T fitting instead of prop. valve, what is the rational? |
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T fitting instead of prop. valve, what is the rational? |
McMark |
Dec 1 2005, 11:38 PM
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#21
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914 Freak! Group: Retired Admin Posts: 20,179 Joined: 13-March 03 From: Grand Rapids, MI Member No.: 419 Region Association: None |
You mean, this graph Dave? (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) It was on the next page. I hadn't turned the page. (IMG:http://www.914world.com/bbs2/html/emoticons/pinch.gif)
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McMark |
Dec 1 2005, 11:42 PM
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#22
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914 Freak! Group: Retired Admin Posts: 20,179 Joined: 13-March 03 From: Grand Rapids, MI Member No.: 419 Region Association: None |
Yes, but locking the brakes is not always under your control, and if they get locked up somehow, you want to plan for that worst case scenario (at least in a production street car). As my dad has said a million times, "It's not if there's going to be problem, it's what's going to happen when that problem occurs." |
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Trekkor |
Dec 2 2005, 12:15 AM
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#23
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I do things... Group: Members Posts: 7,809 Joined: 2-December 03 From: Napa, Ca Member No.: 1,413 Region Association: Northern California |
Good thread. (IMG:http://www.914world.com/bbs2/html/emoticons/clap.gif)
I have been "testing" the stock p-valve and have a tee in tool box. I've found the spongy feel is directly related to heat in the calipers. With normal driving and the first few laps on track I have firm pedal. As I develop heat, the pedal travels farther. I don't like the feel under the foot, but the actual brake action does not suffer. I'm trying to get used to that. (IMG:http://www.914world.com/bbs2/html/emoticons/confused24.gif) I mainly need to shorten the length of time I'm on the brakes and thus, apply them harder and later. KT |
J P Stein |
Dec 2 2005, 12:18 AM
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#24
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Irrelevant old fart Group: Members Posts: 8,797 Joined: 30-December 02 From: Vancouver, WA Member No.: 45 Region Association: None |
S up front, M rear. |
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Dead Air |
Dec 2 2005, 07:15 AM
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#25
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Senior Member Group: Members Posts: 1,268 Joined: 9-May 04 From: Buffalo, NY Member No.: 2,042 |
Our parents wisdom! Mom used to say "hope for the best but plan for the worst" As Dave implies, it's the shift in weight (to the front) that creates the lock-up and that's just a matter of physics. Is that likely to happen in many "street" situations? Not all rear lock-ups will result in uncontrollable over steer, correct? |
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andys |
Dec 2 2005, 10:52 AM
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#26
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Advanced Member Group: Members Posts: 2,165 Joined: 21-May 03 From: Valencia, CA Member No.: 721 Region Association: None |
TK, That's the answer we always used to give at the races...."stop using the brakes!" (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) I understand the fundamentals here, and any racer car I've ever driven had a brake bias adjustment. (But) What do the garden variety 911's use? I have '89 Carrera brakes, front and rear, ready to install. Did that car have a prop. valve? Do Carrera owners mess with their prop valves? Do they experience spongy pedals? Of the street 911's I've ever driven, I always thought they had excellent brakes. Anyone? Andys |
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Dave_Darling |
Dec 2 2005, 10:55 AM
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#27
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914 Idiot Group: Members Posts: 15,048 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California |
True. If you lock the rears while you're going in a straight line, and the road surface isn't too irregular, you shouldn't spin. If you're not near the limit of adhesion in any way whatsoever (except of course for the rear wheels sliding), you've got decent odds of not spinning. Then again, this is a race car (or at least a track car) we're talking about.... --DD |
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J P Stein |
Dec 2 2005, 11:22 AM
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#28
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Irrelevant old fart Group: Members Posts: 8,797 Joined: 30-December 02 From: Vancouver, WA Member No.: 45 Region Association: None |
The corelation between a Carrera brake set up and a 914 with Carrera brakes wouldn't seem to be straight across. The Carrera has more rear weight bais, bigger tires rear to front, undoubtly transfers less weight to the front during braking...The only thing that does work is a reasonable compromise between braking force caliperwise front to rear......it's close enuff to work out on a 914.....if you have a adjustable valve as less pressure will be required in the rear on a 914 due to the reasons above. |
