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My stock k8 gets about 30mpg on highway

You see, that's what I get at about 65 MPH... The moment I go over 70 I lose about 2 MPG immediately. It has to be the gearing of the trans, because I never see those RPMs(3,500) around town.

I'm proud of my city average though, a high 28.

Well it really has to do with your RPM. When doing 70 at what RPM are you at? now what opens up at about that RPM? that is the reason it is such a drastic decrease in MPG. If you swap the 5th gear you should get better as you will be sitting about 700 rpm lower. I have heard ZE guys with mx6/626 trans getting as much as 32.

I just saw a 2mph increase with my new plugs on the last tank. I wish it were not raining or snowing so I could get a more accurate number.

Josh wrote:When doing 70 at what RPM are you at? now what opens up at about that RPM?

OH MY GOD!

How did that never click in my head!? I've toiled over this needlessly lmao, I seriously could not understand why in the hell it was such a noticeable drop at only a 5 mph(250 rpm) difference...

*facepalm*

Thank you Josh. I'm going to do a test with the VRIS disabled on my next road trip... I'll do 3 average tanks around town as well, both with and without VRIS at stock points.

Woooh!

You refering to a VRIS point I take it?

I don't think this will make a difference here, this isn't VTEC where your fuel consumption goes down the tubes, 'yo.

If you look at a dyno with improper VRIS points, you see the power drop before the incorrect point, than shoot back up until the next one, and repeat. Matching ECU VRIS points to the IM make the power delivery smooth with no such drops. Disconnecting the VRIS to be at a position that it wouldn't normally be at those RPMs means you'll lose a few hp. You lose hp, your economy should go down too because your making power less efficiently, unless oddly torque would go up inversely, which could make that claim valid

If you want to confirm this and adjust your driving, a vacuum gauge is perfect for this application and to understand how your motor works and how hard it's working. This was perfect to unstand the attitude of my truck, and wonder why I would go down in mileage when I went slower. My cam profile woke up as I neared 3000rpms, the in-mg would drop (meaning it's working less hard), which also lets me know when the ECM is having a PMS fit. I base my driving of the truck not on the RPM's but on my vacuum/boost gauge; if I'm cruising on flat highway at around 10in-mg, she's good, if less, she's having a fit and I'll turn off the truck (yep while rolling), wait a few seconds and restart and then she's good to go.

0in-mg : engine off or full throttle
20in-mg : standard oem-tuned idle
21in-mg+ : in-gear deceleration.

The best fuel economy I have ever managed with an MX-3 was over 32 MPG in the city...

To do this, I barely use any throttle and shift as low as I can without causing any stumbling. Typically the higher gear is engaged at 1,700-2,000 RPMs when I drive that carefully.

I can't say if VRIS makes a substantial difference, but what you say is only partially true... A car will be pushed along far easier with 500 lb/ft of torque than 400, but that doesn't mean that it requires any more than 80 lb/ft to even move the car in the first place.

If you were climbing a hill, the extra torque would be immensely helpful... You don't need a lot of power to maintain a speed of 35-38 around town.

Now your right, it's not about making power, it's how efficiently you make that power. If you're driving down the highway and you only need 40ft-lbs to cruise along at 70mph, but if you're making only 150 rather than 160ft-lbs, you don't need that surplus, but it still reflects the efficienty of power being developped at that specific RPM, means you have to give it more throttle to mix a bit more air and use a bit more fuel that you would normally need to. Ya only 10hp loss at 160ftlbs is 6.2% loss, which at 40ft-lbs is only 3ft-lbs, but it's not linear either, combustion efficiency does down the less you work the motor hard.

It's like saying you are physically capable of carrying 160lbs (for a certain time or distance, it's irrelevant). Ya, you could carry 40lbs much easier and not work nearly as hard. Say you got lazy, a bit out of shape could only carry 150lbs, that same 40lbs isn't as easy anymore, it's still easier, but not as efficient

Like anything we do for bolt ons (instake, exhaust, pulley, etc), it frees up lost HP, the motor isn't making any more at the crank than it did before, you're just losing less in losses, gaining HP and fuel economy.

I'll concede the point about efficiency, but I highly doubt the losses due to a decrease in efficiency will overwhelm the gains in this instance.

From a purely economical standpoint, I believe there are gains to be made... I'll run an average after I get my '92 back on the road. This is exciting for me, because I've been debating whether or not I should run all open VRIS/all closed VRIS to compare mileage for a while now.

Tuning the VRIS exactly to where I want it will be fun

MrMazda92 wrote: I highly doubt the losses due to a decrease in efficiency will overwhelm the gains in this instance.

Which gains and where are you going to get them? I'm kinda lost...

Nd4SpdSe wrote:

MrMazda92 wrote: I highly doubt the losses due to a decrease in efficiency will overwhelm the gains in this instance.

Which gains and where are you going to get them? I'm kinda lost...

Economic, not horsepower/torque.
Losing a few ponies in the middle of the rev range shouldn't make a dent in your fuel consumption. Reducing the overall "size" of the plenum and keeping the banks separate should reduce the intake velocity slightly, and the amount of air traveling through the manifold, which would in turn require less fuel to burn.

I'm no engineer, and I may be suffering from a misunderstanding, but it makes sense in my head that less air in the CC requires less fuel to ignite. If losing a little bit of torque and HP while traveling at a steady rate on the freeway reduces fuel economy noticeably, then I'll reconsider my line of thinking...