AMC 360 compression/octane questions

[jairush]

Hi- I just joined this forum, looks like you have tons of good information here! I have some general compression questions and am curious to hear your thoughts and suggestions. I have searched

The backstory- I am rebuilding an AMC 360 of approximately 1984-'86 vintage for my CJ-7. I want to do a moderate performance rebuild as my plan is to eventually acquire an AMX or Javelin to swap the engine into, followed by a diesel in the CJ. That's another story for a different day. My goal is to build an engine with a power band in the idle-4000rpm (or so) range, with 350 hp and 400 ft lbs torque at the flywheel. (I know, the jeep doesn't need that much power, but I'm an engineer and the challenge is part of the fun) This is a fairly low budget rebuild.

The engine will be bored .030 over. The ports have been gasket matched, rough spots removed, nothing too extreme. A 3 angle valve job has been done. I will run an Edelbrock Performer intake and long tube in frame headers leading to dual Flowmaster 40 series mufflers. Fuel delivery is via an early '90s GM TBI system with a hybrid TFI/MSD 6 ignition. The ECM also controls the spark (no knock sensor, though). Cam will be the Summit K8600 or possibly the Comp Cams 268H grind. Manual transmission.

Does my target power seem reasonable? From my research it seems legitimate, but I don't know that I can do it without upping the compression a bit. If I can accomplish it at stock compression that would be great, but I am assuming I can't.

My main question- what would you think is the highest dynamic compression ratio can I run on 87-89 octane 10% ethanol pump gas? I have read on some forums (perhaps this one, I can't remember exactly) that 9.25 to 9.5:1 static CR is about the limit. This wouldn't normally be an issue, except as you probably know there is a dearth of off-the-shelf higher compression pistons available for the AMC 360. At the moment, I can't budget the custom forged pistons that Bulltear offers. That pretty much means that the only low budget option I have is to do the cast "343 pistons in a 360" trick. I ran some calculations (http://www.kb-silvolite.com/calc.php?action=comp) and if I could get a hold of the 2V 343 pistons (~14 cc dish volume) it would put me in the right range. (with of course, all the caveats to that trick like clearancing piston skirts for the crank counterweights, balancing the rotating assembly, etc) I can't find those pistons, however there is an eBay seller selling the 4V 343 pistons. With those (no dish, 4 valve reliefs) I would be looking at about 10.25:1 static CR. Dynamic CR is somewhere in the mid 7's:1.

With a computer controlled spark, longer duration cam, and 10% ethanol gas in this area, would I be able to run a 10.25:1 static compression without knocking, preferably on 87 or 89 octane? I know it is probably on the hairy edge, but just curious to get some opinions. If so, with that and the other mods mentioned above, should I be able to hit my torque/hp targets?

Thanks for your assistance and any information you can provide!

[jeepsr4ever]

http://www.kb-silvolite.com/calc.php?action=comp is good. Dont shoot for such a low dynamic compression. Its a crapshoot with the 343 pistons even afer clearancing for the crank throw (sometimes you have to grind the bottom of the piston). The current cast 343 pistons are known to fall apart BUT may work for your application. Egge comes to mind for the pistons. You can get where you want but not with a low low dynamic compression. A summit 8600 cam will get you there with the right compression.

[jairush]

Man- thanks for the quick response! You have a good forum here. (and good quality parts- I used one of the your dipstick tubes on my 304 and fit was perfect. Looking at getting the timing cover)

I don't like the idea of 'rigging' the 343 pistons (or any other parts for that matter) to work, but sometimes unfortunately finances dictate otherwise. I know that dynamic ratio is what really matters, but if I keep that below 9.5:1, can I basically go to whatever I want with static compression? I know there are obviously a lot of factors and no blanket yes/no answer, but it seems to me that the cam makes a huge difference. I doubt I could use the stock cam with 10.25:1 static CR without race gas, but from the figures I have seen it looks like it would be doable to run a higher static compression with a longer duration cam without having detonation.

[jeepsr4ever]

Static CR doesnt mean much. Find the range you want your power and pick a cam with a duration for that range. Thats the easy way of saying it. So if you have a need for power between idle and 4500 you dont want a duration over 230 and with a 360 (now we are talking off the shelf parts) the lift should be no more than .490. Then your ABDC + 15 degrees on that calculator will get you your dynamic. In the end the Dynamic ratio will determine where your octane rating should be (again more to this as well). For waht your wanting you MAY be able to get away with mild quality cast pistons. Your not asking for much here. You can get 450ft lbs and 500hp out of a AMC 360 without breaking the bank too bad.

Yeah dipsticks fit. People always complain that they arent $11.00 though. We often have to explain that we rework the tangs.