few forced induction questions.

[rredalty]

I will be piecing together a turbo system for my '91 4.0l 2wd Cherokee soon and I was needing to know how strong the rods and pistons are. I am assuming that the block and head will be fine for a low boost situation but with no intercooler I will be looking into using a small n2o shot to cool the intake temps. If there is anyone out there that could help me answer these questions I would greatly appreciate it. Thanks.

 

[Cjmartz2k]

Well, the pistons are hypereutectic, so they aren't all that stong. I think the rods are supposed to be pretty decent though. I have seen people on the net running 100 shot with no problems on the stock bottom end. I think most of the blower/turbo kits out there run around 6-8 psi on stock motors as well. I think a lot of it has to do with your tune though. I'm sure Dino will chime in, but I'm betting around 5-6 psi w/out intercooler, and 8-9 psi w/IC. Probably about 75-100 shot of N20.

 

[LS1Jeeper]

I'd get Arp rod bolts at the least .. I doubt you would get much more than 5-7lbs of boost and it live.


and get an intercooler you would be amazed how much better it will run and you under hood temp will be much less ... if you has a NOS kit get the spray bar for the intercooler

 

[streetpirate]

x2
aftermarket pistons and hardware. big intercooler.
side note, where is the best place to mount an intercooler? guessing under the bumper?

 

[Cjmartz2k]

Yeah, ARP rod bolts would be good, and could be done with out too much work. As for the forged pistons, I don't think he wants to rebuild the motor. If, however, you do, Hesco makes forged rods AND pistons. And, as long as your rebuilding your motor with forged internals anyways, you might as well throw a 258 crank in there and make it into a stroker!! Boy, that got expensive quick, huh? A forged internal stroker that could handle 10-15 psi would be SICK!!! You could also throw an aluminum head from hesco on it. Question is, how much $ do you have. I wonder if the block would hold that. That would probably be 450-500hp.

 

[creeperjeep]

get a wiper bottle and pump and mist on the intercooler

 

[Dr. Dyno]

The stock bottom end consisting of cast iron crank, cast iron rods, and cast aluminium pistons can take a 6psi boost without an intercooler or 8psi with intercooler and still be reliable. For nitrous, consider a 100 shot to be the absolute limit.
If you want to use higher levels of boost or a bigger shot of nitrous, you'll need to consider building the engine with Eagle forged rods, ARP rod bolts, forged aluminium pistons, o-ring the cylinders, and add a copper head gasket. The stock 4.0 block has no problem handling 500hp.
The most important thing is to prevent the engine from running lean, as this will melt aluminium pistons in no time. Therefore tuning the fuel curve is very important (you'll need an A/F ratio of ~12.0:1 at WOT) and you'll also need a means to retard timing in proportion with boost to prevent detonation.

 

[Cjmartz2k]

Does anybody out there make a copper head gasket for 4.0's?

 

[streetpirate]

What cr ratios should you aim for for 5-6psi and 8-9psi on a stroker motor, and what has to be done to obtain these ratios? planning on doin the m90 w/sc intercooler on a 4.7 sometime this year. also, injector size and cam profile for a blown stroker?

 

[Dr. Dyno]

For turbos, Roots blowers, or a twin screw supercharger where full boost is in by about 2000rpm, you'll need to keep the stroker's CR not higher than low 9's when running 5-6psi of boost. If you're using an 8psi centrifugal supercharger such as the Vortech, Paxton, or Powerdyne, you still don't have positive boost until 2000rpm and even at 4000rpm you may only have 4psi, so you could build the stroker with a CR in the high 9's.
The M90 is a Roots blower so go for a low 9's CR, 6psi, CompCams 68-232-4 cam, Accel 26lb injectors, and a Vortech FMU to increase fuel pressure with boost.