I'm looking at fuel economy for a given throttle position and rpm range. What's better, 1000rpm WOT, or 3000 rpm, 1/4 throttle, with an engine twice as big.

Let's say you want to build a sleeper using a 1998 GMC RCSB, 416 LS bottom end, 4L70E, AFR 230cc heads, 72cc chambers, 25cc dished pistons. LSXRT intake. 86 octane to make a true challange. I'll use data from this article just to scale things.

http://www.superchevy.com/how-to/eng...-swap-408-ls1/

I'm assuming such a truck has a c.d. of .45, frontal area of 36 sqft. 235/75/15 tires and 3.73 gears, you'll have a 75 mph cruising rpm of 2279.

Using .00204 slug*ft^-3 for density, and after applying the drag force equation, we get cruising power of 40 hp. The engine needs to make 92 lb ft, or 40% throttle on 4.3 v6, or 20% throttle from the 416 if it makes 450lb at 2279. I'm assuming your % of throttle is flow percentage, not radian TPS.

Here's why I'm typing all of this near-nonsense. At 20% ve, the 416 turns into 83 cubic inches, and the 40%ve 262 turns into 104 cubes, thus consuming more fuel than the v8. 68.6 Cfm v6 and 54.7 cfm v8.

If you look at a 3.08 diff, 1882 rpm, the 4.3 goes to 133 cubes, and the 416 goes to 112 cubes if it can churn out 430 lbft at 1882. V8 looks better.
The v6 may want 72.42 CFM and the v8 may want 61 cfm.

The only variable here is if it sucks power to keep the intake manifold under vacuum.

 

Don't forget tire rolling resistance. It can be a more significant percentage of total cruising load than most people realize (especially when looking at the high speed RR models).

I don't have time at the moment to analytically think about the engine comparison, but you definitely do lose a fair amount of power to pumping work with the throttle closed. For pure efficiency (and all else being the same), you'd want to run the smallest engine possible as close to WOT as possible.

Also, I'd try to verify the TPS % is in relation to flow instead of angle. Being that most TPS sensors are rotary potentiometers, I always just assumed it was angle based. I could be wrong, I've never had a need to verify my assumption.

 

You know how you can hold down the gas a little bit and your engine will hold a certain RPM. That region is what kills our gas mileage. I'd just like to make it easier to help us engine swappers find that region before we make a $14K decision.
As long as we have a reasonable amount of load, we're probably in half-decent shape.

I've had motorcycles that were under zero load at 40mph. You could pull in the clutch and the engine wouldn't rev up. The headers would glow Bright red (black light purple in pictures) and the gas mileage was curiously bad for a .45L single cyl.

If anybody has some TPS values for a given speed, in a given car, it would help us make a chart, much like David Vizard has done for compression ratios and Cylinder head port volumes. I've got Maple 15 and Matlab. I just need a bit of data.

 

I think using % TPS is incorrect in this situation.

I've gone from stock LS1/5.3 stage 2.5 heads/LS6, ported TB to a larger NW 92mm TB LS3 heads/intake and found while the TPS of 2-3% seems to be consistent for both setups while logging 70mph cruising, the flow numbers are different.

You can manipulate VE indirectly a lot with simple things like injector timing (EOIT), spark dwell(coils changed to truck) to more complicated things to guage like having an oil coated exhaust piping which has cleaned itself out over time.

If you are using HP tuners, try formulating a MPG calculator to test true "part-throttle efficiency. A real time calculation of how much fuel you are using will really show efficiency results of different combinations. All the necessary PID's are on hand to make this possible in HP Tuners.

 

There is something to this, but as mentioned before % TPS doesn't work so well when comparing different engine displacements in similar vehicles.

The real factor in part throttle fuel economy is part throttle BSFC for the particular engines based on it's particular load.

If you could gather gm/s for both engines, then you can figure which vehicle is consuming less fuel. What sucks here is gm/s varies greatly with TPS and RPM's, but isn't linear either. For example I can be at 6000rpms pulling in 300gm/s at 100% TPS, or 10gm/s at 6000rpms and 0% TPS...

EGR plays a factor here too. If it's open and when it opens based on TPS, RPM, and MAP all play factors. The EGR that holds longer will have less gm/s in cruise conditions for it's displacement.

