Just wanted to help some folks if they ever run across this! Combo: 355sbc, blowthrough carb, glide, big frame 88mm turbo.

Car was running really good last year, got a 5.73@125mph out of it. Found a thrust problem fix it and while I had it apart for repair decided I was going to run a local DR class. I had a 4" downpipe on it that only had a couple slight bends in it but had to add a muffler to meet the DR rules. I refabricated the down pipe so the muffler could be installed, it now included a two pretty sharp bends and one was right off the turbine housing. Car ran like a turd, but it was a fresh motor so I figured something had changed for some reason. After loosening the converter, advancing the cam and screwing with the carb (a lot), I got it back running decent 6.03@121 on the DR's but still with pretty slow spool up.

Tore the engine down again a few months later due to a cracked head. Put it back together again and now the car would idle and drive pretty good but when you floored it, it would stumble really bad like it was really rich. So now I was hunting the same problem as before but now worse! After pulling the carb 12 times, timing changes, cold side piping changes and making sure all the electrical stuff was right it still would not spool.

I decided to yank the downpipe off! I cranked the car and it idled 500rpm higher and spooled instantly! So now we just got done doing a 5" downpipe and it is perfect! Looking at the old exhaust pipe, I would have never, ever thought that it would be that much of a restriction!

Just wanted to put it out here in case anyone else is having any spooling issues! It took us about 20hrs to finally figure the problem out!

 

Cool let us know how it runs . I would not have thought that would be a resrtiction that bad .

 

.... i know i had full 3" exhaust on the powerstroke and by going to 6" doing nothing else it picked up over 10lbs of boost... and ran its but off... ... kinda the same thing... gots to breath.. sweet times too

 

Any pics of the before and after?

 

The ideal exhaust for a turbo is nothing at all. Since that is a tad loud, and would melt everything underhood, a big fatass 5" downpipe is great. Glad to see some real world results, and look forward to seeing how it runs for you now. Might wanna take it easy first time out so you don't tear the rear bumper off.

 

Damn, makes me think a 3 inch pipe is not worth wasting my time!

 

If you didnt have the room for a 5-6" downpipe then could you split an initial 5" into say 2x3" ? I want to run my downpipe out through my inner guard then between the outer side of the guard and my strut, so no way can I fit a massive pipe. Has anyone tried this ? Its still a decent reduction in area over a 5" pipe but they will be straight out of the turbo for maybe 2 feet so that will help a little.

 

two 3.5" has about the same area as one 5".

 

we like to have never figured this out, i think we tried everything in the book twice.but damn does it sound better now.
i think i'll put mine together now.

 

Dude, you are the man! Thanks for doing the bulk of the work last night on the new downpipe and cold side, it looks great!

I'll snap some pics later this weekend and post them up!

It was only out of pure frustration that we even decided to yank the downpipe, but thankfully that was it! Always something stupid and easy, at least on most problems!

 

Well now I feel a lot better about the 5" pipe in my kit. Was gona try and run a 4" and split into 2, 3 inch pipes, but now I am happy to have that fat 5"er! 438 cubes, what was i thinking 4"

 

Awesome, congrats on finding the bug!

-Chris

 

Well thanx for posting it. You saved a few headaches I'm sure.

 

So a 4 inch DP on a street car with 355 cubes would be better off with a larger pipe still right?

 

There is no such thing as too big, so whatever you can fit is what is best. A higher flowing downpipe will also let your WG work better because there won't be as much pressure fighting against it. The only downside is how to package it underhood and how loud it is.

 

damn I just built a dual 3 inch set up off my 4inch down pipe now I'm worried......

Is your car a t4 or t6?

 

Two 3" pipes flow more than one 4". While you can't go too big, a 4" downpipe will still support a LOT of power.

 

Do you have a pic of the 4" dp ??

john

 

I agree, the biggest you can fit will be the best! It had 4" on it when it ran the 5.73, but it was about 8" of straight then a 90 degree bend down. It would spool like that but was slower, so I figured it was just going to be laggy. So the 4" was working (but not as good as it could have) as long as the bend was not right off the turbine housing.

Here is the pic of the one that did not work. It was 5" that I had to neck down to 4" in about 2", thats why it looks like the funnel! Excuse my poor welding skills, I am going to learn how to weld at some point!

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Dont worry about the welds being pretty , as long as they are strong

Its a good find mate and worthy noting before planning any build.