MicroSquirt.com read only forum (Jan.15,2018)

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?

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.

lance wrote: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.

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

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.

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

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.