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andys |
Dec 2 2005, 11:39 AM
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#29
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Advanced Member Group: Members Posts: 2,165 Joined: 21-May 03 From: Valencia, CA Member No.: 721 Region Association: None |
Re-reading my post, I see I mixed a few things together, perhaps confusing the intent. JP, you're absolutely right. Two different cars with two different sets of dynamic conditions. 911 = more rear weight bias = different brake biasing requirements than the 914. BTW, I will be running 255-17's in the rear. Teener's generally seem to complaing about the prop valve, yet 911's use them throughout, ergo my comment "Do they experience spongy pedals? Of the street 911's I've ever driven, I always thought they had excellent brakes." So what's the difference? Andys |
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kdfoust |
Dec 2 2005, 12:19 PM
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#30
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Senior Member Group: Members Posts: 694 Joined: 2-January 03 From: Riverside Member No.: 71 Region Association: Southern California |
I think that in panic stops on the street the worst case scenario can happen very quickly: non-progressive brake actuation, slamming on the brakes, which doesn't allow for weight transfer and front tire loading to acheive maximum traction during the stop. This means the fronts lock before being loading to their limits via weight transfer and even worse the rears may lock. This is why ABS is so effective on street cars but a little less of an advantage on race cars. Every good driving instructor I've had always emphasized not standing the car on it's nose with hard braking. The reasoning is that even on a fairly balanced car you don't want to unload the rear tires anymore than necessary in case you need to do some steering. At Willow Springs I can be reeling the "other Porsche" down from 130 and those brakes will stand the car on it's nose if I ask them to. At those speeds you really don't want to "upset" the chasis anymore than needed. (IMG:http://www.914world.com/bbs2/html/emoticons/burnout.gif) Later, Kevin |
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J P Stein |
Dec 2 2005, 01:39 PM
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#31
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Irrelevant old fart Group: Members Posts: 8,797 Joined: 30-December 02 From: Vancouver, WA Member No.: 45 Region Association: None |
I'm easily confused so don't let that worry ya. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) My old stock brake set up had a hi hard pedal. I never could figure out what folks were talking about ...."spongy?" back in the day. See. gettin' confused is nuthin' new for me. (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) |
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nick mironov |
Dec 2 2005, 02:04 PM
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#32
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nickm Group: Members Posts: 204 Joined: 12-June 05 From: San Francisco, CA Member No.: 4,264 |
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horizontally-opposed |
Dec 2 2005, 02:06 PM
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#33
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Advanced Member Group: Members Posts: 3,443 Joined: 12-May 04 From: San Francisco Member No.: 2,058 Region Association: None |
Eric... (IMG:http://www.914world.com/bbs2/html/emoticons/blink.gif)
Did you remove your reply? Inquiring minds -- that have M calipers up front but still run the proportioning valve -- want to know. pete |
andys |
Dec 2 2005, 03:14 PM
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#34
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Advanced Member Group: Members Posts: 2,165 Joined: 21-May 03 From: Valencia, CA Member No.: 721 Region Association: None |
Some like to free the car up upon entry into a corner with some rear brake bias. It's all in how you like the car setup. Karts with rear brakes only require some degree of forced technique, since rear brakes is all you have to work with. Ever try a direct-drive kart? Here again, it forces a technique change or adjustment. You are correct, that many often forget to modulate the brake pedal to get the desired result. A street car needs to be capable of dealing with a very broad range of conditions. I inagine everything is designed to err on the side of safety. Andys |
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Eric_Shea |
Dec 2 2005, 05:20 PM
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#35
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PMB Performance Group: Admin Posts: 19,289 Joined: 3-September 03 From: Salt Lake City, UT Member No.: 1,110 Region Association: Rocky Mountains |
First Post... (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif)
