BIC Divorced Downpipe

[Moy]

I ran an open dump DDP for about 8 months. I never had boost creep issues. I had my MBC set to 10psi, and the needle on my gauge would never move past that setting. I switched over from a bellmouth-style downpipe and felt (note I did not have it dyno tested), that it picked up some power, and positive pressure came on a couple hundred RPM's quicker than it did with the bellmouth.

3" ebay DDP, 3" Raptor Racing testpipe, 3" Sheepdog/Prof Sport Exhaust

 

[Insidious Surmiser]

there's also been reference to the divider plate not sealing entirely... it is possible that an improper seal can cause scavenging to carry over to the dump tube, causing a vacuum and loss of flow....

 

[suprarx7nut]

I appreciate your replies however it is my understanding that A)a 3" exhaust with internal wastegate only becomes restrictive well above 400HP, at wich point the stock turbo isn't enough anyways and B) suprarx7nut sick photos but racecars are a different game and it looks like that ddp has a bigger pipe for The wastegate gas to flow thru. if you can't do a full pull in 3rd without overboosting thank you for proving my point. ddps are cool, and i understand their point, but on my car i get more problems (fuel cut, popped a motor) than benefits (none).

"Boost creep occurs when the wastegate flow is restricted more than the exhaust"

If exhaust and wastegate go thru the same pipe the restriction is the same, but when the pipe coming out of the wastegate is different and of an insufficient diameter boost creep can occur. You can't stand up for ddps on mk3s if you have boost creep issues.

A: Exhaust restriction doesn't occur with the flip of a switch when you hit a certain power level. Pressure drop (IE flow restriction) is exponential and will increase more and more as flow increases, but there are still restrictions at the low end. I doubt the restrictions at my power level are very noticeable between my 3" recirc'd DDP and a standard bellmouth DP, but I'm an engineer and the benefits, even if minimal are just nice to know I have. I get a kick out of the design and theoretical function, no matter how prevalent the benefits are in my cars typical daily driving duties.

B: Racecars are indeed a different ball game. No argument there. A little background on my experience. My boost creep isn't new. With the factory downpipe with a huge crack right off the elbow and my old CT-26 I had boost creep as well. The CT-26's internal wastegate flow path isn't designed for flow much beyond factory spec. I think creep occurs because of the turbo, not the ddp. If you make anything over ~300whp with a stock wastegated CT-26 i'd be shocked if you didn't have some boost creep. The dimensions of the wastegate compared to the turbine outlet make it difficult to control creep as you raise the flowrate over factory spec, no matter the downpipe.

Show me somebody with an unmodified CT-26 wastegate NOT having creep issues on a prolonged pull to redline in the higher gears. I dare ya!

 

[Insidious Surmiser]

got a point there... also 89+ had a slightly updated wastegate design... the 89 I spoke of was sold early in the year, meaning it was assembled in 88...

 

[pipelinewill]

sorry but I've seen dyno charts showing ~30whp gain over a standard 3" DP... gain comes from elimination of turbulence. The turbulence was a factor taken into consideration when the engineers designed the turbo/wastegate as the wastegate clearly creates turbulence, restricting flow which helps regulate turbine speed. my ct26 on a DDP wanted to just keep spinning past 15psi, when I had it set @ ~half that amount of boost. (keep in mind I'm talking about creep, mainly in 4th and 5th gears) With the exhaust split the wastegate does not flow enough, and turbine flows too much. It would be highly recommended to have some fuel system upgrades (IE 550's + lexus afm) if you're intending to run this downpipe. either that or have your wastegate ported out some... I'm sure that would've helped.


just so you know... the 1.75" dump tube is not the restriction... it's the actual wastegate

all in all it's a great mod, and it's a fact that it will make more power than any other downpipe setup... BUT just like anything else, you have to have the right supporting parts/modifications

I have a lexus maf, afc and 550s, upgrading to a 320 lph pump and fpr to enrich my mixture but untill then I am not comfortable running 15 psi, and my creep is more like 17psi, which is too much. if the ddp causes boost creep then maybe the +30whp is from increased boost. either way i don't want a part on my car that stops me from having control over my boost pressures. that's not worth 30whp to me

 

[suprarx7nut]

I have a lexus maf, afc and 550s, upgrading to a 320 lph pump and fpr to enrich my mixture but untill then I am not comfortable running 15 psi, and my creep is more like 17psi, which is too much. if the ddp causes boost creep then maybe the +30whp is from increased boost. either way i don't want a part on my car that stops me from having control over my boost pressures. that's not worth 30whp to me

The biggest point you seem to be missing is that the DDP should LESSEN boost creep. Once again, it should decrease creep. Not sure how else to say it.

A ct-26 won't do much good beyond 13-14 psi anyways. Beyond that you will definitely have creep issues and you're also just blowing hot air. Is yours trimmed? 57? 60?

You're well beyond the turbo's intended use range. Its not the ddps fault you have boost creep at double the turbos intended flow rate. Lol. Look at the ct26's compressor map for your given flow and it'll be obvious that your next mod needs to be a turbo.

