70 911 batteries

[kissov356]

The two Interstate batteries finally gave out in my 70T and I've decided to replace them with a single battery in the smugglers box. Should I replace the weight equal to the battery weight in the "wheel well" battery boxes. Thank you.

[304065]

That's going to be hard to do given the available space.

Suppose you decided to stay with the factory configuration of two up front. Sonnenschein batteries for that application weigh 15Kg each. To accomplish the same impact on polar moment you would need to fit MORE than 30Kg in the smugglers box, insofar as the SB is closer to the CG of the car. Add to that the need to re-wire: from the Factory, the car has a positive wire running past the gas tank on the port side and up to the fuse box and port side battery, then there is a crossover cable that runs to the starboard battery. You would have to re-route the positive cable to the SB and then back to the fuse box. Not a bad approach for a race car where you have removed the factory wiring and are adding back six circuits with ultra light weight TKT aircraft wire-- more tedious when you are trying to carry current for most all electrical loads back to the fuse box.

Given the additional weight of your 3,2 over the 2,2 magnesium cased 911T motor, I think you would want as much weight as possible as far from the CG as possible. Even on the race track this works better-as in, I can detect a handling difference with a full fuel tank at the start of a race vs. at the end with an empty one. And it definitely handles better with more weight up front.

Or do you mean putting the battery in the SB and putting lead up front?

[kissov356]

Thank you for your reply. I'm waiting for the battery and holder to put the battery in the SB I've redone the wiring. I was wondering about loosing the weight of the two batteries upfront, I know Porsche had their reasons. I could replace that weight with lead or just put the disconnected dead batteries back in the battery boxes for the weight. Or am I just obsessing about something that will not make any difference. Thanks.

[ttweed]

Or am I just obsessing about something that will not make any difference.

That depends on how hard you drive your car and how often you come close to its grip limits. Are you racing? Do you have a min. weight spec for your class where you need to ballast up anyway?

In general, lower overall weight is better--I would not add any ballast if I didn't have to. Weight distribution is always biased rearward by about 20% in a 911, and that can be problematic in some situations, but helpful in others. Anything you can do to shift weight to the front in the car will improve balance, but you are never going to get to an ideal distribution with an extra 80 lbs. of ballast in the nose. At 7 lbs./gallon, the fuel load in a 16 gallon tank amounts to more than that between full and empty. Do you notice the handling difference between a full and empty tank like John does when racing?

Extra static weight on the front tires can increase grip on that end of the car and prevent some understeer, and adding weight at the extreme ends of the car can increase polar moment, which may help you recover from a potential spin by allowing more reaction time to countersteer, but any increase in weight also reduces acceleration, braking, and cornering performance across the board, so it is a compromise. You can tune the suspension to dial out understeer, or drive around it by using techniques like trail braking to accomplish dynamic weight shift, but you can't get back the lost performance of unnecessary added static weight, which is there 100% of the time. Lower polar moment is actually better for transient steering response, as there is less weight to get moving at the end of the car every time you move the wheel. The reason that Porsche put those batteries up there and increased polar moment was to help protect unskilled drivers from their tail-happy, rear-engine design. It was the same tactic as adding the cast-iron bumper weights in the SWB models. With the right setup, good driving skills and fast enough reactions, you don't need them, even if you drive at the limit of grip constantly, which is rare in a street car. I took the 50 lb. bumper weights out of my '67S and never regretted it, but it also had wider rims and tires than stock along with a performance alignment and corner balance. Your heavier engine is going to increase your rear weight bias, but with the right setup, you can achieve a reasonable handling balance without resorting to leaving the useless batteries in the car as ballast.

YMMV,
TT

[kissov356]

Tom,
Thank you for your answer all makes sense, I'm never near the limit with my weekender but there is no reason I can't try it both ways to see if I can tell any difference.

[HarryD]

I replaced my dual lead acid batteries with a single optima on the drivers side. I have not noticed much of a difference in handling on the street or at Autocross.

YMMV

[boxster03]

I think the dual original setup is kind of cool

[merbesfield]

Boxter03, what is your battery setup? I see they are Interstate brand. My local suppliers sells Dekka batteries and I know this is old info, just wondering if there is anything new.