Home porting of 236-series heads

[Michael]

The 454 big block formerly in my Z (and for the past 18 months on an engine stand!) came with small oval port heads; it’s from a 1978 Suburban. In an effort to decide whether aftermarket aluminum heads are the best approach for my purposes, of if the stock heads can be modified with reasonable success, I “ported” one of the heads with an electric grinder and a Dremel. Results were measured on a “superflow” flow bench at the local machine shop. I did not take measurements of the stock heads, but here is a comparison of the data in Chevy High Performance (available on their web site) with the after-porting results:

 

lift int_stock int_ported exh_stock exh_ported

0.200 125 129 87 95

0.300 183 196 111 125

0.400 215 242 130 146

0.500 233 259 144 162

0.600 235 269 150 182

 

Both data sets are at 28” water, with no pipe on the exhaust ports. This is for stock 2.07”/1.72” valves with stock (rather poor) valve job and 3/8” valve stems.

 

The Superflow data set include numbers for effective flow area, flow coefficient and a number called “valve velocity”.

 

In the opinion of the guy at the shop, a 454 with ~9:1 compression, a cam “with around 230 degrees duration [at 0.050”]”, Edelbrock Performer RPM intake and 750 cfm carb will make around 420 hp, with around 520 ft-lbs in the 2000-3500 rpm range.

 

The next day I commenced disassembly of the short block. Since none of the cylinders had appreciable ridges (carb cleaner took care of what little carbon deposits there were), the rods came out with minimal persuasion. Rodless, the crank is easily spun by hand. Oil passages in the rod journals were mostly clogged, but the journals themselves have no scratches or deposits of bearing material.

 

So perhaps by next spring I’ll have a running engine again!

[grumpyvette]

those peanut port heads you ported will support about 480hp/520tq with those flow numbers and matched to a good intake make a fine torque monster engine if you get the compression up to about 10:1

[pparaska]

Michael - Looks like all that aerodynamics engineering schooling paid off! You were probably imagining you were the mixture flowing through the ports as you were spitting iron chips . Nice work!

 

Looks like progress!

 

Dude, you need to stroke that thing!

[Michael]

Bad news!

 

One of the heads is cracked. I took them to the machine shop for evaluation, decking, new valve seats and guides. First, the shop pressure tested the heads at 35 psi (probably more than necessary, but evidently that's their standard procedure). Then they installed new seats and guides. After pressing in the new guides, they repeated the pressure test. And this time one of the heads failed - in two valve bowls.

 

So after spending $600 on machine work, one of the heads is worthless. And if I can't find another 236-head, the set is worthless! But even if I do find another head (so to speak), 40 hours of port work (I work slowly) is ruined.

 

On the bright side, AFR just came out with oval-port heads for big blocks. Their rectangular-port heads were out for around a year now, but those are too large for my application (305 cc ports and up). Their oval port heads are 265 cc. But aluminum BBC heads... $2000... ouch!

 

So what do you think, guys - should I force the machine shop to eat their $600, since technically the heads were OK until they messed with them?

[grumpyvette]

"Then they installed new seats and guides. After pressing in the new guides, they repeated the pressure test. And this time one of the heads failed - in two valve bowls

"

 

sounds like they failed to machine the heads correctly for the valve seats and guides, they used,(something thats not hard to do)and cracked the head casting when pressing in the valve seats or guides, CRACKING the heads

 

 

Michael

if your willing to pay all shipping costs Ill donate a set of iron peanut port head cores, but I will tell you right now its basically wasted time and effort.

 

youll be way ahead getting a set of merlin iron heads at $718 each or EDELBROCK aluminum heads at $860 EACH from JEGS OR SUMMIT

 

and don,t think those 305cc rectangle ports are too large... trust me, in a light car with the correct combo the slight loss in low rpm torque is a good thing, and you more than gain that slight loss in low end power back in the upper rpm range if the correct cam, compression,headers, and gearing is used

[Michael]

hmm... the machine shop's claim was that excessive porting of the heads might have "weakened them in critical places", so the formation of cracks upon pressing in new valve guides was a problem waiting to happen.

 

 

Grumpy, what is the "correct" procedure to install new valve guides - I mean, what methods should the machine shop have used?

 

Your offer of a donation is very generous - thanks! But as you point out, the lure of aftermarket aluminum heads is too great to forego. With that in mind, what is your impression of the new AFR oval-port heads? As far as I know, they have not yet been featured in the automotive press.

[grumpyvette]

if your going to get those AFR bbc heads ID PERSONALLY buy the 315cc small rectangle port heads or the 290cc oval port heads NOT the smaller 269cc oval port versions,they are currently some of the best on the market

 

http://www.airflowresearch.com/performance/westech_figures.htm