Mods for Gasser Towing Economy?

I know, I know - slow down. But let's ignore that one for a minute. 2004 Suburban 2500, 8.1, 4L85E. Still on narrow 245 tires, 3.73 gears. Stock everything except roller rockers, Raylar cam, and tune, because the stock cam and lifters failed. It gets 7.5-8 mpg towing my mini toy hauler trailer at 70-75 mph. I do let her lug back on the hills and generally am able to keep it locked up in 4th turning 2200-2300 rpm.

It's a great truck, but I tow a lot and even a small improvement makes a big difference. Specifically, would an intake and exhaust directionally help, or would they just give me more power with which to burn fuel and lug back less in the mountains? There's a lot of opportunity for power it seems, as I have ~10+ kpa drop vs. ambient pressure even at 3500 rpm and WOT on the intake side. I don't have any good WOT logs all the way out to 4800 revs yet, but that part of the range isn't so important to me. The exhaust is a similar story. I don't have any numbers, but it's a single 2.5" for an 8.1 liter engine, and it's actually even smaller than that inside the muffler. Cats are staying regardless., and keeping the stock manifolds. 

Online, everyone just looks at the dyno numbers, or has turbodiesel trucks where the rules are a little different. Intake is good for 25 horsepower, exhaust is good for closer to 40 from the results I've seen. Both have gains all the way down to my towing engine speed. Anyone have experience with reducing pumping losses to improve economy? Or is it just going to be a case of more air, so it can use more fuel? Mostly a thought exercise right now, but putting 20k+ miles a year on this thing, and being a long term keeper vehicle, it might be worth some effort. Also, like most of us, I can't leave well enough alone.

The beast in question:

I know that we often see a small (1-2 mpg) increase in cruising efficiency when turbocharging Miatas, and I've always attributed that to basically eliminating any pumping losses in the intake. I don't know if that's legit or not, but you can look at the power increase as an efficiency increase. 

Keep noise as a factor, though. I had an intake on my diesel and the nice rumble it added wasn't as much fun on the highway for hours. 

Basically, whatever lets you open the throttle less. Remember that restrictions stack up, so less restriction to overcome allows less throttle opening, less fuel/air, makes for more mpg. Although, it's complex. You do it wrong and kill velocity or resonance driven efficiencies and loose out. (The engine isn't steady state flow, it's a mess of pulses that interact)

Rolling resistance, tire type, and big attention to tire pressures. Lube bearings well, adjust brakes. 

Aero. Little things add up. 

Keith Tanner said:

I know that we often see a small (1-2 mpg) increase in cruising efficiency when turbocharging Miatas, and I've always attributed that to basically eliminating any pumping losses in the intake. I don't know if that's legit or not, but you can look at the power increase as an efficiency increase. 

Keep noise as a factor, though. I had an intake on my diesel and the nice rumble it added wasn't as much fun on the highway for hours. 

One of the big things forced induction does is increases cylinder pressures which makes it more thermally efficient... that is to say, more of the BTUs stored as chemical bonds in the HC are released as force on the pistons.  It's one of the reasons that increasing compression ratio increases power - more of the BTUs are available as force.  That, alone, CAN increase mpg, but not always.

You could re-curve your fuel and spark maps.  Lean out cruise lambda and bump up spark timing, but you're likely very limited in how much you can do that before switching to 93 octane which would negate any benefits.  Also, not very helpful if you add timing only to have it taken away by the knock sensors.

An intake and exhaust are a good idea.  Yes, you'll be moving more air which requires more fuel, but you'll need less foot to maintain the same acceleration.  The bottom line is that topping the hill at 70 requires X amount of energy.  Your foot might have to move less to provide the same amount of air and fuel necessary to climb the hill if you reduce those airflow restrictions.

Honestly, though... you have 8.1L of hungry big block to feed.  Take a hard look at what the mods cost vs how much fuel it would save and how long will it take to recoup the costs.  I think you're in "just embrace it" territory.

Have you compared using tow/haul and not?  That camper doesn't look like it weighs much, and you have an Allison 1000 that isn't being taxed by that load.  Tow/haul uses more fuel.

Edit... you said it has a 4L85E?  I thought all the 8.1s got the Allison.  Still... try it with tow/haul off.  I wouldn't think a 4L85 would have trouble with that load.

