Carb sizing for turbo 250

[Anonymous]

I am trying to decide on a carburetor for my turbo 250 project. The turbo that I am using is from a 3.8 GM in a draw thru configuration. I was thinking of a 500CFM carb, but there isn't one in a spread-bore configuration to match the intake carb pad. So I thinks to myself - the GM must have had some cousin of a quadrajet on it and I thought all quadrajets were in the 750 to 825 CFM range, with real small primaries. Since the 250 is 4.1 liters and the GM is a 3.8, wouldn't a quadrajet be way too big for the engine? I know I could use a smaller squarebore carb, but I really didn't want add to the clutter with another adapter for that.

 

[xctasy]

The Q'Jet is a mission to tune, but is certainly the right kind of carb!

The worst thing you can ever do with a turbo is run it out of fuel if it's a draw through. They used 4MC Q'Jet'S or Rochester 2-BBLS.

Option 1: Some of the 2-BBL Rochesters are a Holley #2300 spacing throttle wise, the likes of a Holley 500 cfm 4412. There are a brace of different kinds, some WITH DIFFERENT BOLT SPACINGS!

Remeber too...Detroit rates most of it's 2-bbls at 3.0"Hg pressure, while the 4-bbls are rated at 1.5"Hg pressure drop, so a 500 cfm 4-bbl is not the same as a 500 cfm 2-bbl...the 2-bbl 500 cfm is really more like 355 cfm. That limits thimgs to around 175 hp as a draw through.

Option 2: Get the 4-bbl carb that came with the Pontiac 301 Turbo or 3.8 Buick Turbo. A 750 cfm Q'Jet can flow enough air to make 460 hp on an Aussie 308 GM Holden V8 race engine, and it wasn't the limiter to the maximum HP!

The four-barrel Quadrajet is not an easy carb to tune, so you will have to work on the hanger and metering rods and mains.

Option 3:The other option is the 430 cfm Holley, which is a little easier.

Option 4: is to use the two 32/36 DGAS or Holley/Weber carbs form a (gasp) 1979-1981 Carby 2300 Mustang/Fairmont Turbo. Mount them on a Clifford style adaptor on either side of the existing YF Carter carb hole. Then use the TRW turbo as a blow through and limit boost to 6 pounds. Should handle it if you boost refrence the throttle spindles, more if you seal the carb. They run richer, and have a good (cheap) parts supply.

A 250 is wasted above 5000 rpm, so 6 pounds and a good turbo will hit the 220 hp barrier pretty easily!

 

[aussie7mains]

one carb that wasnt mentioned that works very well for this is the DCOE weber or DHLA dellorto, they flow very well and are infinately tunable.
A7M

 

[Anonymous]

Now the Q-jet is a pain to tune but there are some books on the subject.

It's not oversized because the engine only sees the tiny primarys untill the demand for more air and fuel makes the secondarys open.

Just run the secondarys a little rich, and it should be fine.

John

 

[Anlushac11]

8)

I would go with the Holley 500cfm 2 bbl. The 200/250 log head doesnt breath any better than a stock Ford 2.3L head and on a draw through carb turbo one of the guys swapped the Holley 5200 for a Holley 2300 500cfm 2 bbl and he said it ran like a champ with almost no throttle lag.

You may have to get the kit that downsizes the accel pump shot for 4 cyl and 6 cyl engines so it doesnt flood them