GMT800 Suburban Towing issues: Gearing or power?

ExplorerTom

Explorer
Octane requirements also decrease with increasing altitude. Ever wonder why the gas pumps at sea level have 87 as the minimum octane yet in Denver and above the lowest is 85? And likewise at sea level the highest octane is 92 or 93 yet in Denver it's 91? It's not a conspiracy against those that live at higher elevation, your engine just doesn't have the same cylinder pressures as they do at lower elevations to be able to properly combust the higher octane fuels.

And I'll guarantee that any owner's manual will say that it is designed for 87 octane (except those designed specifically for higher octane) because the high altitude markets are such a small overall percentage.

newer engines were made to work well on a certain amount of octane such as bmw Cadillac Porsche Ferrari etc all of which have to have premium in order to not knock,
Do you even know what specifically about these motors is causing the need for premium gas?

Going from 87 octane to 85 then back to 87 I felt the difference without even thinking about it. If you believe in wiki then Read the last sentence of the wiki page pretty much says it all
And where were you when you switched octane each time? I run 85 octane most of the time here in Denver. On occasion I've run 87 when I've suspected an issue with the engine. Zero change in performance. However, when I go down to a lower elevation, put some 87 octane in it, my truck feels like a racecar in the thick air.
 

Martinjmpr

Wiffleball Batter
Octane rating is a measure of how much compression the air/fuel mix can handle without detonating. It is not a measure of the energy content of the fuel.

Engine compression determines what octane ought to be used.

Ethanol has a higher octane than pure gasoline and is used to increase the octane of gasoline. Ethanol contains less energy than gasoline so more ethanol means less energy.

https://en.wikipedia.org/wiki/Octane_rating
Right, I've got that and I've never been one to believe the "more octane = more better" myth (probably the first time I noticed this was when I filled the tank of my motorcycle with 92 octane and got both horrible performance AND reduced MPG! Instead of 150+ miles on a tank I hit reserve at 95 miles!)


Newer flex fuel trucks will adjust timing with higher octane fuels. I can feel the difference between 87 and 91 octane with 10 000# behind me. I run higher octane for towing in hot conditions.
^^^^^
This is my understanding and why I'd like to at least try a tank of hi-octane just to see if it makes a difference. The 5.3 is a Flex-fuel motor.

I think probably the worst-case scenario is that I get no benefit and it costs a little more. I doubt that high octane would damage the engine in any way.
 
Martinjmpr, you are correct. A higher octane rating than what you need will only lighten your wallet. No engine damage will result.
 

jonathon

New member
On the flip side lower octane than required can cause pinging, especially under load. Newer vehicles have knock sensors that detect this long before we can hear it. If it detects a knock the computer retards spark timing which in turn robs power.

Personally I will never run any fuel with a lower octane rating then specified in the vehicles owners manual regardless of elevation.
 

DailyExpedition

Active member
Octane requirements also decrease with increasing altitude. Ever wonder why the gas pumps at sea level have 87 as the minimum octane yet in Denver and above the lowest is 85? And likewise at sea level the highest octane is 92 or 93 yet in Denver it's 91? It's not a conspiracy against those that live at higher elevation, your engine just doesn't have the same cylinder pressures as they do at lower elevations to be able to properly combust the higher octane fuels.

And I'll guarantee that any owner's manual will say that it is designed for 87 octane (except those designed specifically for higher octane) because the high altitude markets are such a small overall percentage.

newer engines were made to work well on a certain amount of octane such as bmw Cadillac Porsche Ferrari etc all of which have to have premium in order to not knock,
Do you even know what specifically about these motors is causing the need for premium gas?

Going from 87 octane to 85 then back to 87 I felt the difference without even thinking about it. If you believe in wiki then Read the last sentence of the wiki page pretty much says it all
And where were you when you switched octane each time? I run 85 octane most of the time here in Denver. On occasion I've run 87 when I've suspected an issue with the engine. Zero change in performance. However, when I go down to a lower elevation, put some 87 octane in it, my truck feels like a racecar in the thick air.
I first switched when I was going through Santa Fe New Mexico, found some lower priced gas that was 85, filled it and my Gerry can up and almost immediately noticed the difference, filled up again I think in Durango, put 87, felt an improvement, so I used atleast 87 for the rest of my trip.


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rayra

Expedition Leader
what matters is what octane your vehicle is tuned for or self-calibrated for. Besides the altitude. Older carburated vehicles were especially susceptible to these changes. 11k feet is no small difference. Hell they used to truck aviation piston engines up Pike's Peak to get their high altitude performance data / tweaks set.

With a heavier trailer, people, gear and other contents, martin's likely at or above the 12k# max GVWR rating of the 1500 Sub with tow package. And that's without any altitude considerations.

Still thinking gearing is the best option given his set of circumstances, to help the vehicle stay in a better power band. And it's one of a handful of reasons I've kept factory-sized tires too, not wanting to make the effective gear ratio any worse. Even with my far lower mountain passes.
 

DailyExpedition

Active member
what matters is what octane your vehicle is tuned for or self-calibrated for. Besides the altitude. Older carburated vehicles were especially susceptible to these changes. 11k feet is no small difference. Hell they used to truck aviation piston engines up Pike's Peak to get their high altitude performance data / tweaks set.

With a heavier trailer, people, gear and other contents, martin's likely at or above the 12k# max GVWR rating of the 1500 Sub with tow package. And that's without any altitude considerations.