One other thing... in my world, 245mm rubber is still pretty fat.  If you don't need foul weather traction, maybe consider some 215/85-16 H/Ts?  I lost 1-2 mpg on my Express van when I went from 235mm E-range H/T rubber to 265mm A/S on 20" forged wheels.

Multiquote would sure be useful right now...

Keith - yep, noise is definitely something I'm paying attention to. In this case, the stock airbox allegedly isn't much restriction, it's actually the manifold itself plus the 78mm throttle body. I am going to take some vacuum measurements at some different points in the intake to verify, but my hope is with the stock airbox and resonator, noise would still be minimal. Same with exhaust, I've got a couple giant RV Magnaflows on my K1500 that are pretty quiet and would do something similar with this. Boost would be fun but I'm not going down that road with this thing. Apparently these things make some pretty impressive torque with a few psi.

Apexcarver - all good calls. Aero is definitely not great on this thing, but I'm not sure there's a ton of low hanging fruit. The frontal area is massive. I probably should check the alignment, even though it doesn't really have any bad habits or odd tire wear.

Curtis - I've already played with the timing quite a bit, it's about as aggressive as I can go without getting into KR on 87 octane. The stock map was actually already pretty close to the edge, which kind of surprised me. the internet has a lot of theories, one of which is that the exhaust backpressure causes some challenges. Several people have reported being able to comfortably get a few more degrees at high load after an exhaust. Stock they are barely more than 20 degrees all in, I'm around 25 right now primarily because stock doesn't use PE, and adding fuel helped a ton. That was one of those things were it made a huge difference in power...but now you can get it down to 5-6 mpg on the readout before it kicks down from 4th. Right now it's optimized for timing vs. knock reduction, but I guess I could back off on open loop fueling and pull the timing back out.

I could lean out cruise if I wasn't running closed loop...but with going through the mountains and such, I do want to keep closed loop enabled. You're right, I'm on the edge of "just embrace it" especially since this seems to be right in the ballpark of what other people in much newer trucks get towing a trailer like this.

I generally only use tow/haul in the mountains where it needs 3rd gear. It locks the converter very early in 3rd in tow haul, and generally doesn't unless you put the shifter in 3rd and lift off the throttle in normal mode. That lockup is worth at least an mpg on the instantaneous readout.

The trailer is about 4500 with all the stuff in it. Not that heavy, but it's 10.5 feet tall and the body is maximum width, so it's a huge frontal area.

pickups got the Allison, the suburbans and avalanches got the 4L85E.

I just bought new tires, so I'm on 245s for a while...and my usual routes take me on I70 through Denver, so having some capability in snow is pretty important.

One other aside...when I reflash the tune, the fuel economy is GREAT for maybe a tank of gas, and then gradually drops down to normal. Like, 2 mpg better with no notable change in drivability, loaded or unloaded. I need to get some better logs of fuel trims or maybe try leaving LTFT turned off and see what happens, but it doesn't seem to change dramatically and I really don't have a good explanation for this. Ignition table is staying 100% on the high octane map, or very close to it.

In reply to Curtis73 (Forum Supporter) :

My reference to increased efficiency at cruise means the engine is still running in vacuum. The turbo is spinning just enough to basically remove any restriction on the intake. 

Keith Tanner said:

In reply to Curtis73 (Forum Supporter) :

My reference to increased efficiency at cruise means the engine is still running in vacuum. The turbo is spinning just enough to basically remove any restriction on the intake. 

Agreed, but how does that play with the extra pressure on the exhaust side?  I wonder which is the bigger player - the reduced intake restriction, the higher cylinder pressures, or the increased exhaust restriction.

In reply to gearheadE30 :

Yeah.  Denver.  I remember crossing Vail pass in June and it was 31 degrees.  The day before I had been in Baker CA where it was 108.

This would be dancing around KS activation again, but what's your thermostat temp?  Can you go higher?  Maybe thinner oil if it can maintain proper pressure?

Keith Tanner said:

I know that we often see a small (1-2 mpg) increase in cruising efficiency when turbocharging Miatas, and I've always attributed that to basically eliminating any pumping losses in the intake. I don't know if that's legit or not, but you can look at the power increase as an efficiency increase. 

