For mostly track car. what batteries are you using. And images of rear trunk installation.

Not a great photo but it's the best that I have right now. Car is stored and not readily accessible.
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Look into Antigravity Batteries. They are lithium batteries with protection electronics, and are very lightweight. I have one in my spec boxster, and so far it's working great.

They aren't cheap though. But they do run sales about once a quarter, so sign up for their email list to get the announcement .

I got my lithium battery at advance auto 2 years ago....autocraft LFP-8 for like $89. Evidently they have been discontinued, can't find a current price or availability. It starts my /6 just fine even after cranking with ignition off for 20 seconds to build oil pressure. It's listed as 500CCA and weighs about 4 pounds--remarkable.
Here's the closest I can find currently....Amazon listing for about $200...
https://www.amazon.com/Lithium-Battery-Cran...4853&sr=8-3Click to view attachment

'nother pic....
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Ditto. I'm running one in my street 2L /4, but mounted mine on the left side above the starter to shorten the positive lead run. Very disappointed they're not available any more.

On the race car I run "total loss" without an alternator, so a larger Honda-sized lead-acid in the standard location.

Edit: the one major downside to this battery on a street car is the lower reserve capacity. If you lose your belt or alternator you're not getting as far, especially at night.

The plus side, of course, is that it weighs nothing; in fact, both I and the Advance Auto cashier has to open the box to ensure the battery was actually in there...we were both pretty impressed.

Another plus side is it can be mounted in any orientation. As did John, I mounted mine laying on its side.

That's a pretty good deal for a lithium battery. I've heard that some of them require special chargers to avoid ruining the battery. Is that the case for the Autocraft battery?

No idea.

I do note that my alternator tends to run the car at ~14.5V instead of the expected 13.5V. Car doesn't seem to care (I'm running Microsquirt and no radio, no other electronics to worry about).

Vision AGM model HP 12-116W,commonly used in speedway and track cars in my country,very low discharge rate when unused and very long life ,I purchased in the middle of 2015 for $58nzd (mates rates) and still starts my 6 easily,very impressed with it and it only weighs 9lbs from memory.
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Frankly I don't care for a huge heavy battery when you don't need it,the only downside is the terminal connections being so small.
I custom built a battery tray and can also take the higher capacity version of this...needs be.

I've got the Odyssey PC680 which is the tiny AGM similar to above, which is mounted on its side in the factory battery tray. It holds charge well - I disconnect for storage - and cranks my fairly high compression 2.7/six just fine.

I have this in both my 914s. Cranks the high compression 3.0 six with no problem. The one in my four cylinder is seven years old and is finally starting to show signs of slow cranking on start up after the car sits for a few weeks.