My rule of thumb for port sizing is double the port size and that is the CI the heads will feed without restricting a sub 6500 RPM 1 to 1.2 HP per CI high performance street build. 151 CC heads are used on a 305 HO from GM. Your C3 350 used a 150-170 CC head (used on 190 to 370 HP because it was cheaper to only have limited castings and most evolved from 327's, damn bean counters!). OTC Bowtie heads are 178 CC. 170 CC Vortec heads are used on 350's. A 383 needs 190 CC. A 400 needs 200 CC heads. A 427 needs 215 CC. A 434 needs 220 CC. If your running a lower RPM and under 1 HP per CI you can drop a size. If your over 1.2 hp per CI and running high RPM you can go the next size up. This is my method of choosing port size and has worked for me based on 40 years of experience. I know others will have opinions toward smaller or bigger being better. All I know is what has worked best for me. This is based on trial and error having to small and to big of heads on various builds and power levels.

 

Very informative - good info!

 

Exactly why I bought the 195's for my 383. Thanks for the info

 

I hope you're right, I also bought the 195s for my 383. Good info for others that are working on a new build.

Bud.

 

I had some AFR 210's I was going to use on a 383, are they too over sized to were they would be determential to performance, on a more street minded motor?

 

That is an easy rule of thumb to remember . . . simple enough for my old brain to remember. Thanks for sharing the information. I have 200cc heads for 383 build. Maybe I should be looking at a 396.

 

Great info! Thanks! I have RHS 200cc on my 383...nothing like just a little extra overhead

 

Man..us big block dudes are going to need more motor! My old 2Xtra's were 377cc!

Hmmnnn.....754 cubes....sounds fun!!


No such thing as too big of a head..just not enough motor under them!


JIM

 

BBC different animal. Just trying to guide the small block guys here as I have a lot more experience weith them. We will leave the big blocks to you.

 

Any loss would be on the bottom end. This is not carved in stone. There are guys out there that have used 200 CC heads on 350's and vortec heads on 406. The thing is both are probably not the best match. The 350 is giving up lower rpm power and the 406 is giving up top end power. If I was building a 383 and had the AFR 210's sitting in the garage I would use them. If I was planning that 383 and planned on buying AFR's I would buy 195's.

 

HP (vs. CID) provides a consistent, reliable reference for port sizing, especially when flow and port cross-section are considered in the design. Heads are selected for a power level based on flow, and this selection is typically the starting point for a build - the old adage that an engine is built around the heads is 100% correct.

I use the SuperFlow rule of thumb for estimating power from intake flow, taken directly from their flow bench manual:

Peak HP = (Intake CFM * .257) * # Cylinders

Intake CFM needs to be adjusted based on the intake used, with induction loss of around 20-25% for a stock or low-rise dual-plane down to about 5% for an optimized high-rise single-plane. Failure to adjust the intake flow results in unrealistic estimates. I've found this approach to be very useful and reasonably accurate with realistic flow numbers.

Intake velocity is a strong consideration based on the intended RPM range of the engine - don't make the mistake of buying a big port cross-section for an engine that's never realistically going to see over 6K RPM...or is being put in a car with restrictive intake and/or exhaust. This is also a good time to do some comparisons - the best head is the one that delivers the flow required for your power targets with the smallest port cross-section. Unless you're going racing, don't get seduced by high dyno HP numbers for a street-driven engine - street power is more about torque and the power range of the engine that a slightly bigger peak HP number

As an example, Brodix IK 200's deliver about the same intake flow as AFR 180's; should you make the selection by port size...or by flow? I would offer flow as the better selection criteria, if the 260 CFM intake flow offered by both heads is right for your power targets.

This is why Vortecs can be perfect for one 383 build...and really a poor pick for another.

David Vizard has written a number of articles on the topic; this is a good one with back-to-back comparisons and a balanced perspective:
http://www.dartheads.com/tech-articles/port-volumes/

 

210 afr's were the smallest heads I used on my 383. It is not just the head cc. Bigger heads have bigger port sizes which require single plane intakes and bigger diameter primary header pipes.


