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Race Car Master Cylinder

 
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 733
Location: Maysville, Colorado

PostPosted: Thu Sep 17, 2015 4:42 am    Post subject: Race Car Master Cylinder Reply with quote

Project is to build a 1980 Dprod racer for local vintage racing. Considering my age and health it may never get done, but that's life. My first real problem is with brakes. I have a set from a 944 which are legal and don't want to use a booster as the entire existing system is shot. The 944 apparently had 23.8(15/16") and 1902mm(3/4") bores on the master cylinder.

Should I just get a pair of MC's with a balance bar of those two sizes? Go to two 7/8" with balance bar or a single with a proportioning valve. Or am I completely off base here? I assume I'll adjust the pedal pressure by changing the pivot point distance on the pedal arrangement.

PS: problem I had was convincing the local authorities that people used the 931 head on the motor as the 1981 GCR theyhave a 933 PN on the head. Don Meluzio and his father helped on that one.
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hayeslewis  



Joined: 21 Aug 2011
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Location: Alexandria, VA

PostPosted: Thu Sep 24, 2015 12:41 am    Post subject: Reply with quote

I'm probably telling you what you all ready know, but since no one else has responded maybe it will help.

All the 924s racing in the US that I'm aware of, all 4 of them (including mine) and soon to be a 5th, are using 944 brakes and MC parts with the booster.

I'm confused, If your older and your health is an issue, why wouldn't you want the most "modern" brake option available? I can't imagine it would violate the spirit or letter of the rules.
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Hayes Lewis
1979 924 FP/DP Bibi
1979 924 ITB #6 "Pam"
1990 Miata "Blossom"
2007 Armada
2014 Hyundai Elantra


19?? Cheap Chinese Pedicab with a 1.2HP electric motor "The MULE"
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Fifty50Plus  



Joined: 28 Feb 2008
Posts: 1008
Location: Washington DC area

PostPosted: Thu Sep 24, 2015 12:48 am    Post subject: Reply with quote

As Hayes says, go with what the factory designed. Get another 944 booster and new master cylinder and be done with it. Trying to balance your own designed bias system might take too many years of your time.
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car
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Fasteddie313  



Joined: 29 Sep 2013
Posts: 1559
Location: MI

PostPosted: Thu Sep 24, 2015 5:04 am    Post subject: Re: Race Car Master Cylinder Reply with quote

MikeJinCO wrote:
The 944 apparently had 23.8(15/16") and 1902mm(3/4") bores on the master cylinder.

Should I just get a pair of MC's with a balance bar of those two sizes?


The 2 different size bores in the 951 master have nothing to do with bias between the front and rear whatsoever..

In fact both front and rear circuits are driven ONLY from the rear piston the the master cylinder and the front piston simply floats between the chambers serving to seperate the 2 ircuits in an event of a line failure..

The different volumes between the master front and rear are tuned by combined calliper displacement volume so if one set fails the "front" piston can float one way or the other and provide brake pressure to the existing good circuit..

As an example the 924 has corner to corner split with one big and one small caliper on each circuit, therefore oth circuits have equal caliper volume dispacement, therefore same side "front" piston diamiter and rear.

951 is front/rear curcuited as apposed to corner split so it has 2 igger front calipers and 2 smaller rear calipers, the circuits have different volume displacement, therefore the difference in front/rear pistons in the master to tune the failure mode for what calipers it is made for..

Simplified with either master you will get equal pressure to both circuits because they are both pushed off the rear master pisont, the difference has no effect on performance at all and is only there to tune the master to the failure mode of one of its circuits depending on caliper size difference between the circuits..



Read here starting at post 8..
http://rennlist.com/forums/944-turbo-and-turbo-s-forum/886294-rear-calipers.html
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 733
Location: Maysville, Colorado

PostPosted: Thu Sep 24, 2015 9:24 am    Post subject: Reply with quote

Thank you very much. I just got a mindset that I couldn't use a boosted system and the more I looked into it the uglier it got. Between playing pedal ratio games(and then reinforcing the firewall), The difficulty of trying to set up a single MC, proportioning valve system or in a stalling a two MC balance bar system was getting rather mind- boggling. And also more expensive.

Again, thanks, a simple solution is generally better.
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hayeslewis  



Joined: 21 Aug 2011
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Location: Alexandria, VA

PostPosted: Thu Sep 24, 2015 10:54 am    Post subject: Reply with quote

Cant wait to see the results. What's your plan for the fuel cell?
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Hayes Lewis
1979 924 FP/DP Bibi
1979 924 ITB #6 "Pam"
1990 Miata "Blossom"
2007 Armada
2014 Hyundai Elantra


19?? Cheap Chinese Pedicab with a 1.2HP electric motor "The MULE"
Multi Use Lazy-ass Electric vehicle
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Fasteddie313  



Joined: 29 Sep 2013
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Location: MI

PostPosted: Thu Sep 24, 2015 1:11 pm    Post subject: Reply with quote

Check this out too.. http://924board.org/viewtopic.php?t=41323
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MikeJinCO  



