hows ur mpg? for all you boosted guys out there

[no_doz]

im just curious as to how much mpg is going to be lost by going turbo, those of you out there running turbos, what kind of mpg do you get? what engine do you have, in what vehicle, with what trans, & what mods as well as how many psi are you running?

[Joseph Upson]

im just curious as to how much mpg is going to be lost by going turbo, those of you out there running turbos, what kind of mpg do you get? what engine do you have, in what vehicle, with what trans, & what mods as well as how many psi are you running?

You're leaning towards a lot of old school theory, if your turbo is properly matched to your engine, under normal driving conditions you shouldn't note much if any more loss in fuel economy than your catalytic converter and muffler would cause.

The earlier stock turbo cars have noticeable reduction in fuel economy in some circumstances because of the rich factory tune as a protective measure. The Turbo Grand Prix for instance if I recall correctly has some areas in the tune that drop down to the 10:1 AFR range so everytime you hit that area in the appropriate fuel table you're going to dump fuel for the sake of the engine under load.

Richer AFR helps keep the combustion temps from getting to high which in turn reduces the chance of engine damaging detonation and is why the factory does it.

Your engine efficiency will play a role also. Early turbocharging teaching taught us to drop the compression ratio when boosting no exception, when what they should have said is, run as high a compression as you can get away with for the boost level you intend to run. This works particularly well in fuel injection because you can optimize off boost fuel and spark vs. under boost fuel in spark, as opposed to running a low compression across the entire engine range.

My previous setup was a get this, 3100 with cast iron heads and Fiero intake with what most would have called a rock bottom compression ratio (7s maybe?), .60 compressor, .63 turbine T3 set to 7psi, modified front wheel drive cast iron exhaust manifolds and a 4 speed muncie transmission that put me at a 70 mph cruise rpm on the interstate of about 3000 rpm. Best mpg noted after new ignition parts and some high performance tires were installed ~28 mpg. On that day I stopped early to fuel up during a 225 mile trip because I thought my fuel gauge was broken since it appeared I was about to make that trip on a full 10 gal tank with fuel to spare.

If on the same motor I had a compression ratio of around 9.5:1, a more efficient exhaust system (better contours and transition), an overdrive gear that pulled cruise rpm down around 2200 rpm and a proper tune (ran the engine with a 1 Bar 2.8 program with wot timing and fuel changes, no datalogging, all guesswork) I could have easily broken into the 30 mpg range. I eventually beat the engine and turbo down to around 25 mpg before my swap.

There are fieros running the 3800 SC getting over 32 mpg and it's a known fact that the supercharger causes a continuous variable parasitic drag. If you use a wastegate that has dual vacuum ports, like the external wastegate, you can possibly drop your exhaust interference from the turbo with the proper controls to near 0 by allowing the vacuum pressure when off boost to open the wastegate to allow exhaust gasses a bypass route, this would only work for as long as the vacuum is greater than the spring pressure.

So an 8 psi spring such as what I'm running would allow the gate to remain open as long as vacuum is greater than 8 psi. when you go WOT it would close and not open again until the bottom compartment of the gate approaches 8 psi, this ofcourse is if you have the proper controls in place to cut off the signal to the top compartment of the gate, otherwise the forces will be equal and opposite and the valve will not open leading to overboost.

Here is a good read on the AFR subject and there's another in their outstanding Tech area offering guidance on Lean engine tuning to increase fuel economy; http://www.autospeed.com/cms/A_1595/article.html

[timg]

If you have a small turbo which makes boost at cruise with a FMU, you will probably lose fuel economy. With decent tuning and a reasonable turbo, your fuel economy shouldn't change. I hate to admit it, but I haven't driven my Monte Carlo in long enough to remember what the gas mileage was. My S2000 with 443 whp gets better gas mileage than it did from the factory. I can easily hit 30 mpg at 80 mph.

I hate to say it, but the L67 (3800 SC) cars are the fuel economy champs. They get ridiculous mileage out of enormous cars while still making tons of power. I think the 3.4 DOHC is a better engine for all out performance, but the L67 seems to be more reasonable for a mildly modded beater DD (better fuel economy, cheaper to repair, dime a dozen, etc).

Tim

[no_doz]

how does a car that wieghs more than a v8 camaro, has a pretty good sized engine (3., and ALWAYS has boost being pumped to it-get 30mpg? ive really been looking for ideas to get more power but not kill my gas mileage and ive always wanted to sc something, but i cant picture an sc 3.4 getting 30mpg-even in a car weighing 2500lbs total...

[LeftVentricle]

Gearing and torque, my friend. The automatics have a tall overdrive, like 0.70, and a tall final drive, like 2.97 or 2.73. And a stock 3800 S2 makes 235 ft lbs.

[no_doz]

does all that torque really mean that much for gas mileage? and if so how do these little 1.x 4 cylinders pull around heavier cars with good mileage (2.2 in a 3050lb honda car, 30 mpg if not brought into vtec) & etc...? and what if that kind of tq was in a car that weighed around 2400-2500lbs & awd-wouldnt the potential for mileage be a LOT better if the vehicle was driven mildly? also using a digital boost controller to allow for less boost in order to get better mpg-how well does this work on a supercharged engine?

