minor exhaust mod...huge turbo lag - Hemi Truck Club
 
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Old 10-02-2007, 10:52 PM   #1
 
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minor exhaust mod...huge turbo lag

I recently modified my exhaust tubing from the y-pipe and sts turbo piping. Since I'll be pulling the turbo off in the winter months, I thought I'd make life easier and flange the connection between the turbo piping and truck exhaust. Cutting and weldingg is a pain in the butt.....The rest of the exhaust is stock, including the factory manifolds....and I'm still running a stock 5.7 hemi.

I bought some 3" diameter stainless pipe (.062" wall thickness) and machined up some flanges and welded them on to the pipes. Made up a gasket and bolted the pipes together.

The original exhaust pipe size from the y- pipe to the sts turbo pipe is 2 3/4" in diameter and about 17 inches long. The turbo pipe shrinks to 2 1/2" diameter at about 12 1/2" long before the first bend. I cut this section out, and installed the 3" diameter sections in place. After welding the pipes in place it was time for a test drive...

I went to the same place I do all my test runs, and the truck seemed to run fantastic! I brake stalled at 2500 and floored it. As soon as I lifted my foot off the brake, the turbo was building boost at a rapid rate. I felt that everything was great.....but then....

When I was at the track in NY ( similar elevation and temperature) I noticed a huge difference in turbo lag. While at the track, I couldn't keep the turbo hot enough to build boost off the line. In each pass I made, I never saw any boost until just before the 60ft mark, and then it would rise and stay at around 6psi for about half a second and then jump to 10 psi after that. My 1/4 mile times were at lot slower than before, as I could only run a best of 14.0 at 99 mph.

The wait times before each run down the track were long, and even with the truck idling...it just wasn't enough. I even tried Brake stalling longer in the burnout box to build heat with little effect....

After my first pass, I took the truck to the back of the field on a little roadway about 200 ft long, and I practiced the launches. The first launch was the same as before....no boost until about 4500 rpm.

The second launch proved to be quite better, and by the third launch, I was sqwakin the tires and boosting off the line.....So what happend?...well, I never realized the problem when I put the new pipes on the truck. By the time I got to my test site, the egt's and turbo was hot enough to spool the turbine and perform as it should. But waiting in line at the track, the egt's drop and turbo cools off. The increased diameter in the new pipes reduces exhaust pressure drastically, so I'll be putting the 2 1/2" turbo inlet pipe back in place to cure the problem.

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Old 10-03-2007, 03:50 AM   #2
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minor exhaust mod...huge turbo lag

Sounds like a small drop in backpressure really did chance the flow to the turbo. Is there any possible wastegate adjustment that can be made to help with this?
I know that when I had stock manifolds on my truck and replaced the factory Y pipe with the JBA high flow pipe I could feel a little lose on the low end since there was less back pressure.

Any chance that installing headers for higher flow from the heads would help?

Bill
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Old 10-03-2007, 06:23 AM   #3
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minor exhaust mod...huge turbo lag

Yea, it's designed like that for a reason. You need the concentrated force of exhaust air blowing on a centralized point to get those impellers moving properly. It's like being in a gusty wind storm or having some one put a leaf blower pipe to your chest. When you open the exhaust up before the turbo you're losing the "velocity".
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Old 10-03-2007, 08:44 AM   #4
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minor exhaust mod...huge turbo lag

so.....just to verify.....having headers with the sts turbo kit would only cause more turbo lag? No help whatsoever?
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Old 10-03-2007, 11:44 AM   #5
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minor exhaust mod...huge turbo lag

Hey Steve... Can you post up a pic of what you did????
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Old 10-03-2007, 06:01 PM   #6
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minor exhaust mod...huge turbo lag

Its called the signal. It all has to be right for the turbo/engine/exhaust size you have to work right.

If you increase the diameter of the pipes that feed the turbo, you slow the turbo down.

The term "signal" is a big thing in the NA carb world. The engine/carb/headers, etc. It means it all has to be right or it don't work as good.
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Old 10-03-2007, 08:04 PM   #7
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minor exhaust mod...huge turbo lag

Hey Keni... Not necessarily... I was told by Dave from STS... as long as you keep the diameter of the pipe all the way through the system... It's good to go..
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Old 10-03-2007, 09:44 PM   #8
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minor exhaust mod...huge turbo lag

Well I do agree the pipe needs to stay the same diameter. However, if you put a 5" pipe from your y-pipe collector to the turbo, you won't get the response when compared to a 2.5". Ask Dave what would happen if you put a 5" pipe there. I'd be interested in hearing his response. My turbo books talk about getting "concentrated" flow.
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Old 10-03-2007, 09:50 PM   #9
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minor exhaust mod...huge turbo lag

Take this with a grain of salt, it's an exerpt from my book called "Maximizing Boost".

Tubing size : The choice of tube sizes will usually be dictated by port size and the size of the turbine inlet on the turbo. Where a clear-cut choice does not exist, it is best to select the smaller of the sizes avail, thus increasing exhaust gas velocity.

Rule of thumb : always opt for the smaller, to keep gas velocities high.
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Old 10-04-2007, 05:28 AM   #10
 
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minor exhaust mod...huge turbo lag

It was worth trying anyway. I'm gonna leave the pipes in place and try it again once the stroker goes in. Different cam...bigger head ports...

For now, I'll split the flange and put in the 12 1/2" section of 2 1/2" pipe in front of the turbo. I'll weld a lip on the end and put the flanges back together. If there is still some lag, I'll replace the section right from the Y-pipe
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