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Clutch pressure 480 psi

Posted: Sun Jan 28, 2018 12:23 am
by Ozzsp250
I'm preparing a new engine with Toyota W58 5 speed transmission. I have gone to a Austin Healey 3000 pressure plate and elected to go to a Quartermaster hydraulic throw out bearing. I have it assembled in my test stand have run it as a unit. However I'm using a hydraulic pressure hand pump in place of the clutch pedal to test the system prior to installing in SP250, so I'm using a Ralston pump [] and have found it takes ~480 psi to disengage clutch.

My plan was to use the original clutch master cylinder but now question if the system is capable of developing the needed pressure. Has anybody actually measured pressure to standard clutch slave cylinder?

Any advice appreciated?

Thanks, Dennis

Re: Clutch pressure 480 psi

Posted: Sun Jan 28, 2018 2:28 am
by daimlersteve
Hi Dennis,
Just looked up my test records -- as follows ---
Healy 1400lb clamp, 290lb. release .
Cosworth -1700/305
toyota Dyna -- 590 lb release. this is a tight short heavy clutch but manageable with the right system.
These measured in a press with appropriate range gauge .
To get clamping press. -- clutch cover and plate assembled with 3mm free clearance cover to f/wheel, then pressure to mate surfaces = clamp .
release pressure -- with unit bolted up ,pressure to just free plate.

It is possible that you have the std. replacement unit for the jaguar , looks the same but has 2200lb clamp.
Various suppliers have standardised the selection down to a single unit to cover all.

Just this week had a replacement toyota unit ( aftermarket ) Old cover was 1100/320 , replacement -- 1700/460 lb. Good for this application of a modified sp.
This application is similar to yours , a concentric for a T5 conversion . will sort out the hydraulics when it is installed next week could be interesting to see what m/cyl is needed seeing its a little complex to change the pedal ratio.
Another thing that effects release pressure ( though not in the healy case with the flat release boss ) is where the release bearing actually hits the cover fingers. If is on a radius larger than the minimum the the leverage is going to be reduced and the pedal harder.

Its a great learning curve if ( like in all things ) you just want to know how thing work.

There are websites to walk you through the ratios , both hydraulic and mechanical. Google them.

Are you measuring the release at the point when the driven plate can just be moved ?

Re: Clutch pressure 480 psi

Posted: Sun Jan 28, 2018 2:30 pm
by Ozzsp250
First thanks for the prompt reply Steve,

I measured with engine / trans in my test stand unit not running. Turning tool [socket and 1/2" drive ratchet] on output for transmission, transmission in gear, pressure applied via hand pump until transmission can be rotated by turning tool.

As I still have the original engine and transmission in the car -- i'll see if I can set up and measure the existing set up - it's totally stock.

Thanks again

Re: Clutch pressure 480 psi

Posted: Sun Jan 28, 2018 2:45 pm
by Ozzsp250

I forgot I modified the NOS Austin Healey 3000 9.5" pressure plate to remove the collar to enable the concentric system to fit in the "Dellow" bellhousing. I also had to drill the flywheel and add the additional dowel pins.


Re: Clutch pressure 480 psi

Posted: Sun Jan 28, 2018 7:42 pm
by daimlersteve
Ah, we are not talking apples /apples then, My test is directly onto the clutch (not through another cyl)
the cross sectional area of the concentric comes into the calcs then.
The conversion i'm 1/2 way through had the std flat bearing supplied by the makers in the US . This is suitable for a rolled finger diaphram ,your healy now has straight fingers. The correct bearing will be radiused and of the dia at the end of the fingers for the ideal contact. These are available also as a seperate part# . The fingers are further hardened at the tips .


Re: Clutch pressure 480 psi

Posted: Mon Jan 29, 2018 9:49 am
by daimlersteve
The concentric in this case is a tilton 6000 .
In the case of the toyota Dyna currently in use, its fairly firm , just as a comparison to yours with a direct push--- If the leverage is taken into consideration ---- 70mm clutch side of pivot /120 mm on the slave side (std. toyota ) and the pressure was 355lb. at the slave, giving the clutch cover has a release figure of 591lb. Quite a bit higher than yours so you'll be ok even if you have to play about with master cyl sizes.

'The reason i went to the trouble to learn all this is the hassle to remove the box /clutch etc in my car with all of its chassis stiffening , bolt in seats etc. A digital ram and power pack was hired and replaced the slave for the test-- very accurate piece of test gear. It told me i had to alter the pedal box ratio or replace the clutch, i chose the former.


Re: Clutch pressure 480 psi

Posted: Mon Jan 29, 2018 1:12 pm
by Dragon
Hi Dennis and Steve,
Please clarify for me, is the 480psi figure the pressure in the hydraulic system?
The driver experiences clutch release pressure as effort on the clutch pedal, and changes to the master cylinder bore affect this directly.
Dennis, did you obtain the 480psi figure in the hydraulics, using an hand pump that has a different bore from the SP's standard 3/4"? I doubt if you are reporting pedal pressure!
Using an off-the shelf Austin-Healey BJ8 diaphragm spring clutch with the standard SP hydraulic system reduces the pedal effort from 30lb for the original SP coil-spring clutch to about 15lb, and even a competition BJ8 pressure plate needs no more than 30lb on the pedal.
I can send a conversion article to anyone who requests it by e-mail (not PM).

Re: Clutch pressure 480 psi

Posted: Mon Jan 29, 2018 1:55 pm
by Ian Hastings
Ozzsp250 wrote:
Sun Jan 28, 2018 2:45 pm

I also had to drill the flywheel and add the additional dowel pins.

When I was sourcing a clutch plate and cover for my conversion from a local clutch supplier who specializes in competition clutches, they told me that the pins were only there to facilitate location of the clutch cover whilst you put the bolts in, and as such were dispensable.

Re: Clutch pressure 480 psi

Posted: Mon Jan 29, 2018 6:43 pm
by daimlersteve
Ozz, old chap, dont forget the pedal stop to be used with the concentric unit.
Dragon, the pressures involved are directly on the clutch , not through the entire system --- a whole different kettle of fish. Just now out to get a new speedo cable for the w57. catch ya all later

Re: Clutch pressure 480 psi

Posted: Mon Jan 29, 2018 11:42 pm
by Ozzsp250
Unfortunately I don't have the fittings to hook up to the old system slave cylinder bleeder port to my hand pump at house, it will probably not be till next weekend before I get to duplicate the test with the original setup in the car. Photo below shows the current set up with the hose going into the concentric throw out system and the bleed hose from it bolted on bell-housing where slave cylinder would normally mount.

The intent to measure the pressure it takes to release the clutch would be to use this known pressure once assembly is in car for fabricating the clutch pedal travel stop. Earlier test bed testing, transmission on work bench I over-pressured the system thus over extended the travel and the system leaked. Replaced "O" rings, added temporary stop on transmission output shaft and tested travel at 500 psi leaving system pressurized for 6 hours with no noticeable loss of pressure more importantly no loss of fluid.

Thanks for listening