Briggs 2.50 HP confusion

[creia]

1970 Briggs 2.5Hp, horizontal shaft, "Koolbore"
Model = 80102
Type= 0159-01
Code= 7007311

This engine came on an edger I picked up on Saturday. (Good running engine and only $ 30.00!)
I have never had a Briggs 2.5 HP. The Briggs decal on the shroud (stating 2.5 HP) appears to be original. As I started to take a closer look at it I noticed that it has the exact same bore (2.375") and stroke (1.75") as a Briggs 3 HP engine.
Sooooooo.... What makes this a 2.5HP? The only thing I can think of is that this 2.5 HP engine has a "Vacujet" carb with a (smaller) venturi of 21/32", vs. the 3HP that has the "Pulasjet" carb with the (larger) venturi of 23/32". Does this 1/16" difference "derate" it by 1/2 HP?

Can I assume that the internals (e.g., crank, rod, cam) are the same and interchangeable with it's 3Hp counterpart?
As always, I welcome a appreciate your replies.
Michael

[RoyM]

They were pretty liberal with their horsepower ratings, it was whatever the customer wanted. That is why after a lawsuit they started using torque ratings that the average consumer didn't understand lol. It's all a marketing game.

[bgsengine]

Often the HP ratings were also based on the RPM the engine was built to run at , and yes a restricted intake can indeed cut the power output of an engine quite a bit (Think of the Restrictor Plates they mandated at NASCAR superspeedways) So often enough changing out the carburetor for a 3 HP one would give you a 3 HP engine. (Less air/fuel = less power, no?) The lawsuit thingy I think actually came about because of the more common lawnmower engines - Way back when, mowers were built to run at 3600 RPM with whatever blade size the mower maker wanted, but then the Gummint (CPSC) stuck its nose in, and started mandating Zone controls (deadman bail levers, and the like) , and right on the heels of that , started mandating the "Blade Stop Time" - Because a bigger 22" pushmower had a higher blade tip speed at rated RPM (and more mass to the blade, too) they had to change out governor springs to get a lower Top No Load (That's when it became necessary to have to stock 10 or 15 different governor springs instead of just one or two) - lower top no load allowed them to use the same blade brake mechanism to achieve the mandated blade stop time. However that lower top no load also resulted in reduced power output, so the 3.5 HP engines no longer actually produced 3.5 HP (not surprisingly - they were rated at 3600 RPM, and for a 22" mower, RPM was cut to as low as 2800.. More common was the 3400 RPM 20 inch mower, but still not making a full 3.5 HP)

Upshot of it is, they never bothered changing their labelling (You'd have had to have a couple dozen different H.P. labels for any given base model, which manufacturers deemed not necessary, assuming folks would have the common sense to understand that 3.5 HP rating was at 3600 RPM) which led to the lawsuit. I blame it all on the Gummint that couldn't leave well enough alone and let natural selection weed out the idiots.

[KE4AVB]

Did you measure the actual stroke.? If not you probably find it is 1.5" instead of 1.75" as that will drop the one cu/in difference between the 08 and 09.

[creia]

Did you measure the actual stroke.? If not you probably find it is 1.5" instead of 1.75" as that will drop the one cu/in difference between the 08 and 09.

I removed the head on the 2.5HP and measured the actual stroke- it was 1.75", the same as a (known) 3Hp that I measured side-by-side.
I can only assume (as Brian has mentioned) that the smaller carb venturi on the 2.5 HP "Vacujet" accounts for the loss of 1/2 HP.
Michael

[KE4AVB]

Just trying to figure to out here. Besides I bored.....raining...I think just crawl back in bed and sleep half the day. But smell of the Christmas dinner may keep me from doing that.

[RoyM]

Sounds like a plan.

[bluemower]

I checked the ipl. The 80100 has the same bore / stroke as the 80200 and the 80300. All have the same cylinder head and camshaft. Carb is different on all three models. I checked the torque charts that are published in the 1970 Engine Spec Catalog.

The 80100 has 4 ft lbs torque or slightly higher at 2700 - 3100 rpm. In 1970, the list price for this engine was $57.70.

The 80200 has 4.6 ft lbs torque or slightly higher at 3000 - 3300 rpm. This has the pulsa jet.

The 80300 has 4.6 ft lbs torque at 3000 -3200 rpm. This has the 2 piece flo jet.

A comment in the 1977 engine spec catalog indicated the 80100 was discontinued in 1977.

[Merkava_4]

Sooooooo.... What makes this a 2.5HP?

Shorter connecting rod. If you take the longer connecting rod (and crankshaft) out of a 3HP and install it into the 2.5HP, you'll have yourself a 3HP.

If you have a 2.5HP that's smoking while it's running, you can install a 3HP rotating assembly and chrome ring kit. That's what I did and made a very lively engine out of it. Just make sure the ring gap check is within specs first. No need to hone cylinder when installing a chrome ring kit.

[bgsengine]

Sooooooo.... What makes this a 2.5HP?

Shorter connecting rod. If you take the longer connecting rod (and crankshaft) out of a 3HP and install it into the 2.5HP, you'll have yourself a 3HP.

Couldn't be - longer con rod with more throw on crank would change the stroke spec - as noted originally, both the 2.5 and the 3 HP have a 1.75" stroke, and stroke would be a function of the crank's throw... putting a longer rod in without changing throw would result in the piston poking through the head... and using a longer rod with shorter throw results in shorter stroke... Both engines also have the same displacement (8 CID) - What changes the HP output will be the same as what changes the Torque spec - Reducing the amount of air/fuel the engine is able to draw in, and altering its top no load governed speed.