Downpipe. to step, or not to step
Downpipe. to step, or not to step Details:
366ci, t4 76gts, 3"DP, 3" magnaflow catback
i am building a nice solid exhaust with v-bands etc over winter. however, not alot of money to go crazy.
im not too concerned with closed exhaust performance. as of now, with the 3" cutout, the exhaust is like NIGHT AND DAY with it closed to open. reasons for redoing the exhaust is to have it high and tight, v-band to eliminate shitty clamps and leaks, etc. i need to use at least the first 2 ft of DP, do to fitment and difficulty of duplication reasons. stepping teh exhaust is good anyways for scavenging. refuse to ditch the magnaflow at this point as well.
SO..... the question is:
for that short space before my cutout, is it worth it to step my exhaust to 3.5"...it might only be 2'. (then i will have to reduce it back to 3", which i know isnt optimal, andid never race with the cutout closed). is it worth it, will there be noticale gains?, or should i just keep the whole thing 3"?...
dont have alot of money to go crazy. maybe in a year or so, i will get a custom upright mounted radiator so i can run a full 3.5 or 4".... but either way, i want tail pipes running out the back of teh bumper, because i dont like interior drone for normal driving...
thanks for the input
366ci, t4 76gts, 3"DP, 3" magnaflow catback
i am building a nice solid exhaust with v-bands etc over winter. however, not alot of money to go crazy.
im not too concerned with closed exhaust performance. as of now, with the 3" cutout, the exhaust is like NIGHT AND DAY with it closed to open. reasons for redoing the exhaust is to have it high and tight, v-band to eliminate shitty clamps and leaks, etc. i need to use at least the first 2 ft of DP, do to fitment and difficulty of duplication reasons. stepping teh exhaust is good anyways for scavenging. refuse to ditch the magnaflow at this point as well.
SO..... the question is:
for that short space before my cutout, is it worth it to step my exhaust to 3.5"...it might only be 2'. (then i will have to reduce it back to 3", which i know isnt optimal, andid never race with the cutout closed). is it worth it, will there be noticale gains?, or should i just keep the whole thing 3"?...
dont have alot of money to go crazy. maybe in a year or so, i will get a custom upright mounted radiator so i can run a full 3.5 or 4".... but either way, i want tail pipes running out the back of teh bumper, because i dont like interior drone for normal driving...
thanks for the input
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On one hand, we got some pretty phenomenal backpressure/boost numbers on a 377/T76GTS using a 2' long 3.5" downpipe - like 18 psi boost and 26 psi backpressure which is unheard of for a GTS and 350+ cid.
However, it is MUCH worse to go up to 3.5" then neck back down to 3". The restriction is in the connections, elbows, and size changes - not the straight runs of pipe. 3" the whole way is way better than 3.5" -> 3".
Mike
However, it is MUCH worse to go up to 3.5" then neck back down to 3". The restriction is in the connections, elbows, and size changes - not the straight runs of pipe. 3" the whole way is way better than 3.5" -> 3".
Mike
MIne is less than ideal too...Dual 3" to a section of ovalized 3", back to round 3", where they then join at a "y" for 4 feet of 4", then back to a pair of 3" to the tips. In the end , I suppose that racing with the cut out open is always best, and other than that, its probably not worth alot of money and effort.
well on the boost controller, it stays constant...but ya i know what u mean. with it shut, around 5k i would peak, and by redline i was down 3-4psi. ya!....
i just want to clarify tho...
with the exhaust closed i will NOT be doing any type of WOT. i know that its no good when u have a stepped the reduced exhaust. still dont do it then for possible gains with the cutout open?......
i mean as it is, with the cutout open, i have instaboost, that holds to redline, and theres an extra 60-80hp...
im considering running 3.5" thru a bullet, and splitting it to (2) 2.5" tailpipes (magnaflow) after the axle.... (2) 2.5" pipes have about the same cross-sectional area as (1) 3.5" pipe.... i dunno. decisions decisions
i just want to clarify tho...
with the exhaust closed i will NOT be doing any type of WOT. i know that its no good when u have a stepped the reduced exhaust. still dont do it then for possible gains with the cutout open?......
i mean as it is, with the cutout open, i have instaboost, that holds to redline, and theres an extra 60-80hp...
im considering running 3.5" thru a bullet, and splitting it to (2) 2.5" tailpipes (magnaflow) after the axle.... (2) 2.5" pipes have about the same cross-sectional area as (1) 3.5" pipe.... i dunno. decisions decisions
Originally Posted by engineermike
You have to be careful with a cutout and a turbo. Sometimes, the boost jumps up ALOT when you relieve the backpressure. Ask me how I know. . .
