Guest bastaad525 Posted March 28, 2004 Share Posted March 28, 2004 Well... this is gonna be hard. See, ever since I've put the turbo motor into my Z, I've been getting a lower-than-expected 17mpg's average. Well.. I might have thought that was okay, but everyone on here with their turbos always talking about 20+ I'm thinking either a) my EFI is still not running the way it should be, or I need to stop driving like such a nut... So... the hardest thing I've had to do since getting this car together I full tanked it last night and am going to drive for the next week or so WITHOUT BOOSTING AT ALL!!!! Well... okay I'm gonna TRY! I want to see how much of a difference it really makes in my mpg's. Honestly, I'm not expecting much, but considering that I do get on boost a lot usually maybe I am going to find a few extra mpg's hiding there. I just hope I can stick to it. I'm just curious... I"m sure some of you have gone down this path before... how much difference have any of you guys seen from just staying out of the throttle for a bit? Optimistically? Quote Link to comment Share on other sites More sharing options...
fl327 Posted March 28, 2004 Share Posted March 28, 2004 Never been able to get through a full tank without some WOT. Car can be economical, but why? Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 28, 2004 Share Posted March 28, 2004 stop n go kills mileage. the biggest gains are when you drive steady for long distances. I get 17 around town regularly, and when I took the Z cross-country was amazed that I could hit 26mpg if I drove sanely. Matter-of-fact, the LOWEST mileage I got on the whole trip during highway driving was 19mpg while I averaged 102mph over 1100 miles from Oglalla NE to Grand Rapids MI. So when my in-town mileag combined is 17, that gives you some indication of what you are up against compared to highway cruising! Quote Link to comment Share on other sites More sharing options...
Guest ZmeFly Posted March 28, 2004 Share Posted March 28, 2004 In town driving gets me about 17-19 mpg, on the highway I get 28-30 mpg. Like Tony said, in town stop and go traffic really kills your mileage. When I bought my 71 240Z that Jeffp was holding for me, driving that car back from LA I was getting 25 mpg averaging about 80mph the whole trip. It was really nice to see a 71 240Z get that kind of mileage. Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted March 28, 2004 Share Posted March 28, 2004 Keep in mind too that when cruising, contrary to popular belief, lower RPM doesnt always mean better gas mileage. Typically you want to be as close to the peak torque for the engine when cruising at any given speed. In my honda, I can average 28mpg when I cruise at 4200 RPM and have normal around-town driving too. If I drive the same way, except shift up any time I am cruising and cruise at 3000 RPM instead, I only average 21mpg. Also verify your timing. As far as setups go, timing is one of the things that is most likely to affect gas mileage. Speed is going to affect your mileage too. As your speed increases, so does your wind resistance. Cruising at 80MPH will get worse mileage than cruising at 65-70. The highest speed that you can cruise at without losing gas mileage depends on the vehicle. With a truck, for example, anything above 55 will probably start to drop your gas mileage down because of the resistance. Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 28, 2004 Share Posted March 28, 2004 Oh EXCELLENT point Nic! I keep harping on another site about being close to peak torque. My mileage quoted was in an L28 powered car with an early five speed and a 3.90 TOWING A TRAILER at about 3500rpm steadily. (You figure out that speed LOL) Anyway, the lowest I got with that setup was 22mpg---but that wasn't with O2 feedback, it was on an early EFI system. The 260 got much better fuel economy, but of course it wasn't towing 800# and didn't have three people in it! Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted March 29, 2004 Share Posted March 29, 2004 Wow... I always thought you wanted to cruise at the lowest RPM that you could get away with. I try to keep my cruising RPM on the street to between 2000-2500rpm. But, this thing you guys are saying brings up a whole NEW set of questions. For instance, my torque does peak at 4100rpm (and I gotta mention I dont know if I like the idea of cruising at 4100rpm all day long) but figure that's at WOT at full boost. Of course the power curve of the engine is probably completely different off the boost... or is it? How do I determine what is the optimal RPM to cruise at or where my peak torque is off boost? I'm really curious and interested in this idea though as you guys aren't the first ones I"ve heard to say that... but I have to wonder, WHY exactly would you get better gas mileage at higher revs? Well I understand the revs actually, because you're aiming for higher torque, but why do you want to be as close to peak torque as possible? A few other things you guys brought up. I rarely if ever really hit any stop and go traffic. I pretty much only drive my car to work and back, dont do much around town driving in it, usually run errands and such in the wifes car. My drive to work every day is about 25 mi., and I'd say only about 7 of those miles are on the street before I hit the freeway and after I get off. Further, it's a pretty straight route on the street, and I usually only hit two or three red lights, plus a couple of stops to turn, and I'm doing 45-50 pretty much the whole way. Also, once I get on the freeway, it's smooth sailing all the way to work. That's one of the best things about working night shift NO TRAFFIC! I may sometimes hit a LITTLE traffic on the way back home in the morning, and a few more red lights, but as far as freeway traffic in the AM it's just a little slow, say average 60-65 vs. my usual 80. So, I dunno how that all figures up but to me it doesn't