Old tractor with modern engine.

Recently there was a thread about a Farmall H with a 50 hp Honda motorcycle engine installed, with what looked like the MC clutch and transmission. So there was lots of gear reduction, to allow the tractor to use the 50 hp at slow speeds. Has anyone ever seen something like this at a tractor pull, or on a dyno? It seems to me that with the right gear reduction 50 hp is 50 hp, - the loss in the gear reduction.

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Horsepower is horsepower, true, but I can't think it would have the torque of the original engine, even though it has more horsepower. I may be wrong though, that happens from time to time.

I have an F12 with a 2.8Liter V6. Sounds cool at 4000 rpm. But has zero torque.. My original F12 with the (12 horsepower)IH 4 cyl gas will acctually do better on a sled.The slower rpm longer stroke has much more torque than the high speed V6,even tho the V6 has much more 'horsepower'.

Yup, if you get your gearing right it'll put out the same as any other 50 HP engine. As you spotted, the difference will be that the Honda engine HP/torque curve is probably such that it makes its 50 HP when really wound up. If you had each engine running at their optimal RPM where they make 50 HP and geared them such that the final output was the same RPM, you'd have the same torque (minus some minor losses in the reduction efficiency).

The more concerning aspect for me would be the duty cycle each engine is built for (if you intend to work the tractor). A tractor engine is built to put out all of its 50 HP pretty much all day. Pull into a field for a day of hard plowing and it's going to be used to capacity non-stop.

A car/bike engine, on the other hand, seldom sees that kind of work. When cruising, it's using only a fraction of that 50 HP. Acceleration and really, really fast cruising speeds would be the only time that engine's going to be using all of its 50 HP. Imagine pulling a 1500# cargo trailer behind a 50 HP Honda bike and heading across the country with it. It would probably have the HP to pull it, but you'd be overheating and burning out the engine in no time if you ran like that constantly. It just wasn't built for it. The cooling capacity would probably be the most limiting concern.

I should add to my previous post: The other concern would be the slope characteristics of the Honda engine's torque/HP curve. A typical tractor engine might put out its full RPM at, say, 2200 RPM. If you drop it to 1600-1800 RPM, the torque/HP curve are probably such that it's still putting out at least 70-80% of its full HP, and thus still has a good amount of torque for pulling wagons, field work, or any of those other jobs where we typically don't wind our tractors right up to full PTO RPM.

The curve of most automotive engines, however, would be quite different. Even if they put out their max HP at 2200 RPM (most would be much higher), when you drop it to 1600 RPM you're likely down to 50% or less of your full HP.

There's no magic between HP and torque - it's a linear relationship. The same HP at the same final output RPM will produce the same torque. It's rather that the way the engine's optimized/designed will vary at what RPM you see full HP/torque. And because HP is proportional to toque by an inverse unit of time (550 ft.lb. per sec.): to put out 50 HP and 5000 RPM means very little torque from the engine, while 50 HP at 2200 RPM means a lot more torque at the engine.

In summary: If you gear the 50 HP at 5000 RPM Honda engine down to 100 final output RPM output through some sort of transmission, and gear the 50 HP at 2200 RPM tractor engine down to 100 final output RPM through a different transmission, the output torque will be identical. But what happens when you throttle them down a little will vary greatly from engine to engine.

This post was edited by DanielW on 12/01/2023 at 10:24 am.

When I was in Jr high school Dad and I put a 4 cylinder 153CI Chevy 2 engine in a Farmall H. We put a belt drive governor on it and set it to 3000 RPM. We used it heavily on the farm until we bought a 190XT in 1972. At Farmall RPMs the Chevy 2 engine had about the same power as a stock H but run it up to 2800-3000 RPM it woke up considerably. We ran it with 6 IH suitcase weights up front and five wheel weights on each rear. It was on 15.5-38s full of fluid. It would play with 3-16s at 3000 RPM in 2nd gear, which was going pretty fast. It also spent a lot of time in front of a 12 foot disk and 12 foot field cumtivator also. All the nay sayers said it would never work and at first it was just a fun project but we kept weighting it down and pulling more with it. It was very dependable and used less fuel doing the same work as our Super MTA. I have an 8MM movie of it leading the SMTA, both pulling number 55 John Deere 3-16 plows.

YES, HP is HP but there is more to it than that.

An over the road truck engine might make 400-500 HP with 1550-1850 lb ft of torque at 1250 - 1350 RPM
Typical F1 racing engine might make 750 HP with around 200 lb ft of torque at over 18,000 RPM

If you gear that 18,000rpm down to 1350 rpm you need a 13.3 reduction gear. That would multiply
your torque by 13.3. 13.3 x 200 = 2660 lb ft.

I realize this is an extreme example, but the reason they don't use small high RPM engines is life span spam and reliability. An F1 engine only has to last about 90 minutes. Then it is totally rebuilt.

Same with tractors, slow and steady wins the race when plowing.

Certainly, if you want to make a swap, go for it. Sometimes we do things just because we WANT to!!!

You have a better handle on the torque/RPM/HP relationship than most. The Farmall H in my post below had 2 useful field gears, 1st and 2nd because the engine had to run almost 2X the OEM engibe to produce the required HP. Gear selection would have benefited from a 2X reduction in front of the H transmission. The Chevy 2 engine was much more durable than we ever expected. We kept pushing the envelope until we could pull the same equipment at the same speed as our SMTA which dynoed at 61HP. Near the end of it's field work days we were running it at 3500 RPM under load and at night the entire exhaust manifold was cherry red. That tractor made fools out of the won't pull a plow because that takes torque crowd who will never figure out that flywheel torque does nothing but register on a dyno, rim torque is what pulls a plow.

Chevy 2.8 V6 never made power. A 4.3 V6 would have been a better choice.

For sure the Honda (I own and ride that exact bike with 25K miles on it) would be unable to stay at 50 HP for hours coolan temp would be the issue, then engine wear at that RPM. It runs at 9000 to get that figure. It is powerful and quick from about 3000 to 9000. In lower gears one must be on open road to twist the throttle in lower gears. It is a short pushrod 4 valve Pent roof engine over square. Jim

How far does eight ice cream cones get you? How about hot dogs?

Quick way to make and h into a cub

CID and RPMs along with two constants is all you need to predict HP. However, If you can't maintain your RPMs HP drops off sharply HP can be made with either or, or a combination.

Thanks for all the good comments! I wouldn't expect one to go out and work all day like the original engine, but for a short burst it might be impressive! It's interesting that the Chevy 2 engine worked out so well.