Chevy 261 verse 235 engine differences

What are the differences between a 261 and 236 mid 50's 6 cyl engine?

Will or do the inlet and exhaust manifolds interchange ?

Is the 261 longer ?

Higher/taller block ?

Bore and stroke ?

Thanks in advance

 

Same outside. Are close enough they will swap with no problems. Couple of cubic inches different.

 

Go to Inliners International site, more than enough information on both engines.

 

Bigger bores..."siamese" cylinders with no water jacket between them...and steam holes ala Chevy 400ci V8. Bigger valves, hotter cam, heavier duty rods. That's most of it anyway.

Yes, the intake and exhaust manifolds interchange.

 

1955 Canadian Pontiac w/261 (single carb/exhaust) = 150hp

1955 U.S. Corvette w/235 (3 carb/dual exhaust/cam) = 150hp

No replacement for displacement as they say. One of the heads for the 235 (3836848) has smaller combustion chambers and will wake up a 261 if the steam holes are added to it.

 

-----------
Another tidbit - the 261 Canadian
Pontiac passenger car engines had
a bit hotter cam that was roughly
the same grind as the '53-'54-'55
Corvette 235 cam, except that the
261 Canadian Pontiac passenger
car cam was hydraulic and the
'Vette 235 cam was solid.

Mart3406
================

 

Canadian Pontiac was basically a Chevrolet in Pontiac clothing. In 1955 the US Pontiac went to an all V8 model range. Canadian Pontiacs, which were somewhat smaller, got the 261 six cylinder introduced in 1954 for GMC trucks. This was the standard engine, Chev V8s were available on the usual, optional at extra cost basis.

In street drags a Pontiac with a 261 six would beat a Chev or Pontiac with a 265 or 283 V8. As long as the race did not go on too long. Up to 30 the Pontiac was faster, 30 to 50 more evenly matched, over 50 MPH the V8 would pull ahead.

The 261 was a popular swap in older Chevs (pre V8 models). They were good engines, cheaper to buy and easier and cheaper to swap in, and practically as powerful as the V8. A lot of them were hopped up for street rods and for dirt track stock car racing. There used to be a class for flathead eights and OHV sixes. The 261 gave a good account of itself against the Fords and Mercuries, and won many races.

Canadian Pontiac used the 261 from 1955 to 1962. After that they got the new Chev 230 and 250 engines.

 

The block and rods are basically the only differences. Cranks are the same (3 15/16" stroke). Rods are beefier but same length. Cubic inch increase is due to larger bore size (3 9/16" vs. 3 3/4") which is why the bores are siamesed. NO, you cannot bore out a 235 to a 261. The oil sending unit port on the lifter bore oil gallery is larger on the 261 than the 235. That's the only visible external difference that I know of.

Heads, manifolds, water pump, etc all interchange. The 848 head is the smallest combustion chamber. Valve sizes are all the same.

Of course, we're also talking about pre- and post-'54 engines. '54 has a different water pump, etc than the '55 - '62.

HM

 

Oh yeah, the exhaust valves are basically the same as a small block (1.5"). You can install 1.6" exhausts which helps the thing breath a little more. Intakes are huge and like no other valve, but they aren't the problem with breathing anyway. It's the deeeeeep pocketed combustion chamber that the exhaust sits in that chokes it. Can't spin them that tight anyway since the rotating assembly is massive, so plan on making your horsepower at LOW RPM.

If you want a six that breathes, invest a fortune in an aftermarket head or switch to a Jimmy. At least with the Jimmy you get a more conventional combustion chamber and you can go up to 302 inches (320 when bored out).

Ahhh, I'm giving away all the secrets.

HM