1G Clutch Pedal ratio

[GRNDSM]

Has anyone ever figured out 1G or 2G clutch pedal ratio?

I am trying to figure out why ACT2600 pedal is so hard on my TR7 project.

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[pauleyman]

Leon I don't know the ratio but don't forget there are two different diameter slaves. What hydraulics are you using?

 

[GRNDSM]

Paul, I have narrowed it down to the pedal ratio.

I have Bill’s T5 bell housing with Supra slave cylinder (.81, same as DSM) and 3/4 Wilwood master cylinder (vs .81 on DSM).

So I have some mechanical advantage and yet, my ACT 2600 feels much stiffer! That leaves pedal ratio

 

[We're on Boost]

I have a measurement for 1G turbo that pretty much corresponds to your "Y" number.
It is 2.469 inches.
That is measured off a new master lever with no wear, from the center of rotation to the center of the hole that the clevis pin goes into for the clevis on the master rod. I measured right along the lever, not projected to a vertical line like you have it shown.

Here is a diagram I marked up showing the master lever with 2 red dimension lines. That is the dimension that is 2.469 inches. The lever is #17 in the diagram. The clevis pin is #13

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Here is another diagram that shows the whole clutch pedal assembly with the "turbo" style lever in the lower right. Just so you can see the whole picture. The lever is #18 in this diagram. (The non-turbo lever is #14)
Also note the "turn-over" spring #19. That is a spring that goes over-center when the pedal is about half-way down. It reduces your pedal effort in the bottom half of the pedal stroke (near the floor). The non-turbo doesn't have that. I suppose because it's a lighter clutch? I think the 2g turbo also has a spring that goes over-center but it's different.

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[GRNDSM]

Thank you for the Y value!

I stole that image off the net, it should be the way you measured it and not a vertical projection. But it is close enough to get the point across

Do you also happen to have the pedal around to get X value?

 

[We're on Boost]

Do you also happen to have the pedal around to get X value?

Yes the X is about 14 inches.
I don't have a pedal lying around here off of the car, but I was able to get in there with it on the car, with a stiff tape measure nice and straight.
It kind of depends on where you want to measure to on the pedal pad, which is about 2.5" long top to bottom.
My 14 inches is from the center of rotation to a little below the center of the pedal pad. About 1 inch up from the bottom edge of the pedal pad.

 

[We're on Boost]

Your hydraulic ratio is enough different from normal 1G to make a noticeable difference too I think.

With your 0.81" slave and 3/4" master you've got an area ratio of about 1.17 slave to master. (0.515 sq in to 0.4416 sq in)

My 1G has a 3/4" slave and a 5/8" master which is an area ratio of about 1.44 slave to master. (0.4416 sq in to 0.3066 sq in)

That's, what, 23% difference? Quite a bit.
Your hydraulic ratio would be better than mine for maximizing slave rod travel but worse for minimizing pedal effort.
On a 1G with an ACT 2600 clutch we are usually more trying to maximize travel of the slave rod, rather than reducing pedal effort.
But that is mostly because our 1G pedal assembly parts flex too much and can have loose slop too. Flex plus slop. Not good.
That's why I even have any of these measurements. Was trying to look into where the travel gets lost. So, I have more measurement numbers that might come in handy for you, if you need them.

 

[We're on Boost]

What components are you using?

Well the 1g slave was available 2 ways that I know of.

The 3/4" slave I use is the MD733623 which has a gold piston (older ones had a red piston) and this is the slave you get when you go looking for one for a 1990 - 1994 DSM AWD turbo, from STM or RTM or the Mitsu dealers.

The other slave that is a 0.81" has a green piston and I don't know what the part number is. But that would be worse for pedal travel so I knew I didn't want it. The only reason I even knew it exists is because "GST with PSI" did a nice post about them in 2020 when he was changing from a green to a gold to get more travel at the slave, and he showed some good pics. That is in post #392 here: https://www.dsmtuners.com/threads/gstwithpsi-galant-vr4-1837-of-2000.482807/page-16#post-153750177


The 5/8" master cylinder I'm using is the MB555133 which again is just what STM and everybody else sells you for a 1g DSM master cyl. Holy cow those things are getting really expensive now. They say 5/8 right on the outside of the casting on the passenger side, and the bore does measure 5/8. The old one that was on my 1990 was that also, just like the new one I bought a couple years ago.

A bigger bore master is something I thought about trying to get when I was delving into all of this, because that would have given me more slave travel (and higher pedal effort). But I didn't know what to get. Is there one? I'm ok now with the twin disk clutch because that clutch doesn't need as much travel at the TO bearing to fully disengage. But I'd still be interested in knowing. Does the Wilwood master fit right up to everything in a 1g without modification? You mentioned above a 0.81 DSM master. What DSM ever had that?

