BB Head Differences

[Kevin Whiteley]

From Colvin's book, I've been looking at the differences (and similarities) between the following 3 heads.

3873858 - 66 - 427/425 - 2.19/1.72 - 108.989cc
3904391 - 67 - 427/435 - 2.19/1.72 - 106.8 cc
3919840 - 68 - 427/435 - 2.19/1.72 - 106.8 cc

I believe all are rectangular port closed chamber (if I'm wrong, please correct).

How do these numbers and heads translate into performance (hp/torque) assuming all other engine components are the same?

Are there any other differences which would affect performance? Assume also that I'm looking to build an L-71 type engine and not worried about numbers correctness

[Joe L.]

Re: BB Head Differences

Kevin-----

All of the head castings described are cast iron, rectangular port, closed chamber heads. The differences between them are relatively minor. I would expect any of them to produce equivalent performance.

[Duke W.]

Re: BB Head Differences

If you're building an engine from scratch and don't care about the casting numbers you're probably better off looking at aftermarket heads. Most rectangular port aftermarket heads have 2.25/1.84" valves and though the open chamber type won't take as much compression as closed chamber types, they flow better. I would limit CR to 9.5:1.

BTW, the L-71 cam is an excellent street high performance street cam. If I were building a BB I would go with aftermarket heads, pocket port and port match them for maximum flow and use the L-71 cam. This will make for a bullet proof valvetrain, decent idle and low end torque, and a strong top end. Peak power will arrive in the range of 3500-4000 ft/min mean piston speed. The stroke will determine actual RPM at this piston speed. A "short stroke" 427 would be a real top end screamer, but you're better off building a 4" stroke and going for maximum torque bandwidth and limit top end revs to about 6000.

I'm sure Joe L. will have an opinion on this one. I recall I ran some simulations for Joe on some of the crate motor roller hydraulics and they were also pretty good. He probably kept the data handy, but I prefer the sound of a mechanical lifter cam.

Duke

[Kevin Whiteley]

Re: BB Head Differences

Thank Duke and Joe,

The reason I ask is I have an L71 car (NOM), and through the various sources, I have seen more 66 and 67 heads available than the 68 840's (which would be original to the L71). I was thinking that I could acquire the 66 and 67 hi-po heads and use them until I can get the 840's (for a decent price). I actually want to stick with authentic era parts except for the internals.

And Duke, since you went into an area in which I was going to inquire in the future. I want to build a "torque-monster". I want it to launch off the line, but won't take it to highway speeds for a long time. The car came with 4.11's and that's what I will put in it (past owner put 3.73's in). I have the original M-21 also. If I'm incorrect, please advise, but I've always equated "speed" with HP and "acceleration" with torque.

I really don't have to stick with L71 internals or specs, what I'm after is really something that looks like an L71, but just a little more. I put "assume" in my previous post so I could get base-line answers. Sorry for confusion.

[Frank H.]

Re: BB Head Differences

The first two 858,391,+392 heads you lised have no temp. sensor boss as the last 840,or(842) head does have a temp sensor boss or tapped hole,your 68? (my guess)
will be visually not the correct heads.
New aluminum GM performance heads L-88 ,L-89 would cost out about the same as the 391,392 heads go for.
How about a L-89?
FH

[Mark #28455]

Re: BB Head Differences

If it's a torque monster you're looking for, don't stop at 454". I have an original L89 that I rebuilt as a 489" (4.310 bore, 4.25 stroke) since the stroker cranks have become so cheap now. I used .25" longer billet rods but if you use the stock type I beam rods you can frequently get away without grinding in the 439 and 512 casting blocks. JE sells pistons through their SRP line - forged for about $500 a set. I picked a brand new set on e bay for $300, or you can go with the Keith Black pistons - cast hypereutectic for $300 a set. Make sure you get the flat tops and your compression will be perfect. On one motor, I went with the SRP mini dome and the resulting 10.3:1 with closed chamber heads runs strong, but diesels when you shut it off unless you add a few gallons of racing gas to each tank - a real hassle when you only get 6-8 MPG. I have the 4.10 rear too as you can tell!
Externally, the 840 closed chamber heads look nearly identical to the cast iron LS6 open chamber heads that I bought in 1984 (with the open chamber, I would use the mini dome piston). You can pick up the open chamber heads for about $500 a set used. If you want to see, I can show you at the winter regional in Orlando.
Good luck,
Mark

[Duke W.]

