The Stovebolt Site Forums The Shop Area Engine Shop 261 Chev. -V- 270 GMC comparison

Thanks for the information
Al

What is the possibility that a 1.5 ton, 1959 GMC BOX truck would have either a 270 or 302 engine in it? Is it possible that it has a 50-50 chance on it being a 302? I am going to travel 150 miles to look into this GMC truck, (it has been parked since 1982). I may buy it if I can get enough donor parts out of it to put into my current projects.
Al
PS: Panic, it appears that you have a broad base of experience with inline engines. The straight 8 engine, I have, is the 385 (later) version.

If I was a betting man... it will be a 270. Too bad the owner cant scrape and clean the flat boss to the left of the distributor for a serial number beginning with 270xxxxx or 302xxxxx. Might save you a trip.

Panic, your cam lobe placement looks screwed up 10 exhaust on an L8? and 5 Intake??

Tom, That idea of yours is a good one. I will push the fellow a bit and see if he will check numbers for me. What I thought would be a 150 mile trip one way, I guessed and guessed wrong. The trip is a 200 mile one way trip, so If I can glean some information that is very relevant to my needs, so much the much better for me. (Especially this close to Christmas). I think I would even consider a 270...maybe...??!!
Thanks for your nudge.......
Al

nothing wrong with a good 270. .

Yup! Thanks, that's what happens when you cut-n-paste instead of using brain.

Cam lobe placement: Ex In Ex Ex In Ex Ex In Ex Ex In Ex

Panic, Do you have any experience with how much cam work can one of these inline engines, 235, 261, 270 and 302 can take and still be drivable and run on normal gas? And what might be a good target for the compression ratio?
Al

No real experience, still working on a 235.
The maximum CR is very limited in the 235 by the chamber design - except for a $$$ custom piston even getting 9:1 takes some work.
The GMC has very different limits due to the more conventional chamber shape, but again a domed piston will be needed for anything big.
How much cam? Above perhaps 230° @ .050" the idle quality begins to decay, the power band moves up, but 235 peak power is disappointing, 240°+ will add peak power and need to be driven like a Honda, no more "slip the clutch to get away at idle" that made them good truck engines.
Vizard and others suggest that an engine with limited intake valve area should try a much narrower LSA than any available cam, like 104° or less (108° is a hot cam now). Big $$ and may produce little.
I doubt that a 250 HP 235 is possible. The head design is a dead end. This means that it needs either a 5-speed or a loose torque converter not to be a misery to drive in traffic. Or make a Rajo?
The GMC (H head) is much better in port size and port design.
Either one needs a tube exhaust but, with siamese ports, real sonic wave tuning isn't as effective as on a "modern" engine (Slant 6). For intake my first choice would be fabricating 3 separate (but alike) runners, 1 to each siamese port, ending in a HD6 etc. SU. Large enough for 250 HP in the XK-E.
Much work can be done on windage, pumping loss, case pressure management that has never been tried but can be "borrowed" from Jaguar, Toyota, etc. (an L6 is an L6 to some extent). Santucci doesn't discuss in his 250/292 book - but I'm sure he's done stuff.

Thanks for the "tid-bits" or "nuggets" of information, as it relates to Chevrolet and GMC. I enjoy the unusual, I suppose that is why the straight 6 GMC and Chevrolet appeal to me. I also have a few other orphans that fit into the same category, those being a 258 AMC with custom 4 barrel intake, 250 Studebaker President straight 8, early Nash 278 dual ignition overhead 6, Pierce-Arrow 385 straight 8, Chrysler 265 flat head 6. All of these engines are on "HOLD" while I get with the 302 GMC project and fit it into the '47 GMC dump truck and a solid 235 into a 1946 Chevrolet dump truck. If I could run down a 261, that is what I would use in place of the 235. However, as it now stands, I probably will not travel long distances for a 261 and just use what I have.
Al

Don't get too excited about whether on not it is a 302 or not, any excitement might suggest to the seller to raise the price if it is a 302.

Ed

Ok, here is an update on the GMC truck I went to look at. I did drive across the state to check out the 1959 GMC 1.5 ton. It is actually rather sad, from a condition standpoint. It was originally a milk delivery truck and still has the cooler box on the back. Glass is bad, rust in both fenders on the eyebrows, one fender is mangled. The rest is kinda straight. The truck shows 88,000 miles. Now for the confirmation, yup it does in fact have the 270 engine. (I rather surmised that). I will probably work on some kind of a deal, but I need to figure out who can haul it for me. With the box, dual rear wheels and weight, I can't move it on my trailer. More later.
Al