Jeep98cherokee

Hi i have a jeep with a built 4.0. It has forged pistions and eagle rods and a comp cam. The compression ratio is 10:1. I'm only thinking of doing 6 to 8 psi. The jeep red lines at 5200 so not a lot of rpms. I was thinking of getting a fat head gasket to get the compression ratio down to like 8 or 9. Or Can I just leave it at 10:1 What do you guys think? Thanks for the input.

mstngsrule68

if you keep that boost down around the 6/7psi youll be fine.

Jeep98cherokee

Thanks for the info mstngsrule68. I called Aem and they side that it can't be done. That was a let down I have my kit done and was going to start the Turbo Manifold this weak.
What do you all think the target afr is on a 10:1 motor. Is it 14.5? I'm going with a 12:1 fmu with a mbc to get my afr right also have the Msd Btm for timing and a walbro 255. Will this work?
Thanks for the help.

RideaT1

Can be done easy with a GOOD tune, definatly would not run an FMU in your case.

busa4

it can be done. my stock 2.2 gm ecotec engine is 10:1 compression running 10 psi boost.
stay away from the thicker head gasket and the shitty fmu.
have it tuned and run 93 octane and you should be fine. if your setup doesnt include an intercooler then dont boost it past 6 psi.

donnie m

Ive been trying to figure out what all these manufacturers are calling a fmu. Its basically a fuel booster pump with a pressure regulator spliced in and routed back behind the fuel booster pump, is it not

Jeep98cherokee

Yes I do have a intercooler in my setup. So the the fmu with a mbc to get the afr right won't work? I'm not trying to race it. just need some more get up. 35" tires take it out of a 4.0 jeep. Thanks for the in put.

busa4

an fmu is not a tune. its a mechanical fuel enrichment unit that exponentially adds fuel pressure to your fuel rail. dont use it, dont waste your money and time. it will cost you in the long run. you will need a custom flashed tune. this will give you the most reliability and power.
a manual boost controller only controls the amount of boost. it has nothing to do with a tune.
if tuning is not at option on your vehicle then you may have to go with an extra injector controller which will add fuel to your intake system based on tps and boost pressure. this is the way i went as my car doesnt have many tuning support.

busa4

an fmu (fuel management unit) is a generic term for rising rate fuel pressure regulator. they were popular in the 90's when there wasnt any other way to enrich a n/a vehicle converted to turbo. it mounts after your stock fuel pressure regulator or on some higher priced fmu's, it replaces your stock fuel pressure regulator. all it does is mechanically restricts fuel going back to the fuel tank depending on how much boost it receives. fmu's create dangerous high fuel pressure, damage to injectors, cylinder wash out, and poor economy and power. they cannot adjust afr's dependent on engine load and rpm because they are mechanical and therefore they are not considered a tune.

donnie m

I have a 98 jeep tj 4.0. Dont know if you have the same mpfi system. If so you have probably already done some timing/ mapping adjusting for the mods. If not im supprised your check engine light isnt burnt out. You mention the msd boost timing controll, what about the mapping, split second psc1-001? anything? Larger injectors and higher fuel pressure is the only way to get more fuel without burning out your stock injectors and you must bypass the stock map sensor cause it does NOT read boost. This is where a split second map adjuster would be used, or similar. Also where is your fuel pressure regulator, by the intake or above the fuel tank or on top of the fuel sending unit on the gas tank?