how much power would I lose using manifolds?

I am a couple weeks away from installing a 351w into my 77. Im under the impression that my 6901 headers will not work. So my question is, what would the power loss be (rough estimate) if i used manifolds until i can spring for the crites headers? This is a modestly built engine 350hp at the flywheel.

thanks in advance for your input.

 

I don't think you would notice it if you weren't drag racing and watching an ET clock.

 

if you are going headers...why worry about it? (10-15)

 

It will sound different, quieter, less "pingy". Otherwise, I doubt you would notice much difference.

 

Depends to a degree on the heads and cam. On a stock engine, I would expect the losses to be in the 10-15 HP range as suggested. Throw in a good flowing set of heads, more aggressive cam timing, RPM limit and the loss would be significantly higher staying with manifolds.

We used to see this with marine engines all the time.

 

If you have to use manifolds, smooth them out and port them. 15 hp on a 350 hp mill sounds about right.

 

I agree with scooper77515 above. If this is strictly for street use I doubt you'd notice the difference. It will be quieter inside the car. I have had cars with headers on the street before and, trust me, the resonance can get old after a while.

 

I have done a couple of these swaps and I will bet that a 351W will not fit unless you have modified the spring towers.
Lee

 

I have notched the shock towers and done a few other mods on top of that. Im thinking ill just have to bite the bullet and order the CRITES headers after I try and modify my hooker 6901's (and fail) of course.
and thanks to everyone for the replies... most helpful! This is one of the best forums on the net. Thanks Guys

 

That's a good looking job on those towers.

 

thanks jasonwthompson, i drove myself nuts trying to get them perfect! Had to force myself to be happy with it lol

 

If you're NOT currently running a very high flow and correctly sized exhaust system?(most aren't with cookie cutter sized bolt on parts and crimp bent exhaust pipes).. the manifolds will cost you a solid 25 horse up top and cause your peak to fall off MUCH quicker. This of course cuts the rev range and more importantly.. AVERAGE power.

IF.. you already have a better sized system that still flows sufficient mass for these small motors?.. the losses will be even greater. IMHO, very rarely does a 302 need anything more than a 1.5" - 1.625" primary pipe.. matched to a 2.5" collector. Anything larger gains flow with a major loss in gas velocity as the compromise. Never good to lose velocity right near the start and towards middle of an exhaust system.

Oops.. too many tabs open. A 351 can extend that optimum sizing towards a 1.625" primary matched to a 2.5" collector. If you ever intend to stroke it, and/or run more compression, and/or rev it higher?.. step up to a 1.75" primary and a 3" collector.

 

Good info groberts101, I agree on primary and collector sizes. I currently have zero exhaust so I'm free to do what i want! I have some exhaust plans I'm going to keep under wraps until completed but for now, probably just 2.5" pipes,quietest mufflers i can find and dumps. I also decided to buy several used sets of headers(not manifolds) for various makes and models under 50 bucks a piece (craigslist). I thought i would see what would be the easiest to modify, and re-sell the others. Im hoping to find a cheaper option than CRITES. I spent too much money on the 351 to justify using manifolds and losing 25 hp. I guess we will see how it goes.