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With the 235/261 head being such a limiting factor on performance has anyone thought about adding a small Turbo.

The compression these engines produce would be just right for the addition of say 5-8 pounds of boost. The RPM range is close to that of a small Diesel. There would be some challenges, exhaust, carburetor and overall mounting. The oiling system of a 235/261 is an issue itself.

I'm sure the bottom end if the engine can handle the stresses if it was properly built. The Camshaft and intake would take some thinking.
This maybe just Bench Racing right now, but I do have a 261 on the engine stand.

Craig - Here's a "turn back the clock" very old on topic thread to help your thinking cap process and conversation.

Some good reading, I didn’t recognize a single member on that thread, my how time flies.

Unfortunately, most of that information is no longer relevant with the Turbo system we have now. I have been studying this for a while now and do believe it worth a least a try.

The mounting and plumbing are probably the largest issue. With the 235/261 having both the intake and exhaust on the left side of the engine space is at a premium. A cross over tube on the exhaust could move the Turbo to right side and allow for more room. The oiling is a major factor, 235/261's are not known for their oil pressure. Some modifications would be required.

Camshaft selection and valve springs need to be researched. Head work as well.

The carburetor is another concern. Using a Blow thru design would require a much larger CFM. It would also need an adjustable regulator and stronger fuel pump. All of these are workable nowadays. The Holley 500 2 Barrel seems to fit the bill closely. It has 375 CFM, the metering block allows for easy modifications and an adjustable power valve is available.

I have 2 of the Turbochargers pictured above, they are from a 4 cylinder Cummins Diesel. These are Military Surplus from Planet.Gov (cheap). They already have a waste gate mounted and are rated up to 8 LBS of Boost.

I may do a mock up of this system over the winter with the 261.

I don't have time to dig for it this second, but there was a pretty detailed discussion on this topic 18 months ago or so.

Edit: roughly June 2024 and it was on a 36, according to my easily searchable notes.

If you are running a stock transmission, I would expect significant turbo lag after each up shift. A turbo needs to be spooled up fairly quickly to provide instant boost.
That might take a while with a 3 speed.
With a T-5, you will likely avoid significant turbo lag.
Also, if you put the T-5 in to begin with, you may find that you don't really need a turbo.

One of my Rube Goldberg modification dreams and schemes with a long stroke crankshaft uses 1K+ HP rated H beam connecting rods and pistons for a Japanese diesel engine in a 261. It's about 300 cubic inches +/- a little, depending on the bore size.
Jerry

The 261 is destined for my '58 Chevy 4400 (Thor). I just completed a NV4500 Transmission swap. I do have a 235 going in a '60 Apache, that will get a Lugnutz prepped T5.

All seems super doable, Craig. The general lack of “stuff” in a task force engine bay should give lots of options for plumbing the turbo.

For inspiration, you might look up a semi-recent video from YouTuber “Sleeperdude.” He plumbed a turbo on a Ford 300 in a 70s Maverick. If memory serves, he had to cross over the exhaust much like you describe. Although on a 300 the intake/exhaust are passenger side.

As long as your bearing clearances are good, is the oiling really a constraint? Never given that much thought.

Oiling would be my concern, the turbo needs a fair amount to keep the bearings alive.

Oiling is probably my biggest concern. The Turbo requires constant pressure and a large return line. The return line is not an issue, the supply line is.

Routing an oil line to the Turbo is pretty straight forward. Having adequate pressure to supply both the engine and the Turbo is where I have my doubts.

I read on the HAMB that the 270-302 GMC oil pump supplied more volume and pressure. The discussion was adapting it for use on a 235/261. That's where things got a little fuzzy.

Maybe HRL or Dragsix can shine some light on this.

A GMC oil pump is much larger than one for a Chevy- - - -which would probably involve some oil pan modifications for clearance. Another option might be to use some type of dry sump oil system, with an external pump driven by a Gilmer belt. NASCAR has run those systems for many years, so parts are available. That would supply plenty of oil volume to lubricate the engine and oil/cool the turbo. The high oil volume is needed to keep the turbo bearings cool.
Jerry

I have a super-vague recollection (so vague it might be just my imagination), of HRL outlining a procedure whereby some commonly available pump gears (SBC?) being modified/machined to work in the Stovebolt pump housing. I could also be conflating some procedure where one makes a 216 into a full pressure-oiler.

I do know that some applications use an electric pump to separately oil a turbo. I don’t know whether that would be viable in this sort of application.

There is a way to use SBC V8 gears in a 216 pump housing with a spacer between the base and the pump body, but it's simpler to use a Melling M29 pump designed for a 54/55 235 and rework some of the input/output plumbing. A turbo would need a lot more oil volume to work right.

Here's a dry sump pump- - - -it would need a reservoir tank and some oil pan modifications to work right:

www.ebay.com/itm/353234660795?