Still thinking gearing is the best option given his set of circumstances, to help the vehicle stay in a better power band. And it's one of a handful of reasons I've kept factory-sized tires too, not wanting to make the effective gear ratio any worse. Even with my far lower mountain passes.
Agreed, Gears would be a better option, better than stock exhaust could help too since it could unleash another 30-40hp (supposedly) the biggest thing though too is he doesn’t want to dump a lot of money into it, gears will be expensive, exhaust will be too, starting out, a tune may help a little bit and be the least expensive.


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Buddha.

Lurker
what matters is what octane your vehicle is tuned for or self-calibrated for. Besides the altitude. Older carburated vehicles were especially susceptible to these changes. 11k feet is no small difference. Hell they used to truck aviation piston engines up Pike's Peak to get their high altitude performance data / tweaks set.

With a heavier trailer, people, gear and other contents, martin's likely at or above the 12k# max GVWR rating of the 1500 Sub with tow package. And that's without any altitude considerations.

Still thinking gearing is the best option given his set of circumstances, to help the vehicle stay in a better power band. And it's one of a handful of reasons I've kept factory-sized tires too, not wanting to make the effective gear ratio any worse. Even with my far lower mountain passes.

1500 suburban GVWR is not 12k, that's dually 3500 territory.
I suspect you meant GCWR
 

Martinjmpr

Wiffleball Batter
With a heavier trailer, people, gear and other contents, martin's likely at or above the 12k# max GVWR rating of the 1500 Sub with tow package. And that's without any altitude considerations.
Oh, I'm not sure I agree with that! 12k? Let's add it up: Curb weight of the 'burb is 5500 per Edmunds. Figure 2 adults + dog is 500lbs more. 2 kayaks at 60lbs each. I find it hard to believe that all our camping gear is more than 600lbs but let's used that for the sake of argument. 3500 is pretty much the fully loaded weight of the trailer (we've never put water in it so the water tank is always empty, ditto for the black and gray tanks.) "Shipping" weight for this model is ~2900 so I think 3500 is a generous number considering that we don't carry a lot of food in the fridge and we never have food in the cupboards (to keep critters out.)

So 6800 lbs for the truck - hell let's round up and say 7k, plus 3500lbs of trailer is "only" 10,500. Which is still more than I've ever hauled (except when I was in the Army driving a 5 ton expandable van pulling a 60kw generator - And doing this through the narrow cobblestone streets of little German villages!)

Still thinking gearing is the best option given his set of circumstances, to help the vehicle stay in a better power band. And it's one of a handful of reasons I've kept factory-sized tires too, not wanting to make the effective gear ratio any worse. Even with my far lower mountain passes.
I am thinking that may be the way to go and while it would be expensive it would still be less than 3 car payments on a new vehicle, so there's that, too. ;)

Next question, what gear? I only want to do this once so I want to go low enough. On the flip side I don't want to hurt the daily-driver aspect of the truck by going too low. Is 4.10/4.11 a good compromise?
 

rayra

Expedition Leader
Yes, total combined weight, right on the tag on over the trailer plug on these vehicles.


martin, don't forget the fuel and fluids weights, they add up fast. ~240# for the 32gal Sub tank. ~50 for each 20L of water. PRopane tank and liq. propane. That combined weight is more about braking performance IIRC. But all that weight is still an issue, especially when talking about a 5.3L

we're pushing 7000 with my loaded storage drawers / platform, people and dogs and full tank and 2x20L just for a day trip in the mountains or a telescope trip.
I keep eyeballing my neighbor's camper trailer, one of these days we're going to borrow it for a haul up the mountains and see how it goes. Same model as this



Fleetwood Evolution Series E2, online refs say 'base' weight of 2600#, tongue of #400. That huge ATV platform in front would send it way over our rated tongue limit of 500#, if anything much is packed in it. Hope the water tanks are at or behind the trailer axle.
 
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rcab87

New member
An engine's compression ratio is what dictates the minimum octane. In higher compression engines, more heat is created in the cylinder and the fuel mixture can ignite before the spark plug fires, known as pre-ignition or knocking. In high performance engines, more timing advance is used to make more horsepower, which can also produce higher cylinder temperatures and pressures and cause knocking. The higher cylinder pressure and intake air temperatures of forced induction engines also make them more likely to knock. Higher octane fuel is more resistant to pre-ignition, which is why it is required in these applications. In the flex fuel vehicles, they are able to make more power on E85 because they can run more timing advance without detonation due to the very high octane rating, around 110 octane I think. E85, which is mostly ethanol, does have less potential energy per gallon than gasoline, so the amount of fuel injected into the engine is increased in order to make the same or more horsepower.
Since the manufacturers try to avoid building vehicles that require premium fuel, the timing maps they program into the ECU are generally pretty tame. An aftermarket programmer or tune is optimized to use more timing, even if it is at the expense of more expensive fuel or decreased fuel mileage. Programmers and tuners both also have the ability to change when the ECU can command a shift, to give you better control of what gear the transmission is in on those hills. Also, for a newer LS V8, higher rpm for a reasonable amount of time shouldn't be an issue.
 

Martinjmpr

Wiffleball Batter
In the flex fuel vehicles, they are able to make more power on E85 because they can run more timing advance without detonation due to the very high octane rating, around 110 octane I think. E85, which is mostly ethanol, does have less potential energy per gallon than gasoline, so the amount of fuel injected into the engine is increased in order to make the same or more horsepower.
Interesting...I think maybe instead of premium, next time I'm headed to the mountains with the trailer I'll try a tank of E-85 and see how that does.
 

rayra

Expedition Leader
E85 is gonna suck hard, iirc it's about 10% less energy potential than 87 octane. You're going to get lower power AND lower mpg from it.
 
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