-3 or -4 clutch line; Does it matter?

[swiftdrivr]

Hey guys,
I just found out that my clutch has a -4 line from the master cylinder back, and I have a shiney new [and expensive] -3 line to replace it with. Does it matter? Will it change the feel of the clutch? Any idea if it will cause a problem if I use -3, or should I go ahead and buy a -4 line?
Thanks for your thoughts,

[Charles Warner]

More pedal movement required - probably have to adjust your pedal stop. Easier modulation. If you swap to the -3 you'll have to change fittings (unless they are banjo fittings).

[BeerBudgetRacing]

More pedal movement = finer control. You can feather it easier on starting....

But I'd think about all the other times you press the pedal. Do you really want to have to move your foot more on track?

[BLS]

Pedal movement is dictated by the master cyl bore, pedal mechanics, and the slave cyl. The -3 line should be fine.

[R. Pare]

The only real difference will be in flow resistance - the -3 is a bit more restrictive than the -4. -3 will "swell" less at any pressure level, but in a clutch system, I doubt highly that you could feel the difference. That said, however, with the pressures required by some clutches, it may be just enough fluid volume difference ( a decrease) to require the stop to be raised a tiny amount.

[yellowesty]

You should feel no change in clutch operation by switching between AN-3 and AN-4 lines. In operation, either line will be full of fluid whether or not the clutch is depressed. There will only be two differences:
-- The AN-4 line will be slightly heavier and will hold slightly more fluid than the AN-3 line. Ounces.
-- The velocity of fluid movement when the clutch is depressed (or released) in the AN-3 line will be slightly faster than in the AN-4 line. No matter.
In a brake application, if the calipers have a large capacity, some difference might be noted. Or might not.

[Brian C in Az]

If someone had changed the line from a -4 to -3 without telling you, you would never have noticed any difference.

[swiftdrivr]

Thanks guys. As always, loads of ecperience on this forum, and a willingness to share it. Much appreciated.

[Brian]

The theoretical reason for -4 over -3 is to reduce the velocity of the fluid in the line and thus reduce the possibility of cavitation as the fluid rushes past restrictions in the line (like through a drilling in a slave cylinder or through a banjo fitting). Is it a real problem? I have no idea. Lola used to install -4 stock, so when I replaced mine I used -4. Most people use -3 these days with no issues There is no discernable difference that I have been able to discover between the two in terms of pedal feel or response.

Brian

[DaveW]

http://www.apexspeed.com/forums/show...ght=brake+line

[Brian C in Az]

The theoretical reason for -4 over -3 is to reduce the velocity of the fluid in the line and thus reduce the possibility of cavitation as the fluid rushes past restrictions in the line (like through a drilling in a slave cylinder or through a banjo fitting). Is it a real problem?

If there is no air in the system, then how can cavitation occur? Cavitation requires air to be mixed with the liquid.

[Brian]

Cavitation is boiling of a fluid due to reduction in pressure rather than increase in temperature. As the fluid rushes past a restriction, Mr Bernoulli kindly reduces the pressure and localized boiling can occur. The boiling produces a gas, which does not reduce back into liquid but does remain and cause a poor pedal. Cavitation occurs in the fluid and does not require any gas (air, vapour, who knows what gas is produced by boiling brake fluid) to be present before it happens, but gas is present after it happens. Like I said, I think it's more theory than practice in our clutch systems.

Mr Google - Cavitation is defined as the process of formation of the vapor phase of a liquid when it is subjected to reduced pressures at constant ambient temperature. Thus, it is the process of boiling in a liquid as a result of pressure reduction rather than heat addition.

Brian

[Roland V. Johnson]

Submarines at depth can have propeller cavitation problems. Not a lot of air down there.

[DaveW]

Cavitation is boiling of a fluid due to reduction in pressure rather than increase in temperature. As the fluid rushes past a restriction, Mr Bernoulli kindly reduces the pressure and localized boiling can occur. The boiling produces a gas, which does not reduce back into liquid but does remain and cause a poor pedal. Cavitation occurs in the fluid and does not require any gas (air, vapour, who knows what gas is produced by boiling brake fluid) to be present before it happens, but gas is present after it happens. Like I said, I think it's more theory than practice in our clutch systems.

