Does NB ABS generate reverse pressure?
 

Anyone know if the NB ABS system generates reverse pressure when activated?

If you do not know with 100% certainty, do not post your guess

TC Designs mentioned some older BMW platforms generate reverse pressure while newer BMW platforms do not for example. Tilton doesn't know which car has what type of system so they generically advise against using the compact 75 series with any ABS. Shopping for Tilton master cylinders for our S1 build. I much prefer the more compact 75 series over the longer 74 series with internal pressure baffles (to protect against reverse pressure).

Jump to 2:20

By "reverse pressure", do you mean applying pressure backwards up the hydraulic lines towards the master cylinder?

I don't know, I'm not going to guess, but here's some data from when I was testing my brake pressure sensor. Sensor is installed in the front outlet line from the MC. Car is a 99 with an NB2 ABS system installed, 929 master, NB2 booster, Dynalite fronts, Powerlite rears. ABS was engaged during this test.

http://www.codrus.com/miata/fm2r/brake-pressure-log.png

--Ian

The miata abs works pretty standard for ABS. During abs operation the master cylinder will see pressure pulses from the system but they should only be current line pressure and a drop in pressure. If you really want to be super duper sure, take one apart, its just a hydraulic valve manifold with integrated solenoids, and an integrated pump.

I have a pressure sensor before my ABS pump. I don't have it wired up right now but I will try and get some data off of. Would a log from a 0-1000psi pressure sensor while activating ABS be enough?

Or is there more to it than that.

Edit: Ian beat me to it. Ive got a car on stands though with the setup, Let me know if i can help.

Assuming I have my sensor calibrated properly, 1000 psi probably not enough. I'm seeing like 120 bar peak.

--Ian

he could test on like sand or something where less brake pressure would engage abs.

Car is on jackstands. I was gonna just pull the rear calipers and spin the tires lol.

The guys at TC said systems that generate reverse pressure will usually push the pedal back up during ABS activation, like Ice Mode. I have never noticed any of my NB with ABS doing this. A firmer pedal but never noticed the pedal rising. I have noticed the pedal kicking back and rising during ABS activation in other (non-Miatas) though. Not conclusive, I know.

Not sure how to test for it. Perhaps manually triggering ABS without pressing the pedal and observing line pressure. It should, in theory at least, show zero pressure during activation. We don't have a running ABS equipped car in our shop and our engineer is on vacation anyway.

Thats actually way easier than any of my ideas. I'll see if I can get some data tonight.

This is purely anecdotal but I've been using an NB abs unit with 76 series master cylinders and haven't seen any evidence of it being a problem after a few track days and some street driving. I'll be following this thread though, as I was not aware of using ABS with those particular master cylinders could be a potential problem.

Potential being the operative word.

If someone here can manually trigger ABS in an NB while observing line pressure, we will have our answer. Really hopping it's non- rising as that allows me to use the 75 series masters I already bough that are 2.4" shorter than the 74 series. With that big downpipe and heat shielding, real estate is scarce in that vicinity.