Jerry

Here's a screen shot of a GMC oil pump I just bought on Ebay- - - -from a seller who didn't know what he had, or what it's really worth. There's a considerable size and volume difference to a Chevy pump, but the mounting method is similar. That's why a GMC oil pressure dash gauge has a 60 PSI full scale reading.

www.ebay.com/itm/136443940508

Jerry

I think that modifying the Oil Pan to accept the larger GMC pump would be a lot easier than adding a external Oil Pump.

One advantage to an external pump is the ability to drive it at a different speed than 1/2 crankshaft RPM, thereby changing the volume output.
Jerry

I need to research Camshafts as well. Dragsix suggested contacting Schneider Cams and give them my stats. With the engine destined for a 1.5 ton truck low and midrange torque are key.

The engine will probably never see any RPM north of 3500. It will however live in the 2500 - 2800 range. The last trip I made was just shy of 500 miles round trip, it did fine at 2800- 3000 in 4th, going into 5th the RPM would drop to about 2000. Once it regained momentum it stay around 2400-2500 and ran 62-64 MPH.

With a 4.88 rear gear and 33 inch tires I should be right where I need to be. The little 235 just doesn't have the grunt.

How would a 295 cubic inch "261" with a 4 3/8" stroke work in that application? With an 848 head, that one should be a bit north of 9:1 compression, also.
Jerry

Just looking around and came across these cam specs. From reading all I can find, a Turbo camshaft for low to mid range Torque needs less Duration, less valve overlap, wider lobe separation and moderate lift.

I'm one step closer to having most of the parts to get the project going. I found an intercooler at the Moultrie Swap Meet. While I wasn't really considering an intercooler, the $50.00 deal was hard to pass up.

Adding an intercooler will help with the heat under the hood and add a cooler / denser charge to the carburetor. It will require more piping and and bends to get everything connected.

If on the off chance I don't use the intercooler, I'm sure I can sell for enough to cover my expenses.

Back to the oil pump issue. Will the GMC pump bolt right up to the Chevy block?

And then it’s just a matter of plumbing that pipe to the galley, and modifying the pan to work with the oil pick up?

I haven't been able to find a side by side comparison. I'm looking for the factory specs on the GMC, I can get the measurements from the Chevy pump I have.

The GMC pump housing is MUCH larger that the Chevy, and I'm pretty sure the volume is higher. Since I have examples of both pumps available, I'll post a photo shoot of the two in a side by side comparison once there's enough daylight to get good photos. It was a pretty common modification back when stovebolt engines were used in round track racing. The GMC pump mounts the same way- - - -with a single tapered setscrew in a bracket on the block. It's definitely NOT a bolt-in swap! The oil pan modifications are needed to clear the pump housing, not the pickup.
Jerry

From what I can find, the GMC oil pump produced about 50 - 55 psi at a considerable more volume. That should be just what I'm needing.

Either way I brought a GMC Oil Pump off Ebay. For $45.00 I don't feel like I could go wrong.

Just going off pictures, the oil pan “flange” of the Chevy vs the GMC look quite a bit different.

I knew you couldn’t just bolt a GMC pan to a Chevy, but looks like a fair bit of creative fabricobbling would be involved.

No problem for a guy like Craig.

I tested an oil pump from a '57 GMC 270. The pump regulated max oil pressure at 58 PSI.

My '52 GMC 228 (with unknown mileage on the engine with original pistons, I've put 42000+ on it in the last 12 years) typically has about 50 psi at operating temp and road speed.

I just measured a spare GMC oil pan

Thank you Bill, that's just the type of confirmation I was needing. I'm sure it will take some FAB work, but if Jerry says it can be done than that's good enough for me.

The little bump out there at the 9-7/8” dimension….is that the clearance for the oil pump?

Yes- - - -the pump drives off the bottom of the distributor, which is approximately at the middle of the passenger's side of the engine. The late 55 to 62 235s had a similar bulge in the pan rail to clear the Melling M-45 oil pump, but the GMC offset is a bit larger. I believe the vertical position of the pump can be made to clear the Chevy block, and more clearance can be built into the oil pan sheet metal.
Jerry

Before you commit you should verify that the camshaft and oil pump gears will play well together.

By the way, the Melling pump for GMC engines is an M-23A.

Obvious comparo:
Pump gear height vs. OD
Number of teeth?

The late 235 gear (and Melling M-45) is 1.00" tall, 1.50” OD with 7 teeth.

Bill, the drive gear is on the distributor- - - - -the oil pump drives off a tang on the bottom of the distributor shaft.
Jerry

Short between the keyboard and my chair! Sorry.

I meant to say between the camshaft and distributor gears. I know the mid 50s Chevy and GMC distributors will interchange, but I don't KNOW that other years will.

I'm currently running a Small Cap HEI. No doubt that this same distributor will make its way into the 261.

I believe the change in distributor gear material coincided with the switch from steel to cast iron cams at the same time the cam bearing diameter also changed. That would have been in 1954 or so. The steel cams with the small diameter bearings were used in the 216s and spray-oiler 235s.
Jerry