Too large of a throttle body?

[lagfish]

Hi everyone,
Will I run into problems if I use too large of a throttle body? The intake port on the engine I want to convert is about half the diameter of the throttle body I want to use. If I machine a funnel that tapers from the throttle body diameter to the engine port diameter, will I run into any problems? Some problems that I think might occur are:-
-Difficult to control throttle, because the throttle at "half open" might be full throttle. This isn't that big of a concern to me since I can just map the engine to inject the full throttle amount at half throttle and lose some resolution.
-Air isn't moving fast enough through the large throttle body to "carry" the fuel and air mixture

By the way, going with a smaller throttle body would be ideal but isn't an option for me.

Thanks and I look forward to your inputs!

[Mike_Robert]

Search the forum for the term "ITB". It is doable but you have to be intuitive as to how the engine responds to tuning changes. Your biggest hurdle may be AE; TPSdot typically doesn't work well in this scenario and MAPdot requires a fairly clean MAP signal to work correctly. It can be done - I have a Mazda 12A rotary with a 2x50mm TB running fine.

-Mike

[lagfish]

Hi,
Thanks for the reply. Sorry, I'm new to this, but I read the manual.
Did you mean TBI? What's AE?
Did you find that you had a good improvement in power and efficiency when you switched over from a carb? Thanks

[Mike_Robert]

Hi,
Thanks for the reply. Sorry, I'm new to this, but I read the manual.

Nothing to apologize for!

Did you mean TBI? What's AE?

ITB = Individual Throttle Bodies; after rereading your message it's now clear to me (I think) that you are referring to a larger throttle body supplying a plenum manifold. The potential problems you described are a concern but can be worked around. I also think you're referring to TBI, right? If so, at idle and very low throttle opening angles you'll probably be OK as far as fuel atomization is concerned as the fuel is basically dropped onto the top of the throttle plate and sucked around the edges atomizing it pretty well. All throttles are non-linear in respect to plate angle, MAP, airflow and engine demand. A large TB in relation to actual engine size exacerbates that nonlinearity but, if somewhere within reason, can be tuned around and compensated for. AE = Acceleration Enrichment, a process providing extra fuel in certain regimes similar to the carb's accelerator pump.

Did you find that you had a good improvement in power and efficiency when you switched over from a carb? Thanks

Assuredly YES. 22mpg vs 28mpg, 135 HP to 187 HP - though other things were done at the same time (exhaust, portingm etc.) so this is not a direct comparison. The MS is an integral part of my entire powerplant scheme and has worked well. EFI is just another method of metering fuel into an engine just like a carb is - but EFI is pretty much infinitely adjustable and can compensate for the carb's shortcomings. Jets, emulsion tubes, pump springs, check ball sizes and springs all go away. That said, a poorly tuned EFI system is just as bad as a poorly jetted carb. The user/tuner must understand the physics involved otherwise he will likely be at least disappointed and possibly with a sad bank account.

-Mike

[lagfish]

The injector is in a seperate unit after the throttle body. I'm not sure what the term is, I've included a drawing, including the location of the "funnel" before the entrance of the mixture into the engine.

The nozzle of the injector is about 2 inches away from the valve
I'm worried about the fuel sticking to the walls of the intake at low RPMs, since the air velocity is low. I guess this also depends on the spray pattern of the nozzle - if it's more of a wide spray or squirt. I've tried to line up the injector as parallel to the flow as possible, but there's also the chance that the injector doesn't spray straight but at an angle.

[lagfish]

I know it varies with engines and operating conditions, but any ideas on the typical values for x and tau for cold engines on normal summer days? For small values of x and tau, it's probably not worth it right now for me to implement. This engine is a small four-stroke glow plug model aircraft engine that has been converted to spark ignition.

On an unrelated note, this may be a stupid question, but if all I'm worried about is getting a certain fuel/air ratio, why can't I just use a single lambda sensor? Is it because the response is too slow? What if I just used a TPS with a lambda sensor?

[Mike_Robert]

I know it varies with engines and operating conditions, but any ideas on the typical values for x and tau for cold engines on normal summer days? For small values of x and tau, it's probably not worth it right now for me to implement. This engine is a small four-stroke glow plug model aircraft engine that has been converted to spark ignition.

For that application (driving a prop) I wouldn't worry about X-Tau. I have done exactly one airboat engine (Continental 6 cyl) and it was actually very easy to develop maps for as the load is pretty much very predictable. There's no transmission with multiple gears, hills, etc. I'm sure air density has something to do with prop work/load but this guy is still pretty happy with that install for quite some time.

On an unrelated note, this may be a stupid question, but if all I'm worried about is getting a certain fuel/air ratio, why can't I just use a single lambda sensor? Is it because the response is too slow? What if I just used a TPS with a lambda sensor?

Single sensor is fine. Narrow band sensor vs. wide band is something you may want to look at seeing that you'll be at a significant load during operating conditions - stoich (14.7) isn't going to be optimal for that regime. "TPS only" brings you into the alpha-n fueling method. This is environmentally sensitive (temp, pressure, baro) but a MAP sensor on single cylinder (I'm guessing here, model AC stuff encompasses singles to opposed twins to radials to rotaries and more I'm sure..) can be problematic. I might be looking at something similar for a personal project though mine is a 6.5 HP single cylinder generator; haven't had the time to track down a good source for a small enough injector.

-Mike