View Full Version : Lighter left pedal ?

07-05-12, 11:03 AM
Went to Stoneleigh yesterday and basked in the sunshine ... surprisingly.

I'm going to have loads of Q's .... to get me to my right build spec.
My build is going to be based on V8, and between 350 - 400hp, and with mapability & efficiency of current stock, the LS3 looks a good option. (although cost may influence)

My question though is transmission & clutch ... the consensus on this forum is that Tremec is worth the investment ... unsure if that means T56 to T6060, but sat in a few cars yesterday with LS units & Tremec's ...and the issue is the physical force required on the clutch pedal.

I have had several ops on my left knee ... and while I can change gear all day on my Honda CRV ... I can see that the effort on the cars I tried was going to be an issue, certainly wouldn't want to drive stop-start in heavy traffic !

I asked 2 of the manufacturers and they said that there are options ... now I want to rule out an auto box, and they suggested asking here might also help.
One comment said that a 'spring' clutch will help another said I could add a servo.

So anybody out there any experience of how I could go about lightening up the left foot. ?

Lloyd Barnes
07-05-12, 11:06 AM
easy, just bung a remote servo on it... plenty on ebay for little money and dead easy to plumb in. you can also get different assistance ratios so you can choose how much effort you want.

BTW, its the clutch plate assembly and associated clutch mechanism design, not the gearbox that dictates the pressure required

07-05-12, 06:58 PM
This could have an impact on engine considerations. My limited undertstanding of this is that with a high revving engine you get less vacumn so need a vac reserve. Someone will be along to outline this in more detail, not a strong area for me but seem to remember this can be an issue.


07-05-12, 10:13 PM
Hi Warren.
6.2 Litres on over run will pull plenty of vacuum no problem.
If nothing else, we have cubic capacity on our side with these cars ;)

08-05-12, 02:46 AM
My Cobra had a remote clutch servo in the boot behind the r/h rear wheel. I never knew how light the clutch might have been because the vacuum hose was not fitted, however, the great length of hydraulic the pipework meant that the clutch action was horribly slow and lacked feel so I decided to rip the thing out. I could have re-located the servo into the engine bay but it was going to be difficult to find the space/make a neat job of it, and I thought I'd try the car without it for a while. With a short length of braided hose between the master and slave cylinders the clutch is very positive now, but of course it still has very heavy spring pressure compared to a 'normal' car.

I'm used to the clutch now (but luckily, my left knee is in pretty good shape). However, the car is still absolutely horrible to drive in stop start traffic because the engine hates it, but I live out in the wilds of the Norfolk countryside so it's not a big problem to me. My feeling is that if you anticipate regularly having to do a lot of heavy traffic in your Cobra you really should either keep your engine in a fairly mild state of tune, or invest in one of those modern (LS?) engines which will be much more co-operative in those circumstances. Plus, if your knee's that dodgy, you should seriously consider an automatic gearbox or put a servo under the bonnet or maybe in the passenger footwell.

08-05-12, 08:08 AM
I have an LS3 engined car and the clutch pedal is quite light, not much difference between that and my 120D daily driver. I used a late Camaro slave cylinder on a LS7 clutch.

Neil O
08-05-12, 08:51 AM
The last LS powered car I sat in had a servo on the clutch. I only moved turned it around to move it out of my way but the clutch felt really strange.
It didn't seem to have any feel to it, hard to describe.

Anthony (Absolute Horsepower) has had both my car and the same LS car (without servo) at his garage at the same time and told me the clutch on my car was lighter.
I can't remember what clutch set up mine is, but it seems it's not all about the engine etc. it's probably more about the clutch itself.

08-05-12, 11:35 AM
I've got the remote servo with a vac cannister as I'm running a lumpy cam. The only time you don't get enough vacuum is if you are crawling along in traffic, but the odd blip on the throttle soon brings it back.

08-05-12, 11:40 AM
My ex Dax has remote servo and vacuum reservoir but as Marco said in long traffic ques it still ran out of vac so went heavy until you blipped the throttle. (due to the lumpy cam with lots of overlap.)
No issues with the tiny delay once you get used to the feel of it. :-D
If I were to fit another one I'd go with an electric vacuum pump. (sorted).

08-05-12, 11:40 AM
Sorry double posted, Damn the gods of the WWW.

Lloyd Barnes
08-05-12, 12:48 PM
the servo on my Dax was under the passenger inner wing... no delays that I noticed. Felt just like a normal clutch, perhaps the trick is to keep the plumbing as short as possible.

Happy Jim
08-05-12, 12:58 PM
Clutch pressure plate & Release Bearing/Slave cylinder side/throw & Clutch master all have a bearing. My LS has a standard clutch & box so feels like every other Camero/Monaro/ etc. Standard Cam in the LS will be fine for Vac.

