Inliners International Forums General Discussion Blown and Injected Turboing a 250 for 250-350 HP

I sold my nova over a year ago, and bought a truck with the money I had saved. Now my truck is kind of a pain, and that is another topic entirely.

There is a slight chance I have at buying my old nova back, with the straight six. If I do, I wonder what kind of power/mpg output I can look for with this combination:

250/Turbo, 200r4 tranny, 3.55 gears.

Also interested in a good rebuild setup; Which I am searching for, but would like information in this so I can reference it easier.

I use to post on here, but since I have lost all my sites that I referenced, and have not sharpened up on the knowledge I once learned nearly 15 months ago.

There are many on here that have done it. It works really well and you don't have to lean on the engine as hard as you would if it weren't turbo'd. Probably would be cheaper than trying to get 350 HP out of the engine without a turbo! I'd say go for it....

Do you have Leo's book? I would get Larry's manifold (exhaust, for a turbo, check the classifieds, on the left). Might ask him about what he thinks you should do for that setup.

3.55 and the OD in a 2004R is kinda steep, be carefull as too steep a gear will kill economy just as horribly as too short a gear will (but at least the short gear would have a performance advantage )

Hey Nexxussian, you saying that the OD rate would make the engine lug too hard, even with a turbo?

Depending on tire OD,355's might be a bit high.

If you are running a tire that is 24 inch height,that might work w/355's. Also,, you talking being carburated?????

A nice set of 373's would probably work better,but again,depends on what size tire you will use.

Buick GN's have a 200R4 a 342 rear gear,but they are fuel injected & are sequential SP fuel injected (better for emmissions).

I would think running a stock 250 w/an OD trans,you could get 20-25 MPG.

I got 21 MPG w/my 250,Carter 400 AFB ,4 bbl Offy intake,5 speed manual 10-1 compression,510 lift 230 degress duration @ .050 approx(it's been a while)Driving on the freeway to Famoso dragstrip.Bakersfield.


MBHD

Tire size has alot to do with it. IIRC the Buick GNs had a 24" (ish) tire with their 3.42s.

To know you would have to have the expected vehicle weight (best SWAG), tire size, overdrive ratio (.67 for the 2004R IIRC), and some idea how much drag you have (if you plan on driving on the highway). The drag only has to be a general idea, I have a Model A, it drives like a parachute at speed, a Golden Hawk (just to pick an example) would be much more aerodaynamic (so, what was it you are puting this in, did I miss that?).

Lugging is lugging, a turbo will not help that. For purpose of clarity, for this discussion, I consider lugging anything that would require you to run with the power enrichment on during cruise (carb or EFI). A turbo (or any forced induction) will only require further enrichment.

That would be cruise on more or less level ground, no excessive headwind, not 'drafting' a truck, just regular cruise driving.

I'm not saying you shouldn't use a turbo, I'm just saying you should be careful to match the gearing to the aplication.

I asked alot of people I trusted what gears I should run, and got more or less the same answer (3.55 as the highest number). Pops about had a coronary when I told him I had 3.70's, telling me I shouldn't expect any economy at all (because I would be winding it too high). He was right about the economy (in 5th) but for the opposite reason.

That problematic advice seems to be standard, it's a throwback to 1:1 final in the trans and 23-25" tires on the drive end (I've got 27.8" on the A, call them 28). It goes right along with the whole 'if some is good, more is better' philosophy. As usual, a properly matched setup performs better.


FWIW I'm trying to scrounge a set of 4.11s or 4.30s (9" ford, 31 spline) to try as with that tall a tire, not only is it too tall geared at cruise, it's too tall in 1st, I'm favoring the 4.30's (because of the extra drag, a few more RPMs with the throttle more closed will tend to give better economy). Of course ther's no real way to tell for sure, untill you screw everything together and drive it. Replicating a similar build is close, but unless the build is identical there's no gurantee.

neonova, i run a 700r4/3.55 combo. the o/d in the 200r4 is taller than mine. i also run a 27" rear tire. your car may like a 3.73 better. mine runs 2100 @ 70 mph. the turbo does not kick in at all on highway.
you should expect 250-300hp from a turboed 250 and still be very street worthy. mileage will suffer some. i get get 18 mpg @ 70 mph also.
parts can be had at many sources.
i like:
comp cams
forged pistons with a turbo type top ring land (custom)
modify the head (machine shop)
clifford or offy intake (offy for the 250)
modified carb (lots of books on how to do this)
control the timing ( i can help here)
the rest of the engine parts are standard fair


tom

If you didnt see my other post. Talk to greybeard. he has a turbo setup that came off a 250/nova, claims its rebuilt w/t carbs...the price seems right to me and it is slick as can be, he can give you some times in the quarter on a nova, which might interest you.