Mr Google - Cavitation is defined as the process of formation of the vapor phase of a liquid when it is subjected to reduced pressures at constant ambient temperature. Thus, it is the process of boiling in a liquid as a result of pressure reduction rather than heat addition.

Brian

Just for fun, think of cavitation as fluid particles unable to "make the corner", similar to a car spinning out...

[racerdad2]

Or, the bubbles behind your boat as your propeller cavitates

[J. Salmon]

I think it depends on application. I had a -4 clutch setup that did not work with a -3, fluid transfer was too slow.

[Charles Warner]

You probably needed more pedal travel to fully disengage the clutch.

[J. Salmon]

Line size doesn't determine pedal travel, as has been said, that's master cylinder / slave cylinder. But the line does limit rate of flow. In my case, the clutch would slip as the pressure came off, as opposed to just being open and closed. Worked fine when lazy. Not so much at speed.

[DaveW]

I think it depends on application. I had a -4 clutch setup that did not work with a -3, fluid transfer was too slow.

Line size doesn't determine pedal travel, as has been said, that's master cylinder / slave cylinder. But the line does limit rate of flow. In my case, the clutch would slip as the pressure came off, as opposed to just being open and closed. Worked fine when lazy. Not so much at speed.

That is why I, as I said in the other thread, also use -4 for the clutch.

[R. Pare]

Line size doesn't determine pedal travel, as has been said, that's master cylinder / slave cylinder. But the line does limit rate of flow. In my case, the clutch would slip as the pressure came off, as opposed to just being open and closed. Worked fine when lazy. Not so much at speed.

The smaller line was most likely setting up some high turbulence in a couple spots - like at the ends of the tubes that goe into the hose - which, when that happens, will massively restrict the flow rate.

[Frank C]

Slow release due to increased resistance with the -3 line likely was due to just the higher frictional resistance in the small diameter lines with high velocity.

As noted above, cavitation is the fluid flashing to vapor when the local high velocity produces a static pressure lower than the saturation (boiling) pressure at that temperature. One often sees the word "cavitation" followed by "damage,", for the vapor bubbles formed then collapse violently (eroding props, etc.) when they get to a region with a higher pressure.
Frank C

[BLS]

Does anyone have any documentation of cavitation occurring in brake/clutch lines? I know what cavitation is. I also doubt we have enough speed / fluid movement to produce this. Never heard of it anyway. If -3 is a problem with release of the clutch, why is it not a problem with brake release? There are cars using -2 for brakes.

[Charles Warner]

Does anyone have any documentation of cavitation occurring in brake/clutch lines? I know what cavitation is. I also doubt we have enough speed / fluid movement to produce this. Never heard of it anyway. If -3 is a problem with release of the clutch, why is it not a problem with brake release? There are cars using -2 for brakes.

Ideally there is no fluid movement through the line in the brake system, only pressure.
With the clutch the fluid moves through the line to make the slave cylinder move (i.e. the volume of fluid it takes to fill the slave cylinder.) The master cylinder might move an inch when stepping on the clutch and (hopefully) not at all when stepping on the brake.

[Frank C]

I agree with BLS in being skeptical of cavitation with brake fluid in clutch lines. The temperature is just ambient and the base pressure is high.
Frank C

[DaveW]

I agree with BLS in being skeptical of cavitation with brake fluid in clutch lines. The temperature is just ambient and the base pressure is high.
Frank C

During the depression stroke, I mostly agree with you. However, on the release stroke, the pressure is what ever the clutch spring can generate, so that is relatively low and constant. Therefore, too small a clutch line is more likely to retard re-engagement and promote slippage.

Whether cavitation would be an issue there is not the main point.

Also, confirmation in this previous post...

Line size doesn't determine pedal travel, as has been said, that's master cylinder / slave cylinder. But the line does limit rate of flow. In my case, the clutch would slip as the pressure came off, as opposed to just being open and closed. Worked fine when lazy. Not so much at speed.

[BLS]

Ideally there is no fluid movement through the line in the brake system, only pressure.

You need to get a FV with our brake cylinders

With a "perfect" disc brake system you would be correct, very little fluid movement, so your observation makes sense regarding clutch Vs brakes.

Dave makes a valid point about the clutch spring as well.

I do have -4 on my clutch, but if I were redoing it I would use -3 just to keep the same spare line size and fittings as my brakes. I don't think it would make a difference, but I think the points made here are valid. It might be more important on higher power drivetrains as well.