Out with the old...in with the new...Britain's Racecar Development, Subaru Engine, Lotus Suspension! Time to go racing! Options

[Britain Smith]

Please explain this comment?

The front brakes are 911S calipers and are much larger than the rear M-calipers. Therefore, the front will need a corresponding larger amount of fluid to move to pistons. Moving the larger master cylinder to the smaller rear calipers would only cause them to lock-up even faster.

Correct?

-Britain

[McMark]

From the Brakes FAQ by the brilliant Lapuwali...

The ratio of master cylinder piston area to total caliper piston area determines the mechanical advantage you get in applying pressure to the brake pads. The smaller the master cylinder piston (or the larger the caliper pistons), the more "advantage" you have (more force on the pads for less force on the pedal). However, while you apply less force, you have to push the pedal farther to get the same amount of movement at the pad. With a larger master cylinder piston (or smaller caliper pistons), you have LESS advantage, so you have to push the pedal harder, but you don't have to push it as far. The pedal feels firmer, but you're actually getting less braking. Some people like the feel this gives, so the upgrade isn't entirely a bad thing. However, many people make the upgrade because they think they're actually improving their brakes, when they're actually making them worse, just improving the feel.

[Downunderman]

Yes it is. The issue is weight transfer and line pressure. The front brakes do about 70% of the work under heavy braking and must work much harder than the rears.

With the bigger cylinder on the front you are getting less pressure to the front and more to the rear. 16mm front and 18mm rear should work out about right, then use the balance bar to fine tune. The issue is not fluid volume. Someone with better knowledge of physics than me me should be able to explain it.

The thing must have been fun to drive swapping ends every time you tried to stop it.

Cheers,

[Downunderman]

Apologies. I misread your post. Your proposition is incorrect.

[J P Stein]

You can produce more line pressure with the smaller MC.....IF you have enough volume. The problem with the 17mm MC in the stock 914 system was the lenght of the stroke necessary to produce volume....the first 2 inches of movement had little effect on retardation. You may have had to push the pedal harder to produce the pressure with a 19mm MC but the lenght of stroke was much shorter and it comes up firm . When the wall is coming up it's nice to have a nice firm pedal to modulate......"making them worse" is total...cow flop.

Brit's difference in MC sizes is moot. He can adjust then individually to get what he wants.

[Mike Bellis]

I would just add a proportioning valve in the tunnel by the firewall(coupling location) Then you can bias more pressure back up front.

[charliew]

Did you mention the highest coolant temperature you experienced? It seems to me that the fan needs a complete shroud to pull air through all of the radiator instead of just the center. If it's not getting too hot it must be ok as it is. I wonder how much cfm the hood vents can handle? You know the cfm of the fan. There is a formula somewhere that says the exhaust outlet needs to be a percentage larger than the intake. I think from memory it's 30% but it might be more.

[Chris Hamilton]

I would just add a proportioning valve in the tunnel by the firewall(coupling location) Then you can bias more pressure back up front.

He is running dual master cylinders, this isn't possible.

[charliew]

But I bet as a last resort he could put a bypass valve in the rear line to limit pressure to the rear.

[Britain Smith]

Hey guys...I appreciate the insight.

Regarding the brakes, now that I have bias bar installed correctly there should be no problem balancing the brakes bias front to rear. It will take some trail and error, but it should be possible with the current set-up.

On the coolant temp, the highest that I have seen the engine after a run is just under 200degs and that is running without the radiator fan on and 60deg ambient temps. I switch on the fan while sitting on the grid and the temp drops to ~180 in about 3mins. The current set-up has proven sufficient so far, however the ambient temperature has been pretty low. I will be adding shrouding in front of the radiator when I get a chance. Regarding the hood vents, one thing to remember is that not only is air escaping thru the vents, it is coming out of the cowl vent and the hood actually lifts in the rear at speed and air escapes out of there.

On the radiator air-deflector, I like the idea of the bubble wrap foil stuff that McMark mentioned, I have some of that from makeshift tire warmers I made a while back. I will try wrapping the tank in this before going down the route of an air deflector.

-Britain

[Walter]

A hot fuel tank is usually the result of an EFI engine where fuel is constantly circulated by the fuel pump and heated from the hot injectors. Do you run low-Z (low resiatance) injectors (peak and hold) or high resistance injectors (as most OEM)?
The low resistance have a very high amp flowing through them and get much hotter then the high resistance injectors.

I think insulating the tank from the radiator may not help much, but worth a try at least as its easy and cheap to do and fuel coolers are a lot more work after all.

[J P Stein]

I was told that the front of the tank is hot while the back is cool. I too believe the wrap will do the trick and it is in the KISS mode. (IMG:style_emoticons/default/biggrin.gif) .