Carrera's went to a P-Valve in 84 or 85 (I think it was 84) when they switched to larger 43mm pistons for the first time in ages on the back of a 911. Up until 84 the piston size was 38mm. This is why I don't really recommend the 914 fronts (43mm) back there unless you've got some serious brakes up front. With M-Calipers on the front and stock rears I would use a "T" like Rick has in the Alien. Many people have echo'd that here. I'm not a fan of pulling the P-Valve just because the brakes are spongy. If there spongy there's probably another problem (like proper bleeding or the heat fade that Trekkor is talking about). I'd say, stock brakes, keep the P-Valve. Pete, for-Pete's-sake, I'd probably put a T-in there. (IMG:http://www.914world.com/bbs2/html/emoticons/wink.gif) Mine are exactly the same as JP's w/o the adjustable p-valve. I used Pagid Oranges up front and regular Jurid's in the rear. Basically a 911RS/First Year 930 system. There was another thread where Davep's buddy proved the theory that those are the ticket for the 220-250hp range. You can get a T-fitting at www.cip1.com or from James here I believe... |
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J P Stein |
Dec 3 2005, 10:46 AM
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#36
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Irrelevant old fart Group: Members Posts: 8,797 Joined: 30-December 02 From: Vancouver, WA Member No.: 45 Region Association: None |
Brakes......a decent buy:
http://forums.pelicanparts.com/showthread....threadid=254378 |
horizontally-opposed |
Dec 3 2005, 01:19 PM
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#37
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Advanced Member Group: Members Posts: 3,443 Joined: 12-May 04 From: San Francisco Member No.: 2,058 Region Association: None |
Yipes, I've been rolling this way for 14 years... (IMG:http://www.914world.com/bbs2/html/emoticons/ohmy.gif) I've never run into a problem but was never blown away by the brake upgrade. Now I know why -- I'm probably hardly using the rears! I've locked up the fronts maybe twice in that time, I think. Never had a fade problem, though -- which I did with the stock rotors. I drove a 993 with motorsport Brembos a while back. It had a lot more rear-brake bias and that thing was ~amazing~ under braking on the way into turns. Somehow, I don't think my setup will approach that (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif) -- but a little better is always, well, great! pete |
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SirAndy |
Dec 3 2005, 02:12 PM
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#38
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Resident German Group: Admin Posts: 41,815 Joined: 21-January 03 From: Oakland, Kalifornia Member No.: 179 Region Association: Northern California |
when i upgrade my brake system to a 19mm MC and mid '80s carrera rotors (vented) & calipers on all 4 corners i noticed that, with the stock prop valve still in place, the rear brakes did barely do anything. almost all of the braking was done by the front brakes. i replaced the prop vale with a "T" per brad's advice. he said that they had similar problems with all their race-build 914s and they first tried running adjustable prop-valves but always ended up running those on full open. which is exactly what the "T" does, except it's 100 times cheaper than a cockpit adjustable prop-valve ... now, with the "T" in place, front and rear brakes seem to do about the same amount of work, with the fronts locking up just a bit earlier than the rears. when i hit the brakes hard at speed, the whole car goes down evenly as compared to before with the prop-valve where the front would do a nose-dive while the rear got lifted up ... i'm happy with my "T"! (IMG:http://www.914world.com/bbs2/html/emoticons/boldblue.gif) Andy |
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nine14cats |
Dec 3 2005, 03:20 PM
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#39
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Bill Pickering -- 914-6 GT aka....Leeloo Group: Members Posts: 2,618 Joined: 10-February 03 From: Campbell, CA Member No.: 287 Region Association: None |
Hi Dan,
I'll give you what Doris and I were running in our 914-6 (the one you went faster in than me at Laguna (shhhhh! (IMG:http://www.914world.com/bbs2/html/emoticons/laugh.gif) ) 911T vented rotors / calipers front and 944 vented rotors / calipers rears (no parking brake). T instead of stock prop valve. I did have a cockpit adjustable valve. At Buttonwillow and Laguna Seca I ran the adjustable valve wide open and had not problems with brake balance. At THill (CCW) I would get too much rear brake coming hard into turn 14, so I would adjust that out. I also had the same issues at Sears, where coming hard into 7 or 11 and braking late I would get a little too much brake force in the rear. A couple of turns and I could adjust it out. I like the 19mm m/c pedal travel as a preference because it was easier to heal and toe without resorting to blocks. I'm with JP as far as the brake bias adjustment, especially if you can use it inside your car without it adding too many points to put you in a class you don't want to run in. Bill P. |
J P Stein |
Dec 3 2005, 04:46 PM
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#40
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Irrelevant old fart Group: Members Posts: 8,797 Joined: 30-December 02 From: Vancouver, WA Member No.: 45 Region Association: None |
Basicaly, what Bill, Andys & I are talking about is trail braking
into a corner. With more rear bias the car will tend to oversteer on entry....depending on how deep you like to go....or have to go on a OOOPs (IMG:http://www.914world.com/bbs2/html/emoticons/biggrin.gif). You can have too much of a gud thing as Bill points out. You can either change your driving style (style is not revelant describing mine) or turn the knob... if you have one. |
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