M42 Turbo Options

[Massimo]

Your problem is that you are forgetting fueling. The more air you put into the engine the more fuel that is required. So yes while you are harnessing wasted energy it requires more fuel to be effective.

It is a fine balance between wasted energy and fuel comsumption. It is not hard to do but will require testing and more testing and different turbo setups. In other words it will cost more then it is worth for the DIY'er.

[bwawuz02]

Exhaust gasses are technically wasted power in a NA engine setup. However they are necessary waste because harnessing the entire amount of energy from combustion would require a huge stroke and massive engine and wouldn't work in an automotive application for many reasons.

Exhaust gasses = super-heated, rapidly expanding air. You want a turbo as close as possible to the exhaust valve to harness this waste energy.

A turbo-charger uses that waste gas to spin a compressor which compresses air before it enters the combustion chamber. Now this is me guessing, but pushing compressed air into the combustion chamber is like raising the compression ratio allowing for better performance.

Yes, a turbo harnesses what would otherwise be wasted energy. The equation is simple, but it's not just an increased compression ratio. More air needs more fuel, more of both means more power. As pressure increases heat increases, as heat and pressure increase the mixture is much easier to ignite. Pre-ignition/knock = the mixture igniting too soon and pushing against the force of the pistons movement in a bad way.

That by definition means that you are using waste energy to increase efficiency which is a win win.

true

So the obvious problem is that the M42 was not built to be turbo-charged.

The M42 is actually a great candidate for boost because it has forged rods/crank from the factory, which are very strong and can withstand the extra forces associated with turbocharging. A lot of people will tell you that putting a turbo on an engine w/10:1 compression ratio is not a good idea. In reality it only limits your max boost PSI. And for your "efficiency" purposes, a higher c/r will yield much better low-end torque.

Wouldn't the only reason for not seeing improved efficiency be tuning issues?

no, the problem is we like to drive fast and most people turbo their cars for more hp, not efficiency. the main difference between the recent turbo trend for efficiency and our M42s is that the M42 wasn't designed specifically for this. New turbo engines use much more sophisticated ECUs and direct injection to squeeze every last kW of energy out of the A/F mixture.

[Threehz]

Fantastic information!

So ideally for efficiency and the M42 you would want a very small turbo because you don't need very much overall and smaller turbos mean less lag too.

Then it's a matter of getting the AFR correct and getting timing down.

In an M42 with pretty minimal boost what kind grade of gas would you have to used?

[bwawuz02]

So ideally for efficiency and the M42 you would want a very small turbo because you don't need very much overall and smaller turbos mean less lag too.

small turbo = less lag = lower max psi at higher RPMs
again read up on the IHI VF38. It's a twin-scroll unit from 2 liter subaru legacy engines that have really awesome low-end torque... I think they hit peak torque by 2400rpm.

In an M42 with pretty minimal boost what kind grade of gas would you have to used?

at least 91 octane, but whatever is the highest grade in your area.

[Vivek]

Turbo can be efficent if it wasn't then why is there a tread for new cars pushing fuel effeceny and turbo 1.6lt?

That's not more fuel efficient than a NA 1.6l. It's just more fuel efficient than an NA engine to make the same power. So if a 1.6lt makes 200hp, it may get like high 20's mpg, but a 2.5l making 200hp would be in the low 20's. So it's more efficiency per HP, not per liter.

[Threehz]

I looked into those IHI VF38's, sounds like a good deal if I ever do decide to turbo the old girl.

Vivek I get what you're saying, for instance many new engines that are turbo-charged do so to have a smaller engine size that creates the same HP as a larger engine meaning it gets better gas mileage.

But if a turbo in fact raises compression then shouldn't you be seeing some increase in gas mileage?

[Danny]

Shit I had my m42 turbo running for less than a grand. Then added MegaSquirt and got pretty good mileage. Fuck da hat3rz.

[Vivek]

As much as you seem to know more than me, turbo's fundamentally push more air into the engine. More air=more oxygen=more gas burned (along with more power). I may be completely wrong, or partially wrong, but at least that's how I see it.

[bwawuz02]

So it's more efficiency per HP, not per liter.

Convoluted wording, but simply wrong. Efficiency is efficiency, it means burning every last molecule of fuel with no excess resulting in a pure output of CO2 and H2O. Maximizing the torque output of this burn process simply means lighting the fire at the right time. (see ignition timing)

The part about fuel efficiency and smaller displacement turbo engines popping up all over the place is that you don't always have to be under boost pressure. At light cruising conditions you may not see much boost at all, in effect bringing the engine back to it's N/A fuel consumption levels. Adding the turbo to the smaller displacement engines is a way to get the best of both worlds, lower fuel consumption when power isn't needed and still having the extra power on tap.

For a BMW example, you could put down S54 (estimated 15/22 mpg) power numbers and still see 20/30 mpg M42 efficiency. Taking into account that you are capable of not mashing the pedal to the floor constantly. The key is setting up a really good tune using something that integrates an electronic boost control solenoid.

[bwawuz02]

lol, now that I'm more awake I realize that I forgot to mention volumetric efficiency (VE). VE is the engine's ability to fill it's cylinders with A/F mixture, i.e. breathe. Many 4-valve/cyl engines have a VE around 85%. Engines that are turbocharged have VE's in excess of 100%, hence the term forced induction. So a turbocharger scavenges the heat energy of combustion to increase the engine's VE and thus increases HP/liter.

The only way you could increase the efficiency per HP would be to increase the length of the lever arm, i.e. longer stroke crankshaft. (A good example is the longer stroke of eta engines 8))

The combustion of fuel has a very specific power output per mol, depending on what fuel you're using. The chemical equation for combustion is very specific and doesn't care about turbo or N/A. It will take the exact same amount of fuel to make 200hp in a N/A engine as it will in a turbocharged engine, given they have the same crankshaft stoke and conrod lengths.

Threehz
Compression pressure is more directly related to power output than compression ratio. Here's a good read for you: http://www.kb-silvolite.com/article....n=read&A_id=36

[shEck3lls0]

Very good info. Thanks to all you informed turbo people.

[trackjunkie21]

Flip the stock exhaust manifold and build an adapter for an IHI VF38 turbo. done. semi-cheap and meets your requirements. You can run 10psi on the stock motor without worry, properly tuned of course ;)

DO NOT DO THIS! An NA exhaust manifold differs in many ways from a turbo manifold in many ways besides looks.

[acolella76]

DO NOT DO THIS! An NA exhaust manifold differs in many ways from a turbo manifold in many ways besides looks.

Care to elaborate on how you came up with that?

[bwawuz02]

DO NOT DO THIS! An NA exhaust manifold differs in many ways from a turbo manifold in many ways besides looks.

yeah, your post is meaningless without some kind of proof. How about this... yes, some turbo manifolds have expansion joints to help cope with the extreme amount of heat they might be dealing with. Aside from that they might be made of thicker steel.

But to debunk your statement, it has already been done on the M42.

[acolella76]

But to debunk your statement, it has already been done on the M42.

And countless M20s, M30s, etc.