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Draw through carb and carbon seal ponderings


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I´m very aware that there are many topics and projects that have something to do with draw through turbo systems, but I was just wondering if there are some basic guidelines to how it should be done?

Carb size for example; what size works best for given cc? Street or drag?

Carb style; flatslide or round? Does a cv carb work at all?

I´ve seen people use S&S Shortys, Mikuni HSR 42´s and whatnot. What would you recommend, and why?

This week I´ve been mostly interested in a streetable carb for a 750cc bike, do you have any recommendations?

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Draw throughs are pretty 'old tech' these days - nothing wrong with that but they are hard work to get them running well. A 750 is quite small and to find a suitable turbo will be tricky as most info will be for litre / 1100+ sized engines - the turbo must be able to be fitted with carbon seals to minimise a 'smokey joe' on the over-run. HSR40 may be ok for a 750 - the larger 42 / 45 / 48 HSR's, 1.7/8", 2.1/8" S&S's are definitely better with the bigger capacities up to 1500+ cc! The HSR's seem to work better for a road bike - they seem to fuel better - S&S are great when drag racing.

If I was to do another one, I'd junk the carb and fit a throttle body in its place and build injectors into the plenum to fire directly down the inlets, add a BOV on the plenum as a priority valve and allow use of a modern BB turbo and decent accurate fuel / ignition control - 'old skool' looks but with decent manners - lovely! 

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I was googling non water cooled injected bikes and came across a similar mentioned setup off the 07-16 XJR1300 where the injectors are separate to the throttle body.

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I Don't know the spacing or sizes.

 

Cheers Andrew

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Draw through setups really died out when fuel injection came into play. There are very few draw-through turbo's made today especially to fit a 750cc engine. Though there are older solutions that would work quite well namely the Rajay's. A Rajay B25 would be what you want they are good for engines up to 1000cc and can efficiently flow up to 20lbs. I have one off a running bike if you decide to go down this route it's an ATP badged unit as well which are hens teeth these days.

An HSR42 or a Super B/E would suit the setup well. Either can be setup and work quite well just try to avoid the Bendix carbs if you decide to go with an older style setup they make the bike a chore to ride on the streets.

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10 hours ago, Gixer1460 said:

Draw throughs are pretty 'old tech' these days - nothing wrong with that but they are hard work to get them running well. A 750 is quite small and to find a suitable turbo will be tricky as most info will be for litre / 1100+ sized engines - the turbo must be able to be fitted with carbon seals to minimise a 'smokey joe' on the over-run. HSR40 may be ok for a 750 - the larger 42 / 45 / 48 HSR's, 1.7/8", 2.1/8" S&S's are definitely better with the bigger capacities up to 1500+ cc! The HSR's seem to work better for a road bike - they seem to fuel better - S&S are great when drag racing.

If I was to do another one, I'd junk the carb and fit a throttle body in its place and build injectors into the plenum to fire directly down the inlets, add a BOV on the plenum as a priority valve and allow use of a modern BB turbo and decent accurate fuel / ignition control - 'old skool' looks but with decent manners - lovely! 

Thank's for the info.

The reason for the interest in draw through setups was the simplicity compared to fuel injection and the placement of the turbo. And of course the fact that it is truly old skool. Power increase isn't that relevant, the puny 750cc engine is not the best foundation for big power.

I saw someone using an IHI RHB5 VJ11 in a draw through setup, there was no mention of changing the seals. Maybe he did change them, or he has a smoke machine.

Just the idea of a system without added fuel or scavenge pumps and a semi-hidden turbo is intriguing.

 

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Draw Thru may be a bit agricultural in today's age of fool injection and clever black boxes 9_9

But they make a lovely noise..:banana:

S&S on Garrett T3 into a 1216 oil boiler  250 beans in round no,s 

 

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Posted (edited)

That does sound nice!

The problem really seems to be finding a small enough turbo that could be converted to carbon seal type.

I saw a T25 from a Saab 9000, perhaps buy one from where this world wide flu came from and maybe one could fit a cheap carbon seal back plate on to it?

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That map doesn't seem that much off, flow and pressure ratio are nearly acceptable. Ofcourse it is very unlikely that a chinese turbo would perform as that map indicates.

Edited by Samikoo
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6 minutes ago, no class said:

the only cv carb with accelerator pump that I can think of ..... is the first gen twin cam ditch pumps....

My bad, those were supposed to be two separate questions. Cv carbs in general in a draw through system, and flat or round slide carbs with accelerator pumps vs. ones without.

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In theory you may be able to use a CV carb - they are designed to regulate airflow based on actual engine demand and not throttle position but how they would react once boost overcame vacuum would be anyone's guess! Due to the tortuous inlet tract of a draw through you usually need yo flood the plenum with fuel to get the damn things to fire up so a carb without a accel pump will be at a disadvantage IMO. Flat, round or square doesn't make much difference except on hi boost applications as the actual airspeed pushes on the slide 'wedging' it open - it's why you need strong return springs or a twin cable throttle to pull it shut!

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Posted (edited)

That's what I had in mind when I thought about the cv carb; it could possibly make the bike more streetable... Plus they are selling these knock off Keihin 40mm cv carbs here new for 240€, and these have an accelerator pump too.

Could you use a manual primer pump with a check valve connected to the plenum to aid in starting if a carb wasn't equipped with a pump?

