Gas grade: Regular, Mid or Premium?

[Stekay]

Hey Jim,
Just out of curiosity and bc I'm too lazy to do the math right now, what's the compression ratio work out to when you open the bore up to make the 2.7 liter?

[Jim LaGuardia]

Increase in bore does not increase compression ratio, that requires increasing piston deck height.
In short, stock engines with long cylinders(130mm) are 7.5:1, stock engines with short cylinders(128mm) are around 7.8:1(usually found in 6x6 AMB after mid 1972)
2.7L forged pistons are generally 8.6:1, although I have had higher compression builds done but they require premium fuel only.

[Stekay]

Increase in bore does not increase compression ratio

I disagree.
Unless you're increasing deck height or combustion chamber volume, you're squeezing a greater swept volume (.05 liter) into the same combustion chamber volume (+just a fraction for the greater bore diameter).
Is that not true?

[SAM. C.]

Jim

Thats interesting about the two different engines long and short cylinders. What was the reasoning behind that? A bit more grunt to lug the Ambulance box around?

Is the cylinder length the only difference in the different builds? Same pistons, heads etc?

Sam

[Jimm391730]

I disagree.
Unless you're increasing deck height or combustion chamber volume, you're squeezing a greater swept volume (.05 liter) into the same combustion chamber volume (+just a fraction for the greater bore diameter).
Is that not true?

Greater swept volume, true, but it is compressed into the same proportionally greater volume of the combustion chamber. This gives the same compression ratio was originally. The bore diameter is the same for both the swept area and the compression area. Unless you reduce the compressed area by 1) taller pistons, 2) shorter cylinders, or 3) heads that have less volume ("shaved" heads as the old hot rodders would describe them) the ratio remains the same.

[Stekay]

it is compressed into the same proportionally greater volume of the combustion chamber

The combustion chamber volume did not get bigger except very fractionally at TDC, unless you increased it by machining, raised the deck height, or increased head gasket thickness. These are all common methods of tuning compression ratio. Meanwhile, the swept volume increased significantly. (equal to the difference in the bore diameters squared) Displacement = bore x bore x stroke x .00031416 (4 cyl)

[Jimm391730]

The combustion chamber volume did not get bigger except very fractionally at TDC,...Meanwhile, the swept volume increased significantly.

This is correct. The swept volume is 7.5, 7.8, etc. times larger than the increase in the compressed volume. This is why the compression RATIO remains the same. The increase in volume in the swept area gets compressed, and this compressed volume exactly fits the increased volume in the chamber at top dead center, IF the goal is to keep the compression ratio the same.

[Stekay]

what's the compression ratio work out to when you open the bore up to make the 2.7 liter?

Yes, you can do that, but that wasn't my question. I'll run the numbers for myself assuming some fixed combustion chamber volume.

[Stekay]

OK, here are the numbers

Stock Engine: 2449 cc, 92 bore x 94 stroke, .020 head gasket, .035 deck clearance, 87cc combustion chamber = 7.49:1 compression ratio.

****Big Bore: 2722 cc, 96 bore x 94 stroke. .020 head gasket, .035 deck clearance, 87cc combustion chamber = 8.00:1 compression ratio.

[Jim LaGuardia]

Tip:Pinzgauers do not use head gaskets, cylinders mate directly to heads

[Stekay]

As always Jim, thanks for the tip. In regards to compression ratio however, whether or not the Pinz uses a head gasket is inconsequential.

[milesdzyn]

Getting back on subject of gas grades, does altitude affect the choice of gas grade for high altitude driving?

Miles

[Jim LaGuardia]

Proper jetting is a larger part of the solution to high altitude driving although higher grade fuel will also improve power due to longer burn times(less pre ignition under load).

[krick3tt]

All this considered...how about ethanol in the gas? It attracts water and that causes corrosion, right?
If there is more ethanol in the gas, is it possible that power will be somewhat reduced? As the government is now acting to increase the percentage to 15% ethanol.
Other than this increase will take from the food and grain supplies and cause my Big Mac to cost more, what will it do to my fuel system?

Should I be adding a fuel stabilizer like the ones by BG? http://www.bgprod.com/products/fuelair.html

If I do not let the fuel sit for long periods the ethanol may not cause a huge problem. The acids caused by the action of the bacteria may not be as drastic.
I usually fill the tank as it gets to the half way mark on the gauge (have run out of fuel before). Am I causing, by allowing the tank to never get to empty, an issue
with the ethanol in the tank?

???? So much to know and still I am not sure.

[Jimm391730]

Am I causing, by allowing the tank to never get to empty, an issue with the ethanol in the tank?

Generally, keeping the tank full is better than almost empty -- every day, as the temperature changes from day to night, the air in the tank expands and contracts. As it contracts, it draws in outside air and since this is evening into nighttime the air is more moist (higher humidity) and brings this moisture into the tank; this is the major mechanism for adding water to the fuel. A full tank means less air volume, so the change in temperature causes less air pumped in and out, drawing in less moisture.

If the tank was empty and dry, the air exchange would not have any fuel to absorb moisture, so there is no big problem. But just a little fuel in the tank will be more likely to absorb the greater amount of moisture and air; whereas a full tank has more fuel absorbing moisture from a smaller amount of air.

Ethanol or not, a tank stored full is better than a tank stored less full.