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Cast Iron Heads

I love my Chevy Chevette!
Posts: 3
Joined: 07/07
Posted: 05/05/08 10:22 AM

I have two set of heads:

1. Casting #39447041

2. Casting #3973487x

I like to get some more info on these heads, and which set would you use for a mid 80's chevy short bed truck?

Is it worth putting in 2.02/1.60 valves in and about how much difference will it make power wise?  

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My first time was in a Chevy
Posts: 214
Joined: 03/08
Posted: 05/05/08 02:28 PM says 041's are 67cc chambers and 487x are 76cc both would be heavy castings.Bigger valves in these heads wouldn't be worth it because of low cfm #'s compared to most aftermarket intake runners.  WIEDER  

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I mow my lawn and find Chevys
Posts: 1985
Joined: 10/03
Posted: 05/05/08 04:20 PM

Both are pretty good heads to start with but the #041's are probably more desirable for performance with their tighter combustion chambers. With this closed chamber some valve unshrouding may need to be done after installing larger valves. The #487X heads also have the advantage of a slightly larger port as-cast than the #041 heads.

How much are larger valves worth? Let's look at some ratios, assume some efficiencies, and use some basic formulas to approximate it...

Let's say that either one of those heads will flow 200 CFM at .400" lift @ 28" of water. Since this is through a 1.94" valve .400" off the seat, this equates to 169 CFM per square inch of curtain area. Now, keep the lift the same,assume that the discharge coefficient remains constant (which will require valve unshrouding, bowl blending, a good valve job ectera), and increase the valve size and this increases the curtain area to 1.282 square inches. That equates to approximately 217 CFM. Now, let's assume that a hot street/strip 350 can make about 2 horsepower per cfm and that is a possible 34 horsepower gain. This looks great but I can safely say that it's unlikely that you will see this kind of gain. This only holds true if the cylinder heads are really being 'maxed out' and the velocities are very high.

I say the safe estimate is a 20-25 horsepower gain. And this isn't as easy as it sounds, like I said to make sure the valve efficiency remains the same requires some blending done by someone that knows what they're doing, an accurate valve job (of the proper widths & angles), and sufficient valve unshrouding. This also assumes you have a good aftermarket valve that has the proper back angle and seat width for a performance engine.

If you're dead-set on production castings you may want to go this route but as Wieder said, aftermarket heads provide a much better horsepower per dollar gain.  

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Big Block power for the win
Posts: 895
Joined: 08/07
Posted: 05/06/08 06:20 AM

The other advantage to an aftermarket head is you can go with aluminum, and that'll help guard against detonation in your motor, depending on what compression you're running. And the lighter weight of aluminum will shave about 40-50 pounds off the front end.

If you decided to stick with the iron heads you've got, as far as going to bigger valves, if the heads need new valve seats, might as well have them enlarged to go with bigger valves.  

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