13-05-12, 01:27 AM
Been talking to a few that know (compared to me) .. at Transmission sellers .. the issues are caused by 2 things .. spring pressure has to be high to avoid clutch slip on large HP powepacks ... I could go for smaller 350hp matched clutch rather than clutch that is rated to 600 hp ... or double clutch.
The 2nd problem is length on clutch pedal in Cobras .. compared to typical car clutch arm is short = less leverage = heavier feel.
Not sure of options of longer pedal.

The fitting of servo is not recommended as it can lose the feel of the pedal.
Change the bore of teh master cylinder .. instead of small bore needing lots of pressure, you could have a larger bore requiring less pressure ... as long as end result is same volume of fluid getting to slave cylinder.
RoadCraft are currently talking to AK & DAX about producing such a unit ... with number of people now going for LS3 & Tremec boxes there may be enough business to produce one.

I suppose somebody who has spares knowledge may know of a 'donor' master cylinder with right characteristics.

13-05-12, 03:12 AM
The problem is that when you start experimenting with different size master cylinders you can get a mismatch in the throw of the slave cylinder. What normally happens is that you move the biting point to the very bottom of the pedal travel or the very top making the clutch action jerky.

The biggest influence on pedal pressure is the clutch pressure plate and this is more pronounced on clutches that have to deal with high torque engines, to prevent clutch slip. One solution is to fit a multi-plate clutch (twin or triple plate) because these have a larger contact area they require less force from the pressure plate to stop them slipping.

Or you could use a 'crash' box like my Jerico which allows changes without any use of the clutch although when doing this under full acceleration you have to be devoid of any feelings of mechanical sympathy !

13-05-12, 11:34 AM
Sorry double posted again.
I hate Gremlins.

13-05-12, 11:35 AM
Actually Rick, it's the other way round.
A smaller bore master cylinder will give you a lighter pedal, but of coarse then needs a longer stroke to move the same amount of fluiI'm using a matched pair of 7/8" cylinders behind my 500 bhp and 500+ lbsft. :D
Mcloed clutch etc.

13-05-12, 01:27 PM
These are hight torque low reving engines.
You do not need to rev the engine to the red line before dumping the clutch to make a good getaway.
Get off the clutch as soon as possible (Low revs) and use the engine's torque to launch you down the road.
Once a clutch is slipping it could continue to slip, even a super duty one, or a twin plate (I have seen this).
Use a light clutch, once the pedal is up and all slip is gone the engine will find it much more difficult to start a slip if the clutch already has full bite.


If you are going to play around with cylinder sizes then bear in mind how much pedal travel you want/can handle. this will usually be dictated by the pedal ratio as most master cylinders have around 1.25" of operationg range. The Dax pedal has a 10 to 3.25 ratio to the cylinder which gives around 4" of pedal travel to fully operate the master cylinder.
If you run a 5/8"slave cylinder (Repower size) then at best you are going to get 1.5" of movement on the clutch fork at the point where the slave cylinder pushrod connects with it.
Depending what the lever ratio is on the clutch fork will tell you how much you are actually going to press the fingers of the clutch in. If we assume that you have a 2.58:1 (Std repower) fork ratio then the 1.5" travel at the slave contact point is going to press the clutch fingers by 1/2".
Your clutch may require more or less than this. but this is the Dax/Repower std ratio. So, from pedal to actual clutch fingers you have a 8:1 advantage on pressure to the clutch fingers. If the fingers have a ratio of 5.5:1 then you have an actual advantage of 44:1.
A 2900lb monster clutch is going to require 65lb (4.6 Stone) of pedal force to operate it.
A stock 1700lb clutch will require 38lb of pressure (2.7 Stone)Assuming zero friction etc. A pedal with 3" of travel on the above set up will require 86lb of pressure for the 2900lb clutch and. The 1700lb clutch will require 50lb of pedal pressure. The numbers above are mostly guestimates but you can soon see the difference between 4" of pedal travel and a light clutch (38lb) vs 3" of travel and a street strip clutch (86lb)......And both will slip if you try 5500rpm drag starts.

13-05-12, 04:50 PM
These are hight torque low reving engines.
You do not need to rev the engine to the red line before dumping the clutch to make a good getaway.
Get off the clutch as soon as possible (Low revs) and use the engine's torque to launch you down the road.
Once a clutch is slipping it could continue to slip, even a super duty one, or a twin plate (I have seen this).
Use a light clutch, once the pedal is up and all slip is gone the engine will find it much more difficult to start a slip if the clutch already has full bite.


That is an interesting technique and I look forward to trying it out.