The Stovebolt Site Forums The Shop Area Engine Shop 261 Dyno Results

Thought maybe others would be interested in the Dyno Results of my 261 chev truck engine which has a brand new 1961 fitted block but nothin else special, single carb , Fenton Headers and HEI ignition. Total Horse Power 128 see full report attached

Many many thanks for posting that. Was that done on a chassis dyno or a static dyno?

Is the engine as a whole a 1961 model? I am not sure from what you wrote if you put the guts in a 61 block, or if it is an all 61 engine.

I am interested to compare that to the plots that GM provided for the engine originally.

It is a 1961 Canadian fitted block from Oshawa Ontario that included piston and rings right out of the crate. The rest is a 1956 235 eg crank sheet metal carb. New cam for a 261 and rods for a 261.Head is from a 235 (high compression version)

Ok, being a hybrid like that it isn't really apples/apples to compare to the stock curves.

That said, the 56 235 has a peak of 210ft-lbs (2100rpm) and 140hp (4200rpm) according to the Vehicle Information Kit.

The 61 261 was 218ft-lbs (2000rpm) and 150hp (4000rpm) according to its Vehicle Information Kit.

All of those are the gross numbers, not the net.

It is somewhat interesting that your numbers are right in the ballpark for torque (though shift up the rpm band a bit) but a fair bit lower on power.

For comparison, my 261 got 192 hp on the dyno (rolling road dyno).

Decked 261 .04 over, milled and ported 848 head, Bulldog 3/4 cam, electronic ignition, Fish carb, dual headers, Clifford intake.

Well that is a dyno sheet I would love to see!

Seems like the dyno results I got were similar to the 1955 GM reported net hp. But the gross torque comparison seems similar between the Gm and the shop dyno.

Makes me wonder if the apple to apple issue is more like the difference between the technology of GM 1955 factory dyno vrs the 2023 shop dyno ;or the 261 head is better in terms of hp over the 235 848 head ? ; or possibly hei ignition and fenton headers are both all candy

There is no real flow differences between the 261 head and the 848 head. There were two versions of the 261 head, one was 86.2 cc and the other was a little larger. The 86.2 cc head is the same as the 5913 head in terms of chamber volume. the 848 head is 79.1 cc so it brings just a bit more compression.

As for the GM reported net, keep in mind that when GM did those evaluations, they did them under absolutely optimum conditions designed to drag out every bit of HP and torque so that they could advertise that figure. No accessories, no water pump, belts. So who knows what a real full dress production 261 would do. In fact, it would not surprise me to see a bone stock 261 dyno out on a modern dyno at or under 100 HP. Frankly, I think the carb was your limiting factor.

Thanks for that information as it makes sense. I probably won’t be driving my 1950 truck with the T5 for another two years as I am in final leg of cab sheet metal them paint prep etc. I am sure I will have enough hp for my needs but if not I will for sure look into the carb issue

Either the 261 or the 848 head can benefit from installing the early Powerglide 235 1.94" intake valves and the 1.6" small block V8 exhausts. Then do a little pocket porting to the bowl area under the valves. That won't fix all the 90 degree turns in the manifolds and heads, but it will improve the breathing a little. Inline sixes in general, and stovebolts in particular can benefit from modifications that increase torque, not necessarily RPM or horsepower, such as increasing stroke length and compression and reducing rotating friction with smaller diameter rod and main bearings and lightweight pistons.
Jerry

I have always installed the larger valves, one or both. But I have never had a motor on a dyno so I don't really know how much difference either actually makes. Man oh man, to have unlimited access to a dyno would be pretty cool, lol.

I have an early version of the Go-Power water brake dyno, back when the company was based in Palo Alto California. Dad and I got a tour of their facility back in the late 1970's when he owned the machine- - - -I inherited it several years later. It uses a load cell that resembles a torque converter, and it's filled with water to put a load on the engine. The torque output at the flywheel is measured, and HP is calculated from RPM VS torque, with fudge factors for relative humidity, air temperature, and barometric pressure. I've had engines of up to 700+ HP on it at times.
Jerry