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Thread: Does it harm the engine to use the throttle to lower the RPM's on col water units.

  1. #1

    Default Does it harm the engine to use the throttle to lower the RPM's on col water units.

    This was a question posed on Facebook here; https://www.facebook.com/groups/uamc...comment_follow

    "David D'Eramo

    March 25 at 6:16pm
    Would like to clarify something..
    If I have an 8/3500, and I only run the throttle half way, what am I effecting? GPM? PSI? Both?"




    That question morphed into a discussion about the possibility of harming the engine if you do it. I'm going to attempt to explain the hows and whys of this.

    OK, first this probably came about from dealers such as myself telling users to not adjust the rpms of their pw using the throttle. That was because the vast majority of units we sold or worked on were hot water units. As recently as the late 90's cold water units were pretty rare as compared to hot water units. The market was dominated by hot water units and most available info was related to the use of hot water units. There are multiple reasons why this is a bad thing to do on hot water units. I'm not going to get into that, it is a totally different subject and completely irrelevant on cold water units.

    First I'm going to explain how a pressure washer pump works. It is pretty simple. We use a positive displacement pump that takes in and puts out a specific "amount" of water with every revolution. If the pump is in proper working condition, this "amount" never fluctuates. It is the same on the first revolution and the last revolution.
    We are going to use a TSF2021 as our example pump. It has a rating of 8.5gpm @ 1750 rpms or a discharge of .00485714 gallon per revolution. To get to 8gpm we have to turn the pump exactly 1647.05979 rpms. Here is how that is determined. We take the known rated output of 8.5 and divide it by the rpms of 1750 and come up with .00485714. We then divide our desired flow of 8 gpms by the amount of flow this pump produces with every revolution. 8/.00485714= ~1647 rpms or approximately 1gpm for every 200rpms.

    That amount of water flow we are going to get is fixed in relation to the revolution of the pump. That won't change. What can and will vary is the rpms we are turning the pump and we can do that by adjusting the throttle on the engine.

    Our engine example will be a Honda GX690. Here are the important specs;

    Net Power Output* 22.1 hp (16.5 kW) @ 3,600 rpm
    Net Torque 35.6 lb-ft (48.3 Nm) @ 2,500 rpm

    As we can see, the max horse power is at 3600rpms and the max torque is at 2500rpms. The power curve for the torque is pretty much a straight line, but falls off at the engine revs beyond 2500. The HP curve is a pretty consistent from 10hp@2000 rpms to 22.1@3600. http://engines.honda.com/models/model-detail/gx690 The more rpms, the more hp and the increase is not exactly proportional but it is really close. Certainly close enough considering what is coming next.

    Now we are going to compare the HP requirements of the pump at various rpms, to the hp output of the motor at the corresponding rpm. As previously discussed, the flow per revolution will remain constant.

    We will assume the RPMs of the engine are set to max for the pulley selection. (3600) Lets also assume a 3.0" engine pulley. We are going to want to turn the pump 1647 rpms to get the 8 gpms required. That means we will have a ~ 6.5" pump pulley. (6.55, but the reality is we just get them close).

    So now we have a pressure washer with a GX690 turning 3600 rpms turning a General TSF2021 pump 1647 rpms. No nozzle yet, so we are just putting in and getting out 8gpm.

    Now let's figure out just how much pressure we can get out of our unit. We have 22.1hp to use and 8 gpms. The formula to determine this is a controversial subject and I think I might make that a follow up post. For this post, I will just use the more recent GPMXPSI/1460= electric hp required. Then the multiple of 1.5 must be added for gasoline engines. So 8Xy/1460X1.5= 22.1 y=unknown

    22.1/1.5= 14.73 ehp X 1460 =21,505.8/8gpm= 2,688.225 psi

    So the max we can get is 2688 psi at 8 fpm with a GX690. This mimics real world pressures I see, so I know it is correct. As an aside, a Predator will only do about 2300psi.

    So here is the first dagger in the heart "of running a pressure washer at a lower RPM will destroy the engine" and it is a huge one. Manufacturers are severely under-horsepowering their units in an attempt to make bigger more powerful units at a lower price.

    Guess what this does? Attempting to drive 8 GPM through a size 8.5 nozzle (Actual nozzle rating 7.95gpm@3500. Like the pulley we get these close) to get 3500psi causes...the engine to operate at a load that exceeds it's rating and reduces the engine rpm to a level that allows the volume to drop sufficiently for the engine's available horsepower.

