How does CCs affect blade power and torque?

[Little Fish]

I'm late to this discussion but wanted to add my 2-cents about replacement engines. About 30 years ago I had a small engine repair shop. I found that the vertical shaft B&S engines that Craftsman placed on their push mower decks were unique due to PTO shaft length and blade mounting. They forced most customers to have to get their replacement engine from Sears/Craftsman. Briggs & Stratton would not sell the engine to even repair shops. So if you are planning to replace your engine on a Craftsman deck be mindfully of that.

This is a new craftsman. I think it may have changed since B&D bought them out. But it may he the same, who knows.

 

[TobyU]

Did you read the first reply to this thread? It answered my question. Only issue after that was what would be enough power to get through the grass. Which I would still ask a pro, since they have more experience than I do (I was actually a pro, but I only ran a Honda and did small jobs, so I never experienced other movers and how they work).

And where else would I ask about this? Car guys won't know, and engineers are too busy making money and working to death to reply to some random guy on a forum LOL

That first answer did answer your question but your questions were not the only ones that needed answered.

Cutting tall grass is not just about engine power.
It's mostly about discharging the grass out the side with a side discharge option that many have AND using the discharge deflector attachment.

Mulching and tall grass do not go together

You would be amazed how different the same mower is with flap down and mulching vs lifting it and putting the chute in (always with engine off of course).

It's like a completely different mower.

Also set your engine rpms to about 3000-3100 as some will be down around 2800.

Also you need to cut tall overgrown grass on highest height setting out maybe 1 notch down.
This helps a lot too.

I don't EVER cut grass that just needs to be cut. Everytime I cut it is quite overgrown.

 

[TobyU]

Depending on the blade diameter, the most common engine CC ratings are 159 and 196 for walk behind mowers. These numbers divided by 15 gives you a rough idea of its horse power at maximum rpm. (Back in the old days, this was what was on the engines label telling you what it was rated at.) This answers your question in the simplest terms. If you care to read below, you’ll find the rest of the story below:

What you need to efficiently cut grass (work) is torque and this is dependent on engine rpm. The higher the rpm, the higher the torque the maximum level of which is also dependent on the number of CC’s. Lawnmower engines typically are regulated to run around 1500 rpm by the governor at no load (it’s not doing any work,). At this speed the engine is producing about 40% of its rated torque. When you start to cut grass (load), the engine begins to slow down. The governor senses this and opens the throttle causing the engine rpm and the torque to increase the necessary amount to cut the grass. (You will notice an increase in engine noise.) It will open the throttle until the engine is producing it’s maximum rated torque. Any significant increase in the load after this will cause the engine to stall.

Indecently. A 21” mower with a 140CC, Though that’s a little light, should be able to cut grass 4 to 5” tall. If your mower has a lot of miles on it, I’d take the cover off and make sure the governor linkage is free to move.

I welcome that If anyone notices any significant mistakes in the above to correct me.

I was going to ask where you came up with these numbers....
Not accurate at all.

Dividing CC's by 15 isn't even close to hp (unless you have a supercharger to put on it too).
It's more like divide by 25- almost 30. But there really isn't a common rule of thumb like that.

RPMs - mowers are typically fairly close to 3000 RPMs.
Doesn't matter if it's no load or not as the governor does do what you explained to maintain the same speed under load but only up to the engines torque ability. The most it can do is open throttle 100% and it often does that in taller grass.

Many mowers are set to 2900 +or- 100 rpm. Some are at 3100 etc.
They used to rate the engines at 3600 for hp ratings but that changed 25+ years ago.
Blades also got longer with old mowers being 18,19,20 inches until 21 became most common and then lots of 22s.

This forced them to lower RPMs to keep blade tip speeds under the legal limit.

 

[Little Fish]

That first answer did answer your question but your questions were not the only ones that needed answered.

Cutting tall grass is not just about engine power.
It's mostly about discharging the grass out the side with a side discharge option that many have AND using the discharge deflector attachment.

Mulching and tall grass do not go together

You would be amazed how different the same mower is with flap down and mulching vs lifting it and putting the chute in (always with engine off of course).

It's like a completely different mower.

Also set your engine rpms to about 3000-3100 as some will be down around 2800.

Also you need to cut tall overgrown grass on highest height setting out maybe 1 notch down.
This helps a lot too.

I don't EVER cut grass that just needs to be cut. Everytime I cut it is quite overgrown.

I tried that. I only have the bag for the mower (just found it as I was cleaning out the garage the other day)

That first answer did answer your question but your questions were not the only ones that needed answered.

Cutting tall grass is not just about engine power.
It's mostly about discharging the grass out the side with a side discharge option that many have AND using the discharge deflector attachment.

Mulching and tall grass do not go together

You would be amazed how different the same mower is with flap down and mulching vs lifting it and putting the chute in (always with engine off of course).