Edited by Samikoo
Fixed carb brand
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Why would a CV carb be fitted with an accelerator pump as by its very function, it won't allow more air to be admitted to match fuel flow? And why faff around with primer pumps etc - that's what choke systems are for!

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Posted (edited)
1 hour ago, Gixer1460 said:

Why would a CV carb be fitted with an accelerator pump as by its very function, it won't allow more air to be admitted to match fuel flow? And why faff around with primer pumps etc - that's what choke systems are for!

Beats me, for some reason that Harley Keihin-copy has an accelerator pump. Can't see what good it would do, it would just enrichen the mixture.

I thought you earlier implied that you need to flood the plenum by squirting fuel with the accelerator pump when starting, that's why I mentioned primer pumps.

I need to go through my stash of turbos to find one that's closest to a T2 so I can mock things up on my frame.

Edited by Samikoo
Fixed the carb brand
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1 hour ago, Samikoo said:

I thought you earlier implied that you need to flood the plenum by squirting fuel with the accelerator pump when starting, that's why I mentioned primer pumps.

I need to go through my stash of turbos to find one that's closest to a T2 so I can mock things up on my frame.

You just have to look at how the fuel gets to the cylinder from the carb - it gets centrifuged out through the turbo then dumped into a box with generally poor airflow paths, make a couple of 90 degree turns and it gets to the valves - hence why a wet mixture pre start is needed. Just use the 'right' carb and forget about complicated add-ons. Good turbo the T2, that was my first build - 1186cc, 10lb boost = 210hp S&S Shortie Super E. IHI's are good as well - i'm sure there was a 750 turbo build in the old Turbobike magazine - i'll dig it out and scan it, but I seem to remember it being a blow through!

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10 hours ago, Gixer1460 said:

You just have to look at how the fuel gets to the cylinder from the carb - it gets centrifuged out through the turbo then dumped into a box with generally poor airflow paths, make a couple of 90 degree turns and it gets to the valves - hence why a wet mixture pre start is needed. Just use the 'right' carb and forget about complicated add-ons. Good turbo the T2, that was my first build - 1186cc, 10lb boost = 210hp S&S Shortie Super E. IHI's are good as well - i'm sure there was a 750 turbo build in the old Turbobike magazine - i'll dig it out and scan it, but I seem to remember it being a blow through!

Thanks! The problem with the turbo is, as you stated earlier, to find one small enough. And to find one that can be converted to carbon seal.

BTW, is there something special about these draw through manifolds been always really boxy? Or is it just for the ease of manifacturing?

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Try coming up with another shape that has 4 outlets in a line one side and 1 inlet the other (often not in line) is compact and easy to weld or cast? I've seen tube plenums but cutting radiuses and bevels is time consuming and never perform significantly better so why bother? I guess you could try 3D printing one - how it would withstand heat and pressure could be an issue?

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11 minutes ago, Gixer1460 said:

Try coming up with another shape that has 4 outlets in a line one side and 1 inlet the other (often not in line) is compact and easy to weld or cast? I've seen tube plenums but cutting radiuses and bevels is time consuming and never perform significantly better so why bother? I guess you could try 3D printing one - how it would withstand heat and pressure could be an issue?

Yeah, seems logical. And as there isn't any aid to performance, I see why they are built the way they are.

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Posted (edited)

I found a Garrett T15 (it says so on the side, must be a GT15 as so the part number dictates) from my stash. Based on compressor maps and the rated power of the cars that had this, this might be a feasible starting point. Now if I could just find some info whether or not this can converted to carbon seal.

Edited by Samikoo
typo
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Draw through manifolds have the boxy shape because of the lack of space as well. Not to mention you want the fuel to stay suspended in the air which becomes harder the longer the intake track is. I'd be interested to learn the effects of a split plane draw-through intake vesus single plane. The old ATP kits usually had a split plane just at the inlet whereas the Mr.Turbo manifolds were generally single plane all the way through.

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Posted (edited)

I just got an e-mail from a turbo service company, he said that if he remembers correctly, the Garrett T2-T28, TO3 and Toyota CT26(and CT20 with CT26 dimensions) would be the ones you could retrofit carbon seals quite easily. With enough machining and mismatching I'm sure many others would too, but that's a whole different road.

Vintage Rajays and such are ofcourse an option, but I'd rather not spend a lot on the first attempt. Later on, if it works, and when the motor would be able to take more of a beating, then it would be possible to upgrade to a bigger and a more expensive unit.

Edited by Samikoo
grammar
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  • Samikoo changed the title to Draw through carb and carbon seal ponderings

Today I picked up this T25 from a Saab 9000 for 20 euros, I'll have to see how it fits in there when I get all the stuck bolts undone and can clock the housings.

This one should be easy to convert to carbon seal, we'll see...

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  • 3 weeks later...

So, I was just wondering what diameter link pipes people are using on draw through exhaust? From the 4-to-1 collector to the turbine housing flange that is.

It seems to me that 2" would be overkill, as the distance is quite long and you can't lose a lot of speed from the exhaust gases as the temperature will drop dramatically over the length of the exhaust tract, therefore not giving the turbine the energy it needs.

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Posted (edited)

1.5" or 38mm OD is what I used on mine and it worked OK! This kit was used on my 1186 Kawazuki - the picture was a trial fit on my drag motor that I was thinking about using with methanol - it was high comp hence the alcohol LOL!

GSX1360 turbo 009.jpg

Edited by Gixer1460
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