63Mako 427 small blocks at a minimum require 227cc and they really run better with AFR's new 245 cc

 

Good info, however like any Engine Build there are many things that must be considered. Probably the most important is how will the Engine be used. I used 180 CC AFR's on my 400 CI wich most anyone will say is to small but I have a Twin Turbo Engine and I wanted to make good Torque coming out of the corner on an Auto Cross Track. Then once Boost is made I'm pushing the Mix threw the Ports. Yes my Motor build was not the average I am just pointing out some varibles.

 

It's 2001 all over again, I give up

 

Valve sizing comes into play with bigger SBC of today. When 302,327,350s were built the 2.02 intake valve was still giving good valve area for the size of the engine. Now that 383s, 396,406,427,434 SBC are being built the very slight increase in intake valve now has them with " very poor valve area ". Notice the big intake valve put in the 7.0 liter 427 LS SBC street engine.

Gen 1 builders trying to build big cubes are now suffering the same problem BBC has always had under valved for all the cubes.

Then theres the fake airflow numbers all SBC head manufactures use by using a 4.200 or 4.250 cylinder bore to make the head look better. How many are running around with a 4.20/4.250 cylinder bore. The vast majority of gen1 engines being built a 355/383. Last time i checked they only have a 4.030 cylinder bore.

Even on a 396/427 size big block you use the factory oval port head install the 2.19 1.88 vales the head really wakes up. The original 2.07 valve way to small for 396 cu. The size of the rectangle heads on this engine can be argued as to big for anything but high rpm use. But everyone knows the size valve used in the oval port head was to small for lower rpm builds. You can't even find anything smaller then the average 2.25.1.88 valves in the smallest of any aftermarket BBC head.

 

I was just thinking the same thing about my 315 cc heads; 630 inches!

Not questioning the rule of thumb for SBC's, but valve curtain area must certainly have a lot to do with how it relates.

 

You did not build a 632 for the 315 heads you have to start over lol.

 

With most of the above. As I posted this is not carved in stone but is a great rule of thumb for designing a SBC build. As I stated if your under 1 HP per CI and running lower RPM you can go a size smaller. If your over 1.2 HP per CI planning on a lot of high rpm use you can go a size bigger. The same applies to the 427 Gkull was talking about. If your planning on running higher rpm quite a bit and your high HP you need to upsize. 427 /2 = 213.5 so 210 is a little small but about right if your running 427 HP. If your 550 HP your over the 1.2 and look at 220 or 227. If your pushing 600 HP which is common on these big CI builds you go bigger. Flow is definetly a factor as is port velocity but the reason for proper sizing is to maintain port velocity. a 305 with 151 heads will have similar port velocity as a 400 with 200 cc heads if you are running a cam with similar operating range for each engine. The 400 cam will be bigger with more lift. As billa pointed out a better flowing head you can lean toward the smaller side while poorer flowing heads you lean toward the larger port. No arguement from me with either of these guys and the comments contribute to better understanding of the hows and whys to those needing help.
I have read the article Billa referenced and it is very good. It shows the 200 CC head is the best match for the 1-1.2 HP 383 in the build. It is a typical street type 383 build and if you look at the hp and tq graphs it follows the reasoning. The 180 head gives up some top end power and torque to the 200 and is a hair more torquey at the lower rpm, the 215 makes the best peak hp but gives up 10 ft lbs of torque at the bottom to the 200. Best compromise of the 4 heads is the 200 even though the dyno show best peak HP from the 215. Properly matching your intake and exhaust to your heads and CI is a given.

 

But your running high rpm cams and over 1.2 hp per CI, single plane intake. Requires a step up on the head size as pointed out in the original post.

 

We need to be careful generalizing - it shows that, within the Dart product line their 200cc head delivered the best overall power curve. The Dart 200cc Pro 1 heads flow about 270 intake CFM from the measurements in the article...so I would offer that a set of AFR Eliminator 180's would have offered an even better curve with much less loss at the lower end.

The article is a good generalization to address the concept and impact of port sizing but it's important not to read it as a size recommendation by CID, which it is clearly not.

As an aside, it's interesting to use the SuperFlow estimate formula to compare the dyno results