1 5/8 vs 1 3/4 primarys

[moose]

Sorry if I overlooked it, but a search didn't turn up what I was looking for...

Had my heads machined to match the header gaskets i'm using, the gasket holes measure 1.568"

A mechanic said I shouldn't be afraid to go with 1 3/4";
.75 - .568= .182
.625 - .568= .057
.182 - .057= .125" difference

I know .125 isn't a big difference, but I also know that if the primarys are too big you can have leak issues. What size would you guys recomend? I've decided on Schoenfeld headers and have been looking at the 1 5/8 - 1 3/4 option as well. Is this a better option and is the extra $$ worth it versus using a single sized tube??

 

[jrm81bu]

I'd be more concerned with how much power you are making, to decide which ones to go with. Anything below 500hp i'd just go with 1 5/8" primary tubes.

 

[moose]

400ish horsepower range...so this guy is correct about the 1 3/4 IF i'm making over 500hp. I thought the idea was to match the tube size to the exhaust ports as close as possible with the tubes never being smaller than the ports. Which, I assumed, is why Shoenfeld has the 5/8 into 3/4 option.

Also, would the 1 3/4 hurt me in any way with an underpowered motor? No plans at the moment, but if I get a wild hair up my *ss in the future and decide to add nitrous I'd want the larger size. And since both sizes are the same price...

 

[84sbcregal23deg]

id stick with the 1 5/8. even if you're making power in the low 400 range, an 1 5/8 inch header will make more low end torque than an 1 3/4. the step header design is a very controversial one. the company i work for (vette doctors performance) has done testing on some pretty stout LS engines with step headers only to find that in MOST (how can i emphasize that more?! lol) cases, they really don't make any more power. the goal with them is to get the low end torque as well as preserve the power up top. needless to say that i'm sure there are applications that would benefit from having a stepped primary, but in most hot to even wild street apps, it won't hurt, but probably won't make a difference for the better either. Anyways, 1 5/8 inch gets my vote, even if you do decide on a bigger cam/nitrous down the road.

 

[MD13ailey]

I'm in the 450 ish range and I went from a 1 5/8 headman to the 1 3/4 schoenfeld. although I have not raced the car since I made the change I did notice an obvious difference in how the car sounds and rpms. I have drivin it on the street sicne the change and it seems to rpm better and faster under load, or atleast as much load as I can put on it. downshifting and spinning the tires at highway speeds doesnt really count as a "load"

 

[larryo454]

X2 with M.Bailey on the 1 3/4 schoenfeld headers if your port can handle a 1 3/4 tube go for it. A engine is a big air pump what goes in must go out. with a tube smaller then the port causes REVERSION on the exhaust.With the smaller tube your are making a wall for the exhaust to start the flow back in to head at the flange with is a power lost.

 

[moose]

I have a 383 so I think I'm fairly covered as far as torque goes. I agree that the air needs to flow out with the least resistance possible and a bigger tube makes sense. As of now I'm leaning toward 1 3/4.

 

[jrm81bu]

But there is also a good reason you don't want to run 3" duals on a stock 305. The larger primaries may allow the exhaust gas to cool too quickly and loose speed in the pipe, slowing down the flow.

 

[moose]

Thanks to megaladon6 I read this article; http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/index.html

I have a better understanding with mufflers and exhaust pipe size (I think), but am still uncertain about headers. I now agree that 3" exhaust is too large for most systems and am going with 2.5". Magnaflows are half the price as Flowmasters, but does the filler material blow out or deteriorate over time?

As for headers, this chart confuses me; http://www.superchevy.com/technical/engines_drivetrain/exhaust/0505phr_exh/photo_04.html

 

[moose]

Well I chose the 1 5/8. I was at a reputable shop getting a price on pipes and the owner recommended the smaller size based on my compression...10:1.