View Full Version : Generic Q about aluminum pistons, engine theory.

06-07-2007, 01:00 AM
Suppose one replaces the stock pistons and con rods in a four cylinder engine, with a displacement of 2 liters or less.

The stock engine is not a turbo, nor is it prepped for a turbo. The stock engine is normally aspirated with a factory compression ratio of that for a typical engine using 87 octane pump gas.

The pistons and con rods are being replaced with forged 2618-T61 aluminum parts. They allow for the exact same compression ratio as stock.

(so, nothing is changing except the fact that the pistons and con rods are now aluminum)

1) Assuming the driver is driving to conserve fuel, not a lead foot, how much do you think the modification would effect fuel mileage?

2) Could a driver effectively raise the redline of the vehicle without killing the motor? As in, rev past the factory spec redline? If so, what would be an approximately reasonable redline to raise it to, and not have the engine blow to pieces relatively soon?

3) Ballpark, what kind of horsepower gain could be expected from this mod?

06-07-2007, 05:50 AM
thats a very out there question, as with most engines its the valvetrain that does not allow for a very high rev limit. not the lowerend. but if you want to conserve gas why would you be worried about hp?

06-07-2007, 08:02 AM
Theoretically I don't think the weight reduction would do much gas wise. So unless you change the way the engine works, not gonna do much that way either.

06-07-2007, 09:06 AM
Raising the redline would have more to do with the valvetrain. It also involves balancing the rotating assembly. Changing just the rotating assembly, without balancing, would not affect redline at all. Also, without changing cam profiles, raising the redline would do no good. The engine doesn't make power past redline. That is where a new cam profile would be needed.

You would be strengthening the internals as well as lightening them.

It would not affect power if you were going back with the same C/R. It would be virtually identical, but would rev slightly faster due to the reduced weight.

As far as fuel economy, you would be better off getting a header, free flowing exhaust and a free flowing intake. That is about as far as you can go to increase your mileage.

06-07-2007, 11:45 AM
Well if hes going to go as far as opening the engine, higher compression pistons and some really nice head work would get him some mpg, problem is you'd need to find a head specialist who knows how to do velocity work, and not just flow porting.

06-08-2007, 12:21 AM
I do believe that the lighter internals would result in better economy, but it would only have an effect while accelerating (less energy required to rotate the lighter mass), but it wouldn't be a big difference. I also think there would be a torque increase as well for the same reason (same energy produced but more goes to the wheels because the engine needs less), but once again, it would be very small. So small you probably wouldn't be able to tell without a very senstive engine dyno.

You would get far better results, much cheaper by focusing on the car's breathing first.

06-08-2007, 02:36 AM
Thanks, everyone!

This was one of those "what if" scenarios. As for the question why would you want mpg, it's just a matter of efficiency. A mod that would increase power may in fact bring back better gas mileage. (intake, exhaust modifications, for instance allow for more power but also greater gas mileage)

But, this has me interested in something else:

Horsepower per horsepower, what would get the better gas mileage- a high compression engine, or a turbocharged engine?

Will either potentially allow for greater gas mileage?

06-08-2007, 03:32 AM
I think a high compression engine because there are fewer breathing restrictions.

There is a limited amount of energy in every drop of fuel. Some of the energy goes to turning the wheels, some is wasted as heat caused by friction, some goes to the alternator, power steering pump, water pump, oil pump, a/c, finally some of the energy goes to drawing in air, and pushing out exhaust. By improving intake and exhaust flow you are reducing the amount of energy required to move the air, thus freeing that energy up for the other uses (increasing power) or requiring less energy to give the same performance (increasing efficiency).

Turbos use exhaust gasses to compress the intake air allowing more fuel to be burned, but that means you have to stick a big hunk of heavy metal right in the flow, increasing the amount of energy required to move the air, thus reducing efficiency.

The high-compression engine, on the other hand, doesn't have these restrictions.

06-08-2007, 05:28 AM
Well if you consider what a turbo does then you come to the conclusion that there is no way it could be considered a fuel efficient mod(in general). Consider it- fuel efficiency means using less fuel. Ultimately, a "fuel efficient" car would run as lean as possible while still remaining within the confines of good clean operation. This is contrary to everything a turbo is. I'm not sure of the characteristics of the 4afe, but as far as the 5sfe goes, I've been dieing to get my hands on an safc or better just so I can get the thing to stop running so rich all the time- that would be a definite fuel efficiency mod for the 5sfe.

06-08-2007, 05:38 AM
Well, turbocharged gasoline engines are tuned to run richer to help prevent knock hence the lower gas mileage. That and sometimes you have other accessories like AWD systems or other features adding weight and rotating mass to the equation.

High compression engines do indeed get better economy. An interesting example is the Toyota Prius, it's 1.5 liter has a 13.5:1 CR, but it is an Atkinson cycle engine and the intake cam hold the valves open for a split second into the compression stroke, lowering the cylinder pressure as the engine runs.

More than anything, your speed will affect your mileage. slowing from 65 to 60 will create a nice increase in mileage.