Efficiency potential of the L28 (MegaSquirt)

[FastWoman]

Hi all,

As fuel prices continue to rise, I'm wondering what the potential is of our L28 (or L24/26) engines is. I currently get about 19 mpg combined. I didn't worry about that sort of mileage as a kid, when gas was cheap, but these days that's what you would expect from a pickup truck, and 19 mpg will run you broke in a daily driver.

I have to believe that an engine is an engine is an engine when it comes to basic operating efficiency. So long as they're able to pull approximately the same vacuum, they should be operating with roughly the same efficiency, right -- at least for cruising? Maybe the hp:weight ratio is lower on our engines than on a modern 24 valve engine, but weight doesn't impact highway miles.

And of course the Z has sexy but inefficient lines. A more aerodynamic shape would be that of the egg-shaped Prius. That's obvious in hindsight, I suppose. But is that enough to limit us to only 20'ish mpg -- maybe the mid 20's on the highway?

I'm wondering how inefficient our EFI systems are. Could we squeeze several more mpg out of a MegaSquirt system, not to mention better performance? Is there an efficiency/performance incentive out there for making the switch?

[John Coffey]

Engine vacuum is not a good indicator of engine efficiency. Air fuel ratio is a much better indicator - to a point. Generally the biggest factor is the driver.

From BMW's eta engine studies in the late 1970s/early 1980s large throttle openings at low rpm are the most efficient way to accelerate. Shift early and get into top gear as quickly as possible. I experimented with this technique, never running the engine past 2,750 rpm in any of the lower gears, and saw a 8% increase in fuel mileage for around town driving in my street 2.4L 240Z. Then I stopped because I couldn't stand to drive the car that way.

Assuming you ahve more patience then me... tuning your car for that kind of use requires an emphasis on low rpm torque. As much compression ratio as possible and as much timing advance early in the curve as possible without low rpm detonation. Stock cams and smaller intake valves. Gearing also need to be high (the OEM 3.36 gears are good).

[LeonV]

I have heard multiple people getting 30mpg using MS. In fact, I know a guy who did a turbo swap using MS and did just that. Just takes some tuning and a precise fuel delivery system (not carbs).

[Oiluj]

Given the weight and drag of our cars is fixed, I agree with John that it's more a function of engine displacement and a good fuel mixture ratio, but mostly the driver.

My 2.7 L stroker w/ round-top carbs gets 26 MPG on the freeway if I drive in a sane, (but not boring), fashion. I havn't measured "in-town" milelage, but I'm guessing it's between 15 to 17 mpg. This is actually better than what I get from my 1997 ES300 (Lexus V6 Camry). But then the Z is a much lighter car...

Out of curiosity, I played with advancing the timing and leaning-out the idle mixture a bit and saw no measureable difference. Perhaps there is some small amount to be gained by minor ajdustments of valve timing, as done in modern engines.

Edited July 16, 2011 by Oiluj

[Zed Head]

Here is a recent post from zcar.com along those lines. Tony D has posted on this forum in the past. Most of the Megasquirt threads you see are for turbo or modified applications, so a straight AFM/ECU to MegaSquirt comparison is hard to find.

Reply #4 has the relevant comments -

http://www.zcar.com/70-83_tech_discussion_forum/micro_squirt_qestion_899882.0.html

[doradox]

Hi all,

As fuel prices continue to rise, I'm wondering what the potential is of our L28 (or L24/26) engines is. I currently get about 19 mpg combined. I didn't worry about that sort of mileage as a kid, when gas was cheap, but these days that's what you would expect from a pickup truck, and 19 mpg will run you broke in a daily driver.

I have to believe that an engine is an engine is an engine when it comes to basic operating efficiency. So long as they're able to pull approximately the same vacuum, they should be operating with roughly the same efficiency, right -- at least for cruising? Maybe the hp:weight ratio is lower on our engines than on a modern 24 valve engine, but weight doesn't impact highway miles.

And of course the Z has sexy but inefficient lines. A more aerodynamic shape would be that of the egg-shaped Prius. That's obvious in hindsight, I suppose. But is that enough to limit us to only 20'ish mpg -- maybe the mid 20's on the highway?

I'm wondering how inefficient our EFI systems are. Could we squeeze several more mpg out of a MegaSquirt system, not to mention better performance? Is there an efficiency/performance incentive out there for making the switch?

Modern engines have higher compression, lower internal friction, and ignition/fuel systems that are extremely precise. Add aerodynamics and low friction drive trains to that. I think your mid 20's highway goal is achievable though.

Steve

[aarc240]

If you are needing to rebuild an engine then you can very nearly equal the "modern" cars.

Fit late 280ZX narrow ring flat top pistons and assemble the engine like it really matters to you that it's actually built the way the factory meant it to be.

Use modern lubricants in the drive train, there's really no advantage to a "modern" drive train.

Our road 260z 2+2 is set up this way, 86mm bore, works 1972 rally cam advanced 4 degrees, 10.4:1 compression, precision assembly, 280z injection manifold with 250cc injectors, GM 65mm throttle body, headers with 6xEGT & EGO, Megasquirt3 with sequential injection & EDIS ignition, Chrysler Voyager coil pack, 5 speed C/R, 3.54 gears, 185/65R14 tyres (600mm or 23.6" diameter).

Gets 27mpg (converted to your gallons) on the highway, no idea around town, and will still turn the tyres happily (and doesn't get babied on the highway either).

