Guest bastaad525 Posted November 28, 2003 Share Posted November 28, 2003 Just wondering if anyone has an accurate idea... or one of those spiffy-hard-to-understand-the-why's-or-hows-of- mathematical equations to figure out, just how much less miles to the gallon you get when boost comes on? Figure, I know my L28ET, when driven sanely, and mostly freeway driving, get's 22ish... on the streets more like 18-20, I think. However... I find it harder and harder lately to keep my foot out of it... and with 75% freeway driving I find I'm getting only 18 and sometimes as low as 15mpg's.... this is at 10psi of boost. So how much more fuel needs to be used by the engine, in %, at WOT, at 10psi of boost, compared to say WOT if it was a N/A motor? Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted November 28, 2003 Share Posted November 28, 2003 The question would be related to the HP you are making. If you can figure out what HP you are making at that RPM at that level of boost, you can get a pretty accurate idea of your fuel consumption rate. Keep in mind that this is not what the MAX HP would be, like what you would see on a dyno during a run as that would be the consumption rate under acceleration. If you can do a dyno run and see what HP you are putting down cruising at a specific RPM, that would be the data you would be looking for. (I am not sure if dynos can run like that though.) Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted November 28, 2003 Share Posted November 28, 2003 So there's no easier way to get an estimate just based on airflow? Lets make it easy and say that the fuel amount follows the air flow EXACTLY... so that a 10% increase in airflow would be perfectly matched by a 10% increase in fuel. How much of a % increase in airflow does 10psi of boost give you, or having NO boost? I know 14.x psi is one bar, which is double standard atmospheric pressure... so by my crude math, 10psi is something like 65% more air than just N/A... is that about right? Man my math is so rusty.... there's got to be an easy way to guesstimate increased fuel consumption as a percentage difference when boost is on, vs. trying to figure exact mpg's from exact hp #'s. I already have my starting point, around 20mpg's... conservatively driven, off boost, regular driving, mid throttle... I know that's not exactly accurate, really just a generalization, but that's all I'm going for Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted November 28, 2003 Share Posted November 28, 2003 65% sounds about right. At 10PSI, arent you making about 65% more HP than N/A? If so, then your fuel consumption rate is about 65% more. That means that if an N/A is getting 24 mpg at that RPM, then you are probably getting 45% of the mileage (11mpg). Of course, I believe that would be the consumption rate under acceleration, not crusing. EDIT: BTW, this is all speculatory right now, I might try to figure it out better later. Quote Link to comment Share on other sites More sharing options...
zolorin Posted November 29, 2003 Share Posted November 29, 2003 It does not matter what your boost level is If you cruising at 70mph down the highway without the acceleartion aka steady state (ss for short) and with Z-aerodynamics it requires 45 HP to maintain that speed (that is about right for 70mph and requires 180HP to do 140mph which sounds about right) and say that your engine it 90% volumetric efficient for both NA and boosted then you are consuming the same amount of fuel. Why? because those 45HP produce the required torque to drive the wheel which equates to the force produced by the tires which negate the drag force. Now to produce that torque a specific amount of air/fuel is required and it is constant for both engines. Now say that you have a very high compression engnie then your efficiency is much higher thus you will burn less gas to produce same HP (i.e. your throttle plate will be more closed). Also if your efficiency is higher you will consume less gas. But the problem of the High HP engine is that it is so tempting to step up the gas pedal just to "feel the power" and thus you will burn a whole lot more and your gas milage will decrease. Thus a turbo engine will seem to burn more while being more efficient. Do this experiment. Fill up your gas tank and drive on the interstate at 65mph for say 100 miles and try to avoid stopping and accelerating i.e. and then fill up and see the gas milate. That is only cruise control on you foot then you would get a Steady State gas milage and this is the one I have quoted for my vehicle (and that is on perfectly tuned carbs which last about 1-2months and then they go bad). For comparecent go much faster on the way back and have a "fun" ride and see how bad will your gas milage become Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted November 29, 2003 Share Posted November 29, 2003 It's almost that simple, but not quite. You cannot forget about Brake Specific Fuel Consumption (BSFC). At any given RPM, an engine consumes a certain amount of fuel, even without load. That is the BSFC. Engines that produce a higher Torque and HP output typically have a higher minimum fuel consumption as well. In a situation with forced induction, fuel and air are being forced in at a rate higher than the engine would normally consume. Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted November 29, 2003 Share Posted November 29, 2003 BTW, the easiest solution might be to just hook up one of those nifty gauges that tells you what MPG you are getting and just watch it and keep your foot out of the gas when you can. That's what I plan to setup on my Z Quote Link to comment Share on other sites More sharing options...
