Okay guys. this one is throwing me off big time. Ive got a 69' 427 with oval port heads. The intake that came on the motor is an old Edelbrock X-c96 Cross ram (for rectangle port) I****umed right away that this was a no-go, Being the physical differences in size and a seemingly MASSIVE restriction there. But I cam across more than one article about running mis-matched stuff. quote from a HAMB thread "Research on the net says that it can be beneficial due to the increased turbulence causing the fuel to stay atomized longer.While this is great for a tunnel ram intake a low rise may not benefit". So whats the deal with this? it goes against my own thoughts, and my other car buddies who are more experianced than me plain call BS. My opinion is: the therory of helping fuel suspension with increased turbulance is certainly intregeing. But on the other hand wouldn't that slaughter airflow and signal to the carbs??? anyone actually one this?
FORGET it...! Whomsomeever wrote that hans't a clue just how much off the two port square inches are...OR....just hasn't got a clue. Now between the small port and the peanut (later, even smaller) ports...while I still would not do this, is a much closer port size between the two and...could (!?) be done. BUT as I said, I still would not do this either. Going from the small square manifold to the oval port head is a disaster. If you can get it to seal...will it run...yea. The higher the rpm goes...the worse it will run. Think about it, if it were ok to do this, there would be no peanut or oval port manifolds on the market..! Again, if you'r looking for decient milage and performance...the manifold port and the heap port need to match within about .030" to .040" with the roof being the most important to be matched as close as possible. Mike P.s. - I port heads for some side money and would NEVER recommend this to anyone to do for any reason.
"Whomsomeever wrote that hans't a clue just how much off the two port square inches are...OR....just hasn't got a clue". Well yea thats what i was thinking, if you even look at them next to each other, or set a rectangle gasket on the head and i say to myself "No f*ckin way can this work right?!" like i said, i understand that turbulance isnt always bad...AKA (Leaving some rough/ burring) while porting. to keep the air from becoming lazy. but i dont think to THIS extent. being a huge, dual plane plenum. combined with that i could see a very unresponsive carb. still somewhat interesting. Id love to run the cross ram i have. I could live with killing 20hp or whatnot for the cool factor. But i dont wanna wreck havoc...
Run the manifold that matches the ports; if your heads are rectangle port and it will be primarily street driven, find an intake that matches the heads. The oval ports got a bad rap but they are not that bad in the r.p.m. range of street engines; the velocity at lower r.p.m. is better through the oval port which will help throttle response and mileage on the street.....a rectangular manifold on a oval port will cause the port flow to basically "stall" at some point in the rpm range.......fats
Saw this to many times done and everytime told them not to do it and all they ended up with was no descent idle at all,hard to tune and just basic running like*****,find you a matching intake and be done with it.Whoever wrote that is just somebody that believes if it looks good on paper it will work not a clue as to a running engine.
All the 396's, 427 and 454 hi-po motors came with the LARGE square port heads... The lo-po heads you have are 390 and 400 horse (with 3x2 carb setup) on a 427... Those oval port heads were also used on the 396 325/350 hp motors. Hope that helps!!
Well im keeping the oval ports for sure, being that its a street car i think that would be better. As for the cost of getting ahold of rec heads, just to use the cross ram... Gezze i could almost buy a small blower for that coin. thats basicly where im at. Wind it out, or keep it fat and flat I do appreciate the responses, i like to hear all sides and opinions. I ask my friends...but their all pretty much********es. (not to say im a master of course)
my 427 actually was running like this when I got it. Oval port heads and square port manifold. I took it all apart and ordered new gaskets based off the heads I was looking at (oval). Put it back together and it wouldnt run worth a*****. After a week of trying carb, dust etc.. someone noticed the old square port gaskets in the trash and asked me if I had purchased new square port gaskets , I said no they are oval port heads! He said thats not the gaskets that were on there, so I pulled the manifold and almost*****ped! It was a square port manifold. I ordered the right manifold (oval)and now it runs great, but I have to tell you before I tore it apart it ran real good with the wrong manifold. Might be cuz the A is so light but it ran great! I just couldnt put it back that way and did it right!
I happen to have the makings of a rectangular port 427, the only thing I'm missing is a neat intake..... But the best thing for you to do is to find a set of the old closed chamber rectangular port heads. If you take your time you might find a pair for a reasonable price. The two I have cost me less than $150 total, but it was a while back that I got them. As far as the gasket sealing, if you run the rectangular port gaskets, it should seal fine with the mismatched ports.
I'm going to against the grain here. Back when I had no money I ran an old eldelbrock rectangle manifold on oval port heads. It ran fine, I'm sure it was costing me HP but if you're just going for the look I say go for it. If you do decide to do it make sure you use rectangle head gaskets. The oval ports leave a gap.
Not sure if I remember this correctly...I seem to recall that (and this is before all of the aftermarket heads became available) oval port heads with the big rectangle port valves made for a fantastic street head. That aside, for the cost of an intake gasket set... toss on the crossram and see how it runs (assuming you have the carbs and linkage). If it doesn't work out then it is decision time (either oval intake or rect heads). I say give it a shot...install the crossram.
I normally wouldn't mismatch ports but a long time ago, when I was broke, I had a '396, 2 bolt main, cast crank and pistons that was sitting in a dirt lot. The engine was froze, but hey, it was free. Pulled the engine apart and beat the pistons out with a chisel and mini sledge. Honed the cylinders, used the same pistons and reassembled the motor with new rings and bearings. Added a Sig Erson Hi-flo II cam with large oval port heads. Had a rectangular factory hi-rise lying around and used it with an 800 Holley double pumper. The clearances was like a 100K mile motor and boy that motor would rev. I really push that motor since it was junk and I really didn't care and it was fast. Eventually, after a couple of years, it spun a bearing. Put a good forged crank and tightened the clearances and it really slowed the car down. Don't know why it ran so good, but it did.
We did it on a 454 in an 80 chevy short box stepside 4x4 because we had the intake lying around and anything is better than that lo rise cast iron boat anchor that is stock on anything after 1970. The thing ran great and we finally switched it to the correct intake later but it didnt seem to run any better. I would give it a try but the cross ram might give you low speed problems because of the design any way. Give it a shot. Gaskets are cheap.
i have also ran a square port intake on a oval port head on a 468 ci in a 73 camaro and it ran great not a lot kept up with it..
We ran an alcohol injected 468 with oval port heads in 85' and it ran faster than the square ports.....
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