"sweet Spot"

[skidoo_j]

Okay so I would like to know what RPM range people are keeping their 4.0 renix in to get the best mpg and performance.    Their "sweet spot".  

 

 

I swear I've ran and entire tank at 2k   and and entire tank at 2500 and one at 3k   several different times.   The reason i ask it it seems Neither is better then the other.    And I'm still only getting 12 mpg at best when i'm trying to get good gas mileage.    So i'm not sure if it's my gear ratio is incorrect and i'm just straining the motor....  etc    I live in the country so very few stops, and average driving speed of 40mph.    Alot of my driving is hauling my dirtbike up to the riding area where there are a few decent grades, but from my house to staging area is about 4mi. one way.  

 

 

 

My current set up:

4.0

rusty's TB spacer

Bored TB (HO butterfly)

The spray injectors (http://www.precisionautoinjectors.com/product.sc?productId=7&categoryId=3)

Dyna max headers

no egr

stock intake

new plugs. new 02  new sensors all around. 

all electrical test out.  all idle issues are resolved.  all grounds are good.  most of all cruisers mods done.  

3.54 gears ax15  w/ np231

31 15" by good year duratrac 

Noslip in front,  lock rite in rear   d44  SOA  

 

[Pete M]

have you confirmed that your speedometer is accurate?

[mjtjnj]

If you have a smartphone, download the free Ulysses Speedometer, and it will calculate your speed based on the satellites used for GPS..

 

 

[JFAComanche6]

Go faster :banana: Seriously, see what 60 mph does for you.

[JeepcoMJ]

2250rpm

[comanchedude]

22 TO 24K seems  to be best on my renix system.  I used to get as high as 19-20 hwy with 33" tires and 4.10 gears..now with 35" rockers best I can get is 15-16 hwy.....

[ParadiseMJ]

have you confirmed that your speedometer is accurate?

I just got my speedo fixed after over a year of either not having one or having one that was 15-20% off.  I know it's a small thing but it makes a huge difference knowing my speed / RPM ratio.

 

First tank showing about 18 mpg overall.  My usual freeway/highway speed is ~65...at 2200 RPM,  31's, AW4, 231, 3.07 gears ;(

[skidoo_j]

have you confirmed that your speedometer is accurate?

I'm planning to put a gps in it for a few days to make sure,  but based on tire size and gear ratio it should be very close to stock.   As it came with 3.07.   

[skidoo_j]

Go faster :banana: Seriously, see what 60 mph does for you.

There comes a problem with the possibility of the non-existent police ticketing me if I'm going 60...  It would be my luck that the day I go 20 over posted speed both cops in town would be on duty.  

[92tanMJ]

22 TO 24K seems  to be best on my renix system.  I used to get as high as 19-20 hwy with 33" tires and 4.10 gears..now with 35" rockers best I can get is 15-16 hwy.....

 

22-24K gets me 22(ish) mpg hwy in my 98 xj with 4.5" 31's and 3.55 gearing...I know its not an MJ, but its the best info I can offer. lol

[Eagle]

2250rpm

 

Yep. For a Renix 4.0L that's about the sweet spot.

 

However, you seem to have conflicting priorities. The things you do to improve performance generally won't help gas mileage. Larger, freer flowing exhaust? Nope. Bored throttle body? Nope. 3.54 gears struggling to push 31" tires? Nope.

 

Many years ago, when the factory exhaust in my '88 XJ 4.0L bit the dust, my brother was managing a Speedy Muffler shop. He arranged for me to get his shop price on a custom-bent, 2-1/2" exhaust system with a high-flow turbo muffler (don't remember the brand, and it doesn't matter). The result was horrible. I LOST low-end torque. My gas mileage instantly dropped between 2 and 4 MPG. And it was noisy. That was the first and last time in my life I was actually looking forward to an exhaust blowing out so I'd have an excuse to replace it. Since then I have gone direct OEM replacement from Auto-Zone.

 

For starters, you need more gear. I ran 31s on the '88 MJ for several years, with 3.73 gears. The overall final drive ratio (as shown by actual road speed to engine RPM) was exactly the same as factory tires with 3.54 gears. I would have preferred 4.10s and I think 3.10s would have prodiced better gas mileage, but I already had the 3.73s (bought when I planned to run 30" tires), so I used them. Not horrible, but 4.10s would have been better.

[skidoo_j]

 

2250rpm

 

Yep. For a Renix 4.0L that's about the sweet spot.

 

However, you seem to have conflicting priorities. The things you do to improve performance generally won't help gas mileage. Larger, freer flowing exhaust? Nope. Bored throttle body? Nope. 3.54 gears struggling to push 31" tires? Nope.

