The Stovebolt Site Forums The Shop Area The HiPo Shop How make my 250ci inline6 a bit more peppy?

I have bought a '57 Chevy Suburban that came with a 250ci inline 6 and a Turbo350. It goes well (75/80 mph all day on the highway), but I would like to give it a bit more umph, it is especially a bit slow in town, with a head wind, or accelerating (interstate on-ramp...), for instance.
I do not want to open/rebuild the engine, which is in very good condition; there is a HEI distributor on the engine.
I have read stuff here and there, and would like real-life opinions;
what about exhaust: split stock manifold, headers (which brand?), dual/single line, muffler or no muffler...?
Intake: does one QJet on an aftermarket work well (bad flow to far away cylinders...), what carb set up work well (or not!), what about EFI (would it solve the problem of the distance between cylinders?).
I believe a camshaft would not be needed, am I right?
Should I go for a 4-speed auto trans?
This is for a true all-weather daily driver, no racing involved, but certainly some camping trips!
Any input is welcome. Thanks

Use the search feature on the site. There has been alot of topics on carburation, exhaust etc. From what Ive read More carb tends to be more problems.I personally like the idea of maybe some fenton headers and a nice exhaust ran out the back. Search that subject. Good luck. Others will pipe in soon.

Ken

I would have to believe what you're after is torque. You have a heavy vehicle with a small engine by today's standards. I would aslo guess that you have a very high-geared rearend, since you can cruise at 75-80 easily. Do you have a tach? If so, what is your rpm at 65 mph?

The type of modifications you're looking at will not likely get you the performace you're after. A better flowing exhaust will make a little more power by itself, but the seatofthepants'ometer will probably not notice a difference. Swapping to multiple carbs or a larger single will expand your rpm range, but not gain you anything where you normally drive. It's far more likely to create a hesitation or bog within your normal driving range.

You may be looking at a gear swap of some sort to solve this. You may want to investigate a 700R4 trans with it's low first gear plus overdrive.

Hope I've been of some help

Devin

I believe your easiest approach, considering what you have, would be to go with a small 4bbl, like the 390cfm Holley, headers, and dual exhaust. There are many brands and combos out there. Check the vendors in the links section. Don't overlook Tom Langdon of Langdon's Stovebolt. He's a nice guy and has helped a lot of folks.

Be sure to also use the search function here on the 'bolt. I know there are plenty of discussions on just this subject.

I also agree that Leo Santucci's book is a valuable resource, although most of the information applies to full race and not to warm street.

best wishes,

Les

Contrarily to popular belief, the '57 Suburban is not heavy at all, it is actuaaly quite small! It is indeed really not aerodynamic, though... So I really think the 250ci is a good engine for it, and it is not far off from what I would like.
I have a Turbo700R4 from an 84 Caprice and am thinking this could be a good move, shorter first and longer fourth... But the engine needs to breath a bit better.
Thanks so far, keep the ideas coming!!!

Throw a small turbo on it!

I noticed a big change in how mine felt around town when I switch rear gears from a 3.42 to a 3.08. The better choice would have been to change out the T350 to a 200R4 or 700R4. Joe

The suburban was the heaviest vehicle of the bunch for the 1/2 ton commercial line. Almost 3800 pounds.
As mentioned before, a rear end gear change is probably a good first step.

I've been working on the wife's 58 Burb through the summer, I am behind on posting some pics. The 250 I-6 is the only part of the drivetrain still intact from when we got it. I just picked up a new flywheel & 11 inch clutch (Summit), and aluminum bellhousing (Fall Carlisle) so I can mount the Camaro T5 w/ S10 tailhousing that we picked up locally. We have a 8.5 inch 10 bolt nearly ready to go in, it is running a 3.42 posi ring & pinion. I think it will work well with the 28.5 inch tires but that is based purely on math. I put a used R&P gear set in so we can drive it and get a seat of the pants opinion before putting out money for a new gear set. Still need to switch over to mid-engine mounts, there is a bit more fabrication left to do before it all comes together.

So I am interested in the opinions on this thread. No sense in sinking money into a 4 bbl manifold and headers if the 250 won't push the Burb around. We aren't looking for a hotrod but thought the 250 would out-perform the 235?? I'm pretty sure this motor is running a shaved head as the cylinder pressure is up near 180 psi. Hopefully someone will say that with headers/aftermarket manifold/HEI distributor the 250 will work just fine.

About weight: I've read that the 250 is a bit lighter then the 235. If you add up all the weight savings it could be enough to get the Burb down to 3600 lbs or less: 250 vs 235, T5 vs SM420, monoleaf vs multileaf springs (x4), Al bellhousing vs cast iron, etc. I know I'll be adding weight as it moves toward completion but I've tried to keep it on a diet where I can.

I'm working on a chopped up '49 rod project using the 250.
I'm only at the frame / suspension stage, but from what I've found out, the 250 will be more than adequete.
Alot of info can be gained from some sites specializing in chevy 6's. Little to no info on other sites.

http://www.t6racing.org/index.html
http://cliffordperformance.net/
http://www.sissellsautomotive.com/

The 250 in stock form comes with more horsepower than the 235 (but watch the years)
My '69 250 is rated at 155 hp, vs a 235 at app 116 hp, & that's with a 1 bbl carb., the sprint version is rated at 230 hp !
Mechanics & racers I've talked to say 250-300 hp is an easily attainable goal.

Like the guys said, Leo Santucci's book is also very useful, & a fun read if you're into 6's.

E-mail me with any questions & more info. I have a chart listing the specs on Chevy 6's from '62 - '88. Very informative.

I also got alot of info from the guys on this site- Thanks guys !

Rick.
[email protected]

The 250's suffer from poor intake ports on the heads limiting air flow, and ultimately power. The engines will spin to about 9000 RPM which will get you lots of upper end power, however not much torque where you need it. Not much except a lump head conversion will help airflow for a naturally aspirated engine, however throw a turbo or supercharger on it and you can get a whole lot of power.

Although a 268 Comp Cam, Clifford Intake, Exhaust Header and Holley 390 would be the way I would go.

It is possible using junk yard GM TBI and wire harness and a Megasquirt controller to inject your 250 for maybe $500 to $800 depending on what the junk yard charges and how fancy you want it. I'm in the process injecting a 292 now.

It takes a teardown, but the simplest way to pep up a 250 is to install 307 V-8 pistons. The 250 has dish-top low compression pistons, but the 307 flattops are a direct swap. Same weight, compression height, bore, and wrist pin diameter! The last one I built tht way also got a .050" head shave, and still runs OK on 87 octane gas. A RV-type high torque cam also helps a lot in the low end grunt department.
Jerry

Without going inside the engine to get more pep around town and getting on the Freeway I'd go to 3;55 rear end, 700R4, headers, duel exhaust, and a small 4 bbl. Late 60s early 70s GMC 1/2 tons could be found with 3.55 Dana limited slip 44s. Don't go too big on the pipes 1 3/4" is plenty for the street. I run a 500 cfm Edlbrock on my 270. It's plenty. With the lower gear it will raise your rpms in the lower gears and make more power available where you want it. On the highway the OD will let you cruse 70-75 at around 2200-2500 RPMs depending on tire size.