really count as the kind of serious stop and go traffic that a lot of people deal with daily. I dunno... in the end, for the amount of power I'm making, and the relatively traffic free driving that I usually do, I didn't think 17mpg's was reasonable, but I guess it's not as bad as I thought either. I'm not so much concerned about being economical... I'm more just wanting to make sure my car is running okay... Of course... there are two other factors that throw that last statement out the window and they are that I'm running a) with my O2 sensor disconnected (because even after I bought a new sensor, when the sensor is hooked up the car runs more roughly, a problem I know I'm not the only one who has), and I'm running with my TPS disconnected, hence, I BELIEVE this means I'm running more rich at idle than I'm supposed to be. I found that off idle throttle response seems to be much better with it disconnected though, so I'm leaving it. Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted March 29, 2004 Share Posted March 29, 2004 Around 3300 is normally a good RPM to cruise at. Lower than that, and your car is wasting more gas than it is burning because of poor efficiency at lower rpms. A drastic example of how your engine wastes gas at lower RPMs would be to floor the peddle at 2200 RPM. If the engine does not bog, it is pretty un-resposive. This is because your engine simply cannot burn the fuel and ends up dumping it un-burnt or only partially burnt. Do the same thing at a higher RPM and the car will take off. Basically the torque peak is when your engine is at it's most efficient state. If you pay close attention to your exhaust sound, you can actually hear the RPM band that is optimal for cruising. You will hear the exhaust slightly shift tone to something that sounds smoother, quieter, and more even. You can also try a simple down-shift, that is, when you are cruising, keep your foot on the peddle at the same angle, then down-shift. If the car accellerates then you were cruising in the wrong gear and at too low of an RPM. Playing with the EGR (if present) is another way to improve gas mileage. The following technique works for engines without EGR, but not as well. When you are cruising, let up just slightly on the gas peddle and then re-apply. If you do this in kind of an oscilation lifting your foot for about 5 seconds and then down for 5 seconds, being sure to not lift your foot far enough to lose speed, and not pushing down enough to gain speed, then your engine will actually pull some of the un-burnt exhaust back in and burn it again. This is how I can get better MPG in my Honda than it did during EPA testing before the car was first sold! Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted March 29, 2004 Share Posted March 29, 2004 Oh just another add-on.. In my 240Z with 3.90 rear-end, I almost never used 5th gear. Of course, I dont mind 4200RPM on the freeway If I cruised in 5th gear instead, my gas mileage would drop by about 4 MPG on the freeway. Quote Link to comment Share on other sites More sharing options...
Xander Posted March 29, 2004 Share Posted March 29, 2004 Having a megasquirt system in my car I can read the duty cycle of the injectors in 4th or 5th gear. cruising at around 3000 rpms in 5th the injectors have a lower duty cycle than cruising at the same speed in 4th. This (IMHO) show that in 5th the car is using less gas than in 4th. even if 4th is closer to peak torque. There was a cartest program here in the netherlands where famous people had to drive around a test track trying to get the highest milage. They averaged around 16 km/liter. There was one guy who had research how to drive economical and he got something like 22 km/liter and got the fastest tracktime by far! The trick according to him was not to brake to much. and if you accelerate to like 100 km/h (about 60miles/h) use about 2/3 off full throttle. because thats where the car accelerates most effeciently. hmm... food for thought. Quote Link to comment Share on other sites More sharing options...
wheelman Posted March 29, 2004 Share Posted March 29, 2004 I have to agree with Xander. It seems to me that if an engine is pumping a given CFM (cubic feet per minute) of air at a given RPM and the air/fuel ratio is at a reasonable level then if the RPMs are reduced and the air/fuel ratio isn't drastically reduced (richened) the amount of fuel injected into the air stream would reduce not increase. I know the mileage a car gets isn't totally dependant on RPM but to say that an engine running at 4200 RPM is burning less fuel than one running at 3000 RPM doesn't make sense to me. For that to work the engine running at 3000 RPM would have to have an air/fuel ratio roughly 1/3 richer than the engine turning 4200 RPM. If I'm missing something here please let me know. I understand that you can't necessarily say that the engine turning slower is running at the same efficiency level as one turning faster but just because an engine is running more efficiently at the higher RPM doesn't mean it burns less fuel per mile. Wheelman Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted March 30, 2004 Share Posted March 30, 2004 See I see it the same way as Wheelman here... while a lot of what Nic is saying makes sense in some ways... bottom line to me is that a engine spinning faster is most likely using more fuel, if only because the injectors are opening that many more times during a given minute/mile. Well... I dont know about cruising around at 4200rpm... but I guess I will try to experiment a little myself and not cruise at 2000rpm either. 3000 sounds good. I can definately agree that the motor does not FEEL it's most efficient at 2000rpm, that's for sure The other thing I dont like though is obviously that you are putting more wear on your engine if you're leaving it in a lower gear.... Quote Link to comment Share on other sites More sharing options...