% TPS isn't a good gauge to assume TQ at RPM because % TPS is not directly linear with % VE. Although % TPS directly effects MAP which leads to VE at RPM, load also directly effects MAP at RPM too. That's why the VE charts are based on RPM's and MAP and not TPS%.

Basically, if it takes 40whp to maintain 75mph in a Silverado then if the Silverado is maintaining 75mph it is generating 40whp.

The question now is, which one has less losses to the wheels in that scenario. Assuming both are 2WD with the same gears and going through a 4L60e then they will both have the same drivetrain losses.

Now it comes down to which engine is more efficient at producing only 40whp with said load. The V8 with more internal friction, but at a lower RPM or a V6 with more RPM, but less internal friction....

At 2279rpms either engine needs to generate 92ft/lbs of torque, regardless of displacement, to produce 40 flywheel horsepower. At a steady state rpms drivetrain loss is negligible at best. So we'll roll with this.

Lets assume the Silverado V8 is the 5.3L in this scenario (I know it's not the sleeper you are using, but it's a known configuration), it would need a Brake Mean Effective Pressure of only 43 PSI to accomplish this; approximately 27% of it's peak BMEP at peak TQ, or peak VE.

In Contrast the V6 4.3L would need 53 PSI BMEP to do the same, approximately 33% of it's peak BMEP at peak TQ, or peak VE.

So this shows that the V6 needs more air per CID than the V8 to produce the same torque at the same RPMs. But it's also smaller so now we need to break it down to how much difference, if any, there is in actual air flow.

Unfortunately this is impossible with the information we have. But, knowing that it takes X amount of fuel to produce Y amount of horsepower tells us that the gm/s will be the same IF the engines are both equally efficient at producing that exact horsepower at that exact speed under that exact load. This is further complicated by the fact that the V8's and V-6 vehicles are rarely geared the same.

But, if we look back at the fuel ratings of a 2008 Silverado SCRB 2WD 4.3 and 5.3 they are rated exactly the same. But, the question now is what gears were in the rears when GM did the EPA ratings?

In short, it really depends on how efficient your 416cid is at making the torque you need to maintain the speed you want given the load it'll have. LOL.

If you cam it for low end torque and gear it high (numerically lower) you can get surprising gas mileage. And it is absolutely possible to get better fuel economy with a more powerful engine in the same vehicle; 5.3L Silverado vs 4.8 Silverado, Solstice/Sky 2.4L vs 2.0T, etc....

Look at the highway fuel economy of a C6 M6(26mpg rated) and a RX-8 M6(22mpg rated) and be amazed....

That's right, the bigger, heavier, faster car with a more powerful 6.2L V8 get's almost 20% better fuel economy than the smaller, lighter, slower car with a tiny 1.3L rotary....

 

I'm a little late to this party but the answer is well known. Low rpm and wide throttle opening (low intake vacuum) is the answer to better cruise fuel economy. This is why we are seeing 8 and 9 speed transmissions. This works up to the point power enrichment begins. Tune so you stay lean during most cruise conditions. A hot cam limits how low the rpm can be and a carburetor is incapable of staying lean at low vacuum.

 

Yep tweak PE so it doesn't trigger till say 85kpa
You can lean out O2 trigger levels on some ECU's to run in the 15.x range no problem and not actually notice any different.
Also there is custom CAX files for EFILive to enable the Holden lean cruise mode in standard GM applications or run open loop for a similar effect.
As stated above you want open throttle for low pumping loss along the lines of diesels.
Lean it out to help facilitate this effect as well as not actually needing as much fuel.
Lean best power is around 15.4 AFR at this point power is not far off stoich.

Then of course good intake and exhaust flow will also help.

 

This is interesting. I'm shopping for a cam for my current project. Its a 99 s10 with a lq4/t56 and a 8.8 ford with 373 gears. it has lt headers and 3" dual exhaust with an x pipe. I'm currently trying to decide on a cam and possibly a head swap. I want it to be fun without killing my gas mileage. I currently drive another s10 with a 4.3/auto that gets about 20 mpg mixed. I'd like to get that if possible while making as much drivable power as I can .How large of a cam can I use before it starts to negatively effect mileage? Sorry if this doesn't belong here, I don't see many posts where people care about gas mileage.
Thanks for any help you can provide.
Kory