crushed header (crossover) pipe= better MPG?

I gonged my factory exhaust x-over pipe either at NACFest or at Uwharrie a few weeks later... not all the way crushed, but approx 50-60% of the diameter.

I haven't really noticed any loss of power (1988 XJ, 210k 4.0l, stock except for an airtube/K&N) but I have noticed a slight improvement in MPGs: The last few tanks getting 20-21 vs the 'historical' 18-20. I haven't varied my driving style that I can tell, and my daily commute is the same route. I also tend to use the same gas stations, 87 octane, and quit filling at 'approx' the same time (adding about 1/2gl after the auto shut-off trips)

The only 'variables' in the mix are my replacing the balancer pulley, waterpump, thermostat, belt, hoses & converted to an 'open 91+' radiator... The motor is running cooler now...probably right at 195* and only a little higher at long stops. The other signifigant variable is switching to 20w50 conventional (dino) oil

Will a restricted exhaust cause a 'lean' condition & thus a bit more MPG?

I'd think the 'thicker' summer oil would drop MPG but :dunno:

prepare the jury. some people disagree with what I say, so getm in line an ready.

putting a smaller diameter exhaust on the old 4.os will restrict the exhaust, causeing a backup of it- since the 4.0s like this, you get better mpg and a better low end(you`ll usually hear this from people right before the cat is completely clogged-same deal). On the older engines that have high mileage, switching to 20/50- in my case straight 50 in the summer, seals the rings up quite nicely reduceing blowby considerably-reduce blowby=more power available, and that usually means better MPG if you can keep your foot out of the floorboard. You`ll also notice that with thicker oil, the oil pressure stays up nicely-when it starts to drop, or wiggle around alot, check it-its probably on its way to being black.

your other reasoning is sound- a cooler engine uses less gas.Several people have noticed a mpg increase switching to the newer open system.

useing all these variables, and adding them up is small, but a difference in mpg.

I am ready to be flamed- throw the gas an get the matches.

It might be possible that thr restriction is causing you O2 sensor to get a better taste of the exhaust, and making the computer run a bit leaner. This would be the case if the O2 sensor was weak or the heater in the O2 sensor was bad. What ever the case more mpg = 2 thumbs up, especially with todays gas prices .

I'm gonna go crush mine....

kewlkatdady said:

I'm gonna go crush mine....

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They're already partially crushed straight from the factory. Some say for clearance reasons. Others say for the restriction factor.

Ed in North Ga. said:

your other reasoning is sound- a cooler engine uses less gas.Several people have noticed a mpg increase switching to the newer open system.

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The cooler the engine the LESS gas mileage you will get. All engines are more efficient the hotter they are. This is one of the reasons the 4.0 runs so hot, Chrysler needed to do something to get this old gas guzzler to pass emissions and MPG requirements. People that use 180 degree t-stats get less MPG.

BTW - it's a "downpipe" or "S pipe", a crossover is a peice that joins the two sides of a dual exhaust system to equalize pressure.

xjohnnyc said:

They're already partially crushed straight from the factory. Some say for clearance reasons. Others say for the restriction factor.

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Looking at the crush spot on my 95, I can't see where it would contribute to clearance, so I'm willing to bet it's for restriction. On the other hand, I suppose it might also be a way of strengthening the pipe at a curve that might otherwise crack. Maybe it's one of those multipurpose things like the little piece of tin between a brake pad and piston - Mercedes calls it an anti-rotation plate, VW calls it a heat shield, and when you go to the parts store they'll find it under anti-squeal shim.

ZmOz said:

People that use 180 degree t-stats get less MPG.

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Maybe other people do but not me.
As far as the crushed S-pipe giving more mpg goes, that's baloney. The effect is similar to having a slightly blocked cat, and we all know that a blocked cat will make your mpg go down the toilet.

Matthew Currie said:

Looking at the crush spot on my 95, I can't see where it would contribute to clearance, so I'm willing to bet it's for restriction.

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Look at how the front driveshaft would travel: IIRC someone posted that the LP D30 models don't have that "bend" in them because the driveshaft takes a slightly different route.

my down pipe is crushed i'd say at least 70% but that 4.0 stil runs like a bat outta hell. mine was in an accident prior to me owning it tho so that could explain the extreme ammount of crush-age. is there any truth in the "pre crushed from the factory"? i'd also agree it cant be a clearance thing

ZmOz said:

The cooler the engine the LESS gas mileage you will get. All engines are more efficient the hotter they are. This is one of the reasons the 4.0 runs so hot, Chrysler needed to do something to get this old gas guzzler to pass emissions and MPG requirements. People that use 180 degree t-stats get less MPG.

BTW - it's a "downpipe" or "S pipe", a crossover is a peice that joins the two sides of a dual exhaust system to equalize pressure.

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-proof is in the pudding- both 89s,
both with semi blocked cats, both with 160 robert shaw `stats-
my last check on the 2wd MJ was a hair under 30mpg.
the old XJ was bordering 27mpg-it was 4wd.Both with 200k+
useing the old xj as a benchmark, when I removed the cat and hit it with a new closed system rad with new plastic bottle, and put in a cheapo 190 `stat, she went down the tubes- 18mpg. Remedy was to put the old cat back on, an replace the stat. she also overheated constantly with the 190.
I aint touchin anything in the MJ unless it wont move under its own power and dies-
In this case, I wish what worked for me, worked for everyone. It just dosent seem to be the case.
The guy at the dealership sold me on the idea, except for the 160stat-he said it would stay in open loop- dosent seem to be the case either. mines a california jeep/minus the extras that came with it-but its possible the computer was different, or it just likes me.