quicker240 Posted March 30, 2004 Share Posted March 30, 2004 I gave up on fuel mileage long ago.I guess I could tune my fuel injection for lean best torque and 2/3 throttle it through a whole tank and...and...oh,the hell with it.The car goes like a bat out of hell and I FREAKIN LOVE IT!!!! I drive it like I stole it and love every minute of it! Man,put 10 more dollars worth of gas in it and go have some more fun.mileage,scmileage.SMILE!!! :flamedevil: Quote Link to comment Share on other sites More sharing options...
speeder Posted March 30, 2004 Share Posted March 30, 2004 With a programmable ECU you can have both, providing you can stay out of the boost. Careful calibration of the cruise regions of the fuel and ignition maps will not affect your WOT performance or partial throttle driveability. I managed almost 30mpg on a road trip cruising at 80mph, probably the only time I ever managed to go that long without getting into boost. But then I remember one tank of gas that was burned up tuning and squirting around town that returned me 9 MPG! My typical overall mileage in the 'Chop is around 14, which is probably an indication of how much I like to work the loud pedal . Dood, I have to agree with Quicker - burn the gas and have fun - It's worth the money, and why you built your Z in the first place. Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted March 30, 2004 Share Posted March 30, 2004 See, the issue is more complex than maintaining any given RPM. The problem is that you cannot maintain an exact same RPM without changing fuel flow. It's the minor changes in RPM and the minor acceleration that consumes the fuel and makes the big difference. If, in theory, you could maintain the exact same RPM without needing to change the fuel flow to compensate for things like road slope, wind resistance, curves, etc then yes, it would be all about RPM. Since you are not driving on a perfectly flat, smooth road, and have to compensate for things like friction, wind resistance, human imperfections (you cant keep the pedal at exactly the same point) and various other variables, then it is not solely based on what the fuel consumption is at a specific RPM. The example Xander provided just shows that 3000 RPMs is closer to where he should be cruising at, however, I am not certain that duty cycle alone will indicate what the fuel consumption is. Quote Link to comment Share on other sites More sharing options...
Xander Posted March 30, 2004 Share Posted March 30, 2004 The results are in. I was cruising yesterday at an indicated speed of 120 km/h. in 5th gear my dutycycle was 15%. In 4th gear it went up to 17%. Dutycycle is the amount of time the injectors are open divided by the amount of time they are closed (times 100). There is a direct correlation between fuel consumption and dutycycle. When you compare the real injection pulsewith in microseconds then you have to take rpm into acount. I didn't look at the injection times (sorry) but my guess is that they are almost the same in both gears. (I'll check today). One more thing: Shift early if you are going for milage. Don't go over 3000 rpm. I found that I can still accelerate comfortably if I keep it below 2500 rpm. another one more things: About peak torque. At 3000 rpm I am already at 90% of peak torque. So the diffence in efficiency is easily offset by the increase in rpm. Quote Link to comment Share on other sites More sharing options...
