I know that 18* heads don't provide good low-end torque, but why? Is it because of their large intake runner size or is it because of their different valve angle? Would 18* heads be a good choice for a large displacement small block like a 440 or 454?

 

The port sizes is the biggest reason that 18degree heads have limited lowend. They also have very large valves (again reducing lowend). They could be used in a large displacement motor to somewhat address this issue. Ligenfelter built a 427 smallblock with 18degree heads. It had a hp peak at a little over 6000 and torque near 4500. It can be done, it's just very expensive.

 

The port size is the reason for low-end loss (if any), but the 18 deg heads are such a better design, with a raised port that they have grat flow numbers, even low lift. Also you can get them from GM as a unmachined (unfininshed) casting, meaning that the ports are unmachined - ascast. They are only around 200cc intake runner before machining, instead of around 250+ when you buy them finished. If you buy them unfinished, 200cc, you can just clean them up and keep a small port, which would give you the low-end you want. Match a 400+cid SB with a 230-240 duration hyd. roller cam and - you got a 600 HP street engine. Chevy HI-PO did an article years ago, they built a 434 SB with a set of ported bow-tie heads it made 560HP, them they changed to 18 deg. heads (+the special intake and pistons) and the engine made 675HP. I wouldnt mind building a 18 deg. head SB for the street!!!

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91 B4C 305 TPI - SOON TO BE 383
TREMEC 5-SP, STOCK 1-BOLT REAREAND w/342 GEARS, K&N, AIRFOIL
EDELBROCK HEADERS, DUAL CAT TO HOMEMADE Y-PIPE & 3.5" SINGLE PIPE W/ FLOWMASTER, CRANK PULLEY, MSD, FUEL PRESS REG, COWL HOOD, WELD WHEELS
14.1@ 98MPH
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OTHER RIDE
67 CAMARO - STREET CAR, BIG BLOCK, PUMP GAS
350TH w/ATI 10", 12-BOLT w/373 GEARS
10.94@124.13 ON MOTOR - ET-STREETs w/MUFFLERS

 

As mentioned already, I wouldn't recommend them for a 350/383 street engine, but with a larger motor, they are awesome. I used Dart 18* CNC ported heads on my SBC 427. It made 500+ ft lbs from 3000-7000 rpm, peaking at 580 ft lbs at 5100rpm. Peak hp was 650 at 6700 rpm. This was with a custom Comp mech roller, 256/264, .690"/.650", 112 LSA.

They can be costly though. But you can get a set of unfinished heads from Pace or Scoggin-Dicky for about $350 a piece. Finished heads are usually about $3500 a pair, but they're CNC ported, titanium vlaves and retainers, good springs, etc. But then you have to add in a shaft mounted rocker arms system. My Jesel shaft rockers were another $1300. Crower makes very nice shaft rockers, as does T&D, for about the same price. Also, you need offset lifters, and usually custom length pushrods. You'll need an 18* manifold, since the ports are raised and the head's intake manifold mating surface is machined at a different angle than 23* heads. You'll need headers with either an 18* or spread port pattern, plus the exhaust ports are raised compared to standard 23* heads.

It can get expensive, $5000+ for the heads and valvetrain, but the 18* heads are much more efficient than any 23* head. That's where the power is.

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1982 Corvette Tremec TKO
<A HREF="http://www.geocities.com/monty_williams/434specifications.html" TARGET=_blank>SBC 427 650hp/580tq
</A>

 

Do 18* heads require a shaft mounted rocker system? Also, I assume 15* and splayed-valve(14 degree) are even better than 18* heads?

[This message has been edited by StCr19 (edited August 03, 2001).]

 

Yes, 18* heads, except for Brodix's 18x heads, require shaft mounted rockers and offset lifters.

15* degree heads and splayed valve heads are not necessarily better. GM Performance tried valve angles all the doen to 12* or so, and decided upon the 18* valve angle as being the better overall design. The 15* heads may offer a slight improvement in flow at very high valve lifts, same for the splayed valve heads. A comparison of 18* degree heads vs. a set of similar splayed valve heads shows less than a 5% difference in intake flow. The exhaust flow is practically equal. In all out, competitive racing, where rpm and money are no object, they do offer an advantage. But the special parts required are not as common as those available for 18* heads, and are even more expensive. For the average street guy, the 18* heads are a better alternative, in my opinion. I considered splayed valve heads, as well as 13* and 15* heads when I was building my SBC 427. I just couldn't justify the cost difference between them and the 18* heads.

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1982 Corvette Tremec TKO
<A HREF="http://www.geocities.com/monty_williams/434specifications.html" TARGET=_blank>SBC 427 650hp/580tq
</A>

 

Yeah, unmachined splayed-valve heads cost almost twice as much as unmachined 18* heads and I couldn't track down a manifold for them.

 

hey monty, your sig file still points to 434specs.html

 

b

 

Doh! I guess it does/did. Thanks I fixed it. I don't really post here much and my input is pretty much limited, since I don't have a 3rd Gen, but it's a good site.

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1982 Corvette
Tremec TKO
<A HREF="http://www.geocities.com/monty_williams" TARGET=_blank>SBC 427 650hp/580tq
</A>