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Going from headers to manifolds ?

badmix

badmix

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If going from long tube headers to OEM manifolds, what will I gain / lose ? Also, will carb need re-tuned?

1984 K5 w/305cid, holley carb, rest is stock. Running dual exhaust with catco cats and dynamax mufflers
 
305 / holley :deal:

i like the quiet of cast manifolds but the power gains of tubed headers stock or fill length is huge .
 
sweetk30 said:
305 / holley :deal:

i like the quiet of cast manifolds but the power gains of tubed headers stock or fill length is huge .
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On the right engine I could a agree with that. On a carb’d 305, I’d say the amount of gain would be measurable on a dyno only. There’s just not enough going on with a smog era 305 to justify the better exhaust flow.

That being said, going back to manifolds after headers are already there is not the norm. If the headers leak, look nasty from rust I could see a reason to change them. But going to manifolds is going to require a need for a pipe to be made to connect the manifolds to the existing exhaust. Not easy for the diy type if you don’t have the tools required to make a pipe up. So farming it out to a pro is needed. Factor that in to your thought process for time and cost.

Since somebody has already put the headers on and the exhaust is setup for it you might consider just getting a fresh set of headers to replace the old ones. Keep in mind the new ones still may not match up perfectly either but it would be easier to massage it to fit.
 
I'd give consideration to using the same model headers, but suggest using Permatex black instead of gaskets at the head
 
TJ1978 said:
Why not just replace headers with ceramic coated ones?
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Or cheap Chinese stainless ones. Almost any change will mean a change to the piping, so the differences are in whether or not you have to trim motor mounts/bolts/etc. Just for reference, I had to regularly re-torque my factory cast manifolds to the heads. The only difference in reliability was the collector gasket, which was good/fair with manifolds and fair with headers.
 
Blue85 said:
I had to regularly re-torque my factory cast manifolds to the heads.
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I am curious why you had to do this. Where manifold to head gaskets used? Non factory bolts with thin washers?

This is one thing that I have never had to redo.
I spent some money on the new improved hooker cast iron manifolds from speedway motors.
https://www.speedwaymotors.com/Hook...aust-Manifolds-2-1-2-Inch-Natural,456618.html

Ok these require new head pipes. And complete exhaust kits would work, but you'll need to weld the flange to modified head pipe
I haven't started to redo my exhaust am still collecting parts.
 
Leaks are often if not always a result of the exhaust system not being solidly mounted. There is no difference I can think of between header bolts and manifold bolts. But when headers are installed, almost always the exhaust back is modified. Stock exhaust mounting is quite solid. Mess with that, and you are bound to have leaks and things loosening up.
 
Wes Harden said:
I am curious why you had to do this. Where manifold to head gaskets used? Non factory bolts with thin washers?
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Because they were loose :whistle: - maybe twice a year. Factory-style gaskets and hardware
dyeager535 said:
Leaks are often if not always a result of the exhaust system not being solidly mounted. There is no difference I can think of between header bolts and manifold bolts. But when headers are installed, almost always the exhaust back is modified. Stock exhaust mounting is quite solid. Mess with that, and you are bound to have leaks and things loosening up.
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I'm not following here, because the engine moves in it's mounts. Having the exhaust too constrained should put even more stress on the parts. The rear-most bolt is always the worst, so that theory makes sense for rear-dump manifolds, but long tube headers should spread the stress out more.

I think we all know it's related to heat cycles. What confuses me is the header flange should be cooler then the manifold body. One advantage manifolds have is the thicker material means longer bolts, so greater stretch.
 
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I never use the head to manifold gaskets on a small block Chevy( gen1).
My piping and muffler/s have been modified many times. On many sbc vehicles, and not had this problem. I have only had 2 vehicles with headers, 31 model A with 350hp 327 and my current dune buggy. I use annealed copper gaskets, retorque while warm. Go to go.
I have even re used the annealed copper gaskets, currently 3x on air cooled vw motor.
Am a firm believer in buying the best header you can get. It saves many head aches.
 
Blue85 said:
I'm not following here, because the engine moves in it's mounts. Having the exhaust too constrained should put even more stress on the parts. The rear-most bolt is always the worst, so that theory makes sense for rear-dump manifolds, but long tube headers should spread the stress out more.
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With floppy exhaust you have leverage. Engine isnt going to move nearly as much as the exhaust will, IF the exhaust mounting sucks. You've got 15+'(?) of pipe hanging off the headers/manifolds if the exhaust mounts suck.

GM from factory used at least what, three quite solid (but still vibration dampening) exhaust mounts? Not to mention, manifolds used springs on the bolts to attach the pipe(s) to the manifolds, those would also serve to ensure some movement was possible, but still maintain a seal.
 
I'm going to take a wild stab that the carb probably wasn't tuned for the headers in the first place. If it's quadrajet carb, I'd say it's even better chances it was never adjusted because of folk's general lack of understanding of q-jet tuning and lack of aftermarket replacement jets and metering rods to tune with.
 
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