Renix VS H.O

[kyleag89]

Just curious if you H.O guys are getting any better gas mileage? I am selling my Firebird to buy a cherokee since I now have the Jeep bug.

[Geonovast]

HO's have a tendency to get worse mileage than the Renix, but they punch out a bit more power.

 

My 88 XJ was getting 26 mpg before I did an unnecessary motor swap.

[jpdocdave]

my renix mj 4.0 gets better mpg than my 96 ho 4.0 xj.

[kyleag89]

Wow that is crazy. I would have thought it to be the other way around. My 89 MJ gets horrible gas mileage.

[mvusse]

Best mileage I have gotten out of my 87 MJ was 24mpg (60 mph).

 

Best I have gotten out of either my 94 or 96 XJ was 21 or 22mpg (70mph).

 

But even though the Renix equipped MJ is quick (or was, on stock tires), the XJs are rockets.

 

If you can keep your speed down, the Renix will get better mileage, but if you're a lead foot, at 75mph the HO will do better (my Renix only got about 17mpg cruising at 75, while my 94 XJ still got 21).

[jpnjim]

Renix 4.0L's have EGR, which (under cruise conditions only), displaces some of the fuel air mixture with (inert) exhaust gas.

Less air/fuel mix is then required to 'fill' the engine, and the engine 'acts' like it has a smaller displacement.

 

Renix 4.0L's have a knock sensor.

This allows the Renix 4.0L's default timing curve to be more advanced than the later HO's.

More advanced timing typically results in more efficient burning, and better gas mileage.

 

Renix 4.0L's have a heat riser tube from the exhaust header.

This only comes into play for a short period of time, during the engine's warm up phase,

using warm/hot air drawn next to the header while the engine is still cold, promotes better suspension of the fuel in the fuel/air mixture, allowing the engine to run on a slightly leaner mixture than would be required otherwise.

(= not much difference, but it helps MPG & emissions when cold).

 

 

Renix 4.0L's also have lower fuel pressure, and smaller injectors than the later HO's.

 

HO's have benefits other than more HP tho,

a real 'check engine light' for diagnosis,

a more modern setup that should have greater parts availability for years to come

(Renix 4.0L's were only used for 4 model years, the 'newest' being 20 years ago)

 

I'll leave the GM vs Mopar sourced alternator debate up to you guys.

(I've had GM alternators fail, but I'll take that over the voltage regulator in the ECM that later HO's have).

 

& I don't think anyone could argue that HO Intakes/throttle bodies/headers & cylinder heads are substantially better across the board.

[Eagle]

Wow that is crazy. I would have thought it to be the other way around. My 89 MJ gets horrible gas mileage.

Not really crazy. Perfectly predictable, in fact. The Renix system uses a knock sensor, so the engine can run at the maximum degree of ignition advance possible at all times. The knock sensor listens for "ping" and retards the timing only when necessary.

 

Chrysler wanted to save a buck per vehicle, so they eliminated the knock sensor. They obviously didn't want 9and couldn't have) a bunch of XJs and MJs blowing the tops out of the pistons on a regular basis, so they engineered the ignition advance curve to always be sufficiently retarded that it will never ping. Which is fine ... except that it means the system can therefore NEVER operate at optimum advance, which is what delivers maximum fuel economy.

 

Newer is not always better. Sometimes newer is just ... not as old.

[Eagle]

& I don't think anyone could argue that HO Intakes/throttle bodies/headers & cylinder heads are substantially better across the board.

True, as far as flow rates and volumetric efficiency are concerned.

 

But Renix heads aren't programmed to self-destruct at 90,000 miles.

[A-man930]

But Renix heads aren't programmed to self-destruct at 90,000 miles.

explain...

[jpdocdave]

there was one or two casting numbers that were prone to cracking, i wouldn't condemn ho heads for it.

[cracker]

there was one or two casting numbers that were prone to cracking, i wouldn't condemn ho heads for it.

 

Head 0331 is the problem child.

[HOrnbrod]

there was one or two casting numbers that were prone to cracking, i wouldn't condemn ho heads for it.

 

Head 0331 is the problem child.

 

This is true on some of the early 0331 castings (not "self-destruct", as Eagle, the consumate Renix dinosaur pointed out as always :D ) but the 0331s were eventually fixed and are generally recognized and proven as the best stock flowing OEM heads ever, especially for low and mid torque. The earlier 7120 (91 to 95) and 0630 (96 to 99/2000) HO heads had no such problems, and had far superior flow numbers than the Renix heads. Not rumors, facts.

 

As far as the Renix knock sensor enabling a more efficient/advanced timing curve, yes and no. It did the job in it's day, and might have worked better than the early HO ECUs in regulating precise timing that resulted in slightly better mileage. BUT (there's always a but :D) they are prone to failure as all the Renix hodge-podge sensors are, are tough to troubleshoot because they all interact, and are getting more and more expensive to replace and harder to find. The Renix sensor supply will dry up eventually. So all you Renix guys continue to on the Renix horse until y'all have to park it.

 

Nope, give me the HO platform anyday. Easier to troubleshoot, much more receptive to HP/torque upgrades, and more plentiful and economical parts replacement. For a long time to come. :cheers:

[Eagle]

HEY! Dinosaurs are people, too, ya know. It ain't easy being a member of an extinct species.

[HOrnbrod]

:clapping: