Dashpot oil viscosity

[Ed_K]

After reading every post on the forum about the dashpot dampers and oils that could be used, I came up with this list. I am trying to put them in order of viscosity but I don't know what several of them are indicated by " ??w ". If you know one or more of them, please post. I want to experiment by using no oil in my dashpots and then work up from 0 viscosity until the car accelerates without back firing or hesitating ( after it is fully warmed up).
Here is what I found in the old pages of this forum;

1.. Use nothing at all 0
2.. Singer sewing machine oil ??w
3.. Marvel mystery oil 3w
4.. Three and one oil ??w
5.. Motorcycle front fork oil 20w ?
6.. ATF fluid( didn't say which type) 5-20w
7.. Expensive oil sold for this purpose ??w
8.. Motor oil 20w (never seen it)
9.. Motor oil 30w
10. Motor oil ( multi viscosity ) ??w-30

Thanks

I think the key to optimum acceleration with SU carbs is to use the lowest viscosity dash pot damper oil that your car will tolerate.


.... I can't get the list to display lined up ?

 

[GregW]

Hi Ed,
Motorcycle fork oil comes in many viscosities (2.5wt, 5wt, 7.5wt, 10wt, 15wt, 20wt, 30wt). To add to the confusion, two brands advertising the same weight are actually different.

 

[Dave Russell]

After reading every post on the forum about the dashpot dampers and oils that could be used, I came up with this list. I am trying to put them in order of viscosity but I don't know what several of them are indicated by " ??w ".
Thanks
I think the key to optimum acceleration with SU carbs is to use the lowest viscosity dash pot damper oil that your car will tolerate.

Hi Ed,
See if this helps explain the "W" designation on oils.
https://www.upmpg.com/tech_articles/motoroil_viscosity/
A 20W oil would have it's viscosity rated at a low temperature.
A 20 oil would have it's viscosity rated at 100 degrees C.
A 20W-50 oil would have a viscosity rating of 20 wt oil at low temperature & a viscosity rating of 50 wt oil at 100 degrees C.


The attached chart should help visualize the differences.
A 10W oil would have a viscosity of about 25 cST at 40 C & about 5 cST at 100C.
A 20W-50 oil would have a viscosity of about 50 cST at 40 C & 18 cST at 100C.

As an aside, you can also see how the various gear oils relate to motor oil viscosities.

Since the carbs are relatively cold, the lower temperature ratings would be closer to the actual damper conditions.
D

 

[zblu]

I wonder where olive oil sans the rest of the salad dressing ingredients would rate?

 

[GregW]

Extra virgin?

 

[zblu]

Definately, first cold pressing only

 

[GregW]

Excellent, you can make a salad for me any time. :thumbsup:

 

[Coastalman]

Ed,
I have used Marvel Mystry Oil since the 70s when I was racing a Bugeye and rebuilding MGAs and other British cars. It seems like that is a popular oil to use in the Raleigh area for SUs. (At least it was and Carl still uses it in his race car). I have found it to work well.
Charlie

 

[Ed_K]

Dave,
As usual, you are a walking library of information. I will save
that chart that you have provided. What I am trying to do is find out what the 3-1 and sewing machine oils were in viscosity so I can attempt rank all of them in increasing order of viscosity. I didn't realize that motorcycle fork oil came in so many different viscosities but I guess it should not surprise me. Which type of ATF fluid is 5-20 ? Ford, dextron III, Chrysler ?

Charlie,
I have a can of marvel mystery oil that my dad gave me so I was
planning on using that in my " experiment ". That is if my car doesn't work well without any oil at all in the dampers like Dave runs his car.
MMO appears to be the lowest viscosity oil available so I will try that first. If my car doesn't do well with it, is 3-1 or sewing machine oil around 10w to 15w ? If so, that would be my 2nd choice. I will keep going until the car runs well under full acceleration without backfiring or hesitating. I don't think my car was getting enough air ( or too much fuel ) under full acceleration so I cleaned out the dampers yesterday and decided to experiment a little.
Ed

 

[Dave Russell]

Ed,
On the ATF viscosities,
Dextron II & type F are rated as 5W-20.
Dextron III, Mercon V, ATF Plus 4, are rated as 0W-20.

