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Old 05-24-2005, 09:26 PM   #1
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Default Piston Equivalency?

Piston Equivalency?

Does any one know the equivalency of a HPI #3 piston in a Losi. All I am wanting to do is go from Losi to HPI. I know the Losi pistons very well but I am not familiar with the HPI. I just want to Losi as close to that #3 HPI. Thanks for the help.
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Old 05-25-2005, 10:47 AM   #2
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Any one have any idea. This is the first time I have had a guestion not answered in the first 5 min. Kinda worries me.
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Old 05-25-2005, 11:30 AM   #3
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if i read your question right, you're intending to swap pistons between shocks. there's no real formula to convert from one to the other. your best bet is to do it one at a time and just feel the two, changing piston as needed until you reach nearly the same feel as the original configuration. it's not likely to produce the same dampening on the track, as the ability of the piston to allow fluid flow contributes to a characteristic of dampening referred to as 'pack'.

in short, it's a tough answer, and you'll have to allow time for the person who's walked down this exact path before to comment.... if there is one.
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Old 05-25-2005, 11:37 AM   #4
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also, if you get impatient, you may try to estimate the closest by taking the area of the piston holes and comparing the various pistons you'll want to use. but there are so many other factors at work here, that this will not get you very far. two of which being piston o.d. and thickness... sorry.
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Old 05-25-2005, 11:39 AM   #5
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Well you sort of have it correct. Becouse I have used Losi shocks for years that is what I know. However I have a HPI car. All I know is that it is dialed with #3 pistens. Becouse I know the Losi shocks better I want to put Losi shocks on a HPI car. But I would like to start with the similer setup as the HPI shocks with the Losi Shocks. So my question is in short is

#3 HPI = what losi [55,56,57 so on]

I plan to use the same oil and just want the pistons to "feel" the same. Thanks for the help.
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Old 05-25-2005, 12:20 PM   #6
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empiricism, my friend. just test the pistons until you get close. let human touch be your guide...
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Old 05-25-2005, 12:27 PM   #7
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Instead of guessing, why not take a losi shock dyno, put the hpi shock on one side and build 4 new losi shocks, one with each size piston and dyno them, It will tell you which one is closest
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Old 05-25-2005, 12:33 PM   #8
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Default Duh

Oh yah. I never thought of that. I have one and use it all the time. Thanks.
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Old 05-25-2005, 01:51 PM   #9
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Quote:
Originally Posted by DustinR
Oh yah. I never thought of that. I have one and use it all the time. Thanks.
Or use digital calipers to figure out the diameter of both pistons divide by two to get the radius, square it...then multiply by pi to figure out the surface area. Then subtract the area of the pistion holes.

# The area of a circle can be found by multiplying pi (which is equal to 3.14) by the square of the radius
# If a circle has a radius of 4, its area is 3.14*4*4=50.24
# If you know the diameter, the radius is 1/2 as large.


Thats another way...
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Old 05-25-2005, 03:41 PM   #10
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the analog losi tool? i have found my hands to be a better indicator using force as the relatively controlled variable and speed as the product. the losi will get you close, but it is sensitive to preload differences at slow speeds. infact, the faster you move the piston the less effect preload has and the better you are judging the dampening at typical operating speeds. either way you slice it, it's a coarse measurement at best.

soviet - while the equation for an area of a circle is fairly elementary, the actual effects of the geometry on dampening are tougher to predict. for your method, note also, the area of the annular ring formed by the difference between the od of the piston and the id of the cylinder. this is often over 50% of the total area for possible fluid flow. however, there is no way of knowing how much oil actually uses this as a pathway (blowby). i've seen differences in piston to cylinder diamters as much as .005 and as little as .002 for various manufacturers. work those out and tell me that doesn't impact the calculation with significance. at what viscosity is blowby negligible? at what "gap"? in my opinion, it's not worth the effort when you can just feel them. i'm telling you, fluid mechanics is not usually fun if it can be avoided altogether.

not to mention there are only four pistons to choose from (i think). the closest might just be obvious to the touch.
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