I'm curious if those numbers with stock ECUs or with aftermarket ones?

AIUI, the biggest pumping losses are through the mostly-closed throttle plate, and the biggest reason why small engines are usually more efficient than large ones is that with less displacement you have to open the throttle wider to make the same amount of cruising power.  Same reason why EGR improves fuel economy.

So, something i read at some point put the idea in my head that the highest manifold vacuum reading creates the best mpg. A lot of times this equates to throttle position more than rpm, at least with my tbi truks. I wonder if adding a vacuum gauge and trying to peg it at all times will improve it for you?

Oil is the GM-recommended 5W30. Oil pressure is 40-45 psi at highway revs, depending on temperature. I'm not too comfortable going thinner really.

thermostat is 180F. Stock is 203F ish. Moving to the 180 stat is not something I would normally do, but looking at datalogs led me to try it because:

• Stock mapping starts aggressively pulling timing over 210ish F coolant temp, which was easy to hit at high load. Normal towing on a warm day with AC on, it was pulling ~2 degrees
• I was getting 2-3 degrees of KR with stock tuning consistently with the stock stat, and this seemed like a cheap way to try to mitigate that. This went away with the 180F stat. Between this and the temperature retard, the engine now stays at full high octane table timing unless the intake air temps start to get crazy heat soaking in traffic.
• Surprise benefit 1: the transmission stays 10-20 degrees cooler, depending on how much you trust the gauge cluster. I hadn't expected this, but with the water to oil cooler in the radiator, it makes sense.
• Surprise benefit 2: the mechanical cooling fan almost never comes on unless it's pretty warm out and you're really working it. The fan on this truck pulls a ton of power, and with the stock thermostat, it was cycling regularly.

The vacuum gauge is an interesting idea, I think a lot of Uhaul trucks have exactly that. I guess I'm not sure if that would help though, if I need some given amount of power to maintain speed, reducing throttle to increase vacuum would cause me to slow down. I could run in 3rd gear and have more vacuum, but a higher rpm. Based on the instant economy readout, it's always more efficient to run in the highest gear I can without triggering PE and going into open loop. So right now the trans tables are set up to fairly aggressively hold 4th locked up, only kicking down just before I would trigger open loop as long as the revs are high enough for the engine to be happy with that kind of load.

the vacuum gauge, or just using the tuning software, would be a great way to compare modifications or tune changes though.

I did also enable DFCO, which is disabled from the factory for some reason. In the mountains it helps a lot, not so much in the flat stuff where you don't really coast.

I just woke up and read this as "Mods for Grass Mower Economy" and thought "well if someone was trying to do this it would be these guys".  Carry on, while I pour my coffee.

Is there emissions testing where you live?

I love the thread title. Makes me think of this:

In reply to eastpark :

That's what I thought too!

In reply to gearheadE30 :

You asked specifically about fuel economy improvements while towing (and most responses are about overall fuel economy). If towing is the issue, I'd look first at that trailer. 
 

It's about the worst aero design I can think of.  It's tall, has enormous frontal area, roof obstructions, and worse than the hole it is trying to punch through the front is the hole it is trying to fill behind. That squared hard back does nothing to smooth out the air coming off the back.  I'd look at some of the aero things tractor trailers do. 
 

For the frontal area, I'd consider a deflector on the Burb similar to the AeroShield wind deflector.  You could build something without too much difficulty  

Next the back.  Maybe something like those aero flaps on the back of tractor trailers?

That rig is really high off the ground. Maybe make something similar to tractor trailer skirts?

One more thing... you have very square corners between the roof and the side walls.  I know UHauls have rounded corners and claim they improve fuel economy by cleaning up the air as it spills off the roof and rejoins with the side.  Perhaps deflectors on the roof corners?

These are small, but I think they'd make a big difference when towing. You've got a great big displacement motor trying to drag a parachute down the road.  I'll bet you could build some aero improvements for the trailer for well less than half the cost of a turbo and see significant improvements when towing. 

Sorry to disappoint, though a big block suburban gasser would definitely be interesting to see go down the track...

No emissions testing where I live.

