3" to 3.5" or 4" or 5" dp how much gain?

What is the best way to test a bigger dp?

  • the dyno test should be done same boost and tune

    Votes: 39 61.9%
  • the test does not conclude anything the car should be raced

    Votes: 11 17.5%
  • it should be tested on a bigger turbo

    Votes: 2 3.2%
  • it should be tested on a bigger motor

    Votes: 1 1.6%
  • it should not be tested at all as we know 5" looks good and must be work

    Votes: 10 15.9%

  • Total voters
    63

norbs

Classic fast, XFI, SPortsman & MS3 programming
Joined
May 25, 2001
I am recently running a ptq71 with a 3" downpipe, how much gain do you think i would get in upgrading to bigger, or is the 3" more than enough. To run lets say 138 mph, high 9's
 
I think with that turbo you are starting to push the 3 inch pipe a little to hard.
Do you have any back pressure readings?
 
Arent there 6 second pro-5.0 turbo'd stangs running with 4" downpipes? Seems to run 9's you wouldnt really need anything bigger...maybe 3.5".
 
VadersV6 said:
Arent there 6 second pro-5.0 turbo'd stangs running with 4" downpipes? Seems to run 9's you wouldnt really need anything bigger...maybe 3.5".


4inch twin down pipes my brother if the have something in the T4 range or GT Series Smaller than GT47. If a single turbo then 5 inch.

Some people actually have twin 5inch down pipes like Tim Lynch.

Chris Chow just redid his twin 4inch pipes.
 
Pretty much all of them run a single turbo. I think it may be the rule. So I imagine a 5inch DP may be what they use. But to make 1600-2000hp with a 5inch DP is a hint to what you really need.
I looked up the rules and here it is:

1.23 TURBOCHARGERS
Only one turbocharger permitted.

SINGLE TURBOCHARGER: Accepted single turbochargers are permitted with the following requirements: Turbocharger must be of conventional impeller and housing design and type. Turbocharger is limited to a choice of four turbochargers (at four weight breaks): [1] maximum size of 94 mm/3.700” - inducer wheel diameter at the point where the leading edge of the compressor wheel meets the inlet housing. Inlet housing is permitted a maximum inlet diameter of 3.780” where the leading edge of the compressor wheel meets the housing. [2] maximum size of 96.5 mm/3.800” - inducer wheel diameter at the point where the leading edge of the compressor wheel meets the inlet housing. Inlet housing is permitted a maximum inlet diameter of 3.880” where the leading edge of the compressor wheel meets the housing. [3] maximum size of 98.0 mm/3.858” - inducer wheel diameter at the point where the leading edge of the compressor wheel meets the inlet housing. Inlet housing is permitted a maximum inlet diameter of 3.938” where the leading edge of the compressor wheel meets the housing.

[4] maximum size of 101.0 mm/3.976” - inducer wheel diameter at the point where the leading edge of the compressor wheel meets the inlet housing. Inlet housing is permitted a maximum inlet diameter of 4.056” where the leading edge of the compressor wheel meets the housing. Compressor wheel to housing clearance must maintain a maximum of 0.080” clearance beginning from a) where the leading edge of the compressor wheel meets the inlet of the compressor housing to b) the transition point where the trailing edge of the compressor wheel meets the volute. All air entering the turbocharger must pass through the turbocharger inlet. Injection of any liquid, gas, or any other substance into the inlet or exhaust housing prohibited. Turbocharger compressor wheel must be constructed of cast or billet aluminum. Exotic material wheels prohibited. The tips of the impeller wheel may not be stepped, cut down, or notched to meet impeller tip-to-tip dimension. (.500-inch will define the tip of the impeller wheel).


I dont really know whats available in terms of turbos, but Im sure you're far more qualified to figure out what it is. I have no idea what exact turbo these rules would point to. The rules say they can run any diameter exhaust tubing they want.
Heres a link to Pro-5.0 rules:
http://www.nmraracing.com/rules/pro/
 
Here's a feature of a Pro-5.0 Car..Bill devine's. That DP looks to me around 5". That is one HUGE turbo! The DP actually necks down to a smaller diameter to mate to the turbo. Look at the size of the "crossover" pipe. Actually pretty damn small. Somehow these guys make up to and sometimes more than 2000hp with no other power adders....and without a sparkletts bottle sized downpipe.

http://www.hardcore50.com/Articles/JasonReiss/BillDevine/WildBill1.htm
 
I recently ran a very high nine second pass (9.95 at 135) with a stainless 3" down pipe and would be interested in knowing if anyone has hard data to support performance improvement with a 3.5' or 4" downpipe. I don't doubt that there might be an improvement. I remember Terry Houston telling me back some 6 years ago that he had conducted back to back tests and concluded that an improvement was noted with the 3.5 inch downpipe.
 