Guest d9 Posted November 29, 2003 Share Posted November 29, 2003 I gotta ask... Who cares. When boost is taking place, I could give a rats a$$ what MPG is. I don't want to take anything away from your question... it's a good one, but I don't want a 280ZT for anything but dusting off those "fast and furious" types and my own personal driving pleasure. I'll spring for the gas! d9 Quote Link to comment Share on other sites More sharing options...
zolorin Posted November 29, 2003 Share Posted November 29, 2003 Well I do since I am in grad school and gas 1.75 a gallon so I would rather spend less $$$$ on gas and more on car parts Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted November 30, 2003 Share Posted November 30, 2003 hey man that's just me... I ask a lot of 'what if' and 'just because' questions... just broadening my base of knowledge. Every day is an opportunity to learn something you didn't know the day before. I ask a lot of questions that really have no practical use, just because I want to know. The MPG thing came to mind because I notice that the longer I've had the car, though it seems to be running much BETTER than it used to, my average miles between fillups has gone down... obviously a side effect of me getting more lead footed with the car as I get more used to it. I saw a dramatic decrease in mpg's, almost 5mpg's less on average between fillups, and could only guess that it's because I've been boosting a lot more. It's not like it concerns me a great deal, I love to drive the car and boost it when opportunity arises and I'm not gonna worry much about the lessened mpg's, but being my first turbo car the question popped into my head... how much does the turbo affect the mpg's. I just want to be sure the lessened mileage IS a result of me boosting more, and not some indication that the car is already getting out of tune. Spark plugs look good, not too rich or anything, by the way, so I assume it's running okay. Quote Link to comment Share on other sites More sharing options...
spork Posted November 30, 2003 Share Posted November 30, 2003 damn...$1.75, I remember those days...the price has dropped a lot here lately. Regular is 1.28 (not that I'd dream of putting that in my z) and premium(91) is about a 1.40ish. James Roraback Salina, KS Quote Link to comment Share on other sites More sharing options...
Guest Nic-Rebel450CA Posted November 30, 2003 Share Posted November 30, 2003 (Ignorance question...) With turbos, cant you turn the boost down/off whenever you want? Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted December 1, 2003 Share Posted December 1, 2003 yeah, just keep your foot light on the throttle but how much fun is that? Quote Link to comment Share on other sites More sharing options...
Tony D Posted December 1, 2003 Share Posted December 1, 2003 On my black car, I averaged 17mpg during daily commuting duites (carburetted blowthrough). I was rather proud that I could get as low as 5mpg when I stayed on the boost at track events... Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted December 1, 2003 Share Posted December 1, 2003 I"m averaging about 15mpg's right now... when I first got the swap done I was getting closer to 20. I drive about 50 miles a day 5 days a week, 15 of those miles being on the street, the rest freeway, and I'm on boost pretty often, a lot of 4th gear WOT passing on the freeway, at least 4 or 5 times pulling from 60-80mph. And I get on it a bit on the street too when opportunity presents itself. I still think my mileage is a bit too low.... I'm gonna pull the plugs again maybe tonite and see if they look any different... the car doesn't feel any different, and last time I checked them they were good, definately not running rich. This was even after I disconnected the 02 sensor (which I thought made the car run more rich?)... however since then I've also disconnected the TPS (it was causing a nasty bog)... I know that richens up the mixture, but only at idle... is that enough to really hurt my mpg's? Quote Link to comment Share on other sites More sharing options...
randy 77zt Posted December 4, 2003 Share Posted December 4, 2003 at thunder hill i get about 5 miles to the gallon.this is a 3 mile road course.you are doing either 1 of 2 things-braking or flooring the throttle. Quote Link to comment Share on other sites More sharing options...
Tony D Posted December 4, 2003 Share Posted December 4, 2003 Heck Randy, you're EFI right? So it goes to prove the fact: Carbs or EFI, HP is HP and when you USE it, you all get the same mileage! I feel better that somene else is happy and proud to say they got 5mpg in their car! I have managed to break into single digits on EVERY car I have owned, including a Geo Metro. I surmise Fun Per Mile is inversely porportional to Miles Per Gallon! Quote Link to comment Share on other sites More sharing options...
SleeperZ Posted December 4, 2003 Share Posted December 4, 2003 Heck Randy' date=' you're EFI right? So it goes to prove the fact: Carbs or EFI, HP is HP and when you USE it, you all get the same mileage! I feel better that somene else is happy and proud to say they got 5mpg in their car! I have managed to break into single digits on EVERY car I have owned, including a Geo Metro. I surmise Fun Per Mile is inversely porportional to Miles Per Gallon! [/quote'] I think you've hit the nail on the head, Tony. On the other hand, EFI certainly doesn't limit power in any meaningful way, and for the times where one MUST exercise restraint on the throttle (like on the street ), EFI seems to deliver up to 40% better fuel mileage. Quote Link to comment Share on other sites More sharing options...
Guest bastaad525 Posted December 4, 2003 Share Posted December 4, 2003 Okay so I'm taking all this to mean that 15mpg's average isn't too bad for my setup? Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.