Necking down a down pipe
Thread Starter
Joined: Nov 2009
Posts: 1,621
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From: THORNTON, PA (NEAR PHILLY)
Necking down a down pipe Due to space constraints, the biggest d-pipe I can run is 3.5", and that even tight. Would bad things happen going from a s475's 5" connection tapered to say a 3.5"? This would be for a mild street build with a mostly stock 5.3.
I did exactly what you are talking about 5" to 3.5 taper on a S480 and the car ended up chocking up top. Car ran 6.50 in the 1/8th first pass off the trailer was data logged my tuner called for most boost on the second pass, so I upped it to 15lbs and ran a 6.45, Tuner adjusted it some more and it ran the same thing on the third pass. So it will hurt performance a lot. We put the car back on the trailer I ran a 5" dumped just under the bumper. Pulled the car off the trailer the next weekend, Ran a 6.35 on 10 psi, then a 5.90 on 15-16 psi on the second pass with an identical tuneup from the week before. Only change was the downpipe.
I ended up installing a cap on the end on the 5" downpipe and taping the 3.5 back into the 5" down pipe and running it out the back. On the street I leave the car on 10 psi, I go to the track pull the cap off and let it eat.
Hope this answers your question.
I ended up installing a cap on the end on the 5" downpipe and taping the 3.5 back into the 5" down pipe and running it out the back. On the street I leave the car on 10 psi, I go to the track pull the cap off and let it eat.
Hope this answers your question.
TECH Enthusiast
Joined: Mar 2012
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x2. I run cutouts on almost everything. Dump the 5" downpipe below the motor near the firewall, and let the smaller exhaust run out the back. That way you don't have to have the full 5" noise all of the time and can still have full exhaust fitment. Open the cutout and the choking is gone.
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
I did exactly what you are talking about 5" to 3.5 taper on a S480 and the car ended up chocking up top. Car ran 6.50 in the 1/8th first pass off the trailer was data logged my tuner called for most boost on the second pass, so I upped it to 15lbs and ran a 6.45, Tuner adjusted it some more and it ran the same thing on the third pass. So it will hurt performance a lot. We put the car back on the trailer I ran a 5" dumped just under the bumper. Pulled the car off the trailer the next weekend, Ran a 6.35 on 10 psi, then a 5.90 on 15-16 psi on the second pass with an identical tuneup from the week before. Only change was the downpipe.
I ended up installing a cap on the end on the 5" downpipe and taping the 3.5 back into the 5" down pipe and running it out the back. On the street I leave the car on 10 psi, I go to the track pull the cap off and let it eat.
Hope this answers your question.
I ended up installing a cap on the end on the 5" downpipe and taping the 3.5 back into the 5" down pipe and running it out the back. On the street I leave the car on 10 psi, I go to the track pull the cap off and let it eat.
Hope this answers your question.
Think this is a s480 thing b/c of its 5" connection? They have a t4 4" connection, I would have to assume its not as suseptible?
I see alot of turbo kits which use the s480 t6 5" and they neck to 4" right off the turbo almost.
TECH Enthusiast
Joined: Mar 2010
Posts: 519
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Neck it down to 4 inch right off the turbo no need for 5, then where it's tight you can just buy a section of oval 4inch pipe then continue on with the regular 4 inch from there back. The exit for the turbine wheel it's self is right around 3.5 inchs as it is.
PPL have made 900whp through a 3 inch dp.....it hurt spool time I'm sure but from the graphs I have seen it wasn't as bad as you think.
PPL have made 900whp through a 3 inch dp.....it hurt spool time I'm sure but from the graphs I have seen it wasn't as bad as you think.
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What size turbo is this? What size exhaust wheel? The DP can be necked down, I doubt you have a 5" exhaust wheel. you just need to keep in mind each elbow added is equivilant to a few feet of straight pipe flow wise... varying with radius of elblow and size. You kind of need to know whats going on after the downpipe also.
Thread Starter
Joined: Nov 2009
Posts: 1,621
Likes: 31
From: THORNTON, PA (NEAR PHILLY)
What size turbo is this? What size exhaust wheel? The DP can be necked down, I doubt you have a 5" exhaust wheel. you just need to keep in mind each elbow added is equivilant to a few feet of straight pipe flow wise... varying with radius of elblow and size. You kind of need to know whats going on after the downpipe also.
Trying to figure out what turbo would work nice for a mostly street 5.3 sn95 with a th400 and 3.27. Want something responsive and can still turn some #'s at the track. I like the s475 t6 but it has the 5" connection. Most other turbo's have 4" and 3.5". The biggest pipe I can squeeze I think is a 3.5" and that's pushing it.
So I'm just seeing what turbo's require what - that will help with turbo selection to some degree. Dont want to drop 1000.00 for a turbo if I can help it
Looking at the gt45, s366, tc 72 or 76, and the BW's
Any input is greatly appreciated.
if your power is that sensitive to downpipe at those lower power levels you may be due for valve springs.
my t/a always was with 3.5" downpipe and thats the 1300horse range
my t/a always was with 3.5" downpipe and thats the 1300horse range
I am running into a similar issue. but mostly just not having room for a 5" elbow. so I bought one of the 5" flanges on about 6" length of pipe, and am splitting that down into 2 3" pipes bullhorned out either side. basically going to use a 3" 180* bend split in half and butted together so it turns 90* out either side
TECH Enthusiast
Joined: Mar 2010
Posts: 519
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What kind of times did it turn Mighty?
If you went with say a s475 or s480 yes thats a 5 inch discharge. Race part solutions sells a s400 series v band flange I believe that necks down to 4 inch pipe. Run that as far as you can.
Once you go down where you need it to be 3.5, neck it down there again.
You could then go back to 4 inch or split it to dual 3 inch. Like others say if it's gonna have more bends that creates more restriction so. That small area of 3.5 inch pipe wont kill it. If you ran a long distance and stayed 3.5 it might.
How much? Who knows.....may not be enough to even worry about.
I vote 4in necked to 3.5 where it needs to be then back to 4 inch if possible if not y it into dual 3 inch pipes all the way out back....
lets say your valve springs are 100# on the seat
then you boost 15psi and say your exhaust pressure is 1:1 for simplicity
how big is your exhaust valve? 1.6? area =piexR^2
dirty math your exhaust valve area is 3.14x.8^2 = 2 square inches
call it 1.9 square inches subtracting the stem to be conservative
15 pounds per square inch x 1.9 square inches is 28.5#
your 100# spring pressure is now... 71.5# significant fo sho! this was a conservative calculation
even if you are on a stock cam, with any measurable boost i strongly suggest valvesprings. again these are rough numbers, same can be said for boost on the intake valve.
my best was 8.49 car weighed 3775 with me in it as raced
then you boost 15psi and say your exhaust pressure is 1:1 for simplicity
how big is your exhaust valve? 1.6? area =piexR^2
dirty math your exhaust valve area is 3.14x.8^2 = 2 square inches
call it 1.9 square inches subtracting the stem to be conservative
15 pounds per square inch x 1.9 square inches is 28.5#
your 100# spring pressure is now... 71.5# significant fo sho! this was a conservative calculation
even if you are on a stock cam, with any measurable boost i strongly suggest valvesprings. again these are rough numbers, same can be said for boost on the intake valve.
my best was 8.49 car weighed 3775 with me in it as raced
its only a mustang, start cutting and keep the biggest DP you can fit. if you can get the 5" in there it will be happier