Here's a reply from Joe Lubrant when I asked him if I should run a 3.5" DP with my 70 GTQ:

The THDP would not be a big issue. 3" is not a big bottle neck. I believe that the 3.5 would only be necessary for maybe a 74 and above. A 3.5 wouldn't hurt for sure but at the power level of the 70gtq i think the 3 would be ok.
I know that we recommend a 3.5 for the PT88 3-bolt as a minimum. Heck back in 2000 TSE trim I ran a 9.13 153.55 with a 3" dp in a 3700# car, with a PT88 3-bolt.
 
I looked at the pictures some more, and it looks like the area where the DP necks down to mett the turbo, is actually about as big, if not smaller than the distributor cap sticking out the timing chain cover. Thats around 4 inches.
 
In my experience going from a 3 to a 3.5 DP gained me nothing. I did run my 9.9 et with a 3.5 but could have done it on the 3 in just as good. I have several runs with the 3in that were aborted for one reason or another.
 
There is a twin turbo guy named John Mihovitz. He's the big hero in the mustang cobra crowd and lives in the same city as me. He runs a little 281 c.i. cobra motor with twin 76mm turbos. Makes about 2000hp on about 30psi and runs 6.60's or a little faster these days maybe, at well over 200mph. Its not easy to see, but his DP's arent that big at all. The "mufflers", lol, are bigger than the piping so you can kind of get an idea what size they are.

0312mm_modular_03_z.jpg
 
I wonder why RJC will not dyno test his 5" pipe vs a 3" dp?
 
norbs said:
I wonder why RJC will not dyno test his 5" pipe vs a 3" dp?
I give him props for making such a nice piece and for the work that must have gone into it, but like I said since the day I first saw it...I dont see the need for it. If you have an engine pumping 2000cfm I could see it, but our motors pump nowhere near that much air. I mean, look at the size of the compressor outlet vs. the size of a downpipe. Even in stock form, the DP is bigger than the compressor wheel. Think of how velocity will keep the exhaust charge moving quickly and will pull the next pulse out. Also, a hotter exhaust pipe will keep exhaust velocity up. Thats why anyone in their right mind wont make an aluminum exhaust system. Aluminum radiates heat too quickly. A ginormous DP has so much area that the tube has no way to stay hot, and the velocity drops to nothing because of that and the giant diameter. Now you have a compressor trying to push that huge mass of air out of the downpipe without any assistance from velocity and momentum. The exhaust is so overly efficient that the fresh incoming air/fuel mix comes into the cylinder and flies right out the exhaust valve during valve overlap cause the exhaust pressure is so much lower than the pressure in the intake manifold. Under high boost/high flow situations, this problem diminishes to a degree, but while cruising, the problem will be ridiculous. Good bye mileage. I know the exhaust charge gets squeezed when it goes through the small compressor wheel, which speeds velocity up, but once it gets to the other side of the wheel (DP), it stalls and actually might become a problem. Especially when there is no high speed pulse traveling through the downpipe to pull the next charge out. That resonant scavenging pulse is dampened to the point of non existence in a huge pipe.
 
Just so I get an idea (I meant to go to the nationals @ Beech bend, but didn't). How many of you guys run straight downpipes or turn your exhaust out short of the rear of the car? I know what you guys run for turbochargers, but what kind of other exhaust work does it take to get deep into the nines? Twin scrolls? Pulse tuning? Etc? If any of you guys have sites I can look at with descriptions of your cars, that would suffice. :)
 
Yeah that would be interesting to see how much difference there would be between the 5 inch verse the 3 inch downpipe.
 
Excuse my possible misunderstanding but I believe that the exhauste flow out of the exducer of the turbo is near constant not pulsating? Aluminium is a good thermal conductor, that will make your engine bay hot, never a good thing, it also has a very large thermal expansion coefficient meaning the thing will distort like mad when you heat it up to 500ºF or more. One of the reasons that flowmasters and other chambered mufflers don't work on a turbo application is the resonance tuning of the chambers is totally ineffective since the exhauste flow is smoothed out so much. However you do make a very good point about the exhauste gas velocity and the pressure gradient caused by the expansion. This is exactly why merged collectors work so well on professional race cars. The sudden expansion does cause the air to rapidly decellerate and loose pressure causing a stalling effect that helps cool but can hurt flow characteristics of the pipe. I'd match the size of the exducer bore of the turbine and gradually megaphone it up to the desired size so you have a less of a pressure gradient at the exducer bore mouth. However if you have to make an immediate 90º bend off of the turbo its probably more benificial to run a larger bend than to try to megaphone it after the bend since gas will tend to slow in the bend.

Just some materials and fluid dynamics ramblings, jeez I love this board!
 
With the tubo spinning at 60-70K rpm i don't think the bigger pipe is going to slow down the pulses to cause a hp loss.
 
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