# Exhaust size question



## skyline_steve (Jul 24, 2006)

This is a kind of dumb question but i need to know.
What is the most power i can run threw a 3.5 inch exhaust before i change the size to 4 inch.
I am talking from the cat back.
thanks guys.


----------



## freakazoid3 (Jan 19, 2008)

I don't think power will be a problem unless you're aiming mega-high like 750 hp and up. What power are you going to run?


----------



## skyline_steve (Jul 24, 2006)

At this point i have 486 whp and i have a HKS super dragger 3.5.


----------



## AlexH (Aug 17, 2008)

id say 3.5" would be good for about 600hp, what are your flywheel output?
486 at the wheels must be around 550ish at the fly????

if it is then id say there isnt much room for improvement with this exhaust
but im sure someone with more tech info should give you a better idea

alex


----------



## skyline_steve (Jul 24, 2006)

Depends on the % loss, but with the 3.5 inch its cutting it kinda close.


----------



## NISFAN (Oct 11, 2003)

If you are running the stock size cat flanges, they are normally around 3" anyway, and where the restriction will be. 

So exhaust diameter after the cat is of little importance.

Even so, 3" is still capable of 700-800 bhp, probably not ideal, but still.


----------



## AlexH (Aug 17, 2008)

id hate to think of the EGT's running 700hp on a 3" exhaust, but i suppose it can be done

alex


----------



## NISFAN (Oct 11, 2003)

Done more often than you think, the HKS T51R and T04Z kits, both downpipes neck down to less than 3" to join the CAT, as I guess most Jap kits will do.

Not much point having a 4" cat back if you have to go through a 2.8" restriction in the downpipe.


----------



## AlexH (Aug 17, 2008)

yea i agree, but i think alot of people are unaware of this exact point
they change everything and leave the stock dumppipe on, causing the restriction
but the faster the exhaust gases get out the less the turbo has to push it out

but i do agree on the restriction of the existing stock pipe flanges

alex


----------



## skyline_steve (Jul 24, 2006)

The system is 2860-5 then A/M dump pipes then HKS down pipe the 3 inch cat and now iam going with a 4 inch exhaust from the cat.


----------



## NISFAN (Oct 11, 2003)

^^^^ 

In that case, I would wonder whether there would be any difference at all between 3.5" and 4". 

The back pressure just after the turbine is what you don't want, and with the 3" cat flanges you already reduce the bore to less than 3". 

According to Corky Bell (Maximum Boost), a maximum exhaust gas speed of 250ft/sec or less is desireable, and he lists a chart, where a 3" pipe will be capable of aproximately 720bhp before the 250 speed is exceeded.

For reference a 3.5" bore would support a little over 900bhp, and a 4" close to 1100bhp


----------



## skyline_steve (Jul 24, 2006)

So 3.5 inch should be fine for what i have ?
thanks
Steve


----------



## rockabilly (Oct 15, 2003)

i run a 3.5" system on mine rb26/30 well over 700bhp and no issues, running a gt4094r at 2bar on track i see egt max at 650


----------



## AlexH (Aug 17, 2008)

thats the turbo i am going to use!!
you say it will run on my 3.5inch exhaust????
i was told to upgrade it to a 4inch job

alex


----------



## rockabilly (Oct 15, 2003)

AlexH said:


> thats the turbo i am going to use!!
> you say it will run on my 3.5inch exhaust????
> i was told to upgrade it to a 4inch job
> 
> alex


hi alex,i was told by the ozzies to use a 4" dump pipe down to the gearbox then reduce to 3.5" its coming out of the rear of the turbo where its most important, done 8000 miles in mine now no issues..bernie


----------



## AlexH (Aug 17, 2008)

are you running a screemer pipe or did you plumb the wastegate back into the exhaust?

alex


----------



## rockabilly (Oct 15, 2003)

here,s mine i made


----------



## skyline_steve (Jul 24, 2006)

That is an impressive set up, What i gather is that 3.5 inch is fine for the power but its the dumps right off the turbos that matters.
So a Tomie or Mines would be the ideal choice for a non custom set up.
thanks


----------