Re: BB Head Differences

It's HORSEPOWER that accelerates a car, and a "high torque" engine will have more power at low revs and more AVERAGE POWER across the useable rev range. To build a torque monster you want as much displacement as possible, so I recommend 454 CI as the minimum. Going to a higher bore or stroke than 4.25 x 4.00, can be expensive, so you have to make a cost tradeoff. If you limit CR to about 9.5:1, it should operate detonation free on pump premium.

I would still recommend the L-71 cam and rework the vintage heads with the normal pocket porting, port matching, and multiangle valve job. I would spare no expense on head prep, and then use OE or OE equivalent parts throughout with one exception. The one area where I recommend true racing parts would be the con rods. As I recommend for mechanical lifter SBs, a set of Crower Sportsman rods will make for a bulletproof bottom end.

With 4.11s and a 454 as configured above, your biggest problem will be finding traction to put the power to the ground, and it will be perfectly safe to rev to 6500.

Duke

[Joe L.]

Re: BB Head Differences

Duke and Kevin-----

From the perspective of performance, modern big block aluminum heads are far superior to the old variety, cast iron or aluminum, in numerous ways. However, for a street engine, the aluminum cylinder heads currently available from GM offer the best performance potential. These heads are actually manufactured by Edelbrock to a GM specification and carry GM casting numbers on them. However, the Edelbrock-branded cylinder heads are very similar. Keep in mind that even in the "old days" GM aluminum cylinder heads were not manufactured by GM, but by Winters Foundry in Canton, OH. In those days, the heads and other aluminum parts were cast by Winters and usually machined at GM engine plants. These days, Edelbrock both casts and machines the heads. The machining is done on CNC equipment and this is ONE of the reasons that the heads are so much better than the "old variety". The "old variety" are great for accurate restoration; the "new variety" are where it's at for performance. It's that simple.

The casting numbers mentioned in the original post differ, as others have mentioned, with respect to the presence or absence of a drilled and tapped NPT fitting for the temp sender. Generally, pre-68 engines used the manifold-mounted temp sender and 68+ used the head mounted temp sender. So, if the earlier head castings are used on a 1968, then the temp sender has to be relocted back to the manifold. The modern GM aluminum heads and aftermarket heads have temp sender fittings on the heads.

You can't go wrong with the GM #3904362 solid lifter SHP camshaft. It's "tried- -and-true". My preference is for an hydraulic roller, though. There are just too many benefits of this design to go with really old technology. This is especially true if one wants to achieve high power and torque levels in the lower RPM range (approx 6,000 RPM MAX) which is the most useful range for street operation. Hydraulic roller camshafts and lifters for retrofit applications are very expensive, though. A 3904362 clone and lifter kit is WAY cheaper.

Going with the above scenario, "expanding" the engine to 454 works best. However, that's very expensive, too. Steel 427 cranks are not too hard to find at a good price. Steel 454 cranks are VERY hard to find at a low price.

To keep things in proper perspective, I'll also repeat something I've said many times before: IT IS POSSIBLE TO HAVE TOO MUCH POWER AND TORQUE IN A CORVETTE. Unless you're willing to seriously modify the drivetrain, my feeling is that 500 hp and 500 lb.ft of torque are the ABSOLUTE MAXIMUM that a stock Corvette driveline can RELIABLY handle. I'm not willing to modify in ANY way the driveline of my car, so I would NEVER install more power and torque than that.