Why 2 Batteries?

911t Fanatic · 911t Fanatic's Garage

Can anyone explain why my 70 911T has 2 Batteries, and what will be my concernes if i decide to go to one in the smuggkers box. Thanks.

Dantilla · 02-17-2010, 08:12 AM

Weight distribution.

My '73 is using only one battery. No problem.

Grady Clay · 02-17-2010, 08:36 AM

The two batteries were introduced in 1969. They are there to increase the polar moment of inertia about the longitudinal axis in the front. These masses (batteries) provide the moment of inertia for the front suspension to react against. For best handling, both batteries should be in place and the clamps adjusted so they are firmly in place (there is no benefit if they are loose).

The big ‘impact bumper’ and bumper shocks provide this function on ’74 and later cars.

For SWB cars (’66-’68), Porsche put large cast iron ‘bumper weights’ inside the front bumper at the corners.

Best,
Grady

dicklague · dicklague's Garage

Good explanation Grady. I am surprised how many people don't know the 2 batteries are for weight and balance and not for cranking power. I had one battery go dead a couple years ago. Shop ordered the wrong batteries so I ran with one battery connected for a couple of months before I got around to installing the second one.

No electrical issues with one battery. but handling is a lot better with 2 in the spots they should be in. Especially since I have fiberglass bumpers.

Dick

HarryD · 02-17-2010, 03:18 PM

While I see the validity in theory, I went with a single Optima and have not noticed much of any difference.

Aurel · Aurel's Garage

So, does it mean that an SC will handle badly if I put a lightweight bumper on the front?

dicklague · dicklague's Garage

depends how fast you are!!!

deathpunk dan · 02-17-2010, 04:57 PM

I am still planning on running an optima in the smuggler's box. Street driven car with modern 205/60 z rated rubber.

dicklague · dicklague's Garage

BUT you will always know the handling is compromised!! [kidding]

skinnerd · skinnerd's Garage

Quote:

Originally Posted by HarryD

While I see the validity in theory, I went with a single Optima and have not noticed much of any difference.

Agreed.
The VAST majority of people HERE would never know or feel the difference.

Same thing with the Al lug nuts.
If you replaced with steel....could you tell a difference in your unsprung weight?

You would have to be a world class F1 (IMHO) to feel a difference in some of these things.

HarryD · 02-17-2010, 08:50 PM

Quote:

Originally Posted by skinnerd

Agreed.
The VAST majority of people HERE would never know or feel the difference.

Same thing with the Al lug nuts.
If you replaced with steel....could you tell a difference in your unsprung weight?

You would have to be a world class F1 (IMHO) to feel a difference in some of these things.

Now Doug, I might not be able to tell the difference but Kermzilla would....

911t Fanatic · 911t Fanatic's Garage

Thanks for the help.

WRGREENER · WRGREENER's Garage

I HAVE BEEN RUNNING A SINGLE BATTERY IN MY 72 SINCE 78. IT IS A SERIES 70 SIDE POST MODEL. I HAVE HAD TWO AND THE FIRST SLID RIGHT IN THE SECOND (INSTALLED THIS YEAR) HAD TO BE MODIFIED. MODIFICATION WAS TO CUT THE EXTRA HOLD DOWNS (PLASTIC EXTRUSIONS EACH SIDE AT BASE) OFF. SIMPLE HACKSAW. I HAVE ADDED TWO FEATURES THAT MAKE THE "LOOK MESSY") HOWEVER, THEY MAKE MY LIFE EASIER. e.g. ADDED A "PRIORITY START" (A LOW VOLT SHUT-OFF) TO THE POS. SIDE AND A MANUAL NEGATIVE DISCONNECT. LOOKS MESSY BUT I HATE CHARGING AND LIKE TO DISCONNECT WHILE I PLAY ELECTRICIAN.
NOTE: IF YOU DO USE THIS SYSTEM, NOT ALL BATTERIES ARE CREATED EQUAL. I PURCHASED ONE (LABEL SAID "PORSCHE 911") HOWEVER THE POS AND NEG WHERE REVERED (+RIGHT/-LEFT) FROM MY SET-UP. BURNT UP MY WIRING HARNESS AND ALTERNATOR.
THERE HAVE BEEN GREAT THREADS ON SMUGGLER BOX INSTALLATION