 

Many years ago, when the factory exhaust in my '88 XJ 4.0L bit the dust, my brother was managing a Speedy Muffler shop. He arranged for me to get his shop price on a custom-bent, 2-1/2" exhaust system with a high-flow turbo muffler (don't remember the brand, and it doesn't matter). The result was horrible. I LOST low-end torque. My gas mileage instantly dropped between 2 and 4 MPG. And it was noisy. That was the first and last time in my life I was actually looking forward to an exhaust blowing out so I'd have an excuse to replace it. Since then I have gone direct OEM replacement from Auto-Zone.

 

For starters, you need more gear. I ran 31s on the '88 MJ for several years, with 3.73 gears. The overall final drive ratio (as shown by actual road speed to engine RPM) was exactly the same as factory tires with 3.54 gears. I would have preferred 4.10s and I think 3.10s would have prodiced better gas mileage, but I already had the 3.73s (bought when I planned to run 30" tires), so I used them. Not horrible, but 4.10s would have been better.

I was thinking the 3.54 with the manual tranny would be about right with 31'   I'd like 3.73's but I think the carrier breaks are around there.   I do have a free flow exhaust on it too.   Custom bent 2.5" with flowmaster delta 40. 

 

Maybe i'm 'getting what I should be with the mods i've done. 

 

 

 

My 98 VW gti 2.0 gets about 15mpg.  and people can't believe how low it gets, but with all the engine work and stuff done to it I don't expect any better with street driving.... on the freeway the vw gets 30 though.      That supercharger i think decreases mpg on short distances with the ecu constantly reacting to extra air.  

[JeepcoMJ]

I'm with eagle on the exhaust. 4.0's NEED back pressure. I've put a couple of larger diameter inlet/outlet mufflers on a 4.0, but always necked it down to stock, and only because they didn't make 'em in my size.

 

and you're right, a 5 speed ax15 with 3.54 with 31's is equivalent to 235/75r15 on 3.07's. Our 88 MJ is on 31.7's with 3.54 gears and an ax15. Last tank I ran through it, got 26mpg with an accurate speedo...I have a huge collection of speedo gears to choose from and actually had the right one

[reprod89]

I hit the sweet spot going up to Dundee,MI the mpg spike up to 26.4 MPG average highway that trip  O.O  i had to run the number three times to be sure all came up the same. not bad form a trip from toledo,OH to Dundee,MI with little red average speed was 65-70 :driving:

[gogmorgo]

"Back pressure" is a myth. You really don't want pressure building up in your exhaust. It will never help.

That being said, fluid dynamics get kind of interesting. Your exhaust system has an optimum flow rate. If you aren't pumping enough exhaust you develop vortexes which slow down your flow. If you're pumping too much exhaust down it, you start building pressure, which also slows down the exhaust. Wider diameter pipes typically have higher optimum flow rates, and narrower pipes have lower optimum rates. If you get your exhaust moving fast enough, it will help to suck air out of your cylinders, meaning the engine doesn't need to work as hard to pump it out, so you get a bit of a power bump. You can take advantage of the power bump by choosing an exhaust system optimized to a specific rpm range, typically around peak torque. The higher the optimized rate, the higher up the rev range your bump, and you can gain low-end with a lower optimized flow rate. If you really want to get fancy, you can use butterfly valves and the like to alter the optimum flow rate of the exhaust to get power bumps at several rpm ranges. You can also use wave harmonics to "wave tune" your exhaust, effectively lining up the individual exhaust pulses from each cylinder to push the exhaust faster at specific rpm.

 

So while the principle of "back pressure" is mostly sound, i.e. freer flowing setups lead to more high rpm power and more "constrictive" systems lead to better low end, the fact remains that if you ever start building pressure of any kind in your exhaust, you're just pointlessly wasting engine power to compress your exhaust. Also, the "constrictive" setup isn't actually constrictive until you're going faster than the optimum rpm, and even a more free flowing system can be constrictive if you're not going as fast as the optimum.

 

As far as mpg's go, though, the best I've ever had was nearly 20mpg at ~1850rpm. I've never really got close to that number since. And I'm pretty sure it wasn't just a tailwind, because it was up and back down the same 100 mile section of highway. That said, 300,000 mile MJ is't going to do quite as well as some of those that have lived gentler lives.

 

Honestly, the best way to improve your mpg is to avoid accelerating and idling (sitting at red lights etc) as much as possible.

[92tanMJ]

I disagree a little with needing back pressure being an myth, Ive driven my xj with just straight headers and it was crap. I lost all low end torque and just power in general. Once the cat was it was much better and then even better when it was straight piped after the cat and then ran out the back

[JeepcoMJ]

...