ZR8ED Posted March 30, 2004 Share Posted March 30, 2004 Well all this talk of RPM is kinda confusing.... lower rpm should equal lower fuel consumption..right? WRONG. At least in the real world. Anyone with a turbo engine can probably get this. Try reving your engine in neutral. Can't get the boost past zero?..why? no load on the engine to build up enough exhaust gas to drive the turbo fast enough to build boost. Now try this on the street. cruise at 50mph in 3rd and note rpms AND boost pressure (or vacuum at this point) then put it in 5th at same speed. now watch rpms and boost pressure. Depending on your gearing boost settings/turbo etc etc.. this will vary, but in 5th you will likely be running some boost, and likely no boost in 3rd. Why? engine load. Different RPM's, and likely in 5th you will be burning a whole lot more fuel than in 3rd at the same 50mph. This is just an example. It makes sense to me. Not sure if you can follow my example though. Engine loading is what drives the mileage. In my 280, I can get about 30mpg at a steady speed. In my case, it is something in the Z31 ecu, but if I keep it at 3000 rpm or less, I get great mileage on the highway. If I take it to 3100 rpm or more, my air fuel monitor reads 10:1 and I can almost watch the fuel guage drop. (normal highway it read 13:1 at under 3000rpm.) On a turbo car you want to be cruising on the highway at an rpm that causes you to run on vacuum not boost. You would want to run with as much vacuum you can get away with and still be able to maintain speed. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted March 31, 2004 Share Posted March 31, 2004 Well... on day 3. Have been drivin it REAL easy, very rarely touching boost at all. I confess I floored it once or twice dont think it's avoidable. It's hard to tell but it seems I am getting a few more MPG's, averaging I'd say about 20 or 21 vs. my usual 17. So getting on the throttle even just a few times here and there every day is definately making a noticeable difference in mpg's, and it's not just my car running too rich or whatever. Even 20 still seems a bit low to me but I don't expect much more than that out of the stock EFI, to be honest, as even my N/A ZX's have all averaged 20-23. I do have one question though, how man gallons does a 240 tank hold? I'm estimating 12 gallons, as when I'm down to a 1/4 or less, it usually takes about 9-10 to fill up. Anyone know for sure? Anyways, in response to quicker240's post... yeah man trust me as soon as this little test is over it's back to my old ways > I guess it was just hard for me to believe that what little opportunity I do have every day to really get on it, that it was impacting mpg's that greatly. I guess 3-4 mpg's isn't that big of a difference but it sure SEEMS like a big difference. Well... it will be nice if/when I eventually get a z31 ecu in there and get closer to 30 hey... driving like a nut is fun, filling up every third day and paying $2.40 per gallon is NOT. But yeah, I agree... why spend the money to build it if I'm not gonna enjoy it! Quote Link to comment Share on other sites More sharing options...
Xander Posted March 31, 2004 Share Posted March 31, 2004 I'm sure this has been said to you before but don't you think it's time to megasquirt that thing. Than you can really be able to tell what's going on with your engine. You are already running on EFI so the swap would be simple. I hope you don't think I'm trying to push you into megasquirting, but reading that you are already going to swap in a different ECU made me think about it again. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted March 31, 2004 Share Posted March 31, 2004 yeah well... right now and for the forseeable near future, MS is just outta my reach. Anyways, I have pretty much finally dialed in the stock EFI and it runs really well, 95% of perfect, so I'm in no huge rush to upgrade to one system or the other. I know some of you guys say otherwise and may have done it for cheaper, but every time I price doing an MS setup the LOWEST I come up with is $400 (for the MS itself, 240SX TB, and a cheap laptop)... for me that's not pocket change. Whereas I know I can do a Z31 setup for about $150-200... much more within reach. Also, I'll be honest, I dont really think MS is for me. It looks to be a great system, but it also looks to be much more than I'll ever need. I keep saying this over and over again... my ultimate goal for this car has always been to get at or near 250hp at the wheels, no more. And MS just seems like it is overkill for that 'low' amount of power. Whereas the Z31 or even the stock ECU should be able to pull it off w/o any problems. All I really need in is my intercooler and a way to get some extra fuel in there and I should be there. I'm afraid if I had something like an MS to mess with I'd probably end up blowing something up... and really... I'd be just too tempted to play with the damn thing at every red light. I find that I do better the less variables I have to toy with Lastly, I really dont think you can compare a Z31 swap to installing MS... With the Z31 and wiring harness all the wiring is pretty much done for you except for a few things you switch around, and just swapping the chopper wheel in the dizzy, and that's it. I know MS is not that simple. It's all moot for now... I have other more necessary things to take care of on the car, like fixing my leaking tranny and rear main seal, and installing new bushings. And at some point I REALLY gotta get this I/C installed already!! Then I'm tapped for money as I have to turn the 'car budget' over to my wife to get some problems with her car fixed. So as I keep saying... I just gotta continue working with what I have. Anyways, filled up the tank last night, and found that for all my conservative driving, I only made a measly 2 miles more to the gallon (19mpg's). So my original suspicion was right... I dont really get on it enough to really make an impact in my fuel usage, and it's just a case of my car being a thirsty car. *shrug* now I know... back to FUN driving! I did, however, hook my TPS back up. I've never been sure if the TPS causes the car to go lean or go rich when it sees that the car is idling, but i"m HOPING it's lean, and therefore will also contribute, if only slightly, to getting a little better mileage since I'll be burning less fuel at idle. I readjusted it and it seems to be working well... still has that little hiccup when it first goes open, but not really noticeable from inside the car, only if I'm standing near the engine bay. Another problem that seems, oddly, to have mostly taken care of itself. Now there's just the matter of the O2 sensor... I want to leave it hooked up and hopefully enjoy some more MPG's overall, but I still can't stand the way it makes the motor seem to run much more 'lumpy'. This is with a brand new sensor, and I know at least one other person, I think BayAreaZT, noticed the same oddity with his. So, I'm still leaving it disconnected for now. *whew* Well, I'm glad that whole thing is over with!!! *Vows to never purposely go out of his way to drive slowly again!* Quote Link to comment Share on other sites More sharing options...
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