[DBCooper]

Would never disagree with KISS solutions but would note that I get quite a lot of heat in my motorcycle gas tanks from circulating the fuel past a hot engine and back into the tank. BMW injectors and fuel lines are hanging right out in the airstream, so that might be a factor.

But you know what I most want to know is if that sucker is fun to drive. Brakes aside, of course, because it seems you've got that done. It just looks like you built a rocket, so does it feel that way in the driver's seat?

[Britain Smith]

A hot fuel tank is usually the result of an EFI engine where fuel is constantly circulated by the fuel pump and heated from the hot injectors. Do you run low-Z (low resiatance) injectors (peak and hold) or high resistance injectors (as most OEM)?
The low resistance have a very high amp flowing through them and get much hotter then the high resistance injectors.

I think insulating the tank from the radiator may not help much, but worth a try at least as its easy and cheap to do and fuel coolers are a lot more work after all.

That is a good point. I do have 16mm phenolic spacers between the intake manifold and the heads which should keep a lot of heat out, but I agree some degree of heat will be coming from the circulation.

That said, the front of the fuel tank was very hot to the touch compared to the sides and back of the tank. I will wrap in an insulation material and determine if that helps with the temps.

-Britain

[Britain Smith]

But you know what I most want to know is if that sucker is fun to drive. Brakes aside, of course, because it seems you've got that done. It just looks like you built a rocket, so does it feel that way in the driver's seat?

The car is a damn rocketship!!

If you watch the end of the AX video above, you can see where I come out of the 180deg turn and point the car towards the finish and hit the gas...the car accelerates like crazy!

Put it this way, the tuner at Cobb Tuning has driven everything from 600hp Subaru's to 1000hp GT-R's...he said he hasn't had this much fun in a car in 10 years.

-Britain

[Downunderman]

Brit,

I suspect that you will have a great time trying to sort out the brake balance, moot as JP says it is, but it looks as though there would not be a lot of work involved in swapping the front and rear lines on the master cylinders and giving it a try. Having gone to all the trouble you have, it would be a pity to spend half a season sorting it out.

Cheers,

[J P Stein]

Brit,

I suspect that you will have a great time trying to sort out the brake balance, moot as JP says it is, but it looks as though there would not be a lot of work involved in swapping the front and rear lines on the master cylinders and giving it a try. Having gone to all the trouble you have, it would be a pity to spend half a season sorting it out.

Cheers,

I didn't mean to minimize the importance of getting it just right to fit ones driving style. Straight line brakers can go with more rear bias than one who likes a to trail brake. Some AXers like the back end to step out a bit under trail braking to help the car rotate. The last time I set the rear bias was so one could trail brake (if you had to ) without upsetting the car tho I pretty much was a straight line guy. It took 2 passes to get it close enough.

[Britain Smith]

As an engineer sometime the solution to a particular problem can be way more complicated than it needs to be.

Case in point -
Problem: Need more HP
Solution: Take out the Porsche engine and replace with a Subaru engine.

Anyway, for the fuel temperature issue I have decided to try out McMark's idea with a simple insulated cover made out of the bubble wrapped foil stuff used on windshield screens in the hot summer. We shall see how it works this weekend.

(IMG:http://www.914world.com/bbs2/uploads_offsite/a7.sphotos.ak.fbcdn.net-364-1304012567.1.jpg)

Regarding the brake bias. All the calculations that have been completed indicate that this set-up should work how it is. I just need a few passes to get it dialed in and if that doesn't work then we can evaluate other options.

-Britain

[sawtooth]

Fantastic video, that looks like way too much fun. We need to get you to mount an iTouch/iphone on your dash and run an app like this so we can see/feel the G's (IMG:style_emoticons/default/smile.gif)

[bfrymire]

As an engineer sometime the solution to a particular problem can be way more complicated than it needs to be.

Case in point -
Problem: Need more HP
Solution: Take out the Porsche engine and replace with a Subaru engine.

Anyway, for the fuel temperature issue I have decided to try out McMark's idea with a simple insulated cover made out of the bubble wrapped foil stuff used on windshield screens in the hot summer. We shall see how it works this weekend.

(IMG:http://www.914world.com/bbs2/uploads_offsite/a7.sphotos.ak.fbcdn.net-364-1304012567.1.jpg)

Regarding the brake bias. All the calculations that have been completed indicate that this set-up should work how it is. I just need a few passes to get it dialed in and if that doesn't work then we can evaluate other options.

-Britain

Doesn't Home Depot sell AC duct wrap? I use it on my test systems that have to go through temperature cycles. It's a foil based wrap with insulating foam with adhesion. Simple to apply. Just another thought.

-- brett