Because an adjustable balance bar can only adjust the front to rear proportion. It cannot put a knee in the prop curve.

It is still desirable to have a prop valve in the system because it better matches the brake proportions to the weight transfer. More rear bias initially, then at a certain pressure, reduces the rear once weight has been transferred from the rear to the front.

A balance bar can't do that. Having both systems gives you full control.

Thought of that but could not find anything from Tilton that dose that.

Adjustable PV:http://forums.pelicanparts.com/uploa...1285295706.jpg

Merlin Line Lock.

http://forums.pelicanparts.com/uploa...1285295763.jpg

Thx Tyson.

I love Pelican. Always something new to learn.

There is some serious wide angle effect in that picture....

Whatever...

Does anyone remember the beemers from a few years back that ran -5 deg or so camber?

In my race car (an improved touring SCCA RX-7) I used the prop valve every lap at say, the Glen. The front straight is down hill, while the bus stop us up hill, so i can get better braking with the rears getting different pressure. And, in the rain, I need to adjust it differently as well. Sometimes the track dries out, so....

as for the Miata vs the 911, well, apples to apples (either a stock Miata vs a stock , oh, 89 Carreara, or a Spec Miata vs a mildly tuned 911), the Miata will own the 911 in the entry, the corner, and off the corner. Of course the 911 will own the straight....that's just engine.

I say this having driven both.
The Miata is a great car. Mazda did a wonderful job with it. It just is greater than the sum of it's parts.

That said, most guys I see in 911s at track days are in the way in corners. Having horsepower makes it too easy. To learn how to really drive, they should have something that needs to maintain momentum.

This is a sign that you may not have the brake balance working together with the prop valve as well as it could be.

The graph below illustrates:
http://forums.pelicanparts.com/uploa...1285350861.jpg
The amount of pressure required to lock the front and rear wheels at the same time depends on how much weight the car is transferring. The harder the car is stopping, the more weight is transfered off the back wheels and the less pressure the back brakes can take relative to the front before they lock up. This relationship is the blue curve in the graph above.

A car with no bias valve, like a 930, has a fixed ratio to the front and rear- see the green line. This ratio matches the ideal (locks the front and rear at the same time) at one point on the graph. The rest of the time it's off, and for safety they err on the side of locking the front brakes too early. In wet conditions, however, the front brakes lock much earlier than ideal, so total braking is reduced.

A dual master cylinder race car lets you adjust the bias for the conditions, so you can adjust to the red line in the wet, back to the green line in the dry and everything in between. Dual master cylinders are reliable and consistent, but they make for lots of adjustments to stay close to ideal.

A good bias valve and brake setup can virtually eliminate the adjustment by being almost ideal over a range of conditions- the purple line. If either the caliper bias or the bias valve itself isn't quite right, however, you're back to adjusting the valve often to compensate for changing conditions.

In the category I race, the rules require stock master cylinders. So, balance is done with pads choice. Calipers and discs are stock as well. We are allowed a prop valve. So, I set the car up with the basic balance to the rear, then dial it back for the 'standard" conditions with the prop valve. Then, in the rain, I have some adjustment left. And I have some adjustment for the steep downhill where the fronts get a lot of weight transfer.

Great info guys!!!

Spec Miata's run DOT street tires. Toyo RA-1's to be exact. Not slicks. Plus, if I put the catalytic converter back on my car, I could drive it on the street.

Is the graph in post 66 above why Porsche went to a PV w larger rear pistons with the 3.2 Carrera?

Or is there some other reason?

For reference, spec miatas are comparable in weight and tires to spec 911s- both ~2400 race weight and RA-1s. The spec 911s run a bit over 100 hp more and look a couple seconds faster per lap on a track like thunderhill on the same day. They can also be street/track cars. The spec 911s are considerably pricier, of course.

The 911 gains an advantage vs a miata as the power to weight ratio of the cars improves. As power increases traction becomes more and more of a limiting factor. The extra weight over the drive wheels allows the 911 to usefully deploy a much higher power to weight ratio, part of the reason the spec 911 is on the lower end of power to weight ratios for 911 race classes.

Yes, that's why virtually all newer cars run proportioning valves on the rear brakes.

yes, and they continue to use them even on cars that never would hve used one back in the '70s

oops, missed Pete's response

Excellent reference info!!!


Looked up a few things.

Spec Miata's run 205/50/15, Spec911 225/245 RA1's.

Spec Miata weigh 2285 to 2450 lbs, spec911 run 2400lbs min.

Suspect Miata puts out about 120whp and a spec911 about 220whp.

Thus about a 19lbs per HP for the Miata and 11lbs per HP for the 911.

2 sec a lap on I am guessing what is about a 2:05 lap time (thus 125 v 123 sec per lap).

