Q45 Proportioning Valve?

[motorhead]

Is there one on these cars already, or do I have to go aftermarket if I need front-to-rear adjustability?

[elwesso]

Can I ask you to describe what your talking about. What is a proportioning valve?

[ScottJackson]

he means a brake proportioning valve. I do believe the q has one stock like most cars that biases most hydraulic brake pressure to the front wheels where 3/4 of braking grip is. I know that the ratio of front grip to rear grip of tires changes with the force you're stopping, but 3/4 is rule of thumb. motorhead, are your back wheels locking up all the time and your front wheels not braking much?

[DAEDALUS]

There is one built into the MC unit. Probably have to go aftermarket anyway if you want to adjust it, since OEM is non-adjustable. Proportioning valve reduces the pressure to the rear brakes at a set ratio relative to the fronts. Total braking force allowed varies from rear to front due to shifting weights while braking.

[ScottJackson]

Oh, I re read the orig post. Yep, I think aftermarket will be needed for front to rear adjustability. Heck, might as well install a line lock too so you can do smokey burnouts better... and some water nozzles in the rear wheel wells to spray your tires to help em smoke (bleach water)

[boomstriker]

A proportioning valve controls the bias of brake line pressure from the front to the rear brakes. They were used cars without antilock brakes. Some older trucks use auto adjusting valves connected by a lever to the suspension to measure the weight in the bed and adjust more brake bias to the rear tires to compensate. This way you will have harder rear braking while loaded and much less when the bed is empty and very little weight is on the rears tires. It's biggest purpose is to limit brake pressure to the rears and keep them from locking in a panic stop causing the car to swap ends.On newer cars with antilock, it is usually ommitted because greater pressure can be pumped to the rears leaving the antilock to keep maximum braking without lock-up, no matter what the load is in the car.I had adjustable brake bias in an old RX-7 I used to race. I think you can still get the mechanical type adjustable prop. valves but it's not something you can use with antilock.

Kirk

[Q45tech]

90-93 Q has a setting of 69%-74% [the aceptable range of the valve]........lets say 72.5% on average..........a bit more rearward bias than most cars due to long wheel base and lower CG resulting in 2.5-5% less weight shift ratio in a 0.9G stop [if such a thing is possible considering the tires currently used].

"not something you can use with antilock" sure you can but why bother since the Q [assuming stock brakes is set better than most].

Changing pads [compounds] has a material effect on the required settings.

A nice thing to consider on a Q is to replace the rear rotors and calipers with the rear from a J30 [what has been done on the [97-03 Q..........same diameter just thicker and vented [9mm solid vs 16mm vented]Cylinder Bore Dia1.727 in (43.8 mm)Brake Pad DimensionsLength 3.835 in (97.4 mm)Width 1.335 in (33.9 mm)Thickness 0.39 in (10.0 mm)

[HeavyDuty]

Then *there's* my rear upgrade, Maxnix. OE & a direct fit.

Yes!

Thanks for the tip, Tech!

[maxnix]

The J rears are not larger, but are vented and probably heavier. Caliper and rotor should be a bolt up. I tried to get rear Stillen Brembos for the J from Joe, but they were not available.

Just as I stated (by quoting a previous post of Q45tech) in the Big Brake for Q45 thread. If only Brembo had them for the J30. Still would like aluminum calipers for the reduced unsprung mass.

[Q45tech]

The J30/97-04Q rear caliper in cast iron weighs little because it is single piston floating unit.........weighs within 8 ounces of stock oem 90-96 rear.

Actually the J30 rotor doesn't weigh much more than the Q rear rotor 9mm solid vs 5.5mm, 5.0mm of air space, 5.5mm......~~ about 8-9% heavier so it should run 8-9% cooler in the same stop not even considering any gain from air vents..........46-60F colder is significant on a 120mph stop..........based on an educated guess probably 4-8 feet less [from 100-120 mph] even considering the light load on the rear brakes.

They didn't spend the extra $3 for nothing. But I'm sure there was hours and hours of fighting over the extra 2 pounds [rotors and calipers].