Mistakes to avoid: fitting wider tyres, fitting a rear spoiler, not fitting a front spoiler / air dam, not fitting a factory engine bay undertray, fitting a sun roof, tuning MS thinking it's a race car, failing to keep tyre pressures at the upper end of the comfort range, failing to clean / polish the car, etc

[Tony D]

Really like John C said is torque and proper fuel management.

The Blue Turd will manage 27mpg if I never go over 65mph and accelerate rationally (not granny-like, but maybe upshifting at 3500 or 4000...) Take that speed up to 80 and the car gets 22mpg---plain and simple, drag increases. At average speeds of 100 I crack to 19mpg...

This is an L26, dome tops, late ZX tranny, and 3.7 out back.

Now what has shocked me was I thought that was really decent mileage. But three weeks ago, on the way to work (45 miles from my fillup point) the fifth gear shaft nut backed off and locked the tranny in 5th. So I trailered the car home, changed to an Early five speed I have, with the .85 O.D. Now I turn 36-3700 rpms on the 60 freeway going to work, instead of 31-3200 rpms.

My First Shock was that the car is a BLAST to drive with that early tranny and it's closer ratios.

SECOND was that my mileage went up from rock solid 22 to 23.6mpg on the first tank, and now monitoring it through four more tankfulls, it's been 24, 23.5, 23.7 average. Now really, IMO I'm really beating the hell out of the car since the ratios seem to BEG me to beat on it more now... It's a pleasant side effect which I COMPLETELY did not expect. Only thing I can figure is I'm in the sweet spot of the torque curve now.

The key for EFI conversion of this car will be to datalog the AFR's I currently have, and then set a TPS.MAP table which is equivalent. Then start to tweak it leaner---towards 17 or 18:1 in lightly loaded cruise conditions. Frankly I may re-enable EGR for this (using a diesel EGR Cooler) to allow more advance if it helps the mileage.

The key is to have MS in closed loop use a WBO2 that either has the provision to shoot for AFR target tables, or a configurable 'switching output' so that you set your desired AFR for something other than 14.7:1, more like 17.5, or 18:1 during closed-loop operation, and then stay in a driving situation that keeps you in closed loop.

An L26 or L28 should easily be able to get mid-high 20's with ANY fueling system calibrated for emissions compliance. If you run leaner, you can improve upon that.

I've seen L24's with 40mm Solex Mikuinis turn in 28mpgs on the run from LA to Phoenix with an Automatic and 4.11 differential (but the speed was strictly kept to 65 as the limit was only 55 then! Doesn't do that at 80, which is 4K+!

[Z train]

Really like John C said is torque and proper fuel management.

The Blue Turd will manage 27mpg if I never go over 65mph and accelerate rationally (not granny-like, but maybe upshifting at 3500 or 4000...) Take that speed up to 80 and the car gets 22mpg---plain and simple, drag increases. At average speeds of 100 I crack to 19mpg...

This is an L26, dome tops, late ZX tranny, and 3.7 out back.

Now what has shocked me was I thought that was really decent mileage. But three weeks ago, on the way to work (45 miles from my fillup point) the fifth gear shaft nut backed off and locked the tranny in 5th. So I trailered the car home, changed to an Early five speed I have, with the .85 O.D. Now I turn 36-3700 rpms on the 60 freeway going to work, instead of 31-3200 rpms.

My First Shock was that the car is a BLAST to drive with that early tranny and it's closer ratios.

SECOND was that my mileage went up from rock solid 22 to 23.6mpg on the first tank, and now monitoring it through four more tankfulls, it's been 24, 23.5, 23.7 average. Now really, IMO I'm really beating the hell out of the car since the ratios seem to BEG me to beat on it more now... It's a pleasant side effect which I COMPLETELY did not expect. Only thing I can figure is I'm in the sweet spot of the torque curve now.

The key for EFI conversion of this car will be to datalog the AFR's I currently have, and then set a TPS.MAP table which is equivalent. Then start to tweak it leaner---towards 17 or 18:1 in lightly loaded cruise conditions. Frankly I may re-enable EGR for this (using a diesel EGR Cooler) to allow more advance if it helps the mileage.

The key is to have MS in closed loop use a WBO2 that either has the provision to shoot for AFR target tables, or a configurable 'switching output' so that you set your desired AFR for something other than 14.7:1, more like 17.5, or 18:1 during closed-loop operation, and then stay in a driving situation that keeps you in closed loop.

An L26 or L28 should easily be able to get mid-high 20's with ANY fueling system calibrated for emissions compliance. If you run leaner, you can improve upon that.

I've seen L24's with 40mm Solex Mikuinis turn in 28mpgs on the run from LA to Phoenix with an Automatic and 4.11 differential (but the speed was strictly kept to 65 as the limit was only 55 then! Doesn't do that at 80, which is 4K+!

Tony,you do know that your EXERIENCE flies directly in the face of Dan B's theories-don't you?

[FastWoman]

Thanks, guys! It sounds like the conversion might be worth doing. I confess there's a side of me that wants to preserve the original Bosch L-Jetronic system, but there's also a side of me that wants a more affordable and better running daily driver.

What would you estimate my time commitment to be in making this conversion, considering that I'm competent with EFI engines, but a newbie with MegaSquirt?

[doradox]

Use modern lubricants in the drive train, there's really no advantage to a "modern" drive train.

It is generally accepted that CV joints are more efficient than Cardan joints. One might argue that CV joints aren't really all that modern though.

Steve

[Z train]

Fastchick-there are two Megasquirt conversions on my horizon-the wifes stroker and my L-24E.Just a matter of $$$$.And making a manifold for the stroker.