carb size for draw through turbo

[Rustang Racer]

I'm putting a stock grand national draw through turbo setup at around 8lbs on a 250 with a clifford 264H cam. I've heard that for blow through the carb should be about the same size as naturally aspirated but for draw through the carb should be a lot bigger. I have Hugh McIness's book and he makes no such distinction on carb selection though. Naturally aspirated I would use about a 390. Should I go with something a little bigger like a 600 or 750?

 

[Anonymous]

The Rochester quadrajet that originially came with the setup was rated at 800cfm. The secondaries were then pined to limit it to about 650cfm.

One of the performance mods is to remove that pin.

 

[Anonymous]

For a drawthrough setup with 8psi you will want the carb to be around 50-60% bigger, a 600-650 should suffice...... too big and you will loose driveablity when you are not under boost.

 

[Anonymous]

You do not need a larger carb. for DT!
BT ppl will say ANYTHING to make their stuff look more attractive;-)
That being said I'd be looking for 600 cfm or slightly smaller. 390's kinda small even w/out a turbo for performance use.
Why don't you use a q-jet so you can use the stock Buick upper manifold?
If you do end up needing more fuel flow add a nos nozzle & inject alcohal.

 

[Anonymous]

I'm putting a stock grand national draw through turbo setup

Grand Nat's didn't come with DT turbo's. Only '78-'83 T-types, which btw should be a breeze to install on the I-6's fuel log.

Earlier years have larger turbine housings & spool a little slower than the '82-'83's

 

[Rustang Racer]

Yeah it was from an 83 T type regal but i thought the gn's had it too and that name would be more familiar. Guess i was wrong though.

It looks pretty easy. I made flanges for the compressor and intake. They're epoxyed together. I didn't realize that the fuel might eat away at the epoxy until I was already finished.

There was an EGR system in the carb base. I blocked that off but now I'm starting to get worried about carb icing. Would it be a good idea to run engine coolant through the carb base? And how about taking intake air off the outside of the exhaust manifold until the carb base gets heated up?

The turbo needs a rebuild including a new impellar on the compressor side. Thats probably the next thing I'll do.

 

[Anonymous]

Actually, now that I think about it, I am wong. There were a few rare (200 I think) GN's made in '82 & of those I believe only 20 were turbo'd.
The wife reminded me about that.

I own two '83 T-types & have 3 spare turbo's. I've been wanting to stick one on my 200/6 for the longest time now. I just haven't had the time, too many projects. They do look like they were made for each other don't they?!
I wouldn't worry about the egr. Mine have been blocked off for a long time & I've had no-problems.
Buick used to use vacc. to close off (SEVERELY restrict) the air intake trak & they used a heat riser from the exhaust manifold. My '83 carb. bases have an electrically heated gasket to help also.
Literally ALL this stuff has been removed from one of my T's & I don't miss 'em one bit. All just crutches in my openion.
How did you run your exhaust? Under the oil pan & over the valve cover or did you just loop it around on the pass. side?
How did the turbo's outlet size & the intake log's hole match up? I was thinking of mounting it using a thick spacer mounted to the log & tapped for the turbo or tapping holes right in the log for the housing but this would put it lower than I think I want it?
The 82-82 turbo's have the smallest exhaust housings, .62 the earlier ones were .82.
There is a person selling complete chra's on e-bay for around $100. That's prob. the cheapest way to rebuild.
Good luck with your project. You'll probabley beat me done.

 

[Rustang Racer]

I haven't done the exhaust yet but i'm planning on just looping it back up to the turbo, all on the passenger side. I'm doing the least expensive stuff first in case I get bored of the project and decide to quit.

The turbo outlet is bigger than the intake hole on the log. The log is 1.75" for a big log head and I'm guessing the turbo outlet is around 2", maybe even a little bigger. I got an exhaust pipe adapter that was the perfect size to reduce from the turbo outlet size to the log intake size, and made it into a flange that bolts onto the intake. I also made a flange that bolts onto the turbo outlet. I ran a hose that I pulled off a turbo volvo between the two flanges.

The carb is going to sit about where the car battery is now. I'm going to move the battery to the trunk. Hugh McInness's book recommends mounting the turbo higher than the intake but also says that this rule can be broken without major power losses. I have a 250 crammed into a 65 mustang and the aircleaner already touches the hood with a stock carb with no adapters. Having the turbo higher than the intake just isn't going to happen.

I guess I won't worry about carb icing until it happens then. An electric heated carb spacer sounds like a pretty easy fix.

 

[Anonymous]

The carb is going to sit about where the car battery is now. I'm going to move the battery to the trunk. Hugh McInness's book recommends mounting the turbo higher than the intake but also says that this rule can be broken without major power losses. I have a 250 crammed into a 65 mustang and the aircleaner already touches the hood with a stock carb with no adapters. Having the turbo higher than the intake just isn't going to happen.

I don't think there will be any power loss @ all. The only thing I would be concerned with is fuel puddling. Easy fix tho, just add some kinda drain valve @ the carb. intakes lowest point.
Have you considered a side draft carb. mounted to the compressors intake housing? That'd be the most ideal way to go IMO. Too bad they're so damn expensive.