jaydiii

Year ago I bought a ap1 with a spun bearing, slowly started tearing it down and rebuilding it with oem parts. The only thing aftermarket were the pistons and connecting rods, Mahle pistons because I heard they worked with the FRM liner in the cylinders. My dilemma is I thought I had ordered Mahle Gold series 87mm 11.7 cr. But it wasn't until recently that I installed them and saw the invoice that I noticed they weren't. The invoice says "S2K181425X09 Mahle Honda S2000 F22C 87mm 13:5:1 Gold Series Piston Kit"
I told them when I was buying it that it was for a F20C, so I looked up that part number and its the same part number for the F20C and the cr is lower around 12.
I bought these pistons months ago without checking till recently when I put them in my block (really dropped the ball on this) My question is, Is it safe to run these pistons? thicker head gasket? Or should I just buy a new set with the right compression ratio?
Any help would be appreciated thank you!

Slowcrash_101

12.5:1 is on the high side as far as compression ratio goes, and even then you need a stand alone engine management as well as adjustable cam gears to truly take advantage of the increased CR especially on 93 octane. You could run 13.5:1 but you're going to need more supporting mods as well as higher octane gas or E85, but you'll also need larger injectors if you're going to go down that route, I'm not sure how much bigger though.

jaydiii

basically I should just order new pistons with the correct compression ratio?

chris_barry

Check the crown thickness. You may be able to have the dome machined off the top of the pistons to reduce the compression. Often the same forging is used so the underside of the piston is the same across all of the available compression.

MM3Kwolik

Very sound advice here. I'm sure if anything you could have enough taken off the tops to bring you down to at least an 11.5:1 which should be fine. Even better if you can get down to 11:1. Pretty much any auto machine shop should be able to do this for you.

Good luck, and post up with what you end up doing.

jaydiii

Awesome! I'll take them to the machine shop this week, that'll be great if I wont have to reorder new pistons

Slowcrash_101

For reference the stock JDM pistons are 11.7:1 I think they use higher octane gas in the land of the rising sun but the JDM F20C is rated at 247bhp whereas ours are rated at 237.

Ruble03

You could run the 13:1 cr pistons, but you are going to need a good tune and probably bigger fuel pump/injectors. I don't know if you want to go that in depth, but your stock computer won't compensate for the knock.

s2klariat

Keep the high compression, get a standalone or Kpro, Walbro fuel pump and ID750's and make more powa! Honestly might not be worth it though. Hard to make power N/A with the F series unless you throw ITB's on there and other supporting mods

jaydiii

Just got ahold of the sale rep I purchased from they say these pistons are 12.6:1 which is still really high... I'm just waiting on my friends dad to get back to me to see if he can just mill the heads a little, hope it works..