Low turbo on a DD V-6 in an MC5c

[luvrbus]

I don't how well that will work out for you the air horn on 6v92TA is only 3 inches O.D with 2.5 turbo I.D it's only 3.5 inches for 500 hp 8v92 I seriously doubt you can get boost using that big of a inlet 

[Craig R]

I'm confused by your observation, luvrbus. what I'm looking at in my engine bay is a naturally aspirated 6V71 hooked to an air shut-down housing with an inlet fitting pointing toward the side. The fella I was talking to indicated that the air shut-down housing & blower on a V92 have a different bolt pattern that won't fit on a V71. 

[luvrbus]

I don't where he is getting that from the blowers are tapped for different bolt patterns is yours 4 bolts? ,on the blower there are 2 rows of tapped holes the inside row is for a 71 series the out side row which has more tapped holes are for the 92 series  

I use the high capacity blower with a bypass off the 92 series on a V71 all the time even being a 1/4 inch larger they work. I never remember seeing a bypass blower made for a 6V71 either we always used one from a 6v92 so I don't know really what he is saying  

[Craig R]

what's a bypass blower? 

[eagle19952]

what's a bypass blower? 

one with a hole in it 

[bevans6]

You might have two air intakes on that blower top, around 4" diameter, going over to the stock air cleaner array (plus the air compressor draws clean air from the air cleaner before the blower top).  To be honest, the air intake is not a big problem, a fabricator can make you one in a day.  The large size of the intake, or even the two intakes, is an issue in that the air from the turbo needs to keep all of it's velocity, and the larger than ideal size slows the air down and that kills boost.  The turbo to intake tube on my 8v-71T is a single 3" tube.  You can take that air intake, cut off the air intake tubes and weld on a 3" one where you want it.  That is what I did on my engine.

The bypass blower thing is a different question.  I personally do not have a bypass blower on my engine, and it did not come with one stock as a military turbo engine, so I am certain that it is not actually needed.  What the bypass does is pass air from the intake side to the air box side of the blower to equalize the pressure from the turbo on both sides of the blower, effectively stalling the blower.  A stalled blower takes only mechanical effort to turn since it is doing no work, so you save the 10 or maybe 15 hp it would normally take to generate the 5 - 6 psi of boost that it normally produces.  My engine, it needs all the boost it can get, other engines have different turbo's that create all the boost desired on their own so they can save some of the HP to turn the blower with the bypass.  The blower is un-stalled at low rpm and during start-up, since the turbo doesn't do anything to speak of at those times, so the blower still creates the couple of PSI of boost to run the engine at idle and startup.

Many things to thing about with a turbo conversion.  Hanging the turbo and running plumbing to it is perhaps the most obvious but not the least of the issues.

Brian

[Craig R]

The air shut-down housing on my NA engine is wedge shaped -- left side 3 or 4 inches tall, right side ~1 inch tall. It appears to be torqued down onto the top of the blower by six bolts; I'm not able to see another row of tapped holes in the top of the blower. The air inlet hose is clamped onto a round fitting that's part of that housing -- rather than onto a seperate air inlet housing that bolts to the tall side of the air shut-down housing. figure 2 on page 3 of section 3.5.2 of the March '91 revision of the DD V71 service manual shows an intercooler equipped 12V71 with a pair of wedge-shaped air shut-down housing, each hooked to a seperately-cast side-mounted air inlet housing.

[luvrbus]

It maybe a revision manual in 1991 with a old illustration the 71 never used a air shut off since 1985 till they left the market in 1995 

[Craig R]

  Sorry; it was a March '81 revision -- and page 3 of section 3.5.2 had "December 1980" on the bottom margin. 

[Craig R]

I'm getting confused by advice about air horn or air intake tube diameters. Is the tube on a naturally aspirated air intake housing larger than the one on an intake that connects to a turbo? Can somebody tell me what the I.D. & O.D. are for the intake on a V71na and a V71TA?

[bevans6]

Dimensions are "about", since the NA MC-5C intake left my hands about 4 years ago and the turbo intake is outside in the shed on the bus, and it's dark and cold out...  NA 8V-71 in my MC5C had two 4" tubes from the air cleaners.  Turbo intake has a 5" intake from the air cleaner and around a single 3" pressurized tube from the turbo to the blower top.  NA engine had 4" exhaust from the manifolds to the Y-pipe into the exhaust, which was a 5" input to the muffler.  The turbo exhaust is 3.5" from the manifold to the turbo and  a 4.25" output (ish) from the turbo which is routed to the same 5" input to the muffler with a 5" pipe all the way.

The point is that Bernoulli's law says that increase in volume creates a reduction in velocity of an air flow.  To keep the highest energy in the air intake and in the exhaust to the turbo, smaller tubes make the gas flow retain higher velocity and energy, resulting in higher boost.  In a natural engine, large volumes reduce "friction" and let the air flow more freely, both on the intake and the exhaust.

So to summarize - NA engine had two 4" diameter tubes from air cleaner to blower, while the turbo engine has one 3" tube from the turbo to the blower.  NA engine had 4" exhaust from manifold to the Y-pipe, and 5" into the muffler, while the turbo has 3.5" from the manifolds to the turbo, and 5" from the turbo to the muffler.

Brian