[torq455]

the newer 4-bangers have more research and technology in them. VVT, more efficient heads/combustion chambers, intake design--variable as well, cam profiles, and now direct injection, work together to make a nice healthy flat torque curve. my 2.0 91 Sunbird had ~130 ft/lbs @ 3200 rpm. newer 2.0s easily have that at 1500 rpm, thus making them more efficient. in turn, more lowend torque allows lower rpm operation with an OD tranny. also have to mention: newer cars are more aerodynamic as well. i never would have believed a 2.0 engine (cobalt SS) turbo, not only would get 260 HP, let alone 260 TQ @ only 2000 rpm, but to get 22 mpg city and 30 mpg highway? no way!

[no_doz]

very true-why the other day i was researching the high tec japanese engines & came across things like the b16b making 180 hp at 1.6l! and a toyota 4a-ge with 5 valves per cylinder & 162 hp at 1.6l!

considering that stock 3400's made at the same time make 170-180, although the v6's make a lot more tq. what ive been thinking pretty hard about is how these high tech japanese engines are designed & how much power they create with so much less displacement because of the whole flow/efficiency combination, as well as things like compression & etc... & some of these make lots of power for their size. like the nissan 350z-3.5l v6 making 280-310 or so hp stock, almost the exact same bore & stroke characteristics-4 valves per cylinder-dohc, etc... if a 3.4 dohc had the head done (any reshaping needed on the heads & porting-valve job, etc...), gasket matching, headers, the works-if the internals were made light & the compression was raised to the same as the 350z-the only real thing stopping you is a very small amount of displacement & the intake/cam-& if a supercharger was used then 70% of that intake tract is gone & replaced with pure unadulterated boosty goodness-then some very serious power could be achieved. what im wondering is if a digital boost controller was used to keep the boost way down under normal driving conditions by just letting most of it go via the wastegate-could excellent gas mileage be achieved & lots of power when needed?

[Joseph Upson]

It's really not that complicated and technical. It's all about efficiency and gearing. Best scenario to date I'm aware of is a 9 sec quarter Turbo LSx Corvette, with 27 mpg hwy according to the announcer at the track and if I recall correctly it has an LS engine stroked to 427 cubes, I'll have to look for the video to confirm it.

If you build, cam and gear the engine for its best efficiency at hwy cruise speeds you will get exceptional mileage in everything you drive compared to the standard compromise settings for vehicles in its class. That's why VVT helps so much, instead of a best all around fixed cam, you have a system that gives you the best cam dynamics over a broad rpm range. Advanced for low end grunt, retarded for high end performance. The engine also has to have a proper power to weight range. A lot of the high mileage imports are light in weight.

If custom gears for your tranny were easy to come by and practical, you could run circles around the factory mileage achievments by optimizing the drivetrain to your requirements. From the factory they have to decide on a medium that includes everyone and dependability over all extremes. The EPA decided the 14.7:1 AFR ratio for emissions purposes. Higher ratios would yield better mileage otherwise. As high as 16:1 or more for some engines during cruise.

The 3800 is not exceptional at all, it happened to receive better gears for economy because it had the extra power needed to still provide a performance feel despite the economy gearing. Put the 2.8 and 3.1 on that same transmission and you will see a tremendous improvement. Recall that the early 2.8 and 3.1 rarely found itself attached to a 4 spd automatic and ran 1:1 with the 3 spd and still managed as high as 30 mpg in some cars, the 5 spd did not have economy ratios like those that could be found in the 4 spd auto.

Again, the car has to be built as best all around performer or it may not sell as well, so the weaker engines get shorter gears so they can keep up with the bigger engines in exhchange for less than best fuel economy. The aluminum head motors when placed in Fieros unless you screw it up usually mean an automatic 30 + mpg. Some are getting up to 35 with the 2.8L.

[bszopi]

This is still my favorite (although it is a 4-cyl import). 1.55L making 487hp and running low 11s in the quarter. Yet it gets 23-27mpg in the city and 33-36mpg on the highway.

That article is from several years ago and the engine has been redone since then. I don't know the current power its making, but it is one hell of a beast.

[Joseph Upson]

This is still my favorite (although it is a 4-cyl import). 1.55L making 487hp and running low 11s in the quarter. Yet it gets 23-27mpg in the city and 33-36mpg on the highway.

That article is from several years ago and the engine has been redone since then. I don't know the current power its making, but it is one hell of a beast.

27 psi, let's see, 14.7 psi makes that 1.55L more like 3.1 L and another 12.3 psi of boost totaling 27, makes the 3.1L about 6L worth of symbolic displacement by the amount of air moved. That's about like having a built small block 400 on tap in that little car.

Note the high compression ratio with high boost and he could probably get even more mileage out of it with a 3 series final drive instead of that 4, but that's not what it was built for.

[bszopi]

The current engine is just insane. You can read about it on their website, www.theoldone.com . Check out the engine on the engine dyno, and it's efficiency...



Yeah, I'd say it gets it done...

[RedZMonte]

I am currently getting the same as stock.. 24mph mixed (hwy/city) on 7psi. I am running pretty rich, if i get some more tuning time in i think i can get +2 or 3mpg and make a little more power.. Im currently seeing about 10:1 a/f mixture...

[Canyonero]

Torque and gearing, read here: http://www.carmagazine.co.uk/News/Se...ontent-block=1

[93grandprixse]

wow, id never run a car at anything lower than 12:1... thats rich as hell redzmonte...

i think its like 12.5:1 at which peak torque can be found usualy with a/f ratio...