fuel injection vs the carb thing

mbrooks · Post by **mbrooks** » Fri Feb 01, 2008 4:29 pm

i haven't made up my mind yet, but is there a difference in cam grinds between fi & carb setup?, also, if using fi, as in throttle body with injectors at end of manifold runners, are the heads sized differently vs. the carb setup?

thanks for all your insight.

Warpspeed · Post by **Warpspeed** » Fri Feb 01, 2008 5:15 pm

Not really. Runners, ports, and cams are all mainly about getting enough air into the engine. Cylinder filling, or volumetric efficiency in other words.

The fuel system just adds the correct amount of fuel to this air. But power, and the shape of the torque curve are decided by the amount of air going into the engine.

David Redszus · Post by **David Redszus** » Sat Feb 02, 2008 11:58 am

There should be a difference in cams for carbs vs fuel injection.

But central point (throttle body) injection performs just like a carb and so a can made for a carb should work.

If the engine has sequential port injection, with tuned runners, then its a differenct story and a different cam.

Warpspeed · Post by **Warpspeed** » Sat Feb 02, 2008 5:03 pm

David Redszus wrote:, then its a differenct story and a different cam.

Why is this ?
And what changes do you make to the valve timing ?

David Redszus · Post by **David Redszus** » Sat Feb 02, 2008 7:04 pm

Warpspeed wrote:
David Redszus wrote:, then its a different story and a different cam.

Why is this ?
And what changes do you make to the valve timing?

The critical valve events are the overlap period. With carburation, the fuel is mixed with the air as long as the air flows through the carburetor. With sequential port injection, a measured, and timed fuel delivery is lost if valve timing is not precise. In fact, the EOI, end of injection, becomes an important mapping parameter. With direct injection, the valve timing is not critical as far as lost fuel is concerned.

Warpspeed · Post by **Warpspeed** » Sat Feb 02, 2008 7:21 pm

I respectfully disagree.

Fuel cannot disappear or appear from nowhere within the intake runner.

Whatever fuel is metered into the flowing air stream must pass into the engine, regardless of what the intake valve does.

If a carby is set up to deliver 12:1 air fuel ratio, and EFI is set up to deliver a 12:1 air fuel ratio, there is no requirement to change the valve timing.

The engine may still behave differently because of better or worse fuel atomization, but that has little to do directly with valve timing.

David Redszus · Post by **David Redszus** » Sun Feb 03, 2008 1:22 am

Whatever fuel is metered into the flowing air stream must pass into the engine, regardless of what the intake valve does.

You are quite correct. But it doesn't have to stay in the engine. The issue does not concern the intake valve but rather overscavenging during overlap. With EFI, if the injection period extends into the overlap period, the fuel does not remain in the engine.
This does not apply to throttle body injection or CIS.

Warpspeed · Post by **Warpspeed** » Sun Feb 03, 2008 2:01 am

Very true Dave,

Exhaust reversion or over scavenging can be influenced my many things, EFI can at least be timed after the exhaust valve has shut reducing fuel wastage.

But that does nothing for the air wastage, and the power that loses. Or in the case of exhaust reversion, EFI can do nothing to reduce it.

mbrooks · Post by **mbrooks** » Sun Feb 03, 2008 8:02 am

so what i'm reading, optimize the cam for airflow as you would a carb, then make sure the injection timing makes use of the cam parameters?

David Redszus · Post by **David Redszus** » Sun Feb 03, 2008 1:25 pm

mbrooks wrote:so what i'm reading, optimize the cam for airflow as you would a carb, then make sure the injection timing makes use of the cam parameters?

Yes. If you have the capability of remapping the injection timing you are on your way. If not, pay special attention to reducing the overlap period but not to the point of reducing performance.

Injected systems often make use of resonant engine tuning which make proper cam timing essential to take proper advantage of the resonant pulses.

Warpspeed · Post by **Warpspeed** » Sun Feb 03, 2008 3:42 pm

mbrooks wrote:so what i'm reading, optimize the cam for airflow as you would a carb, then make sure the injection timing makes use of the cam parameters?

Yes. The valve timing should complement all your other modifications to give optimum cylinder filling over the rpm range required. Carby or EFI will not directly effect the cam choice.

EFI gives some slight advantages if your budget or the class rules allow.

Any advantages of EFI probably have more to do with being easier to tune, improved drivability, and better fuel economy, rather than any large gains in flat out power.