We ussually run a bigger downpipe then put in a cutout.After the cutout go to a 3" catback. Just cracking open the cutout to relieve pressure can give some good hp gains without much noise.
Difference-
408ci,T88 turbo,12psi,Exhaust is 3.5" downpipe to cutout,then free flowing 3" exhaust with 2 bullit's.
closed/open cutout hp difference -80rwhp
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Originally Posted by Slowhawk
I agree,tune the car with the free flowing exhaust,then close it up.
We ussually run a bigger downpipe then put in a cutout.After the cutout go to a 3" catback. Just cracking open the cutout to relieve pressure can give some good hp gains without much noise.
Difference-
408ci,T88 turbo,12psi,Exhaust is 3.5" downpipe to cutout,then free flowing 3" exhaust with 2 bullit's.
closed/open cutout hp difference -80rwhp
We ussually run a bigger downpipe then put in a cutout.After the cutout go to a 3" catback. Just cracking open the cutout to relieve pressure can give some good hp gains without much noise.
Difference-
408ci,T88 turbo,12psi,Exhaust is 3.5" downpipe to cutout,then free flowing 3" exhaust with 2 bullit's.
closed/open cutout hp difference -80rwhp
so your saying i SHOULD step it to 3.5" before the cutout, tune for cutout open. and just never really beat on it with it closed?
just making sure im reading the right thing here...sorry to keep repeating myself.
realistically, i will have 2' of (3") stepped to only 2' of (3.5") before the cutout
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Originally Posted by Turbo LS1 SS
so your saying i SHOULD step it to 3.5" before the cutout, tune for cutout open. and just never really beat on it with it closed?
sounds good....
i just got to think this over. i honestly dont know if theres anymore BP to be relieved with the 3" open. i mean....it spools right up hard, and holds. i roast the DRs past 100 on the street as it is because the power/tq ramps up like a shot of nitrous. i know 3" sounds small tho. one big consideration is if i decide to up the boost. i havent tried it yet believe it or not. i know im getting close to the limit of my turbo, but obviously the less BP the better my chances are of more boost
i just got to think this over. i honestly dont know if theres anymore BP to be relieved with the 3" open. i mean....it spools right up hard, and holds. i roast the DRs past 100 on the street as it is because the power/tq ramps up like a shot of nitrous. i know 3" sounds small tho. one big consideration is if i decide to up the boost. i havent tried it yet believe it or not. i know im getting close to the limit of my turbo, but obviously the less BP the better my chances are of more boost
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Originally Posted by Turbo LS1 SS
... i know im getting close to the limit of my turbo, but obviously the less BP the better my chances are of more boost
Originally Posted by Turbo LS1 SS
so your saying i SHOULD step it to 3.5" before the cutout, tune for cutout open. and just never really beat on it with it closed?
just making sure im reading the right thing here...sorry to keep repeating myself.
realistically, i will have 2' of (3") stepped to only 2' of (3.5") before the cutout
just making sure im reading the right thing here...sorry to keep repeating myself.
realistically, i will have 2' of (3") stepped to only 2' of (3.5") before the cutout
Originally Posted by engineermike
At 141, I think your turbo has a ways to go. How much boost are you running?
through AFR 225s mind u
I would say step it up to a 3.5" for the cutout. One thing you may not have considered is that a 3" electric cutout is not a true 3" diameter. It is really closer to 2.75", and it is a rough flow.
Originally Posted by bowtieman81
I would say step it up to a 3.5" for the cutout. One thing you may not have considered is that a 3" electric cutout is not a true 3" diameter. It is really closer to 2.75", and it is a rough flow.
my last one was 3"ID..... but i snapped the motor on that one. i have a new one. havent taken it out of the box yet tho, becausr i may be getting a bigger one