Did some quick logging today and saw a pretty consistent 8-9 kPa intake restriction at WOT across most of the rev range. Before the cam, it was about 6 kPa for the one WOT run I have a log for that has both MAP and the initial baro reading. I work for Cummins, our requirement is 4ish kPa with a clean filter, 6ish max dirty filter restriction. Different beast, but interesting to compare to. 

Peak MAF reading also jumped a fairly dramatic nearly 15% with the cam, which doesn't matter much for my original question, just an interesting data point.

I wanted to verify my MAF and VE tuning, and they are good - give or take 2% on fuel trims in the parts of the map I hit while running errands. No KR except on tip in, which I think is actually burst knock going active. Which is what it's supposed to do, so all good there.

I accidentally happened upon some information on end of injection timing and how it impacts fuel trims with cams that have some overlap. Mine doesn't have much, but it has enough that it might make a difference. More likely to impact the rich smell at idle than the actual towing economy, though.

Putting a sprint car wing on the roof of the truck is an interesting idea - that's definitely something I could rig up temporarily out of wood or something just to see what impact it has. Same deal with skirts. Staring at the trailer for a bit today, I didn't see a great way to round corners off. I think I would get a new trailer before diving into that kind of tearup.

The rear is a little tough because it's a ramp door for the bikes. Deflectors are pretty likely to get damaged...I wonder if those stick on vortex generators actually do anything?

gearheadE30 said:

Oil is the GM-recommended 5W30. Oil pressure is 40-45 psi at highway revs, depending on temperature. I'm not too comfortable going thinner really.

I wouldn't either.  About your only choice would be 5w20 and that might get you 0.04 mpg.

thermostat is 180F. Stock is 203F ish. Moving to the 180 stat is not something I would normally do, but looking at datalogs led me to try it because:

• Stock mapping starts aggressively pulling timing over 210ish F coolant temp, which was easy to hit at high load. Normal towing on a warm day with AC on, it was pulling ~2 degrees
• I was getting 2-3 degrees of KR with stock tuning consistently with the stock stat, and this seemed like a cheap way to try to mitigate that. This went away with the 180F stat. Between this and the temperature retard, the engine now stays at full high octane table timing unless the intake air temps start to get crazy heat soaking in traffic.
• Surprise benefit 1: the transmission stays 10-20 degrees cooler, depending on how much you trust the gauge cluster. I hadn't expected this, but with the water to oil cooler in the radiator, it makes sense.
• Surprise benefit 2: the mechanical cooling fan almost never comes on unless it's pretty warm out and you're really working it. The fan on this truck pulls a ton of power, and with the stock thermostat, it was cycling regularly.

Just a thought.  Reducing stat temps lets you get away with more timing, but I highly doubt it's saving you fuel.  For any small addition of MPG you get with adding timing, it maybe more than negated by the additional fuel that is being sent for the cooler temps.

Example (which may not translate to your specific truck).  My 96 LT1 had a few different tunes.  I consistently got 20 mpg on a stock tune (Impala SS).  I did a tune with fuel and spark curve changes and picked up 15-ish HP and MPG stayed around 19-20.  I did the common 160-degree stat swap and appropriate change to fan temps and picked up a few more ponies, but I tanked mpg to 16. Sidenote:  although 160 degrees sounds extreme, it really isn't.  LT1 reverse cooling means that the stat opens when the radiator side reaches 160 as opposed to normal flow which opens when the block side reaches temp.

I think you might be surprised at how much more mpg you might find if you raise coolant temp.  Sure, you'll hit KS retard and lose 10hp, but you have plenty of ponies to spare.

If you're really looking at saving fuel, the rest might have to go.  I feel like you might be spending too much focus on hyper tuning the engine that you've lost MPG in the mix.  The stock tune was a dance between CAFE and EPA.  Since you aren't worried about EPA, focus on the CAFE and sacrifice a few horses.  If the engine runs hotter, who cares?  If the trans is a few degrees hotter, no worries.  Add a trans cooler.

