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Old 05-10-2004, 01:37 PM
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turbo draw-thru question

ok i have a small project, im going to mount the garett t-2 that i had on my 1.3L vw (i no longer own the vw) on to my newly aquired 1978 Honda CB 750 sohc (F2 model)

the question i have is that the only thing i really have room for is a draw thru system, im goung to use an SU carb (the bell model found on older jags and rangerovers) infront of the turbo inlet and fabricate a small plenum to distribure the charge air, (boost will be about 7-8 psi as the honda CR is 9:1)

somthing like this setup:



my question is: will a turbo made for blow-thru work on the honda, my concern is that when i idle the pressure in the compressor housing will be so low that i could suck oil from the turbo into the intake chage and foul the plugs and deposit detonation inducing carbon buildup.....

the honda has dry sump oiling so oilsupply is not an issue..

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Old 05-10-2004, 03:12 PM
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Wow, that's a great question man. I'm happy to say that you won't have a problem on a suck through system as it's been done plenty of times before. The oil pressures reached inside the turbo can get quite high and the system is designed to withstand outside forces and to keep the oil over the bearings. I'd suggest to make your own puke container/small filter for the engine crankcase to breathe. Otherwise you'd have blowby oil in the turbo duct and you would be questioning where the oil's come from. Hope that helps. Rob
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Old 05-10-2004, 03:29 PM
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thank you for the reply, i just wasnt sure if turbos for draw thru had some special seals or somthing.. and i am allready routing blowby to a catch can instead of the airfilter box, for that very same reason,

what if one were to always add i liter of alcohol whenever filling the tank (yielding 10%) and jetting the carb richer, would that allow me to run more boost???

sort of like integral alcohol injection...
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Old 05-10-2004, 03:38 PM
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I doubt you will have trouble with oil getting sucked in at idle, if you did I would check over the turbo carefully at the ring seal. Better back cut those gear dogs if your going to boost it, I've owned a few of that model of bike and they all suffer the same fate eventually - rounded drive dogs make the bike jump out of gear under load and no matter how strong your foot is holding it in, it will jump out and zing the limiter. I had one with 9000Kms once and it would jump out when it was new, the newer motorcycles (85/86 and later) have much improved drive dogs making them a better choice for this kind of buildup. My 1981 CB750 SS has the same problem to a minor degree if your clutchless shifting.
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Old 05-10-2004, 04:04 PM
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Some turbos for draw-thru do have a special seal on the compressor side to prevent oil being sucked into the intake. You should be fine though. The only time you might have a problem is when decelerating with the throttle closed.
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Old 05-10-2004, 09:31 PM
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Interesting project.....

I have honestly not done a whole bunch of research(have not had to) about draw through setups. Orangef4 is right about the seal, but the real question is what kindof seal does your turbo have.....I have also heard there can also be issues with fuel contamination of the turbo bearings. I can not quantify this, but the idea is that some fuel will pass the oil ring on the turbo shaft and get to the bearings under boost. Of course, I usually like to fail miserably and go down trying so if it was me I would give it a shot.

As for fueling you do want to jet a little rich since with turbo setups you want about 1 point more on AFR for combustion cooling. The alcohol sounds like a sound theory if you can get it to mix. I have never seen what happens when you mix it with gasoline. If it mixes well you should be okay.

Sorry I couldnt help more.....you have an odd project there. Sounds like fun though.

Chris
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Old 05-11-2004, 12:58 AM
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Willys, you are right on the money on the drivedogs in the tranny, the reason im taking the engine out is that my 5.th. gear is chewed up. And since the carbs are acting up, and i have the turbo on the shelf... well i just couldnt help it.

but the 86 onwards doesnt fit the frame as far as i know, so im kinda stuck with this., but could you perhaps tell me how, and how much to back-cut the drive dogs??

but im not going overboard on the boost, the primary drive chain is not good for much over 100 Hp as far as i know so im keeping it reasonable
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Old 05-11-2004, 01:51 AM
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Hey, if pontiac can do it with the turbo 301, you sure can too.
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