Thus it looks like the 911 has about a 40% hp advantage, more gross tire per lb (maybe 20% more) and probably more braking reserve per lb than the Miata.

However, the 911 is only about 2% faster.

What is up with that?

It would be cool to see some data acquisition info comparing from the Miata and Spec 911 on the same track and see the speeds in the different part of the tracks.

I think Infineon would be a better track to compare handling since it is less of a horsepower track than Thunderhill.

I've run a 1:48.47 at Infineon in my Spec-911, and that was carrying an extra 150# of trophy weight, so 2520# with driver. My car dynod @ 205 RWHP.

What is the Spec-Miata record at Infineon?

Good question. I think they can run in the 1:54s, but the quickest time I saw in a quick scan was in the 1:56s. I'm sure the spec miata guys can correct me...

Infineon puts more of a premium on putting power down than Thunderhill, which helps the 911's cause.

"Handle better" is certainly a slippery question.
Thought experiment: take a Spec 911 and Spec Miata chassis but equalize hp at say 150 hp: the miata would likely win at most tracks. But if all you did was up the power of both cars to 450 hp while keeping the chassis and tires the same the 911 would handily beat the miata. The miata can't use much of the extra power while the 911 can.

You didn't change the suspension or tires- which car "handles better"?

Spec 911's have run 2:00.xx at Thunderhill. I think in ideal conditions one of us is going to break into the 59's there.

So that's about a 6 second delta at both Infineon and Thunderhill.

The Spec Miata is an interesting case, and I think one aspect that is in it's favor is the incredible numbers of them, and the resulting development.
And as a starting package, the SM "kit" (dampers, springs, etc) is pretty darn good, on top of a car that is pretty darn good. Whats interesting to me is that, when you "upgrade" a Spec Miata to Improved Touring allowances, the result is faster, but not by a whole lot. A friend has a well prepped ITA Miata, and holds ITA track records all over the NE. (IT allows 7" wide wheels, which get Hoosier A or R comps, some more engine allowances (mid 130 at the wheels, max), and free damper and spring/bar setups, essentially, at a race weight of 2380) Yet, at a track like Lime Rock, where laps are a minute and change long, it only beats the top SMs of the Runoffs champ level guys by maaaaybe a second or so.

I've driven them, and the cars are telepathic, you think, they do. I find them super easy to get to 98%. Finding the nth degree in speed takes time, and experience.

Tyson, I haven't driven a spec 911, but I have to think it's a much trickier machine to carry speed with. I imagine you've been in both, is that the case?

Can't use it in the corners you mean. But assuming that they are running a bigger tire front and rear, comparable to the 911, they are going to go into the corner faster, and maintain that momentum. The 911 is going to go in slower, and come out faster. On some corners the 911 will have the advantage, and others the Miata. At least that is how it works with 911s and 928s.

I'm sure you're right on the THill times Tyson. I saw 2-3 seconds same day on a couple time sheets, but those were 2:02s for the 911 spec, not 2:00.

The faster the car, the more it changes from a "momentum" car to a "slow in, fast out" car. Consider: in a car that can't accelerate at all, the ideal line is simply the largest constant radius arc you can draw through a corner. In a car with infinite acceleration and braking, on the other hand, the fastest line is the tightest line through the corner, so the exact opposite. Obviously the "slow in, fast out" line would be much faster all else being equal.

Thus while it's true that the high powered miata spec will carry more speed through the corner, it's largely true to say that's because it can't accelerate with the 911. And while it's also true that at some point on the straight the high-powered miata will hook up and be able to deploy the extra power, the time down the straight is disproportionately effected by acceleration at the beginning of the straight.

So as power to weight climbs even a miata (or 928, or 951, etc) wants be be a "slow in, fast out" car. They are limited, however, by having ~50% weight over the back wheels, which caps their peak acceleration off the corners regardless of the power available. A 911's peak acceleration will be more than 25% greater, all else being equal, due to where the weight is, so it's that much closer to the ideal "slow in, fast out" car.

At low power to weight ratios this doesn't come into play, as you're rarely traction limited. The faster the car, however, the more you want the weight in the back and the less mid-corner speed is the deciding factor.

So simplistically we have a two cars that are close on racing weight but the 911 has about a 40% HP advantage, maybe a 20% GROSS tire advantage, better brakes and it is only about 2-4% faster than the Miata.


I am still guessing the 911 is a bit rear traction challenged compared to the Miata. I think 205/255's would get the 911 to about the same traction per axel.

(The Miata I am guessing has about a 1200 lb axel weight w 205 8.4" wide tires.
The 911 rear axel is about 1440 lb with 245 9.6" wide tires.)

The Miata also has less frontal area to try to push down the straights.


It would be cool is some G readings for the Spec911 and Spec Miata.