Big Dave

My Interstate gave up the ghost after 6 years/55K miles. Which Optima Red Top do I need to fit my '85? The Optima website doesn't find one that fits, but I'm sure folks here have them.

Thanks in advance!

BD

davek9

Dave this was posted here by another member however I have not checked it out accuracy.
I have also heard that it will require an extension on the Pos cable.
I too am considering one as I need a replacement battery for my 325is and the 928 battery fits so..!
Optima Red top, Part # 75\35-OR, Model# 7586DT

heinrich

none, basically ... you need one with poitive post at the rear and to the right. and then you'll need to extend the cable or loosen it all the way and pull ...

Iris and Kevin

Hey Big Dave,

As far as I know, the Optima Red Top battery comes in one size:

Red Top Battery
10 inches long x 6.8 wide x 7.8 high.
800 Cold Cranking Amps
Summit link
Red Top Battery


Unfortunately, the stock battery cables have been found to be too short to make the stretch to this battery, which is a couple of inches smaller around than the stock battery, on 3 cars so far. My 84 US, my 84 Euro, and Glen McCartney's 85 US (he bought one from us that we were getting rid of recently in my wife's (Leslie.) FS thread). We're still not sure if this is three hits of bad luck or if this is a model wide issue, having only the three cars to go by, but it's looking like they may be needed for all 928s.

However, if you have a lot of aftermarket electrical equipment, I recommend the Optima Yellow Top Battery. It's the same size as the Red Top.

Summit has a set of cables that work perfectly for the swap. You can get their positive and ground cables here:
Summit Cables

Summit also has a battery relocation kit with hold down (it's a trunk mount kit which also comes with a battery box you won't need or use...it doesn't fit):
Summit Battery Relocation Kit

Hope this helps!

Big Dave

Thanks for the info. I can get a substantial discount on new Optima batteries, so now I need to decide whether it's worth the effort.

dr bob

Dave--

Six years is a pretty good life for a 928 battery. I've been getting four to five years. The latest one I installed is from Autozone, supposedly made by Interstate. Perfect fit, no cable issues, has the vent hose connection needed for the closed battery box, etc. About $60 IIRC. So far (two years...) so good!

JWise

I think SeanR has an Optima in his car, hopefully he will chime in.

Some of the Optima line features an "R" model with the terminals reversed. This would place the negative terminal where it needs to be, closest to the ground strap. I was recently considering one, but was very concerned about the height of the battery. It would have been VERY close to the battery box lid, and you certainly don't want the two coming into contact with each other. I also didn't want to rely on a rubber barrier to separate the two, as those can fail.

In the end, I went with the old standby Interstate MTP-91. I've used this battery before with good results, and its a perfect fit. Keep in mind somewhere during the 80's, I think the battery box size changed. Not sure which year, though.

FlyingDog

If you use the right Optima battery, you should only need a longer ground strap on pre-86.5US/pre-86ROW. On cars with the smaller battery box and separate dual exhaust you need the positive extended.

I'm getting one of Kevin and Leslie's red tops. If it doesn't fit, I'll buy the Autozone battery. I don't want to deal with extending the positive.

Champagne

I have a Red Top in my S4 and both cables fit perfectly. All I needed to do is isolate both posts to keep them from touching the metal lid of the box.

Paul

IcemanG17

I have an autozone "duralast" in mine...works great and is cheap.....7 year warranty....I think the size is 48D

SeanR

Exactly for mine also. I did replace the positive lead on mine because the old one was chewed up real bad and I didn't like the looks of it. As far as extending the cable, I've not heard of that one and didn't do that on mine.

It is a taller battery so it is smart to use some sort of rubber, plastic cover (mine had the stock one in there) just in case.

I've used Optima's for years on boats and cars and love them, just don't get them at Autozone or some place like that. For some reason the price was close to $240.00 from AZ and you can get them at Costco or Sams club for less than $150.00.

The part number posted above is the correct one, but before you buy one, do a test fit. I can't recall which cars have different size battery boxes, but I think some are different.

SwayBar

I am not familiar with the Optima batteries. Why are people willing to pay extra for them versus a standard battery; are they lighter?

Slantnose!

...same here.

Slantnose!

Yes, they are a little lighter, but big advantage is they're sealed and have no acid to leak out.

SwayBar