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Old 07-06-2016, 06:38 PM   #1
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Default prop valve and trailbraking

I've installed a prop valve in my braking system and am having trouble getting my head around what it should feel like during braking.

Long story short, I've installed a bbk in a 2004 msm. The dynalite's have a much smaller piston area (3.00 sq in) than my oem front brakes (2.54sq in) and this has pissed off my abs/ebd. The brakes don't stop well at threshold braking. I've installed a Wilwood prop valve, but have questions about it's adjustment.

This valve apparently cuts the rear brake supply completely until it sees 100# in line pressure, then it is adjustable above that pressure. While I see how this effects threashold braking, what about feathering the brakes/trailbraking? I don't have any real numbers, but I am often on my brakes lightly thru the corner entrance to apex. I'm curious if much braking is done at the racetrack under 100#? and, how putting in a valve that will turn off my rear brakes until I hit 100# would effect my cornering? Can I still use the brakes to help point the car? Or, will it now promote understeer?

Asking this because the car is not licensed for the street. The only test/tune time I have with my local SCCA group is during qualifying... and there's just a lot to think about in addition to how to tune the valve.
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Old 07-06-2016, 06:53 PM   #2
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I'm not sure it's wise to run the adjustable prop valve alongside ABS. Could do unpredictable things.

As for trailbraking and rotation, what you are doing is transferring weight forward with the light braking. For the most part, it doesn't really matter which end of the car is making that weight transfer happen (assuming you aren't locking up or anything).

BTW, you can get most of the same effect by just modulating the throttle. 0% throttle is actually a pretty aggressive way to transfer weight forward and it is the drag on the rear contact patches that is doing most of that transfer. I try to keep off the brakes and use throttle modulation instead because the brakes **** away your energy so rapidly. I save the trailbraking for those parts of the track where I need to rotate a LOT and then I'm mindful of the energy loss. In most cases, I've found adjusting the line to be wider so that rotation with throttle alone does the trick increases the minimum corner speed and buys time.

I think I just saw a picture of your car with an oil cooler and intercooler install with the front crash structure substantially altered (looks like you used the oil cooler to tie the car together at the front). Was that your car I'm thinking of? Any more pictures of that? I'm looking for a way to get air around a FAB9 IC to my radiator, but the front crash bar is in the way. /end threadjack.
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Old 07-06-2016, 07:06 PM   #3
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Originally Posted by hornetball View Post
I'm not sure it's wise to run the adjustable prop valve alongside ABS. Could do unpredictable things.

As for trailbraking and rotation, what you are doing is transferring weight forward with the light braking. For the most part, it doesn't really matter which end of the car is making that weight transfer happen (assuming you aren't locking up or anything).

BTW, you can get most of the same effect by just modulating the throttle. 0% throttle is actually a pretty aggressive way to transfer weight forward and it is the drag on the rear contact patches that is doing most of that transfer. I try to keep off the brakes and use throttle modulation instead because the brakes **** away your energy so rapidly. I save the trailbraking for those parts of the track where I need to rotate a LOT and then I'm mindful of the energy loss. In most cases, I've found adjusting the line to be wider so that rotation with throttle alone does the trick increases the minimum corner speed and buys time.

I think I just saw a picture of your car with an oil cooler and intercooler install with the front crash structure substantially altered (looks like you used the oil cooler to tie the car together at the front). Was that your car I'm thinking of? Any more pictures of that? I'm looking for a way to get air around a FAB9 IC to my radiator, but the front crash bar is in the way. /end threadjack.
Thanks for your quick response. Maybe I should unhook the abs completely... it's surely on the table as it's inputs have me confused... I'm just never quite sure what the brakes are telling me.

As far as removing the front structure... I'd beware of removing that flat-boxed in cross tie (in front of radiator opening). After doing that, my hood shakes... somewhat violently when at speed... it rattles side to side near the base of the window. I'm still trying to figure out if it is because of weakened forward structure, or if it's air pressure that is rattling it.
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Old 07-06-2016, 07:25 PM   #4
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Quote:
Originally Posted by gtred View Post
As far as removing the front structure... I'd beware of removing that flat-boxed in cross tie (in front of radiator opening). After doing that, my hood shakes... somewhat violently when at speed... it rattles side to side near the base of the window. I'm still trying to figure out if it is because of weakened forward structure, or if it's air pressure that is rattling it.
Thanks for the warning. That is the area I was thinking about -- I suspected it was structurally important.

Air pressure can definitely make the hood move around though. Just putting NA headlights up and down makes quite a difference.
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Old 07-06-2016, 09:21 PM   #5
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Originally Posted by hornetball View Post
I'm not sure it's wise to run the adjustable prop valve alongside ABS. Could do unpredictable things.
Just noticed posts from Emilio and Savington recommending the adjustable prop valve with NB2 ABS. Those guys have direct experience, so I'd listen to them. I'm just a basic non-ABS NA guy and am perfectly content with my braking, but I'm also not driving all night in a 25 hour enduro.
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Old 07-06-2016, 11:08 PM   #6
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Quote:
Originally Posted by gtred View Post
This valve apparently cuts the rear brake supply completely until it sees 100# in line pressure, then it is adjustable above that pressure.
Other way around. Adjustable prop valves have a "knee" below which the F/R pressure ratio is relatively unchanged. Above this knee, they start reducing line pressure to the rear.

There is more weight transfer to the front wheels under harder braking.

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Old 07-06-2016, 11:13 PM   #7
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