7 replies to this topic

[aljobo]

I'm suspicious about the level of vacuum in the servo hose. I can get a reading of 0.4 bar on tickover, and 0.6 bar on the over-run, quickly settling to 0.4 again. The one-way valve in the hose appears to need a substantial vacuum to open.
Has anyone the typical theoretical or measured figures (inches of Hg or bars/psi), so I can compare mine and get further with a non-operating servo?
I know that there are other possible explanations for a servo problem, but I'm trying to be systematic and want to eliminate the vacuum one first.

[dklawson]

I have no idea what the cracking pressure of the servo check valve is.  Regardless, the values you measured seem very low.  Once the valve opens the vacuum in the servo should match what is in the intake manifold.  On a healthy engine with a standard cam you would expect somewhere around 15" to 20" of mercury at idle and as much as 30" on deceleration (roughly 0.7 and 1.0 bar respectively). 

 

Before you assume there is a problem with the vacuum in the servo, move the vacuum gauge connection to the intake manifold directly (make an adapter to go where the servo hose would normally connect).  Confirm that your engine is developing the vacuum levels I mentioned above.  Keep in mind that if you have a performance cam that the manifold vacuum is going to be lower than the numbers I listed above.  Let us know what you find.

[aljobo]

Thanks for your reply. You've helped a lot. The vacuum levels are low in my car, and cannot be improved. I think the issue is that the engine (an SC BMW conversion with 4 Keihins) must have too much valve overlap to generate the numbers you mention. My neighbour's classic Mini has 0.8 bar at tick over - I measured, it today - mine has 0.4.
I checked all connections. There are 4 inlet manifolds, each with its own vacuum connector, and they are inter-connected. The check valve is definitely working.
I can't think of any other explanation for the problem.
The easy answer is either to buy a small 12V vacuum pump, such as that made by MES/DEA in Switzerland, or I just live with non-servo-assist! Given the car's performance, I prefer the former solution.

[dklawson]

As you suggested, with the different breathing your car has, your only real option is some form of aftermarket vacuum pump in place of manifold vacuum.  

 

However, keep in mind that the servo does not make the brakes work any better or safer, it just reduces pedal effort.  Your car is no less safe without the booster (as long as you have strong legs).

[Captain Mainwaring]

Thanks for your reply. You've helped a lot. The vacuum levels are low in my car, and cannot be improved. I think the issue is that the engine (an SC BMW conversion with 4 Keihins) must have too much valve overlap to generate the numbers you mention. My neighbour's classic Mini has 0.8 bar at tick over - I measured, it today - mine has 0.4.
I checked all connections. There are 4 inlet manifolds, each with its own vacuum connector, and they are inter-connected. The check valve is definitely working.
I can't think of any other explanation for the problem.
The easy answer is either to buy a small 12V vacuum pump, such as that made by MES/DEA in Switzerland, or I just live with non-servo-assist! Given the car's performance, I prefer the former solution.

 

 

It won't work like that - you need to have 4 individual NRV's, one for each carb or you'll lose to much vacuum because of reverse flow due to cam timing - i'd also think that the flow through the vacuum lines will make it impossible to balance the carbs properly

[Captain Mainwaring]

Change the NRV too....it shouldn't need much to get it to open.

[Pete649]

It won't work like that - you need to have 4 individual NRV's, one for each carb or you'll lose to much vacuum because of reverse flow due to cam timing - i'd also think that the flow through the vacuum lines will make it impossible to balance the carbs properly

 

 

Good point. Without wishing to hijack the thread (the OP's question has been answered anyway) that is how my Z-Cars Hyaybusa setup is currently, with one vacuum pipe per throttle body interlinked then running to the servo. Never gave flow reversals or interconnectivity a second thought. Might see what electric pumps are on e-bay.

Edited by Pete649, 21 May 2014 - 06:45 AM.

[aljobo]

Many thanks to all, any things to contemplate.
I do want to get my servo working, because two things happen : first, I struggle to get the force necessary for heavy braking (I'm over 70). Second, when you drive several cars, some of which have servo assist, it's hard to remember which one needs the heavy foot in an emergency. With the Mini, I tend to leave space between me and the car in front.
The issue of variable induction is something a friend mentioned, too. It seems to make sense that the vacuum can be compromised by valve opening at different phases of the 4 pistons. I need to figure out what the options are on fitting multiple check valves.
Thank you all again. Allan