What CA are needed for battery?

[phunt]

86 3.2L call out for battery cranking amps is 750+. I have seen threads where people say and i guess use the Odyssey PC925 nice and compact and light but only 330CA. So what's the deal? What is really needed? If you are using a PC925 how long have you been using it? is there ever a time when it struggles? Also my car is considered high compression at 10.5. Probably around 200psi at cranking

[Schulisco]

The starter motor of a 911 G has an output of approximately 1–1.5 kW. At 12 V (which isn't the case, as the voltage drops to 8–10 volts when the starter is engaged), we're talking about a maximum current of 125 A. To make the calculation more robust, I would add 100% for cold days and the activation time, which is practically equivalent to a short circuit in the first second (the initial current surge is infinitely high in the first fraction of a second). So you can easily answer the question yourself...

I wouldn't recommend using a motorcycle battery in a car, even if it's an AGM. Porsche preferred large lead-acid batteries in the front to make the car more controllable, as the front end is too light. The large battery, the spare tire, and a full gas tank in the front make the car steer better and less sensitive. That's why older 911s had lead weights in the front bumper, and Porsche always recommends using both spoilers front and rear. Otherwise, the aerodynamic balance will be disrupted, and the car can become uncontrollable at higher speeds. This is mainly due to the body shape, which is reminiscent of airplane wings and creates negative pressure at the rear. Just because the rear becomes lighter at higher speeds doesn't mean that more weight is shifted forward.

If you want to make the car lighter, you should not only reduce weight at the front but also at the rear to maintain balance. You can't compete with modern cars. If that's your goal, a classic 911 might not be the right car for you.

Thomas

[phunt]

When did I say anything about wanting to use a light battery?? Or lighten up the car?? I thought I asked a simple question about cranking amps. I know I did not ask for a math lesson. If you can’t answer then don’t. Always someone here has to shot off at the mouth with unnecessary information. I guess trying to make themselves feel relevant. Schwachkopf

[wazzz]

OK, I'll tune in for rescue (I hope). What Thomas just says is you don't actually need 750 amps, that's overkill. As the math say, max current needed by the starter motor will be around 125 A. The starter motor cannot draw more current than that, since it is rated for 1,5 kW and only uses a low voltage of 12 volts.
Since most commercially available batteries put out more than 5 or 600 A, you can buy a standard lead-acid battery. For a 3.2 Carrera at 10.3 CR, buy a 70 or 75 Ah capacity and you should be good to go.

No opinion about the weight loss. HTH

[Snitzler]

88 3.2 Carrera. I used a PC925 for my car and it spun the engine just fine. I live in a colder climate and early spring and late fall never caused any problems with the battery. I did redo all my grounds and the one that was on the transmission was falling apart so look at yours. Great battery and lots of ways to mount it. I had mine in the car from 2017 until now.

[phunt]

thank you

[darrin]

Phunt -- fwiw, porsche/other car makers specify batteries with far more reserve than is "necessary" to help assure the car will start in compromised conditions. e.g., while many foreseeable drains with relatively low power draws (a dome light or parking lights accidentally left on overnight) can fully deplete a 330 CA battery, a 750 CA battery would likely still have the reserve necessary to start a car after the same drain has been applied. Similarly, as a battery's capacity decreases with age/abuse (i.e. allowing a lead acid battery to fully discharge), the extra reserve amps provided by a larger battery help mitigate the risk of being stranded/requiring a jump start. TLDR version -- if all is working correctly, a small 330 CA battery provides plenty of power for starting, but it provides far less reserve against "the unexpected" than the larger battery porsche specs. I downsized my 86 targa's battery from a group 48 to a slightly smaller/lower capacity group 47 SLA many years ago (primarily because it's a lot easier to maneuver into position) and have no regrets. However, I doubt that a smaller 330 CA battery would have similarly survived (and/or started my car after) the "operator errors" my group 47 battery has weathered.

[ErrorMargin]

Originally Posted by phunt

If you are using a PC925 how long have you been using it? is there ever a time when it struggles?

My 1984 Targa came with a PC925 when I bought it three years ago. The engine in my car is completely stock.

I put it on a small Noco 1A charger at times I know that the car will sit for more than a month.

The car starts right up every time, no struggling.

I fully agree with the comments above about weight, etc.

If my car still had the stock sized battery I would probably just stick with it and not use anything like a PC925.

[josephvman]

I've used a PC925 for nearly 5 years and have left the car off the charger and unused for over a month and it has yet to have an issue starting the car.

[Ian Comerford]

I have had a 925 for just over two years and have had repeated starting problems. But, they were induced by my fitting a Brise coaxial high torque starter motor, not realising that it needed 330CCA. In warm weather it was generally fine but as soon as the temperature dropped it struggled. I’ve now replaced it with a 'normal' Yuasa lead acid battery and all is fine

[jkkarrow]

For a Carrera at 10.3 CR,, You can buy a standard lead-acid battery, buy a 70 / 75 Ah capacity and you should be good to go.

[walfreyydo]

Originally Posted by wazzz

For a 3.2 Carrera at 10.3 CR, buy a 70 or 75 Ah capacity and you should be good to go.

No opinion about the weight loss. HTH

Originally Posted by jkkarrow

For a Carrera at 10.3 CR,, You can buy a standard lead-acid battery, buy a 70 / 75 Ah capacity and you should be good to go.

Report this guy, hes trying to build post counts to defraud people on used parts classifieds