Battery specs?
 

The s4 I bought came without a battery, and while building and mildly modifying the engine we'll also be relocating the battery to the boot (convertible, so not in-cabin). This means I'm not constrained by physical size/measurements of the battery, but I was wondering what batteries or specs people recommend? The car won't be a daily, just track and cruising use.

Some will crucify for this, but I use a 51R in the back cubby. No issues. Many others use even smaller for track cars (Odyssey, Braille, etc.). A good relocation, good battery, and good alternator, you’ll be fine.

A small Odyssey will get the job done, but if you want the car to be able to crank for longer than 5 seconds consider getting something with more than 600 CCA's. I had a Odyssey with 500-something CCA's and it didn't like to spin the motor for very long before dying. This is probably due to a few other factors, I know, but just a personal recommendation.

thanks! that gives me a good starting point at least

I just replaced my old Optima yellow top..with another one!
I think it needed to retire..after TEN years of service!

the 51R was actually a stock battery in Japan for the S4.

I just looked up pricing! yikes
our local pricing is over double what the american pricing is (after currency converting to compare apples with apples)

Ya,I just about dropped some fudge nuggets when I got the price of the battery.
The thing was that I already had a custom battery holder specifically for that type of battery,so I was kind of in a spot.
Plus the fact that I am impatient as hell,so I got a new battery,brought it home and turned the Key..Vroom!

One thing to keep in mind with small batteries is that these cars can flood. If you only get 20 seconds of cranking out of a super light race car battery, you're going to be in a parking lot somewhere at some point in your life looking helpless holding out a set of jumper cables.

Street cars should have street batteries. My personal preference are the Deka Intimidators. Off hand I think they are rated at 60Ah or there abouts. CCA is a mainly useless measurement however I believe the Intimidator is > 600 CCA.

For my daily, I had the stock battery, which is quite big. Always cranked long enough to clear a flood, and each battery lasted about 6 years of abuse.

My TII has the smallest traditional battery I could find at the parts store (26R, 540 CCA). Fits with shims under the strap and cranks great. The car does have good compression and overall health. I'm considering going smaller.

Aaron raises a good point and a decision to make.
The small batteries can only do so much in terms of starting the car and then BLAH..it's done.
Personally the only time I would use a small battery is if it was for racing..or IF your car starts up without hassle.

I bought the Optima as I have a 6000 watt stereo and some other stuff running.