990 head swap?

[454cid]

I know of some 990 heads locally-ish. They're a sqaure port head that go back to the LS6/LS7 454's, I beleive. I don't know what the whole casting number is...... had it, but lost it. I think they're a 118 combustion chamber?

Any idea what stock rebuilt 990 heads would do swapped onto a stock L29 instead of the original 279 heads? I guess I'd loose some compression, and the ports wouldn't match, but I've heard of guys mixing oval and square ports before.

Would the accessories bolt on?

[454cid]

I guess they're 6272990 castings. I've been reading that sqaures ports are better for all out race engines. Still I wonder if it would be any improvement to an L29, even if they weren't being used to potentional.

[454cid]

This is Chevrolet's best cast iron cylinder head for high-performance big-block V8s. It offers exceptional power at an economical price. This casting has 118cc open combustion chambers that reduce shrouding at high valve lift and improve engine airflow. The seats are ground for large 2.19" diameter intake valves and 1.88" exhausts. This high-performance head has rectangular intake ports and square exhaust ports. It is machined for 7/16" screw-in studs and guideplates (not supplied).
Technical Notes: (see "Technical Notes" at the beginning of the Cylinder Head Group section for important information) This head was installed on LS-6 engine assembly P/N 366250 and LS-7 engine assembly P/N 3965774. It can be used with both closed and open chamber pistons to produce various compression ratios. Casting number 14096188 or 6272990. Cannot be used on Gen V engines because of upper water hole alignment. Open Chamber Design.

Found this on GMpartsdirect

[1998454]

I am still trying to find where i read this, but apparantly gen 5 and 6 heads will interchange, 4 to 5 will not, 4 to 6 will. There was some mention that in 96 there was added material around the coolant passages that the gen 5 didn't have. I would compare a head gasket with the heads. The only other hang up would be if the lower intake is too thin in spots to gasket match those heads.

[454cid]

......I would compare a head gasket with the heads. The only other hang up would be if the lower intake is too thin in spots to gasket match those heads.

I probably wouln't try gasket matching. Assuming the ports line up, the intake is smaller and there would be no restriction going from and oval to a square port. My concern would be if the compression would drop too low, valves hitting, or something else that I haven't thought of.

[1998454]

I don't think there would be a valve clearance issue, but the compression drop would be a lot. Machining them down some and using thin head gaskets would bring it back up some. I would think the added airflow would still make more power even with lower compression. Just not sure what it would do to low end torque.

[V3NOM GTO]

They make special head gaskets to go from one type of head to another type of block, just gotta do a little research. From what I've read on those heads, they will make your truck a dog on the street. They are made for much higher flowing motors. If you want some old school heads, find some with smaller combustion chambers and full open Oval ports, that'll be your best bet.

[PonyKiller87]

Yeah the 990 heads are not going to be worth the trouble. The intake isn't going to match up right without a bunch of work and the ports on the intake would still be smaller than the ports on the heads so you would never really see the benefit of the rectangle ports. Over all these things are going to kill the performance of a L29

[durbo.jones]

Unless you plan on building a 600 HP motor the heads will be way overkill and nearly all low end is going to go out the window.

[454cid]

Yeah the 990 heads are not going to be worth the trouble. The intake isn't going to match up right without a bunch of work and the ports on the intake would still be smaller than the ports on the heads so you would never really see the benefit of the rectangle ports. Over all these things are going to kill the performance of a L29

Well, I keep hearing the heads are the restiction, so I was assuming the intake would do well enough.


Unless you plan on building a 600 HP motor the heads will be way overkill and nearly all low end is going to go out the window.

That's what I was afraid of. In at least one place I read that the sqaure ports won't provide the port velocity needed for anything but high rpm power...... or at least that was my interpretation.

[PonyKiller87]

Well, I keep hearing the heads are the restiction, so I was assuming the intake would do well enough.

The heads are a restriction but when you pick better heads you don't want to swing so far the other way that the heads are so open that the rest of the engine won't support them. Then like durbo said, the port velocity slows away down and all your low end power is gone.

Now if you were to port the intake a bunch, bigger TB, bigger cam,.... those heads might not be to bad, probably still won't have great low end but they would make a ton of mid to upper end. Works great for drag racing where you can just swap out the converter for a high stall to keep you up in the power, but for towing it would suck.

[durbo.jones]

The best bet for towing would be to either find a set of older closed chamber oval port heads or spend the cash on some Brodix or Edelbrock heads with smaller 270ish CC intake ports. Anything much bigger than that is going to push the peak torque up towards 5000 RPMs and won't really do much in a towing application. I am not sure if older heads will work on our trucks with the stock intake. The L29 heads don't have a crossover and the older heads do.

[1998454]

I would still be tempted to buy the 990's. What's the price on em?

[454cid]

I would still be tempted to buy the 990's. What's the price on em?

Don't know yet...... they're coming up for auction.

[N0DIH]

Don't forget the drop in compression with them, unless you go with different pistons.

These might be better to go with.

http://www.edelbrock.com/automotive_new/mc/cylheads/chevy/bb_performer1.shtml


(http://www.edelbrock.com/automotive_new/mc/cylheads/chevy/bb_performer1.shtml)

[454cid]

Don't forget the drop in compression with them, unless you go with different pistons.

These might be better to go with.

http://www.edelbrock.com/automotive_new/mc/cylheads/chevy/bb_performer1.shtml


(http://www.edelbrock.com/automotive_new/mc/cylheads/chevy/bb_performer1.shtml)

Well, the idea was that I'd put the heads on my original engine. I did some calculations and I figured it would drop the compression from 9 to 8:1. I read on Google Books that often the higher flow would offset the drop in compression...... I'm not sure what book it was, Though.

I'd rather not go aftermarket. Too expensive for me, and I like the idea of keeping it all GM.

I'll just wait until I find something better suited to the L29...... if it exists. I kind of thought 702's might work...... I think that was the number.