Ok so I decided to set aside my bare 781's for a future rat rod build. and picked up a set of 990's on impulse for a great price(the reason I set aside the 781's)
my question is are the 990's gonna be too big for my street/strip 496?
engine specs:
compression comes out to 10.22:1 or 10.57:1 depending on the piston dome size
cam:629/629 lift 260/260 duration @.050 or 595/595 lift 248/248 duartion @050 lift
head specs:
2.25 intake 1.94 exh. valves
titanium retainers
small port/pocket and polish job
.700 max lift springs
if I can think of anything else I'll add it

no, not too big but very outdated design. Sell them and the 781s, buy a set of modern heads.

I dont have $1500+ to spend on heads thats 3x what I paid for the 990's

The 990 heads will be fine. We replaced a set of 990's with a set of AFR 335's a couple years ago on a drag car, 482-2700lbs - powerglide, and only gained .12 and 1 mph. I had expected more.

The 990 heads had 2.25-1.88 valves, the rough edges in the bowls had been smoothed up but no extensive porting. The 990 works as well, if not better, in all applications both street and strip for 454-512 applications as the oval ports every time I have used them. We had oval ports on a 468 engine that ran 10.05 in our 55 Chevy around 1982. The 990 engine version with the same compression, cam, intake, etc, ran 9.88. For the money the 990 heads are one of the best bargains around, the internet has perpetuated the myth that the rectangular heads have no low end torque. Most that believe this have most likely never had an engine with rectangular port heads. They sure have never done a back to back swap/test with both sets of heads.

The key to making the 990 heads work well on the street is compression between 9.5-10, cam duration at .050 no more than 255, and a good dual plane intake. I think you will be fine. I would limit the compression at 10 for steel heads, regardless of dynamic compression. I know a lot of other folks will say you can do more but every time I do more I end up having to set timing back. I would rather have the correct timing and less compression than more compression and less timing. I consider the correct timing to be 36-38 range total on mechanical and let the vacuum advance pull in 10-12 more degrees.

I am sure everyone will not agree with this advice. This is what I have found after playing with this stuff for the past 40 years, works for me but maybe not everyone else.

I am more or less asking about the validity of this article. If no one has any problems with it and you think this trend would carry over to his setup, he may want to consider selling the rectangular port heads and getting port work done to the 781's.

http://www.compcams.com/Community/Articles/Details.asp?ID=-1989283802

This engine is very different from the one you are building, but I think it is interesting none the less.

Adam

I am more or less asking about the validity of this article. If no one has any problems with it and you think this trend would carry over to his setup, he may want to consider selling the rectangular port heads and getting port work done to the 781's.

http://www.compcams.com/Community/Articles/Details.asp?ID=-1989283802

This engine is very different from the one you are building, but I think it is interesting none the less.

Adam

Not trying to be argumentive but that article would reinforce the keeping of the rect. heads. In the normally aspirated test the difference is 18 ft.lbs of torque and 16 horsepower. In the blower test the difference is even less-11 lbs of torque and 5 horsepower. The max horsepower peak was at 5300 rpm on the blower test. Everything in this test favored the oval port heads. Add 1000 rpm with more cam and the rect ports would look much better. Two or three degrees of timing would have more effect than the difference in either test.

I think the test and article is valid enough. They were trying to show how you could have a very docile street engine with a power adder that did not detract from drivability.

the internet has perpetuated the myth that the rectangular heads have no low end torque. Most that believe this have most likely never had an engine with rectangular port heads. They sure have never done a back to back swap/test with both sets of heads.





I agree. My 990s 60 footed the same as my 049s on my 454 with the same 2.19/1.88 valves. Too often I hear guys say how the rectangle ports don't make power below 3500 rpm, but then they have never had a set on their car. The 60 foots were the same, but my car ran two tenths quicker and three mph faster (1/4 mile) shifting at the same rpms as with the 049s. I later went from 4.10 gears to 3.31s for street driving, and the big ports can still easily boil the tires.

For a true street/strip 496, I would go with the 990's. They wont give up much down low with the 4.25 stroke, but will breath much better up top (6000+ rpms).

I would however look into different cam selections. Neither of those are a very good choice. OEM heads (either oval or rect) have poor exhaust ports and needs the extra duration & lift to support the engine, especially in the higher rpm range. Look into a dual pattern cam with more modern lobe profiles.

I'd go ahead and run the 990's and agree that if you have not bought a cam yet give Harold a call to see if he might have something for you. Also, since springs are already installed make sure to match your spring pressures to the new cam.

Sleeper, no arguementativeness taken. Your response is more or less what I was after. I wish someone would do a test of a more radical combination between the two sets of heads.

Adam

Is the 2.25/1.94 valve combo already in the heads?

Now my 2.19/1.88s feel small...:(

If this engine is for a rat rod, you've got no need to chase down every last pony hiding in it. You won't be able to use it all anyway, and if the tires just start spinning every time you hammer the throttle, you won't enjoy it fully anyway.

Run the 990s and don't worry about it. I'd also recommend some tall gears (like 3.08s) to kill some bottom end and give the tires a chance to bite.

no the engine is for my 2nd gen, I was sayin I have now set aside the 781's for a drag rat rod project. and yes the valves are already installed in the heads. the heads are all ready to go