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Su Dashpot Oil Choice


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#1 mingy

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Posted 31 July 2011 - 08:41 PM

I am going to use an SU HIF 44 to fuel my 1380 engine when it,s rebuilt, i,ve read in various publications that a 5W type synthetic oil is best for the dashpot.
I used some 5W 20 synthetic oil that i was given by my neighbour who has a motorbike and thought this would be alright.
But a friend who used to race mini,s checked the operation of the piston and said that it was too slow in rising and falling.
He told me to use 3 in 1 oil but the advise given in any literature i have read states not to use it.
What type of oil should be used??????.

#2 minimender

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Posted 31 July 2011 - 09:46 PM

I am going to use an SU HIF 44 to fuel my 1380 engine when it,s rebuilt, i,ve read in various publications that a 5W type synthetic oil is best for the dashpot.
I used some 5W 20 synthetic oil that i was given by my neighbour who has a motorbike and thought this would be alright.
But a friend who used to race mini,s checked the operation of the piston and said that it was too slow in rising and falling.
He told me to use 3 in 1 oil but the advise given in any literature i have read states not to use it.
What type of oil should be used??????.

Depends on the age of the carb and how worn it might be. Older worn carbs allow the oil to seep into the carb and be drawn into the engine and burnt off

Heavy oil such as engine oil will dampen the piston so it rises slowly and lighter oil like 3 in 1 will allow the piston to rise quickly when given the boot.

Many arguments about this concerning performance but I would just use engine oil or if you are fussy consult the manual.

#3 Big_Adam

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Posted 31 July 2011 - 10:11 PM

This is like engine oil, its down to choice.

I've heard people use all sort, some guy brake fluid.

I use 10/40 el cheapo engine oil that I keep in the oil can. I tried 3in1 and found I don't like the way my car feels with it.

So try some other, find out what you like.

#4 liirge

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Posted 31 July 2011 - 10:27 PM

I'm a 20w50 man myself! but it really is up to you, lots of people will tell you the disadvantaged of using a multi oil like 20w50 or 10w40 etc. Personally i find that if you have a flat spot at all, try a thinker oil, usually sorts them out.

#5 puggered

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Posted 01 August 2011 - 01:01 AM

I have used 3 in one oil for over 10 years in my BL tins and never once had a problem however you could try this

Cheers, Sean

#6 Ethel

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Posted 01 August 2011 - 01:55 AM

Engine oil is what the Mini makers specified. Thicker oil giver more acceleration enrichment, thinner less.

#7 TimmyG

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Posted 01 August 2011 - 10:42 AM

as above, the oil is there to slow the piston down when you open the throttle fast and this temporarily richens the mixture, just like the accelerator pump jet on other carbs. If you put oil in that is too thick then you will get the enrichment but it could dampen the response as the piston moves too slowly and richens too much. What it really boils down to though in my opinion is that if the carb is set up correctly in the 1st place, then the oil that SU sell for the dashpot should be the right oil to use (funny that) So in other words if your engine is running lean in the first place (causing a flat spot on acceleration) then thickening the oil will help eliminate this. Conversely, if your engine is running rich to start with, then thinner oil will help to liven it up a bit when you pin it as the engine will need less enrichment.

Edited by TimmyG, 01 August 2011 - 12:35 PM.


#8 bmcecosse

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Posted 01 August 2011 - 11:15 AM

The thick oil holds the piston down - so it limits the amount of air getting in to the engine! And guess what - it's an air engine - it NEEDS air to run - air flow is the limiting factor!! We can always add fuel. The thick oil may be fine for poodling about town - it's hopeless if you want performance. This is where a Weber wins - there is no air flow restriction when you floor the pedal - and it pumps in fuel to make the enrichment. So - 3 in 1 is the way to go, but you may need to set the idle a bit rich to ensure it doesn't stumble. Obviously the spring strength comes in to this too. It wants to be just heavy enough to ensure the piston reaches full height at about 5000 rpm on full throttle. I have to say my sprint Mini ran best on track with NO damper in the carb - but it would have been useless on the road like that.

#9 Gulfclubby

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Posted 01 August 2011 - 11:18 AM

There is a modification to the piston dampener that will make it fall quicker. A Dremel an 5 minutes time will do the trick. Simply take off some of the solid disk on the dampener rod. That way, it will still be restricted on acceleration, enrichening the mixture, but the thick oil will flow better on deceleration/falling of the piston and thus make that happen faster.

#10 AndyMiniMad.

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Posted 01 August 2011 - 11:20 AM

+ 1 For the above!! 3 in 1 is what I use and I have not had any problems with hesitation or flat spots while accelerating.

Oh! mines a 1380 with an Hiff 44 as well.

#11 liirge

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Posted 01 August 2011 - 11:24 AM

Right lets think about this a little more, you really have to slow the piston a lot to affect the air flow to starve the engine, its only a temporary effect until acceleration has stopped.