 

the venturi effect only works if you have a properly restricted pipe to create the "sucking vortex".    try again.

 

 

smaller pipes build higher pressure, yes.   Larger pipes build lower pressure.   stock exhaust is ALREADY optimum size for the 4.0 to run.   it's an inline six, not a race car motor.

 

If you build it up, bore it, stroke it, and generally get it to a point where it's pushing way more power and exhaust, then you NEED a bigger pipe to let more air pass.

 

I worked for coca-cola for quite some time installing and maintaining fountain soda equipment.   We had to run 3/8" line for syrup, running 120PSI CO2 @ the BIB pumps to push the fluid.   run low on Co2, you can't push the syrup fast enough and hard enough to build the right pressure.    Run it through 1/2" line, it needed twice as much Co2 to keep pressure built and get the product pushed.     run it through 1/4", you couldn't deliver enough product to the valve in time to meet proper mixture ratio.

 

So.   too small of an exhaust will build too much pressure and choke the engine out.   a clogged catalytic converter does essentially the same thing.

 

Too large of an exhaust doesn't build enough pressure and lets the engine run free...negating any type of assistance from the venturi effect, making the motor do all the work, which will bog it down OR require more fuel to deliver the proper end results

 

The right size exhaust (factory) or just a tiny bit over will result in everything flowing properly, burning less fuel, not bogging down the motor, not causing any harm, and not sucking your wallet dry because it's using more fuel.

 

 

I've had strokers, borla exhaust, etc.    In the end, I would always end up with a borla header and downpipe (2"), oem replacement cat, flowmaster 40 or thrush welded running 2" in and out, and a stock tailpipe.     19mpg mixed, up to 24 on long hauls, could beat a stock subaru wrx

[gogmorgo]

I disagree a little with needing back pressure being an myth, Ive driven my xj with just straight headers and it was crap. I lost all low end torque and just power in general. Once the cat was it was much better and then even better when it was straight piped after the cat and then ran out the back

Did you even read the whole post?

 

A long tube allows the exhaust gases to accelerate, which helps to pull the exhaust out of the engine. Power is of course highest at the point where the exhaust is flowing as fast as possible, but before the exhaust starts building pressure when the engine pumps it out faster than the exhaust can flow in the pipe. A modern catalytic converter doesn't provide much obstruction to exhaust, and you were mostly noticing the effects of the longer straight section of tube. Having the tail pipe go as far back as possible really helps that. As you noticed.

 

When you push air into a wide open space, it slows down, but when you push it down a tube, it accelerates. But if your tube is too big for the amount of air you push through it, you don't move all of the air inside the tube, and the air that doesn't move interacts with the moving air, slowing it down.

As the motor revs higher, it moves more air through it, so you would need an increasingly larger exhaust to be always at the point where you aren't compressing the fluid but have it moving as fast as possible. That's not really possible, so you pick the rpm where you want the most power boost and choose an appropriate exhaust diameter for that rpm. Stock exhaust on a 4.0 is set up for around peak torque, so changing it will mean you lose power at peak and generally in the low end, although if for whatever reason you want more power at higher rpm, you can choose a larger pipe, sacrificing low-end performance, which would work out pretty well for a turbocharged motor, since you want to keep the revs up anyway.

 

 

 

JeepcoMJ... Venturi effect? Try again yourself. It never comes into play, nor did I bring it up. Since your exhaust gases are losing heat as they travel further from the engine, they lose pressure (Guy-Lussac's law). To conserve kinetic energy, the lower-pressure exhaust must then travel faster. So it's similar in result to the Venturi effect, but without needing to waste power from the engine to force the exhaust through a constriction. This also helps explain why Zack noticed a big difference after putting on the cat, since the reactions happening inside it take away a huge amount of heat, significantly reducing the pressure and thereby increasing the speed of the exhaust.

 

To OP, sorry for the thread hijack... my bad.

[92tanMJ]

 

I disagree a little with needing back pressure being an myth, Ive driven my xj with just straight headers and it was crap. I lost all low end torque and just power in general. Once the cat was it was much better and then even better when it was straight piped after the cat and then ran out the back

Did you even read the whole post?

 

....

 

To OP, sorry for the thread hijack... my bad.

 

I did later lol I was on my phone and It was having issues and I just forgot to edit my post on my PC

[JFAComanche6]

There comes a problem with the possibility of the non-existent police ticketing me if I'm going 60...  It would be my luck that the day I go 20 over posted speed both cops in town would be on duty.  

.

Understandable, I honestly don't expect an engine as old and big as the Renix 4.0 getting decent city mileage then. I wouldn't bother trying personally, I'd ride my bike everywhere if I didn't live 10 miles from anything.