First track day w/ 3.6 conversion!!

[mmasse]

Today I took my '76 S with 993 '97 Vario Euro engine to the track for the first time.

Overall I am extremelly happy. I do have some issues to solve...but that's what all of this is about, isn't it?

I had two main issues:

1) Extremelly spongy brake pedal..once hot

I put new Motul racing brake fluid and power bleed the system, before going to the track, and I also had almost new Brake Pads.
What can be the problem? My only guess would be that my rubber brake lines are too old. Any other suggestions??

2) Too much understeering.

I have stock suspension, except for 21mm torsion bars front and rear. I have 245/35 18 in the front and 295/30 18 on the rear.

What should I do to generate more oversteering?

Some pics of today

[mmasse]

For some reason pics didn't upload the first time. Let's try again

[RichMink76]

Wow, your '76 doesn't look anything like mine. It seems you have done a few modifications.
Did you upgrade your brakes to handle the extra power?

As far as the suspension is concerned, I think you need to go heavier on the torsion bars in the rear. You might try 30's or even 31's.

Or with that 3.6 you could try some more right foot to cure that oversteer.

Rich

[Jack Olsen]

Are you running the stock 76 brakes? If so, you might want to consider at leas upgrading to a Carrera setup, if you're tracking the car.

Understeer could be caused by a long list of things. I assume by 21mm bars front and rear, you mean antisway bars. Are they adjustible? If they're not, things like alignment settings and even tire pressures can be adjusted to tune the car's handling. Is it low speed understeer, high-speed understeer, understeer on entry to corners, or on exit, or in mid-corner?

[Jeff Alton]

21mm torsion bars or sway bars??

Your pedal goes soft when the brakes are hot because they are too hot! The fluid in the caliper is boiling and introducing air into the system, making the pedal spongy feeling. Investigate some brake cooling ideas as seen on this forum.

Cheers

[PorscheGuy79]

To echo what the others have said, the reason your brakes are setting fire is because your current brakes are designed to reign in 150 or so horses, and you now have around twice that. Look into some cooling, maybe coupled with slightly larger brakes like the aforementioned Carrera set up.

[mmasse]

Thanks for all your comments.

Yeah, I meant 21mm antisway bars...sorry

Regarding the boiling of the brake fluid. I assumed that the fact of having small brakes would imply less stopping power but not necessary a spongy pedal. Am I wrong?

[JeremyD]

Yes - you are boiling the fluid. Small brake calipers (and more importantly, rotors) will just transfer the heat more quickly. Having a bigger tire patch will quicken the process.

I'm suprised you even had brakes after 10 minutes into a session.

Easiest way to introduce more oversteer is with tire pressures. Go down on the front and up on the rear.

[herman maire]

So.... Where are you getting the Understeer???

[Jack Olsen]

Originally posted by mmasse
Regarding the boiling of the brake fluid. I assumed that the fact of having small brakes would imply less stopping power but not necessary a spongy pedal. Am I wrong?

It's just the opposite. Any Porsche brake will give you all the stopping power you need. Stock brakes will allow you to lock up the wheels just as well as any upgrade. But the chief virtue of larger components is that they dissipate heat better.

[mmasse]

Thanks again for all your help.

With the brakes it seems clear..go bigger!!

Regarding the understerring. this is what I am experiencing: When I go into the turn, the car seem to respond well. i assume the weight transfer to the front helps to reduce the understeering and the car enters the turn nicely. Now, when I start giving some gas, I experience a strong understeering. Again, i assume this is caused by the weight now being shifted to the back.

Doing some research on the forum I found the following chart:


Jeremy, according to this chart in order to increase oversteering I should increase tire pressure on the front and reduce on the rear, which is opposite to what you suggested. Which one is correct?

Any other advice would be greatly appreciated.

Thanks again

Mario

[mmasse]

Question: Will a strut bar increase, reduce or do nothing to understeering?

Thanks

[Jack Olsen]

A strut bar will have no effect on understeer.

What are your suspension settings currently? Camber and toe settings are probably the most effective way to dial out your corner-exit push.

[umgang]

Hello there,
Having a 1975 911S that has a 3.6 varioram in it I understand your debacle. The beauty of Porsches is the unique balance of power, braking and handling-as a package. Though I tried to tell myself that the engine conversion did not really upset this blalnce, after trying different tricks like cooling ducts for brakes, wider rubber, corner balancing tweaks, I realized the power overwhelmed/stressed the other elements. (A photo of the car at speed on the track really woke me up). Suspension was first, and I was fortunate to have the help and expertise of someone who has done these conversions and built many a fine 911 race cars. He explained to me the way suspensions work as an integral unit, torsional rigidity, compression and rebound of dampers, and tying the corners together through anti-sway bars. (Read Fred Puhne's book "How to Make A Car Handle") I did it once the right way and hit bullseye. Later, came the realization of a brake upgrade-I went with Brembo rotors and 930 calipers specially machined for this car's application. If you are not saavy in engineering your car, it is well worth the time and $$$ to talk to someone who is-in my case Steve Weiner at Rennsport Systems. He designed and spec'd the car from a long way away, and I put it together. It's stunning and visceral.
Good luck, you've opened Pandora's Box!

Nick

[Jack Olsen]

Also: does the car have a limited slip differential?

[mmasse]

Jack, no, i have not LSD.

My alignment parameters are as follow:

Front:
Camber = L: -0.18 deg; R: -0.51 deg
Caster = L: 5.32 deg; R: 5.72 deg
Toe = L: 0.09 in; R: 0.10 in

Cross Camber: 0.33 deg
Cross Caster: 0.40 deg

Rear
Camber = L: -0.94 deg; R: -1.11 deg
Toe = L: .19 in; R: .33 in

Are this acceptable? If not, what should i aim for.

Thanks again

[Emission]

Originally posted by JeremyD

Easiest way to introduce more oversteer is with tire pressures. Go down on the front and up on the rear.

I am pretty sure it is reversed.

Oversteer in Porsche 911 is more pressure in front, less in rear.

This makes the rear break free sooner.

[LarryP]

Ditto re the last comments on brakes. Your brakes were NOT "designed to stop 150 HP"; they were designed to stop a 2900 lb car. It does not matter how much HP the car has now; it is still 2900 lb. They are getting hot because you are using them harder (on the track, for one, and at slightly higher speeds for another). Vent them and they will work fine.

[Joeaksa]

You might do a search on this forum. Bill Verburg has forgotten more than most of us will ever know about brakes and he is the resident expert.

He has a SC with a 3.6. Will send him a note and get him in on the thread.

JoeA

[mmasse]

Thanks Joeaksa. I have already done a lot of research on brakes and started a separate post in which Bill already contributed with his great expertise.

Thanks again.