32 replies to this topic

[liirge]

I still don't think you understand, it's been described to you that having a ridiculously thin oil can't work as well....so you are not just giving out bad advice out of being stubborn, I can take a good guess where that trait came from.

Yer I've seen the steam powered engine and quite a few of the diesel engines, interesting pieces of kit.

[MJWarren]

The combination of the piston spring and the viscosity of the oil stop the piston rising too quickly when the throttle butterfly is opened. My guess is that if this wasnt so a the piston rose quickly youd get a weak spot and hestitation until the engine had stabilised. remember webers and delortos have an accelerator pump which adds extra fuel at this point where as the SU doesnt.

The old recommendation was to use the same oil you put in the engine but its worth trying going thinner to see if it makes any difference in throttle response.

I have seen SU's mounted vertically obviously with no oil in the dashpot because it would all run out! This was on a 1300cc ford x-flow and the engine ran fine.

[liirge]

ignore

Edited by liirge, 01 August 2011 - 07:20 PM.

[MJWarren]

ohh ok

[Tommyboy12]

I break the mould. I use 10w40 diesel oil. Only cause its lying around and 20w50 is damn expensive compared!

[jamiestevenbell]

how often should i need to top the dashpot oil up as im currently having to do mine every 120 miles! and thats using 20w50 what is the cause of this

[MiniLuke]

how often should i need to top the dashpot oil up as im currently having to do mine every 120 miles! and thats using 20w50 what is the cause of this

Shouldn't go anywhere really, should it? Perhaps could be sucked in the engine if the carb is worn?

[jamiestevenbell]

could this be detrimental??

[Ethel]

Jamie, my guess is you're overfilling it. Remove the dashpot for a look and you'll get a better idea of what's going on. You only need oil in the inner tube that's part of the piston, and then the brass plunger displaces some of that oil. If you can feel the plunger pushing into the oil when you screw the cap on there's enough.

I hadn't thought of increased gas velocity under the piston as an advantage to filling the cylinders too.

It's the spring in the dashpot that controls how much air the engine gets, as it's compressed in relation to the air demanded by the engine (as modified the throttle). Any restriction on airflow will be proportional to the spring's compression rate, which should ideally allow the piston to reach the top of its travel when the engine's flow rate is greatest i.e. at maximum torque. SU's & Strombergs are known as constant depression carbs for a reason.

The damper, and it's oil, can be compared to the dampers on each corner of your Mini: take them off and it will bounce around the ideal ride height when you compress the suspension, exactly the same as the carb would fluctuate around the correct mixture. The damping will have a similar effect on airflow as it does to suspension ride height.


I reckon the short answer is to use the same oil as was used to set the engine up. Engine oil if it's in original factory spec, or anything you choose if you're prepared to put the time in with an AFR gauge.

Now what about multigrade oils and the effects of temperature on your carb damping

[liirge]

Jamie, my guess is you're overfilling it. Remove the dashpot for a look and you'll get a better idea of what's going on. You only need oil in the inner tube that's part of the piston, and then the brass plunger displaces some of that oil. If you can feel the plunger pushing into the oil when you screw the cap on there's enough.

I hadn't thought of increased gas velocity under the piston as an advantage to filling the cylinders too.

Is this sarcasm? Or has it got you pondering?

It's the spring in the dashpot that controls how much air the engine gets, as it's compressed in relation to the air demanded by the engine (as modified the throttle). Any restriction on airflow will be proportional to the spring's compression rate, which should ideally allow the piston to reach the top of its travel when the engine's flow rate is greatest i.e. at maximum torque. SU's & Strombergs are known as constant depression carbs for a reason.

The damper, and it's oil, can be compared to the dampers on each corner of your Mini: take them off and it will bounce around the ideal ride height when you compress the suspension, exactly the same as the carb would fluctuate around the correct mixture. The damping will have a similar effect on airflow as it does to suspension ride height.


I reckon the short answer is to use the same oil as was used to set the engine up. Engine oil if it's in original factory spec, or anything you choose if you're prepared to put the time in with an AFR gauge.

This is 100% the way to look at this if you think about it.... if performance isn't EVERYTHING!

Now what about multigrade oils and the effects of temperature on your carb damping

Edited by liirge, 01 August 2011 - 09:30 PM.

[TimmyG]

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.........

It maybe the case that engine oil (20w50 I assume) was recommended by the factory so as to allow leaner steady state mixtures without the side-effect of a flat spot on rapid accelerator openings, thus maybe giving better economy? just a thought mind you.
Agreed the carb will continue to increase fuel after the piston has reached the top, but it will no longer be in control as such and will act the same as a fixed jet carb...without the extra jets.... so leaning out as the airflow rises further. This could be a problem on a higher revving engine that produces it's peak power say around 7000 or more if the piston has reached the top by 5000. Lean mixtures at high loads = melted pistons.
Interested in the steam powered a- series! what type of boiler does it use? is it just the short block or does it use the cylinder head and valve gear aswel? i actaully think that there is a future for steam piston engines. In a modern electronically controlled format they could be extrememly efficient!

[TimmyG]

which should ideally allow the piston to reach the top of its travel when the engine's flow rate is greatest i.e. at maximum torque.

Now what about multigrade oils and the effects of temperature on your carb damping

Just because i'm a sucker for detail...sorry...! Maximum torque is produced when the engine is at it's most efficient at filling the cylinders, I.e. peak V.E. (volumetric efficiency) which coincides with inlet port peak velocity. Maximum power is produced when the engine is pumping the greatest volume of air which is when the carb sees the greatest airflow.

Interesting point with the oil....i would of thought to some extent the cold engine would have thicker oil and therefore it would give an increased richening effect when it was needed but as the engine and dashpot oil warm up then the richening up of the mixture is reduced when it isn't required so sort of self compensates maybe.
A multigrade is more temperature stable so would level it all out a bit more making the carb more temperature stable in it's responses, a synthetic even more so. Having said all that, a carb that is used mostly at wide throttle openings such as a race car will run cool, a road car will run much hotter, so by the time you factor in waxstat jets it could all get quite complicated quite quickly!

[jagman.2003]

I tried to play with the dash pot oil once (many years ago). I too worked on the principle of the thinner the oil the quicker the dash pot rises & therefore the quicker it will accelerate. Although the may be some truth in this compared to very thick oil. My idea was to try vegatable oil. The thinnest I had around the house. Something I hadn't anticipated that being a cold day & the freezing point of vegatable oil wasn't far off. The cool air running through the engine bay soon froze the carb up & gave me that one engine speed car..fast! I haven't bothered messing much since...just some basic engine oil.

[liirge]

As a note, when I bought my last mini, the dash pot was filled with....

Lemon juice!

needless to say it didn't perform well

[freshairmini]

I've got a 1275 with stage1 on it (maniflow 1 3/4" straight through twin box)

When I got my mini tuned up at Slark Race engineering down in Salisbury, neil said that 20w50 is best for use in the dashpot, 3in1 oil is far to thin

Edited by freshairmini, 04 August 2011 - 10:29 AM.