AMC Pacer Revival

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Edited by MN Gremlin - Jan/23/2013 at 8:34pm

It works, but... make DARNED SURE the machine shop knows you only want 0.0009-0.0013 piston to bore clearance (minimum is 0.0009, preferred is 0.0012-0.0013, but the tighter the better on a budget stroker).  The reason is the pistons come real close to the bottom of the bore and will rock and accelerate wear if they are very loose. Now this clearance is assuming stock cast pistons. There are some special forged pistons that will allow using the 4.0L rods with the 258 crank (available from www.bulltear.com) that need a bit more clearance, but they are a bit over 1/16" higher up in the bore.

Another problem is compression. If you are using a carb it's no big deal, but the EFI doesn't like much more compression. With stock 4.0L pistons you will have 9.5-9.7: compression and will have to run at least mid grade fuel to control ping, and it will like premium much better (no ping at all on premium, only under heavy load with mid, just about every time you hit a hill with regular).  The 87-90 Renix system is a little more tolerant, but it will keep the timing turned down as far as it can on regular (but you CAN run regular if you control ping by careful driving... not worth the few cents saved over mid grade though!).

My first stroker was a Renix engine with stock 4.0L pistons and 258 crank and rods. The machine shop bored it at max clearance (around 0.0030 -- over 2x stock!) to clean it up at 0.030" over and sleeving only one cylinder. Always had a tick, and it only lasted about 50K before needing a rebuild. I actually ran it to about 70K -- when it would use about a quart of oil every 250-300 miles. I wasn't driving it much at the time and just didn't notice it was THAT bad for a while! When we tore it down it had about 0.016" clearance near the bottom of the bore (that's right, I didn't forget a zero!).

The pistons for the 4.0L rods cost a bit more than stock 4.0L replacements, but for longevity that's the best way to go. Keep the piston to bore clearance tight and the stock 4.0L pistons with 258 rods should last 100-150K though. The 4.0L rod pistons also take care of compression -- you can keep it at the stock 8.7:1 (don't go more than 9.0:1 with the EFI) and the EFI will be really happy on regular. I pulled the pistons from mine after four years running (had to pull and rebuild the head -- didn't when first building the engine as I had a head with only 60K on it) and increased the dish size of the pistons to drop compression back from 9.7:1 to stock 8.7:1. Best thing I did! Could run regular, and there was no loss in power due mainly to timing coming back up instead of being retarded all the time. Remember, the 91+ computers DO NOT HAVE A KNOCK SENSOR AND WILL NOT RETARD TIMING. On those you can plan on running mid grade or premium fuel all the time, they won't run well on regular with stock 4.0L pistons/258 crank/rods (9.5-9.7:1 compression).

That's a good guide. If you have the trans out it's not too hard to mount a Wrangler CPS on the original bell housing, but of course you must use the flexplate from a Wrangler as well for the auto trans. If you're not using the EFI no problem though.

The only thing that article errs on (IMHO!!) is the casting. It has since come to light that the next head (97-99) is a lot better. The ports are slightly smaller but it flows slightly better. The reason is the port castings are much smoother can cleaner inside. It took me 4 hours to pocket port a 95 head (basically all I did was smooth things out, didn't make the ports larger). The 99 head I got later simply didn't need anything, no point in getting the die grinder out at all! That's the 0630 casting. For a street car a 2001-06 0331 casting is probably the very best. It flows better at low speeds, but flow drops off around 4000 rpm. It has the best shaped but also the smallest exhaust ports. Below 4000 rpm it makes a little better power than the earlier heads. For racing it sucks, but for the street it's arguably the best one. Just be careful! Late 99/most 2000 model heads crack easily in the center. This was fixed, but the casting number wasn't changed. If the letters "TUPY" are cast right in the middle of the head under the valve cover then it's the "fixed" casting and you're good.

Google  Mopacer.

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Edited by MN Gremlin - Jan/23/2013 at 8:36pm