Need some help with this cam selection

[Ethyl Cat]

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[xctasy]

Weeellllll. Thank Father X mass you are okay! Happy New year and look after yourself, EC!

There you go "Early" Bronco Junkie, what can we say?

[Broncojunkie]

Yowsa! Welcome back, Ethyl Cat! I'd trade my..well, I'd trade some stuff for one of those early Broncos lol!

Originally, I was planning on keeping the 302 in this 79 Cobra, so I was going to put a set of heads, cam, 4 barrel and intake, and some link bar lifters in it. In the last few months, however, I've acquired this 408 block and rotating assembly, pistons and rods to match a 351w block I already have (machined to .030), and 2 complete running 351w's. So I guess I'll be in the market for some good aftermarket heads here soon.

[Ethyl Cat]

I feel that in my reply I should have stated ways around the big inch small head problem.

A small head feeding a big cylinder will never have very good volumetric efficiency due to its inability to keep up with piston demand. If you think of it as too small a carb you can realize that the cylinder will just not build the pressure it could have due to the restriction.

If you cannot fill the cylinder with the mass of air and fuel it needs on the intake side, what good is more duration and lift on the exhaust side? There is nothing to expel!!

If the OP had the GT40 heads on his 408 shortblock and could physically swap the intake and exhaust lobes on his cam making it a 294/288, 24x/23x .580/.560 cam it would make about 20 more hp than if installed as made. The swap puts the extra assistance where it is needed in this case, the intake side.

Generally if valve sizes are on the small size for the displacement, the cam should have a tighter lobe separation to allow overlap to assist in filling the cylinder. A quick run of Vizards formula for 20+degree wedge type heads comes up with a 104 lsa required for this engine using a 1.9" intake valve, but I think I would have it not tighter than 106. The bigger the valve, the less overlap required. The better the low lift flow, the less overlap required.

Hope this helps.

Happy New Year everyone!

Steve

[Walking-Tall]

Carbs? They work just fine if you are prepared to get into them.

Yep, bolted on, carburetors can give you satisfactory and mediocre function, as always and forever. Carburetors dialed in, better have a good grip on that wheel. Ditto with EFI, as is, mediocre, and a multitude more factory variables and compromises. There's good reason hot rodders and racers have always been able to one-up the factory, and that isn't any different today than it ever was... IMHO, more-so today... those dyno tuners don't charge what they do for what they can do with your EFI "tune" for nuthin'...