    So here we are right off the bat trying to run at 3600 rpms and can't because we don't have the available power to do so. Now this is how you can damage an engine. (We can follow up on this later and there are really easy ways to test this for yourself by getting a tachometer and measuring the rpms)

    Lets go to the low end of the spectrum though since that was what this was about.

    Let's drop our engine to 2000 rpms where it has 10hp available.

    That drops our output to 4.48gpms (determined by our pump output of .00485714gpr X 923 pump rpms which was determined by 2000X3/6.5=923)

    4.48gpm through a size 8.5 nozzle is 1125 psi. Our formula for hp required comes into play here again.

    4.48X1125= 5040/1460= 3.45 X 1.5= 5.17 hp required. That is way less than the 10 we have. Even if you still have concerns about the spike, you still have 10 hp available to drive almost zero volume to 3800 psi to spike.

    To sum all of that crap up, use your throttle to lower your rpms on cold water units. It is completely safe. Hot water units are a completely different story.

    I don't want to proof read all that BS and the math might be off somewhere but it is all close. So I reserve the right to edit any glaring mistake. lol


    Kevin Porter
    EnviroTek Cleaning Systems
    903.759.9461

  • #2

    Default Re: Does it harm the engine to use the throttle to lower the RPM's on col water units.

    So all that being true which I believe, there is an rpm where you would not want to fall below. My guess is like 2000 and then you could begin to fall short of the horsepower/torque to bypass. Obviously the engine will die below a certain rpm, but in theory there would be a point where the engine would not die, but could over drive the valve train everytime the trigger is being released provided the unloader has not been adjusted to lower psi. Is my theory correct?

    Sent from my SM-G920V using Tapatalk

  • #3

    Default Re: Does it harm the engine to use the throttle to lower the RPM's on col water units.

    Quote Originally Posted by Kevin Porter View Post
    This was a question posed on Facebook here; https://www.facebook.com/groups/uamc...comment_follow

    "David D'Eramo

    March 25 at 6:16pm
    Would like to clarify something..
    If I have an 8/3500, and I only run the throttle half way, what am I effecting? GPM? PSI? Both?"




    That question morphed into a discussion about the possibility of harming the engine if you do it. I'm going to attempt to explain the hows and whys of this.

    OK, first this probably came about from dealers such as myself telling users to not adjust the rpms of their pw using the throttle. That was because the vast majority of units we sold or worked on were hot water units. As recently as the late 90's cold water units were pretty rare as compared to hot water units. The market was dominated by hot water units and most available info was related to the use of hot water units. There are multiple reasons why this is a bad thing to do on hot water units. I'm not going to get into that, it is a totally different subject and completely irrelevant on cold water units.

    First I'm going to explain how a pressure washer pump works. It is pretty simple. We use a positive displacement pump that takes in and puts out a specific "amount" of water with every revolution. If the pump is in proper working condition, this "amount" never fluctuates. It is the same on the first revolution and the last revolution.
    We are going to use a TSF2021 as our example pump. It has a rating of 8.5gpm @ 1750 rpms or a discharge of .00485714 gallon per revolution. To get to 8gpm we have to turn the pump exactly 1647.05979 rpms. Here is how that is determined. We take the known rated output of 8.5 and divide it by the rpms of 1750 and come up with .00485714. We then divide our desired flow of 8 gpms by the amount of flow this pump produces with every revolution. 8/.00485714= ~1647 rpms or approximately 1gpm for every 200rpms.

    That amount of water flow we are going to get is fixed in relation to the revolution of the pump. That won't change. What can and will vary is the rpms we are turning the pump and we can do that by adjusting the throttle on the engine.

    Our engine example will be a Honda GX690. Here are the important specs;

    Net Power Output* 22.1 hp (16.5 kW) @ 3,600 rpm
    Net Torque 35.6 lb-ft (48.3 Nm) @ 2,500 rpm

    As we can see, the max horse power is at 3600rpms and the max torque is at 2500rpms. The power curve for the torque is pretty much a straight line, but falls off at the engine revs beyond 2500. The HP curve is a pretty consistent from 10hp@2000 rpms to 22.1@3600. http://engines.honda.com/models/model-detail/gx690 The more rpms, the more hp and the increase is not exactly proportional but it is really close. Certainly close enough considering what is coming next.

    Now we are going to compare the HP requirements of the pump at various rpms, to the hp output of the motor at the corresponding rpm. As previously discussed, the flow per revolution will remain constant.

