Switch 22hp gas to 18hp diesel, will it be less powerful?

[Blackbird_BX]

It does weight more than the gasser but not by as much as you think. About 125 compared to 90 for the gasser.

It sounds like you've worked out a plan and the Yanmar diesel is a far better power plant than the original. Good luck with it.

You might try throwing 35 lbs on the front of the tractor to see how the extra weight affects it. 35 lbs might not sound like a lot, but it is around a 38% increase if my math is right. I would think that steering would be a lot heavier. Maybe the next project will be power steering.

 

[LBrown59]

New diesel mowers are very expensive.
1*I believe the lowest price diesel in John Deeres lineup is over $10k .
2*I'm sure Kubota's is not far behind. .

1*Or about $15000 for the x749.
2* I just bought a new BX1500 for $7750.
Rite before I bought it I'd gotten a price on a BX1850 for $9100.

 

[MossRoad]

18.5HP is 3.5 HP less than 22HP no matter what kind of math you use. The important thing to note is that the HPs are produced at significantly different RPMs. If we assume the 22HP motor is measured at 3600RPM and the 18.5HP motor is measured at 1000 RPMS less, than we can do some figuring.

Torque = (HP * 5252)/ RPM

The 22HP motor at 3600 RPM should produce about 32 ft pounds of torque.
32.1 ft. pounds of torque = (22HP * 5252)/3600RPM

The 18.5HP motor at 1000 less RPMs, or about 2600 RPM should produce about 37 ft pounds of torque.
37.4 ft. pounds of torque = (18.5 * 5252)/2600RPM

The lower HP motor will produce more torque at those RPMs.

But if the 18.5HP engine produces that HP at 3026RPM, the torque is the same as the 22HP motor and if it produces it at something higher than 3026 RPM, it will have less torque than the 22HP motor.

So, you need to know at what RPM the motor produces it's rated HP to see if it actually has more torque. Or, just look up the torque rating of the engines and compare them.

Personally, for $1200 to $1700 dollars just for an engine I'd just buy a brand new lawn tractor and not hassle with an engine swap, pulley changes, possible radiator if it is water cooled, replacing the front axle to handle the extra weight, etc... If however, you already had a diesel engine to use and you just plain enjoy making stuff, then go for it.

 

[allenr]

Often threatened to do the same on my petrol (gas) mower.

I'm sure you'll find it'll have plenty power.

My mower has a Briggs and Stratton engine and while I usually regard anything from the USA as being top quality and reliable I reckon these are a slur on the USA's good name.

 

[MossRoad]

Often threatened to do the same on my petrol (gas) mower.

I'm sure you'll find it'll have plenty power.

My mower has a Briggs and Stratton engine and while I usually regard anything from the USA as being top quality and reliable I reckon these are a slur on the USA's good name.

I have a 23 year old Briggs that has never had an oil change, spark plug change, or air cleaner change and still starts on the first pull. They are good engines for the price.

 

[Superduper]

It sounds like you've worked out a plan and the Yanmar diesel is a far better power plant than the original. Good luck with it.

You might try throwing 35 lbs on the front of the tractor to see how the extra weight affects it. 35 lbs might not sound like a lot, but it is around a 38% increase if my math is right. I would think that steering would be a lot heavier. Maybe the next project will be power steering.

I'm about 160lbs and the tractor weighs about 800lbs. I think that it should be able to handle 35 lbs fairly well.

 

[oliver28472]

Do you already own the Yanmar engine?

 

[Soundguy]

Back to the pully issue, and rpm issues.

I'd think you only have to swap to a larger pulley ont he engine.. nothing else.

Also... I think you guys are probably guessing correctly about the diesel engine rpm operating range.. and I'd also agree that the whizzbang gasser is a 3600 rpm job..

soundguy

 

[Superduper]

Ok guys, looks like I have a project now. I've acquired an engine and once the weather improves a bit, I'll start on it. It will probably be a couple of months before it's complete but I'll document my experience in doing so for those who would be interested.

I want to thank everyone who contributed their input.

I've attached 3 files: The specs on the donor engine, the specs on the current engine, and an image of the guinea pig tractor.

I know some of you think that this is a waste of time or effort but if it works out well, I will consider it a success. I'm actually very fond of this tractor. This being the reason I'm willing to invest the time/work into the swap. Sears does make some very inexpensive tractors, this was not one of them. This is a HST model purchased in 1998 for around $3200. It was their best at the time and is part of their Garden Tractor series with transaxle rated for ground engaging work. The mower deck is a 3 blade system and fairly heavy guage (comparatively speaking, 10 or 11 guage, I believe). Anyhow, I no longer require it to do anything but mow the field so as long as it has enough power to do that, and do it reliably, I'm happy with my time/money investment.

Mossroad: I know exactly what you mean by buying a new lawn tractor rather than dealing with the swap. But I've been there, done that. Had Murray lawn tractor, became unreliable, tired of changing batteries, tired of tires going flat, tired of rebuilding carburetor, etc. So, bought new Craftsman tractor. Old Murray still sitting in the field (10 years now). Result, new tractor needs new battery every year (fixed by modifying battery tray to accept CAR sized Optima battery), tires go flat every year and slips off rim (fixed by installing new Chev Ag type tires, much beefier and never lost a sip of air since). Now, engine is starting to require annual carb rebuilds, etc... No, I've no great desire to have another one of these in my field. When my toaster goes out, I'll toss that and buy another. But the current tractor deserves a little more patience....

 

[mopacman]

In automotive engine swaps gas to diesel they say to compair cu. in. not HP. If this is true you are way ahead in this swap. I am looking forward to this swap.......Larry