?? Turbocharger exhaust tubing size.

[riceburnerfb1]

I am about to begine the process of building a collector pipe for a 1425cc dragrace type motorcycle.

Historically I have always used 1.5" on some of the later ones I upped the size to 1.625" with good results.

I have always used a standard exhaust housing, collector with a common area before the inlet to the turbine. On the motor I am going with a split tangent exhaust housing to keep the exhasut pressures and burn gasses away from the clean combustion.

My question is...drum roll please....I want to go to larger primary tubing on the collector 1.750", (the exhaust size and trim is staying the same, a 58 and a 62 I beleive) would upping the size of the pipe be an increase of velocity at the turbine? Or will the larger pipes kill the velocity because of the increased volume of the pipe itself?

Or will it just be an exercise in making an exhaust pipe special when nothing will change?

Thanks for your input in advance.

s.r.

 

[patprimmer]

Keep it as compact and as small a volume as possible without causing an obstruction to flow.

Primaries should be just a bit bigger than the exhaust port outlet or at least the flange mating surface should be. If the pipe is the same size as the port, it should be flared out slightly at the flange. The flare out or size increase should be just enough so the gas never hits a flat face as it exits the port and allows for possible miss alignment due to bolt hole clearance. Of course if you have dowels or minimal clearance on the bolt holes that improves alignment if not ease of fitting.

The primaries should be as short as practicable with some consideration to streamlining, but not to equal or tuned length, and formed to merge directly into the turbo mounting flange.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[riceburnerfb1]

I guess that would be the question, when does the pipe get so small that it does obstruct flow, 1.375, 1.500, 1.625, or 1.750? It does seem that they do work better the larger diameter they are getting, Yes, short as possible.

 

[Tmoose]

"I upped the size to 1.625" with good results. "

What were the "good results ?"

 

[riceburnerfb1]

more boost on the two step, more boost down track. quicker times, higher mph.

 

[riceburnerfb1]

my thought was by moving the restriction from the pipe so the motor could flow, it put the restriction at the turbine so the motor could breathe easier.

 

[evelrod]

Sounds like a question that only riceburner can really answer...trial and error? What you picture looks pretty sanitary...if, to my limited turbo experienced eyes, a bit large.

Having done hours and hours of building and dyno testing differing headers on Lotus twincams, I have found that the smallest dia tubing that does not restrict flow abnormally (what's that?) tends to give the best tq/hp. However, in the real world of racing, in places like Laguna Seca that require noise attenuation, the actual numbers (by default, the dia of the header/exhaust tubing) get really screwed about. Also, the camshaft timing/profile has a HUGE effect on the exhaust system. I don't see a turbo installation suffering from the noise problem in the same manner, indeed, you may have no limitations on noise in your racing.

Rod

 

[riceburnerfb1]

Thanks for the replies,

Question, how big of diameter are you using and how big is each cylinder generally?

the pictured pipe is the 1.625 on an 1100cc engine and will easily makes in excess of 50psi boost (works best at 35 though), again that is 325cc smaller than the one I am currently working on.

I understand the camshaft thought also. We have generally go for the smaller camshafts, however in this situation with a larger pipe diameter it maybe a good idea to go up on the lift, back the timing on the intake and put more out the pipe.

If it restrict less, put more in (sounds good anyway).

 

[turbomotor]

What is the size of the turbine nozzle? I suggest mapping the expected mean exhaust velocities right up to the turbine. Diffusion is typically fraught with loss, so do as little diffusion of that precious velocity head that comes from the sonic blow-down phase as possible. Try to make sure your only decelerate the gas once (just downstream of the valve following sonic blowdown, and re-accelerate the gas only once (at the turbine nozzle). Typical turbine A/R nozzle sizes can be complicated from an actual gas flow vector standpoint, but if you measure the cross-sectional area of the final fully enclosed gas passage in the turbine housing (sometimes called the T-T section), you will get the most applicable nozzle size dimension in a "nozzle-less volute" type turbine housing. Exhaust bypass port placement can also have a big impact on disturbing the flow field into the turbine stage.

 

[riceburnerfb1]

Thanks for the reply Turbomotor,

After reading it I figured that I jump up at just the right time some of that might just graze just the top of my bald head.

s.r.

 

[turbomotor]

Riceburner,

Sorry - I guess I wasn't thinking through my thoughts very carefully, or how to say them. What I was really trying to say is pretty much what everyone generally says, that is, don't make any unnecessary or drastic changes in the exhaust flow area from the exhaust port to the turbine inlet section.

But although everyone recommends this generally, the intermittent flow and temperature of the exhaust usually makes the system complicated enough so that the most reliable solution is to make some parts and run some back-to-back tests.

The data you already have with the 1.625" is far more instructive than anything I can add...

My suggestion would be to stay with the 1.625" and not increase size to the 1.75", but my suggestion is based on the thought (as you pointed out) that the larger pipes may kill the velocity head. But I really don't know if 1.75" is too large, and the only perfect solution is to test both.

Dick