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[pipelinewill]

The biggest point you seem to be missing is that the DDP should LESSEN boost creep. Once again, it should decrease creep. Not sure how else to say it.

A ct-26 won't do much good beyond 13-14 psi anyways. Beyond that you will definitely have creep issues and you're also just blowing hot air. Is yours trimmed? 57? 60?

You're well beyond the turbo's intended use range. Its not the ddps fault you have boost creep at double the turbos intended flow rate. Lol. Look at the ct26's compressor map for your given flow and it'll be obvious that your next mod needs to be a turbo.

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57 trim. i'm not saying all ddps cause boost creep but i think the cause of my creep is the dp because the previous owner didn't hit fuel cut till he installed it. at the moment i'm not trying to run double the stock flow rate, i'm trying to to run stock boost. i haven't got a boost controller installed and the stock waste gate isn't shimmed. but it just climbs. 15 is a plan 4 later when i can make my car run well. i'll stop this debate until I get a chance to try some fabrication and porting and general shenanigans and i'll post weather i was right or wrong. no way i'm getting a new turbo, I need money for a silly 510 for silly boost.

 

[Insidious Surmiser]

my turbo would only creep terribly in 5th gear... which apparently you're not supposed to even run full-out in 5th (overdrive gear)

4th gear creep would be 14 psi... I had mine tuned to run @ 12 psi, just because I was on stock IC, mainly

 

[suprarx7nut]

57 trim. i'm not saying all ddps cause boost creep but i think the cause of my creep is the dp because the previous owner didn't hit fuel cut till he installed it. at the moment i'm not trying to run double the stock flow rate, i'm trying to to run stock boost. i haven't got a boost controller installed and the stock waste gate isn't shimmed. but it just climbs. 15 is a plan 4 later when i can make my car run well. i'll stop this debate until I get a chance to try some fabrication and porting and general shenanigans and i'll post weather i was right or wrong. no way i'm getting a new turbo, I need money for a silly 510 for silly boost.

If you're running less than 10 psi or so you should not have any creep regardless of exhaust. The DDP allows more flow and will make it more likely to outrun the turbo if you up the boost. This means increased chances of creep and fuel cut.

If you're aiming for stock pressure the DDP isnt really a concern. If your boost is creeping with no boost controller and no shimmed wastegate you've got other problems.

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[pipelinewill]

If you're running less than 10 psi or so you should not have any creep regardless of exhaust. The DDP allows more flow and will make it more likely to outrun the turbo if you up the boost. This means increased chances of creep and fuel cut.

If you're aiming for stock pressure the DDP isnt really a concern. If your boost is creeping with no boost controller and no shimmed wastegate you've got other problems.

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the previous owner had stock boost, then installed the ddp, then started hitting fuel cut.

 

[DeMoN2318]

the previous owner had stock boost, then installed the ddp, then started hitting fuel cut.

No way you are getting 6 more psi from just a DDP.

How is it stock boost with your mods? lexus afm and 550s should put you above stock boost...right? and then add in the 57trim ct26....am I missing something?

Something seems weird...going from ~6.2psi (stock) to ~12psi (fuel cut) with just a DDP??

 

[suprarx7nut]

the previous owner had stock boost, then installed the ddp, then started hitting fuel cut.

There's something else going on here. You shouldnt hit fuel cut without a controller. Something else has gone wrong.

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[Insidious Surmiser]

that's funny, that's what my 89 did w/ no boost controller (hit boost cut w/ stock wastegate, no modification) again... I will note I believe '88 parts were used to build my car (the vin said it was assembled the first month in 89, and a ma70 chassis was used (not ma71)) the wastegate design was revised on 89+ cars, in what way I have no idea... but it did net a 2hp increase

 

[pipelinewill]

No way you are getting 6 more psi from just a DDP.

How is it stock boost with your mods? lexus afm and 550s should put you above stock boost...right? and then add in the 57trim ct26....am I missing something?

Something seems weird...going from ~6.2psi (stock) to ~12psi (fuel cut) with just a DDP??

lexus afm and 550s are so you CAN run more boost, but the wastegate should still control boost. the 57 trim, pipes and exhaust can cause creep, (more flow and a more aggressive turbo), but WITH all that it didn't creep past 10 UNTILL the previous owner installed the ddp instead of the 3" internally gated dp he had on. i called driftmotion for parts and mentioned the problem, they said that they experienced boost creep on EVERY 7m and 1j they installed a ddp on. I think that it's quite clear that the ddp isn't helping my boost creep. on one instance a stock 1j that was on 7psi jumped over 15psi on driftmotions dyno with no new mods except the ddp. and my goal is to run 15 psi, but I want to start out at stock boost and increase slowly to make sure it's tuned well before i start boostin hard. i'll just fab a custom widemouth in the beginning of march and post back if it works. if a widemouth 3" dp stops the creep i'm right, if it doesn't i'll eat my sock. sorry for threadjacking lol