It's like a completely different mower.

Also set your engine rpms to about 3000-3100 as some will be down around 2800.

Also you need to cut tall overgrown grass on highest height setting out maybe 1 notch down.
This helps a lot too.

I don't EVER cut grass that just needs to be cut. Everytime I cut it is quite overgrown.

I tried to open the side discharge. I don't have a chute, so it just clogged immediately (I had a stick to hold it open). Same with the rear discharge, except it only clogged if I didn't set it to fully open. It worked better, but it still bogged down bad.

For all the people saying to set it to the highest setting: Duh. That was the first thing I did. 4" isn't really gonna make a difference to 18-24" grass cutting LOL. Even Hondas only go to 5 or 6"...

 

[eyekue138]

Did you read the first reply to this thread? It answered my question. Only issue after that was what would be enough power to get through the grass. Which I would still ask a pro, since they have more experience than I do (I was actually a pro, but I only ran a Honda and did small jobs, so I never experienced other movers and how they work).

And where else would I ask about this? Car guys won't know, and engineers are too busy making money and working to death to reply to some random guy on a forum LOL

If your question was answered already, just say "Thank you" and don't respond any more.

 

[eyekue138]

I tried that. I only have the bag for the mower (just found it as I was cleaning out the garage the other day)

I tried to open the side discharge. I don't have a chute, so it just clogged immediately (I had a stick to hold it open). Same with the rear discharge, except it only clogged if I didn't set it to fully open. It worked better, but it still bogged down bad.

For all the people saying to set it to the highest setting: Duh. That was the first thing I did. 4" isn't really gonna make a difference to 18-24" grass cutting LOL. Even Hondas only go to 5 or 6"...

Hire a heavy-hitter mower for the first cut, then keep it mowed short.

 

[StarTech]

Push are never meant to be used as a bush hog. HD walk behind string trimmers works better but even they have their limits. Even when I was teenager I when I had to cut high grass it took a lot forward and backward movement of the mowers. And like Sunday here I was using a Gravely HD52 ZTR that keep clogging up the deck in high thick grass. Whenever the deck stopped discharging I would have back up raise the deck and then disengage to PTO so the grass clippings would fall down where they suspended over the blades. Then re-engage the PTO and lower the deck and cut until it stopped discharging again. Every so often I had repeat this cycle but I finally got the tall thick grass mowed.

 

[GearHead36]

I tried that. I only have the bag for the mower (just found it as I was cleaning out the garage the other day)

I tried to open the side discharge. I don't have a chute, so it just clogged immediately (I had a stick to hold it open). Same with the rear discharge, except it only clogged if I didn't set it to fully open. It worked better, but it still bogged down bad.

For all the people saying to set it to the highest setting: Duh. That was the first thing I did. 4" isn't really gonna make a difference to 18-24" grass cutting LOL. Even Hondas only go to 5 or 6"...

So you have a cheap residential push mower that can't be set up for side discharge because you lost the chute, you want to mow 18"-24" WET grass, your mower won't do it... and you think that a larger engine is the answer?

BTW, mowing high WILL make a difference. The lower you go, the thicker the grass gets.

 

[jimbir]

I was going to ask where you came up with these numbers....
Not accurate at all.

Dividing CC's by 15 isn't even close to hp (unless you have a supercharger to put on it too).
It's more like divide by 25- almost 30. But there really isn't a common rule of thumb like that.

RPMs - mowers are typically fairly close to 3000 RPMs.
Doesn't matter if it's no load or not as the governor does do what you explained to maintain the same speed under load but only up to the engines torque ability. The most it can do is open throttle 100% and it often does that in taller grass.

Many mowers are set to 2900 +or- 100 rpm. Some are at 3100 etc.
They used to rate the engines at 3600 for hp ratings but that changed 25+ years ago.
Blades also got longer with old mowers being 18,19,20 inches until 21 became most common and then lots of 22s.

This forced them to lower RPMs to keep blade tip speeds under the legal limit.

Thanks for the question. I got my numbers from an online calculator. I'll admit I was a very skeptical. I did more research and found a number of sites that showed the divisor could be anywhere between 25 and 40. This makes a lot more sense to me. In the future, I'm going to stick with 40 and even that seems a little low

 

[DinosaurMike]

Little Fish, Too many pages for me to read again, too few remaining brain cells to remember.
Did you get the mess cut down? Are the waterlogged lawn and tall grass issues annual? How deep into the year do they remain? Do they reoccur during the summer with heavy rain? Do neighbors have the same issues? Please add some pictures of your bog (not trying to be nasty).
People have posted some good ideas. I hope by now that you have given up on the Tim Allen engine replacement approach. Please answer my questions and hopefully followers will reset their thinking.