360 Build Planning with ?'s

[MrFurious]

I have a stock 360 that's in need of a rebuild. It's in a 78 Cherokee that presently has the TH400, QT and 3.08's in the axles with stock 29" tires. When I pull the motor I'm going to also change over to a TH700R4, a Dana 300 t-case, 3.90 gears and jump up to 31" tires.

Plans are as follows...

Bore: +0.030" (4.110")
CR: 9.0:1
Heads: #502 Cast Iron with 2.08/1.74 valves
(from Areohead Racing)
Intake: Edelbrock Air-Gap (since I have it already)
Carb: Holley Truck Avenger (670 or 770cfm?)
Dizzy: HEI
Ignition Box: ??
(Naturally the usual oiling mods and prep will be included.)

Now here are my questions:

1. Having done some math on the motor, I can't for the life of me figure out how Silvolite and Speed Pro claim a 9:1 CR with their pistons having a 1.58" compression height and a 24-28cc dish. Even milling the decks 0.020" to make up for the change in compression height, the piston will still be 0.013" down the hole and the CR comes out to 8.5:1 (0.045" gasket and 58cc heads).

Naturally I don't want to drop $700+ on custom forged pistons for a daily driver, so I took a look at the Egge L2336 flat-top 343 pistons AMC69PHATTY found. http://www.bulltear.com/forums/viewtopic.php?t=6782

Unfortunately the piston would still be 0.013" down the hole. The up side is that the decks could be milled 0.018" leaving the piston 0.005" out the hole and net a perfect 0.040" quench height with a 0.045" gasket. The downside is my compression ration would be 10.9:1 which won't cut it for a daily driver.

Also note the problem Painted Rat had when installing these pistons in a 360 as indicated in this thread: http://www.bulltear.com/forums/viewtopic.php?t=6620

Would it be possible to enlarge the dish on these pistons to 26cc? A 26cc dish would give you a 9:1 CR, which is much more in line for pump gas and a daily driver. (Note: a 20cc dish will give you a 9.5:1 CR for those running Edelbrock heads.)

Considering they will already need to clearance the skirts at PaintedRat found out, I don't see why enlarging the dish would be a problem so long as it doesn't affect the structural integrity of the piston's head.

2. What cam would you suggest for this combination? My biggest gripe about the motor now is that it has no top end power and falls flat on it's face around 3000rpm. It's pretty sad when you kick it down to pass someone and it makes a lot more noise but doesn't go any faster. I don't do any rock crawling or serious off-roading with this rig, so I'd rather focus on mid to top end without taking too much away from the bottom. Something that will pull well from about 1500 up to say 5000 or 5500rpm.

The heads will be #502's with larger 2.08/1.74" valves from Aerohead Racing (see link for full details: http://www.aeroheadracing.com/id9.html).

I've been looking at the XE262H cam and get decent numbers with it from Desktop Dyno (see Fig. 1 below), but it leaves me about 25HP shy of the 400hp/400ft-lb marks I'd like to attain. The XE274H gets me the HP I'm looking for, but does so at the expense of 30ft-lbs at 2000rpm (see Fig. 2).

http://www.savageshooters.com/images/misc/AMC360_XE262H.jpg
Figure 1

http://www.savageshooters.com/images/misc/AMC360_XE274H.jpg
Figure 2

The change to an OD transmission could cause a problem though as that will greatly reduce my cruise RPM, even with the gear change. My calculations show 2066rpm @ 70mph.

3. What do you suggest for ignition upgrades? I've been looking at the HEI options, but what about an ignition box? And please don't say MSD....if you'd seen how many came back when I worked at Jeg's you'd NEVER buy one either. lol

[jeepsr4ever]

HEI = no ignition control box...I havent heard anything but mild frusteration with egge pistons. You could probly save your money and have new valves put into your heads rather than buy Aerohead replacement stock heads with larger valves. It is hard to say if any of these cast pistons would have enough material for miling a few cc's out of them.

[MrFurious]

Well, from my understanding the Aerohead Racing heads are already setup for stud mounted rockers and guide plates. They want $250/each around here just to recondition heads, so for $100 more it's not that bad of a deal.

As for pistons...that's what I was worried about. Considering how many 360's are out there you'd think you could get a decent cast replacement by now that offers good compression, but that's just not the case.

Much as I'd hate to do it, I'd probably go the SBC route before I forked out the $$ for custom pistons. The money I'd save on the trans adapter plate would cover the cost of the radiator and motor mounts from Novak.

[82Waggy]

As you have discovered, trying to build a good quench motor with a 360 is a problem if using stock style pistons. Aside from the compression heights being too short to get near .040-.045 piston to head clearance, the stock trough head style piston is not good for promoting quench.

If you are going to stick with an off the shelf cast piston, your best bet for quench would be to use the Egge flat tops, and then open up the head chambers to about 65CC by unshrouding the valves to bring down static compression ratio. Do the math and see if this gets you under 9.5:1. Even .050 - .055 quench would be acceptable - after .060 you might as well throw in the towel.

PS: Well, I just did the math and you can get to about 9.7:1 with 65cc chambers and 8cc flat tops if you don't have to mill anything, and that leaves you with about .057 quench. Keep in mind you may lose some rod length and stroke to reconditioning as well.

Using a 24cc piston and 58cc heads I get 8.86:1 compression with lousy quench.

If you are stuck with the high compression, you could try runnning a long duration cam to keep cylinder pressure in check, but you may not like the low end and idle characteristics.

Not much else you can do short of going with a custom D-cup dish piston to bring everything in line.

What we need for a 360 is a piston with a 24cc D-cup dish and a 1.61 compression height. This would allow for some loss of rod length to resizing and some deck milling to establish near zero deck height, good quench, and around 9:1 static compression ratio.