The temperatures you see are insignificant.  We all panic when an engine reaches 235 degrees because that means boil-over and warped heads or severe damage.  The reason isn't because 235 is a magic number, it's just that that is about when the coolant overcomes the pressure and starts to boil.  Once it boils, you get superheated parts of the engine while the rest stays 235 degrees.  Aluminum melts at 1200 degrees.  Combustion events happen at 2700 or higher.  There are two critical parts to engine temperature:  1) that the system doesn't boil allowing a big temperature delta between areas, and 2) the radiator can shed more heat than the engine gives it.  The thermostat temperature doesn't matter.  As long as the stat temp is below the boiling point of the cap and the ratio of coolant/water, the engine doesn't care if it's 200 degrees or 300 degrees.  This is why non-aqueous coolants were all the rage in the 90s.  I had a Caddy 500 in a C30 dually that frequently saw 300 degrees with non-aqueous coolant and no radiator cap and I didn't care.  Sure, I was down 20 hp because the carburetor didn't' compensate for anything, and I had to have a pretty big trans cooler, but it was epiphoral.  Of course, we know that non-aqueous coolants have a litany of downsides, but the important part is to no focus on the actual farenheit number.  You should focus on heat in minus heat out, which has almost nothing to do with temperature.  The only reason that 235f is a scary number is because they industry standard is 50/50 glycol/water and 18 psi caps prevent exploding hoses and radiators.  

The vacuum gauge is an interesting idea, I think a lot of Uhaul trucks have exactly that. I guess I'm not sure if that would help though, if I need some given amount of power to maintain speed, reducing throttle to increase vacuum would cause me to slow down.

That would be true except that throttle position isn't the only thing that controls vacuum.  Advance the cam and you get more vacuum at the same throttle position.  Same goes for advancing ignition timing.  We're not saying that you need to reduce throttle, we're saying that if you set up a situation in which vacuum increases, you'll need less throttle for the same torque output.

GHe30 what exactly are you looking for???? Are you willing to spend$$$$ on 'Burb' mods(CAI, long tube headers. green CATS,3"cat back or NO?!?!?!!!).....iffen not then me thinks your looking at the 'TOY' box.....that being the case SV ReX  is speaking to you.....late

I'm looking to understand if there's a practical way to improve mileage, and specifically if intake and exhaust restriction is something beneficial just for power or if it would have any meaningful impact on economy, especially with an application like this that seems very restrictive.

I'm fine with spending some money. It's a keeper truck, and I'll probably put at least another 100k miles on it over the next 5 years at the rate I'm going. I know it's unnecessary, but I enjoy playing with it so that's part of the draw as well.

SV reX is probably right that my biggest gains for the moneyu will come from the trailer, even though it's a lot less exciting to modify.

@Curtis - all of the warmup modifiers, from the factory, are out by 155-165F depending on the table. That was one of the first things I checked, and logging confirms it never falls back into warmup. If I'm not in warmup, is there still benefit to running higher thermostat temperatures? I had always heard there was for carbureted/TBI stuff, but once you get into dry manifolds and port injection, that the evaporation impact didn't matter so much, but that's all secondhand "knowledge" or worse.

A t stat swap is cheap and easy on this, maybe I should just try it.

Interesting comments on system temperature - I race dirt bikes (hence the toy hauler) and tons of people run the Evans waterless stuff for hard enduros. It's always made me nervous but you're right, even in that environment it's not like there's a rash of engine failures from it. That's wild running an old Caddy 500 that hot....but you're right, I'm just used to panicking over 225ish

You are right, I have lost some mpg, primarily because I think the cam isn't quite as great was advertised for ~2200 rpm towing, and because enabling PE lets me gobble up fuel a lot faster on the throttle. I wouldn't think the other stuff I've done would hurt (retuning for the cam), but it's possible.

I'll have to see if there's enough resolution in the MAP/MAF sensors to see vacuum changes. I can add a gauge, but it would be nice to also have that in my datalogs so I can compare that way or use the bidirectional tune feature to sweep timing at a fixed speed.

As I say that, I realize I probably am too far down the rabbit hole for limited potential gain...lots of time for maybe half an mpg.

In reply to gearheadE30 :

When I mentioned the corners, I was picturing playing with a deflector kinda like this to get the air to dump over the edge better. 
 

Actually rounding the corners would be impossible, but maybe some gains in a deflector 

A brick towing a bigger brick with an 8 liter gas engine at 70-75.  Any aero or friction or fuel mods will not be cost effective  for a long time.  Slow to 65-70 and watch how much more mileage you get per tank.

In reply to porschenut :

Good point