Joined: 08 Jun 2010
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Location: Maysville, Colorado

PostPosted: Fri Sep 25, 2015 10:16 am    Post subject: Reply with quote

I will get an ATL/Fuelsafe one of the approved ones about 8 gal probably from Summit or Racers Wholesale. I think it will fit in the back compartment. Directions on that are a little vague, I look at how the RX7's are set up in late October at PPIR in Colorado Springs. The regular fuel tank has to be removed. Most of the small bores are Spridget.TR stuff with a couple of 914's so fuel cell in the trunk. and one really quick MGA,
.
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Mike
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Fifty50Plus  



Joined: 28 Feb 2008
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PostPosted: Fri Sep 25, 2015 11:32 am    Post subject: Reply with quote

Before you go swapping your gas tank for a fuel cell, verify with the powers tha be in you racing organization that it is mandatory. if not, save yourself many headaches and stick with the stock gas tank. Conversion is a PITA (speaking from many race car experiences).
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 733
Location: Maysville, Colorado

PostPosted: Fri Sep 25, 2015 2:23 pm    Post subject: Reply with quote

Unfortunately they are quite clear n that one. RMVR exceptions to 1981 GCR for Production,GT-2, GT-3, GT-4 Cars.

Appendix A Section 1.5.3 -Fuel Cell Installation

OEM fuel tanks are not allowed and must be replaced with a safety fuel cell. Refer to.....

And that will be about a $800-900 hit.

They also specify 8 point roll cages where NASA Spec944 is at 6. Some things just have to be lived with.
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 733
Location: Maysville, Colorado

PostPosted: Mon Aug 01, 2016 3:30 pm    Post subject: Reply with quote

Hate to bring this back up --but. The stock 924 booster is a nominal 7". I think and I may be wrong, that the stock 924 booster is a nominal 9". If that is the case then the booster will interfere with the throttle cable where it comes thru the firewall and be very close to the clutch cable bracket. Can someone help me out here. Another option might be to just stay with the smaller booster and have quite a bit more pedal pressure required.
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Fifty50Plus  



Joined: 28 Feb 2008
Posts: 1008
Location: Washington DC area

PostPosted: Tue Aug 02, 2016 3:59 am    Post subject: Reply with quote

Mike,
I just looked at my new '79 NA racecar which came from the factory with the 9 inch booster and vented brakes. There is barely clearance at 11:00 oclock for the throttle cable and no problem at all at 7:00 oclock for the clutch bracket.
I no longer have access to the '82 racecar so I can't tell you what was on that one although it was converted from standard brakes to 944 vented before I bought it.
Disregard my PM.
Regards,
Chuck
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1979 924 NA race car
1982 924 NA race car - Sold
1982 924 Turbo almost a PoS
1981 924 Turbo a real PoS, new engine
1982 924 Turbo nice body, blown engine
1972 911 E race car - going to Vintage
Various 944s to become IT-S race car
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MikeJinCO  



Joined: 08 Jun 2010
Posts: 733
Location: Maysville, Colorado

PostPosted: Sun Aug 21, 2016 3:10 pm    Post subject: Reply with quote

The saga continues. I got a used 944 booster. The booster did not have a PN tag on it so there are no guarantees it is the correct one.

I first tried installing it using the firewall spacer that came with it, but it didn't work as it sticks out further and also has a higher angle tilt so that there is no way the throttle cable can get past it. So then I switched to the 924 spacer which is shorter and with lower angle. It now fits -barely- for the throttle cable, but the actuating rod is almost 1" longer on the 944 booster than the 924 so it is too long to attach to the brake pedal lever, even with the adjustment set for the shortest length. So it appears my options appear to be.

1. I can put a spacer of approximately 7/16" in between the booster and the spacer using the existing studs on the booster, then thread the actuating rod a little further (10mm dia) and saw the rod off about 1/2" shorter or as required. This would also ease the situation of the throttle cable interference some.

2. Get a rebuilt booster from FLAPS and hope it is different than what I have.

3. i could also build a spacer between the spacer and the firewall, but that is a much more complex construction and I would only trust using a full ring as just the blocks would not be structurally sound.

I'm going to start off with a trial fit of home made wood spacer on the booster of 7/16" and see how it all fits and go from there.

This whole process gets interesting, but also very aggravating. The information I have read is that the stock 7" booster doesn't have enough power for both front and rear disc brakes.
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MikeJinCO  



Joined: 08 Jun 2010
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Location: Maysville, Colorado

PostPosted: Mon Aug 22, 2016 1:28 pm    Post subject: Reply with quote

Solution: The studs on the 944 booster are slightly longer than the on the 7" from a 924. I made a trial space from 1/2" oak trial fitted and it greatly improved the situation although there was no free play in the pedal. So I cut about 3/8" off the actuator rod and all is well. The throttle cable interference situation is also improved significantly. Since I have to make all new brake lines moving the MC forward is not really an issue.
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