Nobody brought this up but if you want more flow you must keep the pressure up. This can be achived by adding an aditional spring to the bypass valve. If you do not do this all your efforts in modifying the pump will be worthless.

Hi ken

I have a pump modified as described, with a standard non adjustable oil pressure relief valve.
I have much higher pressure than standard, it goes off the gauge most of the time, hence my desire for a different gauge or maybe bigger jets to reduce pressure back to standard and increase flow as some contributors have mentioned.
Given this situation I'm not sure if I would want to up the setting of the oil pressure relief valve. I presume that in my situation I am opening the valve but it cannot flow enough bypass oil to reduce the pressure down to standard, hence my higher than standard readings?
I would be interested in your experiences with an additional spring fitted.

Regards

Steve

Hi Ken,

If you increase the size of the jets as descibed there is no need to change the blow off spring.

Regards

Rob

Can someone please advise the pressure at which the blow-off valve is designed to lift?

Mike

Mike,

I asked my dad last night to take measurements and test the rate of the spring. (I cant do it myself as i am away from home at uni). I will get back to you once i have calculated what pressure it should blow off at.

Does anybody know if any of the blow off valve components changed at any point during the production??

The three bearing engine has much smaller jets and hence the higher pressure but is the blow off valve different?

Regards

Rob

Hi Rob

I think I may be right in saying that some earlier ones were adjustable wereas the later ones were fixed?

Also watch out for the preload when you make your calculations!

Steve.

Hi All,

Dad and i have done the maths

The spring is 3/8 od 0.036" wire and 12 turns
Working from data in Machinerys Handbook i worked out the spring rate to be 5lbf/in

dad measured the preload as 25/32" and the seat diameter as 5/16"

That gives a blow off pressure of 51psi

i then convinced dad to measure the rate of the spring directly, it came out at 4.5lbf/in

That gives a blow off pressure of 45psi.

Given the tollerance on the spring and preload the actual pressure could be anything in this range.

This is obviously much higher pressure than the engine runs at so must be intended to protect the pump in case of blockages etc.

Regards

Rob

.

Thanks for that Rob, and for doing all the maths. You set me going with an old relief valve spring, which I had knocking about, and by experiment I would generally agree your findings regarding the spring rate.

What we are saying is, I think, that the pressure relief valve has little effect on the working pressure even with an over-bored pump. The possible exception might be with a cold engine.

Mike

I fiddled around with my spare crankase today. My measurements pretty much agree with Robs, so it looks like it doesnt really control running pressure?
Were the earlier ones adjustable as I thought? If they were why were they changed?
Did the pressure fed engines have the same relief valve?

Steve.

Steve,
Pressure relief valves on both my "late" 10 stud pressure fed "Ulster" engines are adjustable. I have an 8 stud blown engine where I think it isn't adjustable, will check in the garage tomorrow. As far as I can recall, the relief valve in a Speedy engine isn't adjustable. Don't know about Grasshopper.

Regards, Stuart

I have taken things a step further, i am looking at working out the velocity and mass flow rate of the oil leaving the jets to work out how much oil actually gets to the crank. I think i am going to have to draw the crank and jets to scale to work out it out more acurately. Intial rough estimates suggest the flow is not too bad. I will have to reconsider some of the more significant approximations i have made before i have a confident answer.

Starting to wish i remembered more of my fluid dynamics lectures!!

Regards

Rob

Im so jealous!!

Rob

Hi Rob

Just a thought ref your equations etc etc, dear old Bernoulli or whatever he is called maybe helpfull given you know the pressure and cross sectional area and maybe viscosity or density, BUT I have another more practical idea......

Get a spare crankase with a pump and jets, arrange some Heath Robinson equipment to run the pump at correct RPM and hold a calibrated jug under the oil jet. With some clever thinking you could also heat up some oil and measure the effect of temperature on the aforementioned apparatus.
This would give you a flow rate. Now with some mathematics or a sketch as you suggest you may be able to guess at the amount of oil reaching the crank and thus the bearings. I have read somewere but cannot remember where a figure quoted for the actual working pressure at the bearing/crank interface with the splash feed arrangement, I remember it being a higher figure than I expected?

steve

Steve,

Have been using Bernoullis equation, had to make a few assumptions. Will take some measurements at the weekend and see if i can refine the calcs.

Had thought about the practial approach, not a bad plan at all!

The pressure of the oil in a hydrodynamic bearing can be huge, well worth reading up on the theory. Incidentally the design of the centre main on the three bearing engine is terrible. With a relatively simple mod it can be made much more effective.

Regards

Rob

I have also som problems whith the oil pressure
Sorry but my english are not so good.

I have all paeces in the oilpump new, and also the valvepressurekontrol i new, but my oilpressure is very low, i dont "risk" the engine whith the low oilpressure, soo the nippy is still in the garage