I noticed about a week ago that Briggs now published (part) of the torque curves for their engines on their website. Let's take a look at a few and use the good old Torque x RPM /5252 to figure out the actual HP ratings. Note that this is purely an excercise for lulz, not for any real purpose.
First, the Briggs 875 Professional OHV that we picked up on our pressure washer
http://bsintek.basco.com/BriggsDocumentDisplay/jkqnsNDvrS8lFix2.pdf
So assuming the washer runs at 3,000RPM, that is about the torque peak of 8.75lb/ft- 8.80lb/ft. That would be 5HP. Assuming 3,600RPM, which is what they used to test HP ratings at (I don't know why OPE makers were sued, it was an ISO standard) with a torque rating of 8.5lb/ft, that would be just under 6HP. For lulz, let's keep going. Assuming at 5,252RPM using my magnificent brain and math capability I'm going to guesstimate the torque at about 7.65lb/ft. Which would put it at 7.65HP. Of course, that's probably pretty close to redline for this engine, but it's a pretty decent number.
Let's move on, to the one on our lawnmower, the venerable Flathead. In this case, a 675 Series 190cc flathead (Same displacement as the 875 above).
http://bsintek.basco.com/BriggsDocumentDisplay/jkqssNDvrS8lFix2.pdf
I can already see something interesting here. Notice how the OHV engine seems to peak more towards the middle, and drop off more slowly, the flathead peaks much lower in the RPM range and drops pretty quickly from there.
Let's perform the same calculations again.
So since I adjusted the governer I estimate mine to be running 3,300RPM or so. So let's take that, and it's torque curve puts it at 6.5lb/ft (a peak of 7.05 or so at ~2,450RPM) so that makes it out to be 4.1HP. Sounds about right. So if we take it to the ISO 3,600 standard at 6.0lb/ft, that would be.. Still 4.1HP. Hmmm. Let's take this one out to 5,252RM too, which is definitely close to redline for the flathead engine. Once again using my not at all professional skills, I'm going to guesstimate the torque at 4.75lb/ft at that speed. Notice how torque definitely drops off much more quickly on these flatheads at higher RPMs. So that makes it 4.75HP. Amazing the difference the OHV makes. That's a full 3HP difference at 5,200RPM, from the same displacement.
However that being said, I like the torque curve on the flathead better. Most will never, ever be run that fast, and the flathead curve seems better for OPE use. That is, if it starts to slow down due to load, the Torque is actually stronger (relatively) than on the OHV. I thought it was interesting, anyway.
First, the Briggs 875 Professional OHV that we picked up on our pressure washer
http://bsintek.basco.com/BriggsDocumentDisplay/jkqnsNDvrS8lFix2.pdf
So assuming the washer runs at 3,000RPM, that is about the torque peak of 8.75lb/ft- 8.80lb/ft. That would be 5HP. Assuming 3,600RPM, which is what they used to test HP ratings at (I don't know why OPE makers were sued, it was an ISO standard) with a torque rating of 8.5lb/ft, that would be just under 6HP. For lulz, let's keep going. Assuming at 5,252RPM using my magnificent brain and math capability I'm going to guesstimate the torque at about 7.65lb/ft. Which would put it at 7.65HP. Of course, that's probably pretty close to redline for this engine, but it's a pretty decent number.
Let's move on, to the one on our lawnmower, the venerable Flathead. In this case, a 675 Series 190cc flathead (Same displacement as the 875 above).
http://bsintek.basco.com/BriggsDocumentDisplay/jkqssNDvrS8lFix2.pdf
I can already see something interesting here. Notice how the OHV engine seems to peak more towards the middle, and drop off more slowly, the flathead peaks much lower in the RPM range and drops pretty quickly from there.
Let's perform the same calculations again.
So since I adjusted the governer I estimate mine to be running 3,300RPM or so. So let's take that, and it's torque curve puts it at 6.5lb/ft (a peak of 7.05 or so at ~2,450RPM) so that makes it out to be 4.1HP. Sounds about right. So if we take it to the ISO 3,600 standard at 6.0lb/ft, that would be.. Still 4.1HP. Hmmm. Let's take this one out to 5,252RM too, which is definitely close to redline for the flathead engine. Once again using my not at all professional skills, I'm going to guesstimate the torque at 4.75lb/ft at that speed. Notice how torque definitely drops off much more quickly on these flatheads at higher RPMs. So that makes it 4.75HP. Amazing the difference the OHV makes. That's a full 3HP difference at 5,200RPM, from the same displacement.
However that being said, I like the torque curve on the flathead better. Most will never, ever be run that fast, and the flathead curve seems better for OPE use. That is, if it starts to slow down due to load, the Torque is actually stronger (relatively) than on the OHV. I thought it was interesting, anyway.
