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Some guys never learn.
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So I was reading through some past threads about Chevy heads and the general opinion seems to be that rectangle port heads are only good for real high RPM applications or big cubic inches. My boat came with them stock on a 454 in 1981 and I know that Chevy used them on a lot of LS6s and similar motors. Just curious why Chevy thought they were a good chioce for a lot of 454s in factory street motors.
 

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So I was reading through some past threads about Chevy heads and the general opinion seems to be that rectangle port heads are only good for real high RPM applications or big cubic inches. My boat came with them stock on a 454 in 1981 and I know that Chevy used them on a lot of LS6s and similar motors. Just curious why Chevy thought they were a good chioce for a lot of 454s in factory street motors.
i believe the answer "could" be that u can do more to them than the oval's...bigger valves, better porting, better breathing, a better selection of manif's...i guess it all depends on the individual & what they want to do & the availibilty of each kind

FastRat
 

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the statement, oval heads are better for smaller, lower RPM (read 454 and under, 5500 and under) really only applies to "modified" oval heads like FastRat stated. Stock an oval is pure shit. You start with larger valves and go from there, depending on how much flow you need, until you've spent too much and would be better off with after market ovals.:D Knowing that a shit flowing stock oval could never feed a LS 6 or 7 or a L-88, chevy elected to use a shit flowing rectangle instead.:)sphss



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steelcomp was here
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the statement, oval heads are better for smaller, lower RPM (read 454 and under, 5500 and under) really only applies to "modified" oval heads like FastRat stated. Stock an oval is pure shit. You start with larger valves and go from there, depending on how much flow you need, until you've spent too much and would be better off with after market ovals.:D Knowing that a shit flowing stock oval could never feed a LS 6 or 7 or a L-88, chevy elected to use a shit flowing rectangle instead.:)sphss
Had to laugh out loud at that one...true as it is.:p:p
Most of the reason for big port heads in a Hi Perf. application (like the GM rect port, the Boss 302, the 429CJ, etc) is to support the aftermarket and racing programs that are going to follow production. I don't think the engineers developed these rediculoulsy large heads with the intention that they were ideal for the OEM application. The factory's intention is to build a platform, but leave a huge amount of improvements to be made by the aftermarket industry. The easier the platform is to modify with good results, the better the platform will sell. What wins on Sunday, sells on Monday, etc. It was easier for the OEM's to come up with a good head that would respond to bolt ons like an intake and cam change, maybe a bigger carb, than it was for the aftermarket. It made the OEM's look good, and left plenty of market share for the retail performance parts market, which is a HUGE industry.
 

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Strap on the JATO
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305 Posts
Bocco, I recently brought my 990's to Tim Morgan's in Walnut Creek
and he did his thing with them and set up a hyd roller combo.
He's done years of testing/tuning with these "Big" heads. I finally
got everything together & fired it up the other day. I'm going to
drop it in the water in a couple days & I'll post back, but if you're
looking for some advice, you can't go wrong talking with him. FYI...
 

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Had to laugh out loud at that one...true as it is.:p:p
Most of the reason for big port heads in a Hi Perf. application (like the GM rect port, the Boss 302, the 429CJ, etc) is to support the aftermarket and racing programs that are going to follow production. I don't think the engineers developed these rediculoulsy large heads with the intention that they were ideal for the OEM application. The factory's intention is to build a platform, but leave a huge amount of improvements to be made by the aftermarket industry. The easier the platform is to modify with good results, the better the platform will sell. What wins on Sunday, sells on Monday, etc. It was easier for the OEM's to come up with a good head that would respond to bolt ons like an intake and cam change, maybe a bigger carb, than it was for the aftermarket. It made the OEM's look good, and left plenty of market share for the retail performance parts market, which is a HUGE industry.
Okay S.C. Here it is from the Ol Guy you don't want to hear from. You are very correct. Due to class requirements the big three had to make a production product to meet class requirements so enough had to be sold as a factory item to be concidered STOCK. Maybe more than required for a daily driver but great on a sunday to keep them (buyers) coming in. PEACE. M
 

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steelcomp was here
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Okay S.C. Here it is from the Ol Guy you don't want to hear from. You are very correct. Due to class requirements the big three had to make a production product to meet class requirements so enough had to be sold as a factory item to be concidered STOCK. Maybe more than required for a daily driver but great on a sunday to keep them (buyers) coming in. PEACE. M
Man you crack me up.:)sphss
 

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steelcomp was here
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990's can work very well on smaller engines at reasonable rpm's, 5500+. The trick is to get the flow up on them without sacrificing port size. If you can do that, you pick up the port velocity which greatly helps on the smaller stuff...that and adding compression. I got over 350cfm on the intakes on mine with not much more than a good valve job and bowl blend. They were on a 467 which didn't ever run over 6000rpm, and made very good power with a solid flat tappet. The exhaust is the weak side, but can be woken up nicely, and with the right cam, you're in there. An oval port single plane intake will also help step up the intake velocity for lower rpm apps. If you want power below 5500, the AirGap is a better choice. A 750 will have very crisp throttle response and nice cruising manners but suffer a bit on the top end. An 850 or bigger will make more power, but unless you have some compression and want to turn over 6000, won't get you much except a bigger gas bill.
On the 990's, I've found going to a 2.25 intake allows for a much better valve job and works very well. The 1.88 ex valve is a little large for the port, but with a nice radiused seat will work with the right cam grind.
If the guides are worn and you want to refurbish, then I'd recommend going with thick wall liners and getting 11/32" stem valves as well.
If it's in your budget, I'd be happy to set these up for you. Let me know if you're interested.
 

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WERD!
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990's work very well even on smaller low rpm engines. For instance with my combo which is a basically stock replacement shortblock for the 73-86 BBC GM Truckline.I went from a 049 casting set of ovals and a single plane Holley Strip Dominator to a set of 990's and a Eddy TR2X Tunnel Ram with 2 750's.

You should of heard some of the crap I got when I decided to go this route. In fact there was a thread some time ago on Hotboat about it. Most people told me I was crazy for going to big for that setup blah blah blah. Well not listening to anyone and doing what I knew worked I picked up nearly 8mph to nearly 84mph with those 2 changes and guarantee most of it was from the heads. This was in an efficient jet boat hull. Same engine with NO changes went directly into a V-drive Runner Bottom hull and went 90mph. So yes these can be used on smaller ci low compression deals and work well :)devil
 

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E-7 Sheepdog (ret)
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If the guides are worn and you want to refurbish, then I'd recommend going with thick wall liners and getting 11/32" stem valves as well.
That is a very good way to get arround 15% to 20% of the valve weight removed from a very heavy valve train, and sure won't hurt port flow either.

It took aprox 25 grams off each one of my valves, and that was going UP in size from 2.07/1.76 to 2.19/1.80. Denny weighed them in front of me.
 
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