 

[GRNDSM]

As I expected, DSM has much larger pedal ratio than TR7, 5.67:1 vs 4.25:1, which explains why clutch requires so much more effort.

I was totally wrong about AWD DSM data (that is what I get for being lazy and getting numbers on line, instead of measuring them myself). Slave cylinder is .75” and master cylinder is .625”, I will change my previous post so that no one is steered wrong it it.

 

[GRNDSM]

Using these numbers, I am getting the following overall ratios

DSM 8.16:1
TR7 w/.75 master. 4.95:1 <<<
TR7 w/.625 master. 7.13:1

No wonder I thought that current effort was ridiculous

5/8 master should be much softer, but still stiffer than DSM

 

[bastarddsm]

Leon, 2g slave is .812" and maybe like n/t fwd. I run a wilwood .700" master in my car 2g .812 slave and reworked the pedals for longer travel but raising the pedal and reorienting the arm. Works really well to get the 2900 and 2600's to shift good at 10k.

You should think about all parts when you work on it. The more line pressure the more deflection in the cylinders and lines. (waisted work) Braided lines are not as solid as hard lines.

You need somewhere around 0.400" of tob travel to fully release a 2600.

 

[We're on Boost]

Braided lines are not as solid as hard lines.

Yeah. Anybody who has hard lines still in good shape should take real good care of them.

You need somewhere around 0.400" of tob travel to fully release a 2600.

This is real handy info, but it needs to be measured with everything put together and under strain, which I figure means it needs to be measured from outside the bell housing, as slave rod travel.
To do that you'd have to know the "rocker ratio" or whatever you'd call it, of the clutch fork. Which would be easy enough to measure for anybody who has one lying around. But I don't.
Do you have a number for what that ratio is?

 

[GRNDSM]

I am getting 60mm and 108mm for 1.8 ratio.

So if we need .400” on clutch side, we would need .720” on the slave cylinder side.

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[We're on Boost]

I am getting 60mm and 108mm for 1.8 ratio.

So if we need .400” on clutch side, we would need .720” on the slave cylinder side.

Ahh good! Thanks for those pics.
I'm going to copy them out and save to my "stuff" folder. Which is where all the good stuff is.
0.720" at the slave, yeah I never had that much slave travel when I had an ACT 2700 in the car.
I had somewhere between 9/16 and 5/8 inches of slave travel, as near as I could measure it.

 

[GRNDSM]

I must agree with you, I am used to seeing around .600” at the slave cylinder. Perhaps that .400” number is a bit high?

 

[We're on Boost]

I must agree with you, I am used to seeing around .600” at the slave cylinder. Perhaps that .400” number is a bit high?

Well, I think he is saying that you need that much (.400) to get release that is good enough to shift at very high revs, like he mentioned 10,000 revs.
I could believe it because an awful lot of people have complained that the heavy ACT singles like the 2600 won't let you have a clean shift at even 9,000 or even a little lower than that. So maybe that's because they aren't getting quite enough travel to do it.

I wouldn't know from my own experience, because I've never had my engine rev limiter set higher than 8,000 rpm.
What I found with mine with the ACT 2600 on the stock engine with stock stock rev limit was, no problem. I could shift it cleanly.
Then, with the new engine built by English Racing in 2016, a new ACT 2700, and the rev limiter set to 8,000:
Again I had no trouble shifting cleanly at any point before the rev limiter, with the stock synchro transmission.

Then my transmission broke, and that was the start of various clutch and transmission problems for me which I think are still not settled out.
I did eventually quit using the heavy ACT single type of clutch because I was suspicious of how close to the floor my disengagement point was (pedal close to the floor) and I thought that might be part of my problem. Still not sure. But I would have gladly taken 0.720" at the slave cylinder. Now I don't need it with the twin disk clutch.

BTW the "2700" comes from emails I had with ACT tech support (Eric Weatherwax). In December 2020 I asked him what my ACT MB1-XTSS clutch would be equivalent to in the old terminology (like 2600, 2300, 2100, etc). He said, "We don’t do that form of part numbering anymore. The pressure plate you have would have been referred to as a 2600. Now it would be called the 2700 due to the clamping force increase."

 

[bastarddsm]

Yeah, that's what you need to get them to release good. They start letting the disc loose at 0.250" or so, but for good shifts you need a lot more.

Rememer this is dynamic, that shit moves around a lot. What you see on the bench IS NOT what happens in the car. That little diddy jacks has about checking disengagement is only the very tip of the iceberg.

 

[TuskeeTuner]

I searched this forum because I plan to use an "ACT 2100" plate with the 6-puck sprung disc, and an extended slave cylinder rod I bought back in 2005. It worked with pedal engagement near the top of travel on a FWD with an ACT 2100 from 2004.