Excuse my lack of diligent searching skills. Im sure I have seen this discussed before deep in some threads..
I am ditching my custom boig header/spd collector ( was built for my previous NA beast) for a racing beat header that I just acquired t.o help me gain a more favorable powerband. I will be having a new exhaust built as well because the current one is built specifically for the boig header.
However I am not sure what size tubing I should go with? 2.25" 2.375" (isnt this what the racing beat exhaust is?) or 2.5"?
engine is a 10.5:1 bp4w. stock ports, stock size valves, i am also ditching my itbs in favor of a squaretop that is on its way. I need to rev it to atleast 7500...

FYI: with this engine with my TWM itbs and boig header it made 139whp/116tq corrected on 93 octane

What do you guys think?

 

A 2.25" exhaust should do the trick. If you have a really good ITB setup, a 2.5" might be worth it; however, there's a very, very fine line to walk when N/A since turbo is braindead simple bigger = better.

 

If the cost is the same I'd go with the 2.375 to match the racing beat header you have. What tuning software are you using? I think Emilio said the biggest gains on a BP motor are in tuning and intake side.

 

megasquirt/tunerstudio

 

2.25".

 

2.5 will make more power at the higher end of the rev range and the same power everywhere else, and only weigh slightly more than the 2.25 setup.

 

CSP national champion says otherwise.

 

If I recall, corksport used 60mm or 2.375" tubing for there bp power series exhaust. Claimed they made best usable power with that size n/a. This was many years ago of course.

 

What is on your PTE car Andrew? That should be a trusty choice

 

Since when do you hold auto-xer's opinions in any sort of regards.

 

anyone test out different lengths? drivers side exit vs passenger side exit?

 

Under car dump right before the axle is lighter.

 

Didnt the spec guys go down to 2" because they made more power than the stock 2 1/8?

 

One of my spec buddies said the springfield dyno exhaust made less power than the borla race they put on. we touched on the point of different lengths possibly having an effect, like 2 stoke engines.

 

Well 2 stroke engines are a bit different. They rely on the pulse to go back into the engine. Na on the other hand rely on the pulse to pull the next pulse out.

 

Right, there's still theoretically proper lengths and awful lengths for an exhaust after the header collector, but it doesnt seem to matter much in the real world unless you're using a real converging diverging merge collector.

 

2.5" because my Stahl terminates in 2.5". I wish it were a 2.25" exit. I still have part of a 2.25" exhaust should I ever end up going back to an RB header for some reason.

 

copy that, Thank you

 

How about exit location with aero effect. Centered and up high. sorry if I'm going off topic a bit

 

I think the best scenario might be to put a vacuum on the exhaust, at which point the discussion of exhaust diameter becomes null...

Since the best vacuum source we have is the intake, it makes the whole process rather simple - the best part is that it's already been invented!!!