Why convert EFI to Carbs!?

[LJ851]

I wonder if you can retrofit carbs as a cheap way to get ITB's under EFI control.

Sure, you can just remove the chokes and venturis, as long as you have fuel injectors in the intake manifold.

[u3b3rg33k]

Individual throttle bodies > Single throttle body.

On an N/A engine; 6 bike carbs, or 3 Webers, Dellortos, Mikuni PHH etc will ALWAYS be better than a single throttle injection system.

I'd bet a case of beer that you can't take a stock M20, with a perfectly operating EFI system, factory exhaust and intake, and beat the responsiveness, power, torque, and fuel efficiency with a sidedraft carb setup.

Gauntlet Thrown.

[u3b3rg33k]

Here are some fun facts.

Reason (no resons, just the one) why a carb is better then FI.
Given the distance fuel travels in a carbed manifold it usually has more time to mix with air. This is the sole reason why a carb CAN make more power then port fuel injection at WOT. As it will cause an improved burn in the engine (atomisation)

I think the reason for the hypothetically better atomization is from the pressure drop, not the distance. if you have a carbureted system with ANY angles between the valve and the venturi, then you risk the fuel falling out of the airstream, and travelling along the outside wall. in many systems with single carburetors (take the stock 2002 dual barrel carburetor setup) this can result in cylinders #1 and #4 getting slightly more fuel than #2 and #3, but only after you've reached equilibrium with the amount of fuel deposited and the amount of fuel evaporating off the sides.

In some setups, like those found on older engines with twinned intake runners, this can result in the need for hotter sparkplugs on two cylinders if the engine is not run hard regularly, as it will cause carbon buildup and fouling of the richer cylinders. Here, an injector facing down each runner would provide more even distribution of fuel at the turn, and therefore more even fuel mixture than is possible with the carburetor.

This problem is much less of an issue in aircraft, boat, or mower engines, where equilibrium is reached rather quickly and maintained for long periods of time.

[der affe]

I wonder if you can retrofit carbs as a cheap way to get ITB's under EFI control.

You are not going to be saving any money by doing this. Unless you can do the welding and machining yourself and even at that, you are better off starting with properly sized motorcycle throttle bodies and building and intake manifold to match them and the head.

[SkiFree]

Time for another photo break. Just noticed that there are no pictures of an M20 with carbs yet....

We'll fix that, have a couple shots (both DCOE and IDF's Weber carbs shown below)...














And some ITB's, scratch build or otherwise




Extrudabody

S54 throttle bodies


(if any of these pictures were taken by yourself and would like them removed, please simply PM me)

[gstuning]

I think the reason for the hypothetically better atomization is from the pressure drop, not the distance. if you have a carbureted system with ANY angles between the valve and the venturi, then you risk the fuel falling out of the airstream, and travelling along the outside wall. in many systems with single carburetors (take the stock 2002 dual barrel carburetor setup) this can result in cylinders #1 and #4 getting slightly more fuel than #2 and #3, but only after you've reached equilibrium with the amount of fuel deposited and the amount of fuel evaporating off the sides.

In some setups, like those found on older engines with twinned intake runners, this can result in the need for hotter sparkplugs on two cylinders if the engine is not run hard regularly, as it will cause carbon buildup and fouling of the richer cylinders. Here, an injector facing down each runner would provide more even distribution of fuel at the turn, and therefore more even fuel mixture than is possible with the carburetor.

This problem is much less of an issue in aircraft, boat, or mower engines, where equilibrium is reached rather quickly and maintained for long periods of time.

It´s distance, the same reason why it´s done on FI engines as well. And its not hypothetical.

I´m not comparing any carb setup to any fi setup. I´m comparing the best solution of each, straigh carbs vs straight ITB´s.

[gstuning]

I'd bet a case of beer that you can't take a stock M20, with a perfectly operating EFI system, factory exhaust and intake, and beat the responsiveness, power, torque, and fuel efficiency with a sidedraft carb setup.

Gauntlet Thrown.

Stock M20´s have the WORST throttle response, no matter how good they are. You only need to run any ITB setup and see the difference.

[u3b3rg33k]

Stock engine, never said stock metering.

[nando]

the AFM isn't the primary cause of slow throttle response on an M20.

[u3b3rg33k]

Maybe not, but a worn out 25 year old AFM sure doesn't help things when you compare it to a MAF.

The only way you'll catch me running an AFM again is if it suddenly dies and I need to get home - I keep the old system in the trunk for emergency use.

[nando]

the best thing for throttle response is nothing at all. ;)

[u3b3rg33k]

valvetronic?

[nando]

no, I meant alpha-N or MAP or a combo of the two

valvetronic is cool but it adds valvetrain mass.

[slammin.e28]

Just throw on a slide throttle with triple Webers. BAM!

[LJ851]

Just throw on a slide throttle with triple Webers. BAM!

Double throttles?