Run Away

Hello all, I'm in the process of building a B series for my EM1.
So far the build consisted of:
B18C1 block
B16A2 head
89mm LS crank
LS rods
81mm USDM ITR pistons


According to the zeal compression calculator this would put me right at 11:1 compression.
Now my machine shop has gotten back to me and confirmed there is up to 0.003" taper in some cylinders. This means boring 0.010 or 0.020 over.
My intended use is for daily street driving/autocross duty. I keep changing my mind on keeping it N/A vs bolting on a JRSC but for the past few weeks I've been leaning towards supercharging it.

So since the pistons I have won't work, I'm looking at my options.


That's what I've initially rounded it down to.
Power goals are in the 250whp range ideally, obviously less on motor, maybe more if I make the jump to forged pistons.
I'd prefer 81.25mm because that leaves another step before sleeving should anything happen, but it's not a deal breaker. The Arias 81.08mm option is interesting too for keeping the stock bore but bringing the PtW clearances back in spec.
Also, does it make any sense to put aftermarket pistons on OEM rods? They have ARP's in them but then at the power levels I'm going for it's kinda overkill....

Thanks!

The Ogre

sorry 250whp is very unlikely with an 81mm setup. NA atleast.

Run Away

Please read the 6 words after "250whp".

Goals if staying N/A would be about 200whp, which is not enough IMO for the heavier EM1 chassis hence my want for a JRSC.

Natural Aspirations

Personally I like Wiseco when going forged.

If your set on using the JRSC you may want to look into going lower with your compression to make it more manageable for a first time build. 11:1 area isn't absolutely too high but its a bit above what I would consider ideal. I would look to run a whole point lower 9.5-10:1. A point of compression will not make much of a difference in final output power and will give you a little cushion if you get bad gas or have an abnormally long autocross on a hot day.

OEM GSR pistons should put you in that zone.

Run Away

Thanks for the reply!
I have considered GSR pistons but ruled them out because I was concerned the valve cutouts on the GSR pistons wouldn't be ideal with the different valve angles of the B16 head. The aftermarket pistons are designed to accommodate both obviously. I will have to consult my engine builder and spend some more time searching though if you're saying it's not really an issue. USDM GSR pistons would net me 10.08:1 and JDM GSR would net 10.67:1 (which would be exactly right on the money IMO but I don't know where I'd find new 81.25mm JDM GSRs)

I had also not chosen a wiseco piston as they didn't have an off the shelf design that gave me a compression ratio I wanted. The two closest choices are 9.36:1 and 11.26:1 for an 81.5mm bore.

Langford360

Personally my current build is pretty much exactly what you are looking for. lol I made 255whp and 175 trq on my JRSC b18c1 with gsr crank,stock rods, rs machine usdm 81.5mm type r pistons,.040 cometic head gasket,stock ported p72 head(.010 off the surface),flat faced valves, and the gsr/itr cam combo. Maybe a few little nick nac mods im not thinking about. According to the zeal comp calculator i should be at about 11:1 compression and the SC was making 8.9 psi at the map sensor. All tuned on a hondata s300 with 92 octane pump gas. We even added a few degrees of timing at the end of the day. The car runs and drives flawlessly and i love it