Throttle Body too large?

[AdrianC]

I've got a throttle body sat here - 48mm off a Metro 1.4 K-series - but I can't help feeling it might be a bit big as a replacement for a 26/35 twin choke Solex. This is for a two-pot 602cc Citroen.

What problems am I likely to get if the TB is too large? It shouldn't be like a carb where too big = lousy air velocity = lousy fuel delivery. Should it?

[Bernard Fife]

Adrian,

It's not quite the same, since the fuel delivery is always good with EFI (well, mostly). The problem with too large a TB is that at low rpm, you go from low kPa to 100kPa with very little throttle movement, making driveability 'worse'.

For example, on my car, with a 900 cfm TB (4 x 1.750"), I can easily get 100 kPa at 20% throttle at 2000 rpm. This means if I want to hold it at 40 kPa for cruise, I have to be very steady, and if I sneeze, I burn rubber!

Now that's not a problem for me, I like the feeling (it feels like you have an incredible amount of torque on hand)! But my grandmother definitely wouldn't like it.

Lance.

[AdrianC]

It's not quite the same, since the fuel delivery is always good with EFI (well, mostly). The problem with too large a TB is that at low rpm, you go from low kPa to 100kPa with very little throttle movement, making driveability 'worse'.

For example, on my car, with a 900 cfm TB (4 x 1.750"), I can easily get 100 kPa at 20% throttle at 2000 rpm. This means if I want to hold it at 40 kPa for cruise, I have to be very steady, and if I sneeze, I burn rubber!

Now that's not a problem for me, I like the feeling (it feels like you have an incredible amount of torque on hand)! But my grandmother definitely wouldn't like it.

With 602cc and 30bhp as standard, I *really* don't think melting tyres accidentally is going to be a problem... The throttle's binary anyway, if you want to make any decent progress!

Thanks. I'll stop worrying.

[ami8break]

Hello Adrian,
I'm afraid my reply isn't up to date....

I wonder that there is no formula of TB diameter, I'd estimate that max hp would require an amount of air, proportional to fuel injector flow (AFR) and a an average air speed velocity should not be exceeded (friction --> pressure losses) and not fall below another value to keep car driveable (for me and Lances grandmother ;) ).

I don't know 2CV runners' diameter by heart (34mm?) but this should be a well working minimum TB diameter since on a 4stroke twin only one cylinder sucks in air at the same time (with breaks between!).

I've got a TBI unit from Suzuki Swift 1.0l (~1989), a guy told me that even the 1.3l release had the same TBI Ø. 38mm suits very good to 2CV/Visa manifold, biggger Ø would require an adapter for smooth flow. I like also the Suzuki air filter which looks stock (for an untrained » eye) but has a paper inlet which works not very well with the OEM 2CV crank case valve.
Also I found 2 Daihatsu Charade 1.0 TBs with Ø of 40mm which are pretty small - maybe for single TBs one day....

I'd look for any small TBI engine on scrapyard, maybe Fiat Panda had the smallest one you can find in Europe.

»Horst

[donjr5]

The main problem I had with a too-big throttle body (mine is a TBI setup) is that tip-in was hinkey. Alot of vacuum resistance against the throttle plates when pressing the gas pedal from idle caused a low RPM bog.

[the bubbler]

How about a TB from a Smart Car, Daewoo Matiz, or Hyundai Getz, or perhaps a Fiat Seicento or Cinquecento?

Or perhaps you could modify the standard carb by blocking off the various orifices and mounting the injectors in the manifold.

Andy

[bobnova]

Fuel injected suzuki 1 liter three cylinder (geo metro) throttle bodies are pretty dinky, probably about the right size.
You could get a set of ITB's off a motorcycle, and just use one throttle body.
Or you could make up a restrictor plate for the TB.

Isuzu trucks used a two-butterfly throttle body for a while, you could modify one of those to keep the second plate closed all the time.


I think a geo metro 3cyl 1liter TB would be the best bet though.