Wide Open Throttle Efficiency Challenge

WOT DAT?

All you points make a ton of sense and by all means, this is an older car that I have no desire to modify it. I was just thinking about WOT efficiency when I was driving

In college, I remember having difficulty with this topic - logically, it didn't make sense to me.

It may apply under a specific set of conditions, but normal driving probably rarely meets those conditions.

WOT for modern fuel injected cars will feed the engine an ever increasing amount of fuel until it either hits the rev limiter, you back off the throttle, or the drag and friction creates a terminal velocity.

Even carbureted cars work the same way as fuel quantity is a function of airflow speed.

Yes, this car is a blast to drive. The next lightest car I own is about 700 lbs heavier - quite a difference.

Can you elaborate on max torque is the most efficient place for the engine. In college, I remember WOT was the most efficient.

WOT a year that must have been...

You are probably associating WOT efficiencies with pumping losses- which is correct, as the engine is exerting work to create a vacuum. On a dyno there is a point at which you have the highest efficiency, which is usually around torque peak when tested at WOT (noting on the other comments of VVT etc., I'm not familiar with the effects but would imagine they tend to flatten the curve and broaden the efficiency peak). This is very useful for designing something like a generator that runs at a fixed speed; it can be set up to run slightly above this point under normal operation.

This is maximum efficiency for unit of work output though- not the same thing as MPG. Under real life conditions you usually don't need the amount of power output at this point at WOT, so it starts to become a trade-off between pumping losses and engine efficiency curve. As a practical example, short shifting is not the best thing to do with 2 adults in the car when heading up a steep incline anyways, but will be fine when you're going down the street to the corner store.

It's just no fun on winding back country roads.

WOT is most efficient only if the engine is designed to be that. The other operating parameters( rpm, mixture, available cooling, FI/carb, etc ) can all add major changes to the efficiency.

Agreed that around peak torque RPM is a good aiming point for best efficiency. Or used to be - I wonder at the effect of now-common variable valve timing.

Also, some years ago with carburetors, it was common for WOT to enrich the mixture for peak performance and this would be detrimental to fuel efficiency.

I just looked at the torque curve for this engine and it peaks at around 4600 rpm. The VVT gives two peaks for torque, but the higher rpm one is the higher peak.

If peak torque RPM gives best efficiency it doesn't necessarily give maximum mpg. Almost certainly need to be in highest gear for max mpg, giving high road speed at peak torque RPM. Going a bit slower might gain more from lower wind resistance etc than is lost by lower engine efficiency.

Incidentally, there was recent talk in UK about increasing the motorway speed limit from 70 to 80 mph, and antis (aka busybodies) claimed this would increase fuel consumption by nearly 50% as proportional to speed³. Assuming resistance varies as speed², that's right for consumption per unit time, but for unit distance consumption varies as speed² as less time is spent getting there.

That and you really do not spend much time at the upper speed limit in the UK.

Heck, Britain is only about 650 miles long north to south, a little longer than the state of Florida.

The real fuel waste is sitting at traffic lights and crawling in stop and go traffic.

Yes, but when moving you might as well do it as economically as possible, if that's what you want to achieve. USA might be much bigger, but there's still plenty of stop/go traffic. And in plenty of other places round the world.

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If you had no comment why did you comment?

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You need to improve aerodynamics, drop the weight and add power with a turbo, then you can raise gear ratio to cruise at 1500 rpm@80mph....

https://www.youtube.com/watch?v=t8Xf_lpn0zA

Must be older then 2002 or when purchased the option for the 6 speed transmission was not considered. If it is newer then 2002 you could check the junk yards for a 6 speed transmission.

You're correct, it's a 2000 model. The first few times I drove it, I tried to put it into 6th gear - luckily something in the linkage/transmission blocked me from doing it. It just felt natural to do, because the engine was running such high rpm.

This car is a toy for me and I'll probably just keep it for the summer. It doesn't make sense for me to put a 6 speed in, but thank you for the tip.

If the MR2 was anything like the two Miatas I've had the six speed would not make a difference in the final drive ratio. The first one I had was a 2001 (their second generation model) with a five speed. At 60 mph the engine would turn at 3000 rpm. I did the same as you for the first few weeks that I had it and hunted for the next gear that wasn't there. I have since bought a 2006 (the third generation model) with a six speed. I was expecting it to have a final drive ratio that would allow the engine to turn at a slower speed in sixth gear but found that 60 mph in this car returned the same 3000 rpm. The only thing that adding the sixth gear accomplished other than saving them money on development cost (the transmission came directly from the RX-8 that they were producing at the time) was to space the gear ratios closer together. This allows you to keep the engine in a higher RPM range more of the time and keep it closer to it's maximum horsepower and torque values which, by the way, is a whole lot of fun. I had a friend that had a third generation MR2 that I always wanted to trade with her for a while but she sold the car before we could make it happen. Being a fan of Porsche Boxsters and 914s I have always wanted to have a mid engine car but Miatas are much cheaper to upgrade than MR2s and cheaper by probably an order of magnitude than working on Boxsters.

I think these little Japanese roadsters are so much fun to drive and unlike their predecessors, they are reliable, cheap to fix and won't leave you stranded.

Back in 2004/05, I seriously looked at buying a nice roadster. I looked at the Z convertible, Miata Speed, Crossfire SRT, Z4, SLK and Boxster. When the dust had settled, we had our Boxster. I still have the car and I've enjoyed owning it, although the IMS problem caused a blown engine in 2007 (at 23K miles), and a dead battery problem I still haven't resolved.

So, while the Boxster sits, we're using the MR2 Spyder. Yesterday, we took a very pleasant drive to Ventura. I did make a mistake - the engine runs at 4,000 rpm at 80 mph.

I became a convert to them when I started to see the cars racing in the SCCA's Spec Miata class in the early 2000's. Before then I had always said that I wanted an MGB or a Spitfire but I have since adopted the credo that a Miata is the best BRITISH sports car ever made! I've actually embarrased quite a few 911's on race tracks (albeit the right kind of race track) with a Miata. A 911 will best a Miata on a straight section of track but given the right set of curves a Miata can, in the right hands, hold its own with the best from Stuttgart.

I believe that Porsche has officially recognized the IMS issue. I know they have issued a kit to proactively fix the problem. You may be able to get some compensation.

My car was fixed under warranty. I was told that the new engine they put in had the IMS fix.

Excellent.

I agree 3500 RPM for cruising the freeway is quite high. Probably the reason you've got very little acceleration at this point, is because I think you've got the VVT engine, which is pretty gutless at low RPM anyway. They did do a supercharged version of this engine. It might be worth looking to see if this supercharged engine is coupled to a gearbox with different ratios.

I'd consult or obtain the services of a tuner. In the old days, I upped my mileage 3-4 mpg by creating an early spark advance. That may be a problem to implement on later model cars with all the electronics. But if it can be done, a performance specialist is your best bet.

There have been some studies documenting improvements in fuel economy using what is known as 'pulse and glide' driving techniques, which seek to leverage the improved efficiency at WOT. Improvements are typically in the range of 30% to 70% and interestingly improvements are seen not only in gasoline powered cars but also diesel powered cars, which lack throttles.

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A decent site that strives both to keep upgrade low dollar and make reasonable efforts to test the actual benefit of any modifications is ecomodder.com