Re: Like Duke says, it's a whole system
Thanks, that helped me a lot and has given me something to chew on. Really appreciate it. Terry
Thanks, that helped me a lot and has given me something to chew on. Really appreciate it. Terry
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Re: Like Duke says, it's a whole system
Mark,
Interesting information. Building an L-88 has been a long-time dream of mine.
I hope to be able to get that finished some day.....
The oval port issue has always been of high interest to me. Please define
"well ported" heads for me.
I read a thread on another forum recently that centered on the use of oval
port open chamber heads with the L/78-L/72-L/71 camshaft. Several owners
claimed that this combination would be awesome up through mid-range. Just
wondering what you think about it?
SteveComment
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Re: Like Duke says, it's a whole system
First, you can waste a lot of time and effort on a set of heads and really screw them up! To really work on heads (more than just pocket porting) you need access to a flow bench and a number of scrap heads you don't mind tossing when you're done. I started with a mid 1970's 781 casting 454 head with oval ports and open combustion chamber.
As Duke noted in another post, up to about .250 lift, the valve is the limiting factor, over .250 it's the port. The short side radius in the oval port heads is too sharp - needs to curve more gradualy to the chamber. In addition, the main cut for the valve seat is at a 45 degree angle. So, I went up to a 2.25 intake and a 1.88 exhaust valve side with the valve stems .100 longer than stock. In locating the new seat, I intentionally moved them .100 toward the chamber to elongate the short side radius by that amount. In addition, when you cut the port throat to match the seat (with a slight venturi) that in effect also elongates the short side radius. Then, you have to massage the bowl to get it all to work and finish up by cutting the chamber to unshroud the valves.
All in all, about 40 hours of work on a pair of heads - for street, it's not really worth the trouble, just stick with a pocket ported set of 049 or 781 castings.
John Lingenfelter was testing oval port heads on 496 cubic inch BB engines about 20 years ago and up to about 6500 RPM, the oval port heads kicked the tail off the rectangular port heads (so it's not just in the midrange). The 435 HP cam is a nice cam, but there are some hydraulic roller grinds that really wake up this engine (think short duration like 230 degrees or so with about .580 lift).
For the past 4 years or so, I have been playing with the Gen VI Vortec BB heads. I have seen a 502 based engine with these pocket ported heads and a Vortec pickup truck cam (.480/.483 lift 204/209 duration) make 560 ft/lbs torque at 3500 RPM and 431 HP at 4500 RPM - compare that to a 502 HO with rectangular port heads and cam (.510/.540 lift, 211/230 duration) that made 550 ft/lbs at 3500 RPM and 450 HP at 5250 RPM and you can see that oval port heads aren't all that bad! And yes, you can weld up the one water passage to bolt those heads on an older engine.
As for the L88 - a real blast with headers and open exhaust. It didn't hook up in my 1969 with 4.10 rear until third gear! Unfortunately, when used with a stock exhaust, it was a true shadow of itself.
MarkComment
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Re: Like Duke says, it's a whole system
The valve ceases to become a limiting factor over about 0.25 TIMES VALVE DIAMETER, not .250" lift. So that's about .550" on a 2.2" valve, which means the valve is the limiting factor to some degree throughout the lift cycle with any OE cam. That's why taking a top 30 degree cut on the inlet valve to reduce the unused portion of the seat surface is effective. At low lifts it provides a smoother approach to annualar opening.
Pocket porting does not significantly increase port volume, but it improves FLOW EFFICIENCY, particularly on the exhaust side. That's why it doesn't hurt bottom end torque significantly, while improving top end power by up to ten percent.
Full porting is not necessary unless you ar building a SERIOUS race engine. Pocket porting to improve flow efficiency is very effective for street engines, and I recommend it for all including even "mild" engines like the 327/300 and base 350s in later years.
DukeComment
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just buy a set of either cast iron or aluminum
heads now produced by any of the after market people and you will save time playing with porting as these heads have the lastest port configuration as cast for max power. these head companies are trying to outdo each other and i believe some of the BBC aluminum head sold by GM are edelbrocksComment
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