As near as I can tell, the special SU damper oil is just 20 ND motor oil. Quite profitable.
D

 

[healeynut]

Standard SU dashpot oil is 20wt oil.

Whatever you put in there, you definitely do not want multiweight - the SU uses single weight oil because as the car warms up, it needs a less rich mixture on acceleration - if you use a multiweight it means your mixture will run too rich on acceleraton after the car warms up.

The primary purpose of the dashpot oil is to slow the piston's upward movement when cold, to reduce the requirement for choke at idle (i.e. and waste gas), and to damp the upward movement of the piston just enough when hot to keep the car from getting too lean, skipping and prefiring.

I'd say any sort of 10wt or 20wt should be fine. Going less than that may cause your car to skip when under load (e.g. going up a hill).

 

[GregW]

Standard SU dashpot oil is 20wt oil.

Whatever you put in there, you definitely do not want multiweight - the SU uses single weight oil because as the car warms up, it needs a less rich mixture on acceleration - if you use a multiweight it means your mixture will run too rich on acceleraton after the car warms up.

In the context of this thread, if Ed wants to experiment with multi-vis oils and post the results, I'd say "go for it, and thanks". I have a doubt that the oil in the dashpots would get hot enough to fully change.

Going less than that may cause your car to skip when under load (e.g. going up a hill).

Under a constant RPM or a gradual increase, the oil would not have any real effect. Needle profile and damper spring rate would be a bigger consideration.

 

[Ed_K]

This experiment didn't take very long;

1.. I started with nothing at all in the dampers.
I took it down the road until it warmed up all the way. Then I eased it up through the gears until in fourth. I floored it once in fourth, the result; snap, crackle, and pop through the carbs. I slowed down a little and downshifted into 3rd, then floored it again, the result; pop . End of step 1 of my experiment. Back to my garage.

2.. I filled up the dampers with Marvel Mystery oil as this is said to be 3w viscosity by someone on the web. I have no idea if he knew what he was talking about, but it is a very low viscosity, ( runny ), oil and the lowest viscosity oil I had. Then I went back out on the road.
There was no hint of any backfires and it ran smooth no matter what I was doing. Since it was working well, I kept driving downtown to a radiator shop to check on getting an uprated core installed between my original tanks. There seems to be a desirable side effect of using this light weight oil, the car seems to idle better at stop lights AND I don't smell very much unburned gasoline when sitting at a stop light for a long time ? I have no idea why since the damper oil doesn't do a thing until you step on the gas pedal ? Maybe I was imagining it or there was a breeze every time I was stopped at a light this afternoon ?

... Conclusion and discussion;

I am fairly certain that I have the original dashpot springs and if they are anything like my oil pressure bypass spring ,( old, tired spring steel that has long ago lost it's temper ), then perhaps if I bought some new ones or some of increased strength, maybe I could get away without using any oil in the dampers. But since the car seems to work well with this light viscosity oil, and they put those dampers there for a reason, why not use Marvel Mystery oil in the dashpot dampers ? The car seems to accelerate a little better when I first floor the gas peddle. I should explain that I had been using 30w motor oil in the dampers until I started this experiment. I think that was way too thick to allow the dampers to open back up in a reasonable amount of time after flooring it.
So I guess Charlie ( Coastalman )and Carl had this figured out correctly back in the day when they were racing LBCs . One can of Marvel Mystery oil should last me the rest of my life since I have not used this for anything else so far.
Ed

 

[Dave Richards]

Great to know, but what am I going to do with a <span style="text-decoration: underline">whole</span> bottle of Singer sewing machine oil???

 

[Ed_K]

Hi Dave,
Use whatever you have a can of in your garage.
Ed

 

[GregW]

Hi Ed,
Don't know if you've read it, there's a book called "How to Build and Power Tune S.U. Carburettors" that you may be interested in (or not).