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.