Small Engine RPM?

I have always wondered (and don't have a tach for checking) what rpms a small engine runs at; say a Briggs 3.5 horsepower?

Thank you,
Glenn F.

3600 RPM's has long been the common "standard" at which the average little engine was governed..

Some newer, "politically correct" lawmmowers are slowed down to lessen the force with which they throw projectiles.

3600 is standard I think

I have several small engines that turn in the 13,000 rpm range, one that probably turns about twice that and another that's way down around 9,000 rpm. Displacement ranging from .030 cu in to .45 cu in.

ditto what bob said. which is why gas powered whizz-bang engines and economy lo-pole genny heads work so well together

soundguy

Most small 4-stroke engines on mowers, pressure washers, rototillers, etc. are governed to 3,600 RPM. There are however are some snowblowers, etc. governed a slighter higher RPM (typically 3,800 - 3,900) in order to give a bit more power.

And of course direct-connected, 2-pole generator sets (typical homeowner emergency generators) must spin exactly 3,600 RPM to give 60 hz output.

3600 is what many are governed at now- but many Kohlers and Briggs were governed at 3200 when I worked at small engine parts counter in late 70s, some were set at about 2800 and a few were working at 2000 to 2400 rpm. A few years back many small engines were advertising HP ratings at 4000/4200 rpm while governor that could be set that high was normally set about 3600- some people adjusted governor up to full 4000+ for running to get advertised power. More recently the laws on advertised power has been disclose power at rpm and advertised "normal" running power like a DIN instead of "peak" SAE power or be fined for false advertising. The new overhead valve clean burn 10 hp engines seem to be weaker than old 10hp flatheads is common complaint- reason is the old flathead was run and power rated at 2800/3200 rpm and newer model is at 3600/4000 rated-- while the equipment it is used on is still using the about 3000 rpm setting on governor to run mower blades, hydraulic pump, PTO. Torque ratings at RPM are a better comparison and many newer engines show that figure in small print on tags. Older engines you have to check spec sheets to get torque figures.

Depends on what it's mounted on. Small engines originally ran 3600 RPM. But, many laws came into play limiting engines to lower RPMs. The first was the "blade tip speed" law. When that started, any mower with a direct driven blade had limits as to how fast the ends of the blade could travel. That resulted in - 21" mowere only running around 2800-3000 RPM. Mowers with shorter blades could run higher RPMs. And mowers with belt-drive could work aorund it. Later more regs got involved, including for noise.

This is what led to the many law-suits about false horsepower claims. An engine would be advertised as 4 horse, but was really only "4 horse at 3600 RPM" and only be allowed to run at 2800 RPM and have less power.

After all the law-suits, you rarely see horsepower ratings on small engines anymore. Now they use "torque." Or even better, some are sold as "formerly rated 4 HP", etc.

A friend, now retired from "Briggs & Stratten",
said that when testing, to get the horsepower
rating at 3600 RPM, they fed pure oxygen into
the carbrurater. Under normal conditions they
would never achieve that horsepower rating.

Thanks, feds.

Dean

I don't believe the oxygen story. I went to many Briggs & Stratton, Clinton, and Kohler service schools starting in the 1960s. If Briggs had tried something like that, they would of gotten exposed, fast. Tecumseh, Clinton, Kohler, Chrysler, West Bend, Wisconsin, etc. all rated the same way.

They all rated their engines in various ways, just like tractor and car makers.

For example, take the Briggs 7 3/4 and 14 cubic inch engines.

Tested and proven at Nebraska Tests:
7.75 c.i. Briggs put out 1.7 horsepower at 3600 RPM on the PTO. 2 3/8:" bore by 1 3/4" stroke.

14.1 c.i. Briggs put out 3.8 horsepower at 3600 RPM on the PTO. 2 3/4" bore by 2 3/8" stroke.

Now, how did Briggs & Stratton advertise them?

7.75 c.i. engine rated at 2.1 horsepower continuous at 3600 RPKM with accessories, and 3 horse max with accessories. More if rated NET.

14.1 c.i. engine rated at 5.1 horsepower continuous at 3600 RPM with accessories, and 6 horse max with accessories. More if NET.

Here are some Wisconsin ratings given by the company at the same time.

14.9 c.i. ACN engine rated at 4.8 horsepower continuous at 3600 RPM with accessories, and 6 horse max with accessories. More if NET.

And Tecumseh:
7.75 c.i. LAV30, H30, H25 engine rated at 2.5 or 3 horsepower continuous at 3600 RPKM with accessories, and 3 horse max with accessories. More if rated NET

And Clinton:

7.2 c.i. Model 400 engine rated at 2.1 or 3 horsepower continuous at 3600 RPKM with accessories, and 3 horse max with accessories. More if rated NET

18.6 c.i. Model 412 engine rated at 5.95 horsepower continuous at 3600 RPKM with accessories, and 7 horse max with accessories. More if rated NET

I had a Briggs and Stratton lawn mower engine turned up to 4200 +/- rpm when I was using a mulching attachment and a bagger. It worked really well but fried the rings and probably the bore. I haven't taken it apart to assess the damage.

Cured that problem on my mower by connecting the governor linkage into the proper hole.

Funny the safety nazis had left the "correct" hole there, isn't it?

Also, you can just bend the arm where the spring hooks with pliers and it perks right up. If a dealer does in, in theory, he/she could get sued.