    We will assume the RPMs of the engine are set to max for the pulley selection. (3600) Lets also assume a 3.0" engine pulley. We are going to want to turn the pump 1647 rpms to get the 8 gpms required. That means we will have a ~ 6.5" pump pulley. (6.55, but the reality is we just get them close).

    So now we have a pressure washer with a GX690 turning 3600 rpms turning a General TSF2021 pump 1647 rpms. No nozzle yet, so we are just putting in and getting out 8gpm.

    Now let's figure out just how much pressure we can get out of our unit. We have 22.1hp to use and 8 gpms. The formula to determine this is a controversial subject and I think I might make that a follow up post. For this post, I will just use the more recent GPMXPSI/1460= electric hp required. Then the multiple of 1.5 must be added for gasoline engines. So 8Xy/1460X1.5= 22.1 y=unknown

    22.1/1.5= 14.73 ehp X 1460 =21,505.8/8gpm= 2,688.225 psi

    So the max we can get is 2688 psi at 8 fpm with a GX690. This mimics real world pressures I see, so I know it is correct. As an aside, a Predator will only do about 2300psi.

    So here is the first dagger in the heart "of running a pressure washer at a lower RPM will destroy the engine" and it is a huge one. Manufacturers are severely under-horsepowering their units in an attempt to make bigger more powerful units at a lower price.

    Guess what this does? Attempting to drive 8 GPM through a size 8.5 nozzle (Actual nozzle rating 7.95gpm@3500. Like the pulley we get these close) to get 3500psi causes...the engine to operate at a load that exceeds it's rating and reduces the engine rpm to a level that allows the volume to drop sufficiently for the engine's available horsepower.

    So here we are right off the bat trying to run at 3600 rpms and can't because we don't have the available power to do so. Now this is how you can damage an engine. (We can follow up on this later and there are really easy ways to test this for yourself by getting a tachometer and measuring the rpms)

    Lets go to the low end of the spectrum though since that was what this was about.

    Let's drop our engine to 2000 rpms where it has 10hp available.

    That drops our output to 4.48gpms (determined by our pump output of .00485714gpr X 923 pump rpms which was determined by 2000X3/6.5=923)

    4.48gpm through a size 8.5 nozzle is 1125 psi. Our formula for hp required comes into play here again.

    4.48X1125= 5040/1460= 3.45 X 1.5= 5.17 hp required. That is way less than the 10 we have. Even if you still have concerns about the spike, you still have 10 hp available to drive almost zero volume to 3800 psi to spike.

    To sum all of that crap up, use your throttle to lower your rpms on cold water units. It is completely safe. Hot water units are a completely different story.

    I don't want to proof read all that BS and the math might be off somewhere but it is all close. So I reserve the right to edit any glaring mistake. lol


    Thank you so much Kevin, mind blown...Look forward to your follow up thread.
    Jeff SmithSparkle Works Power Washing LLCAugusta, GA(706) 200-8383 Sparkle Works Click HereOur informative pressure washing facebook page.The easy decision for Pressure Washing in Augusta GA. Sparkle Works on Youtube

  • #4

    Default Re: Does it harm the engine to use the throttle to lower the RPM's on col water units.

    Quote Originally Posted by George Clarke View Post
    So all that being true which I believe, there is an rpm where you would not want to fall below. My guess is like 2000 and then you could begin to fall short of the horsepower/torque to bypass. Obviously the engine will die below a certain rpm, but in theory there would be a point where the engine would not die, but could over drive the valve train everytime the trigger is being released provided the unloader has not been adjusted to lower psi. Is my theory correct?

    Sent from my SM-G920V using Tapatalk

    I'm not sure, I might test one tomorrow. If so I will post what I find out about the lowest rpm. I did find one mistake in my post. The hp output is actually 13 plus hp at 2000 rpms. That means at 2000 rpms it has more power than a GX390 has and those easily drive the spike pressures to ranges that unload.
    Kevin Porter
    EnviroTek Cleaning Systems
    903.759.9461

  • #5

    Default Re: Does it harm the engine to use the throttle to lower the RPM's on col water units.

    This is a cool topic. I only ever felt the need to lower rpm once I went from gx 390 to gx 690 and realized I didn't need 8 gpm for a one story house and it didn't seem efficient to run full throttle or push too much water toward the house.

    Sent from my SM-G920V using Tapatalk

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