Recommended reading: turbos, carbs, & blow-through (not

[MJJ_ZX6RR]

Afternoon all,

I am thinking about a winter project to turbocharge a carb fed bike (4 cylinder, 4 carb) of mine. I have never built any sort of forced induction engine, but I see this as a good way to learn welding, learn forced induction, and have a nice project for the long winter nights.

I have A Graham-Bell's "Forced Induction Performance Tuning" book, and am in the middle of re-reading it. The author does not seem to cover carbs at all, except a single paragraph that says essentially "trust me, do not blow through the carbs, use the turbo to pull through".

Having to engineer the turbo between the carbs and the intake (so it pulls through) is significantly more work than simply feeding a plenum and keeping the carbs between plenum and intake.

Anyone got any advice, or reading recommendations to understand more about why blow-through the carbs is not recommended?

Martin.

[maxfield]

Use a Renault 5 carb

Carbs don't like pressure and when pressure is put through it the jetting alters. Can't really remember much about, get googling.

[TouringTash]

are you going the lengthen the swingarm?

[MJJ_ZX6RR]

are you going the lengthen the swingarm?

No, I am not trying for silly horsepower, more about proving to myself I can do this and hopefully beefing up the mid-range a little. The bike is a big heavy tourer with plenty of weight keeping the front end down.

Martin.

[UweM3]

are you going the lengthen the swingarm?

No, I am not trying for silly horsepower, more about proving to myself I can do this and hopefully beefing up the mid-range a little. The bike is a big heavy tourer with plenty of weight keeping the front end down.

Martin.

the "problem" from what I found out about turbo charging is the amount of horsepower you will end up with. If you do it somesort of right, it will produce quite a bit of bhp, if you want or not. There is no just a little bit more please in the midrange.
Do your calcs right, pick suitable hardware and even with moderate boost you will see good gains.

That's why I stayed away from turbo charging my S14.

[TouringTash]

Why not have a look for a set of flatside carbs instead.
Even big heavy tourers can develop a tendency to lift the front wheel without much effort with a snail.

[E30BeemerLad]

Didn't suzuki do a turbo'd GSX in the very early 80's? Would that have run on carbs?

[TouringTash]

http://www.ratbike.org/docs/194.php

he's called Baz

[GeoffBob]

Anyone got any advice, or reading recommendations to understand more about why blow-through the carbs is not recommended?

There is nothing wrong with blowing through a carb provided the float bowl can handle the pressure.

On any fuel injected turbocharged engine the fuel pressure at the injectors is typically required to be 3-bar greater than the air pressure in the manifold. This means that the absolute fuel pressure must rise and fall with the manifold pressure. This is achieved by means of a fuel pressure regulator (between the fuel pump and the fuel rail) which is referenced to the manifold pressure in order to keep the pressure drop over the injectors at a (typically) constant value.

Now, the same is true of a blow through carb, with the exception that the float bowl must be pressurised to the same value as your turbo compressor outlet pressure. The reason for this is because a carburettor uses a venturi (following Bernoulli’s principle) to create the pressure differential that sucks fuel from the float bowl. So long as the air pressure at the mouth of the carb is the same as in the fuel bowl (which is quite naturally the case on a normally aspirated engine) then the fuel jets will function more-or-less as designed (with an inconsistency resulting from the fact that the pressure differential that results in the venturi is independent of the density of the inducted air). Problem is, pressurise an unsealed float bowl and it will p!ss fuel everywhere, usually causing a fire.

BTW, the 2nd generation R5 Gordini (post 1985 model) had a blow-through carb system that had a habit of catching fire. This is why A. Graham Bell warns against blow through carbs. The earlier R5 Gordini (pre ’85) had a draw-through carb system which is infinitely preferable. Long before the days of electronically controlled fuel injection your only choice was either a blow-through or draw-through carb system.

So long as you can find the right carb (with a tightly sealed fuel bowl) you can do it, but would you want to risk setting your legs on fire if the seals were to fail?

[MJJ_ZX6RR]

Thank you GeoffBob, that makes a lot of sense and gives me some food for thought.

I can work on ensuring the existing carb float bowls are up to the job, or try and find some throttle bodies off another bike to fit and convert it to EFI.

I like the sound of the EFI conversion, that means I get to learn about setting up aftermarket management too aswell as welding and turbocharging.

So, I see two outcomes here:
1) I learn a lot of very valuable skills, suitable for my ultimate aim of supercharging my E30 335i
2) I bankrupt myself turning a perfectly good motorbike into a non-running, bling laden worthless garage ornament.

I am going to focus on option 1) for now

Martin.

[GeoffBob]

I can work on ensuring the existing carb float bowls are up to the job, or try and find some throttle bodies off another bike to fit and convert it to EFI.

EFI would definitely be the way to go. Learning how to set up an ECU on a bike would stand you in good stead for doing the same on your car. One of your biggest obstacles on a bike is, however, trying to find space for all the performance goodies. Regardless of whether you go EFI or keep the carbs, you'll still need an electric fuel pump (preferably submersible so that you can locate it in your tank). There are a few compact ECU's out there as well that don't need an entire dash to hide behind.

Don't underestimate the size of the task though, a bike may be smaller than a car but you still have to fit and/or modify all the same bits as on a car engine. In your case especially so since you state that you have a 4-cylinder engine. Not to mention that you will have to do everything in miniature so as to get it to fit.

[Gunni]

For a bike even a SMT6 can do the fuel as it can run few injectors.

But I´m with GeoffBob on his comments.