And I bet you haven't considered the potential increase in torque, the smaller opening will increase the velocity of the charge flowing through the inlet system.

IT IS NOT HOPELESS if you want performance, I just wonder if you know how to exploit the SU carb Bmcecosse....

#12 TimmyG

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Posted 01 August 2011 - 12:15 PM

The thick oil holds the piston down - so it limits the amount of air getting in to the engine! And guess what - it's an air engine - it NEEDS air to run - air flow is the limiting factor!! We can always add fuel. The thick oil may be fine for poodling about town - it's hopeless if you want performance. This is where a Weber wins - there is no air flow restriction when you floor the pedal - and it pumps in fuel to make the enrichment. So - 3 in 1 is the way to go, but you may need to set the idle a bit rich to ensure it doesn't stumble. Obviously the spring strength comes in to this too. It wants to be just heavy enough to ensure the piston reaches full height at about 5000 rpm on full throttle. I have to say my sprint Mini ran best on track with NO damper in the carb - but it would have been useless on the road like that.

I see where you're coming from with the 3 in 1 - get the piston up faster and add more fuel instead - fair enough as an idea initially but flawed for a number of reasons, and it means you would have to be running rich all the time to avoid flat spots. As you say, richen it up to stop it stumbling. maybe ok on a race car but on the road it would ruin economy. Basically ruin fuelling everywhere just to correct wrong choice of oil!
But, back to the flaws.... say you are cruising along at 2000rpm and then you floor it, the engines demand for air is not going to be such that it needs the piston open that far anyway, as you said your self it shouldn't be fully open until 5000rpm (or more correctly around peak power rpm whatever that is....peak power equals peak demand for air...) so what will actually happen is that the piston will rise at a controlled rate to where the airflow dictates, the 'controlled' part being the damping and temporarily richening the mixture as the piston is open slightly less than its 'ideal' point at any given time. The reason it richens is not because of a restriction as such (like say a conventional choke butterfly on a fixed jet type carb) but because the flow through the carb remains much the same as the piston is rising..a venturi effect.....this is the reason it picks up more fuel....same flow, higher speed =greater ejector effect to pull fuel from the jet. As the piston rises to it's equilibrium against the spring pressure and weight the speed slows down and the normal mixture is resumed. On the other hand, say you are running at about half load, say 4000 odd revs at motorway speed for example, the piston will already be a good way open and again therefore will not pose a restriction as such to the airflow when you pin the throttle. You only have to look at just how little a 1.5 su restricts a big power engine (i'm assuming you have read Vizards book as you have quoted it before) to realise that an orifice that is only slightly smaller than ideal for a small amount of time is not going to affect power to any degree. Also it is not the oil which determines where the piston stabilises, it's the spring pressure. The oil slows it down, thats all.

#13 TimmyG

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Posted 01 August 2011 - 12:31 PM

Right lets think about this a little more, you really have to slow the piston a lot to affect the air flow to starve the engine, its only a temporary effect until acceleration has stopped.

And I bet you haven't considered the potential increase in torque, the smaller opening will increase the velocity of the charge flowing through the inlet system.

IT IS NOT HOPELESS if you want performance, I just wonder if you know how to exploit the SU carb Bmcecosse....

i think you're right, let's see from memory...worn throttle spindles? - doesn't matter, balance pipe? not needed,
dashpot oil? like piss or not at all! Are you sure your not running your a series on steam bmcecosse?

#14 liirge

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Posted 01 August 2011 - 02:29 PM

Right lets think about this a little more, you really have to slow the piston a lot to affect the air flow to starve the engine, its only a temporary effect until acceleration has stopped.

And I bet you haven't considered the potential increase in torque, the smaller opening will increase the velocity of the charge flowing through the inlet system.

IT IS NOT HOPELESS if you want performance, I just wonder if you know how to exploit the SU carb Bmcecosse....

i think you're right, let's see from memory...worn throttle spindles? - doesn't matter, balance pipe? not needed,
dashpot oil? like piss or not at all! Are you sure your not running your a series on steam bmcecosse?


You're response before the quoted pretty much is the reason to run the right oil and not an insanely thin one. I just didn't have the time to write such an expanded explanation.

You know how he does engine work don't you? he LEAVES the transmission IN the car, and splits the engine in situ!! insane, also claims it to be quicker than removing and stripping....

#15 bmcecosse

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Posted 01 August 2011 - 04:58 PM

Ahh - I see you follow my tips....... Well done! They WORK! You should try the engine trick next time you need the engine out - but not the gearbox.... But carry on - put thick oil in your SU - each to their own. Be assured - thin oil works best if set up correctly. And the piston should be fully up before peak power to maximise air flow - the air stream will continue to drag out more fuel as it gets faster and faster as the revs rise even higher. I am aware of a steam powered A series in a Minor Pick-up, but sadly it's not mine......... ;)




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