I've been researching heads for about a 9:1 compression 434ci with a comp xe256h cam I want to build. The big question is how much headflow do you really need?

Most of the builds that I find are from early 2000's with 210-220cc heads. We have even better flowing heads available now. One article mentioned that whatever the original headflow for the engine was, increases that by 15-20% for a good street engine.

Now since you can build a 427 in sb or BB and the original BB came with 260cc intake runner heads. Does it make sense to run a 255cc cnc ported head on a 434 with a mild cam or would that be overkill? Comp makes that same cam for both sb and BB. Lift is .447/454 sb vs .480/.485 BB but all other specs are the same. Thoughts?

 

[bracketchev1221]

With a cam that small the engine will be out of breath by 5000 rpm. No need to put heads on it that can flow to 7000. You can put 200 cc heads on it and it will live at that rpm.

 

[lilbowtie]

As 1221 said - very bad match up. What do you want to do with the car?? Heads, cam, compression, intake, carb, convertor, and rear all have to complement each other for what you are planning on doing.

 

[Marketmerlin]

I'm not married to that cam. Just an example that they make the exact same one for both BB and sb with the only difference being lift. Ultimately though I want to build a sleeper with a fairly smooth idle that can run on 87 with aluminum heads. I'm still a long way from motor work but picked up a scat 4" crank last week for $220. Hosed it down with chain lub and wrapped it up in a pillowcase to keep it from rusting in the corner of the bedroom in case I happen to drool on it. So I'm pondering a little bit.

 

[Marketmerlin]

My old El that i had in 99 had a
NP440 trans with 1st 3.09, 2nd 1.67, 3rd 1.00, 4th .73 gear ratios with 2.73 and 26" tires. Im going to have that trans rebuilt and upgraded. I'll probably drop down to a 3.08 or "maybe" a 3.23 in this one. I just kinda figured I'd build an engine good for about 5500-6k redline and figure out what gear i like from there.

But this post wasn't really about my car. I'm trying to figure out why sb's are running smaller 210cc heads where BB's were running 260cc runners minimum from the factory for the same displacement. That part doesn't make sense to me.

 

[bracketchev1221]

HP is based on headflow and engine size. So if the head cannot flow the air, it will never make hp. Big block Chevy's used cylinder heads that were oversized at best. More on the intent that they couldn't get velocity and flow, so they just went big to compensate. An oval port big block chevy head can actually handle a low 600 hp, sub 500 inch engine at 269 cc port. So they don't REALLY need to be large if the flow CFM is adequate. However, this stands true for a equal rpm, hp small block. A 434 small block still needs the same amount of cfm airflow to make the same power as a 433 big block. So basically a 325 cc big block head that can flow 350 cfm will support the same power as a 255 cc small block head that can flow 350 cfm. However the 255 head will most likely do it at higher velocity.

 

[DRIVEN]

Respectfully, there's a lot of contradictory stuff going on here. "Smooth idle 87 octane high RPM sleeper" is a tall order. I think we'd all like one of those. Also, the gearing isn't quite optimal for anything other than because-I-already-have-it budget concerns.
The NP440 / A833 is a great muscle cruiser transmission but not too good for stoplight shenanigans. Those big sturdy gears have a lot of rotating mass and they just don't shift very fast. Plus they have pretty wide gear splits. The upside is that you get to enjoy that extra gear. Don't get me wrong, I like them a lot and have run a few -- just pointing out some facts. I'd pair it with some 3.23s and cruise the freeway at a comfy 2000rpm.
If I were in your shoes and building around that giant stroke, I'd focus on torque and just not spin it that high. Then you can enjoy lower compression, cheaper fuel, "smaller" heads, and a tamer cam. Nothing wrong with being all done by 4500 rpm if you've got a big fat torque curve that gets you there really quickly. It will also help disguise the shortcomings of the NP440.
The alternative is high flowing heads. Problem is, now you'll need high compression to make them shine...and premium fuel...and a lumpier cam...and a bigger carb...and deeper gears...

 

[fleming442]

Or, a bunch of nitrous!

 

[Marketmerlin]

bracketchev1221 I honestly didn't realize those 255cc heads that i looked at flowed so much less than the 200cc afr's. 261cfm vs 300 at .500.

 

[Marketmerlin]

DRIVEN I agree that the 2.73 and np440 are not the best for performance. I like the np440 because of the wide ratios. It still gives you the control over the backend like any other stickshift when you want to play and your not shifting constantly in town. Then cruise 80mph on interstate at 2k rpm as a daylily driver. Like you said, good muscle cruiser. With a mild 434 i think it would still be plenty of fun.