Turbo on 2v versus 4v motors

[Quadrajet]

This is a new twist of a question. And since I've never messed with Turbos I had no point of reference for an answer.

It was mentioned to me in a discussion that Turbos will make bigger gains on a 2 valve head motor as oppsed to a 4v motor. It seems counter intuitive to me.
But at the same time I was thinking that if you were making a very simplified comparison, of say an M10 versus the M42, it kinda makes sense. By virtue of the M42 having 4v's per cylinder and thus having more efficiency the gains wouldn't be as great, with a Turbo.

Thoughts? Experiences? Anyone been there and have the data?

[wagonizzle]

Stick to your guns; I think you are correct. Power is made in the head, and more valves make that easier. In your example, the only reason to make an argument that the m10 is better for turbo is that a) it has lower compression than the m42, and b) there is no other way to get serious power from an m10 (and c), they are a dime a dozen; blow that shit up!). Old, antiquated engine designs have turbocharging as their savior. Basically, there isn't enough 'adjustment' to do to a single cam 8 valve to make it worth the money spent on a cam, head porting, etc. - turbo is the only decent value. If you had the money to do it right, you could make more power with an m42 I think.

Look at honda - some of the best flowing heads, they can make huge numbers from a sub 2-liter engine - sophisticated valvetrain. And I think at least a few people have added turbos to them...

[nando]

I think the M10 will be more reliable though, they're pretty stout little motors. the M42 is a little bit fragile.

the M10 is also cheaper to build by a long shot. you could probably do a basic M10 rebuild for what the M42 timing chain system costs alone..

[xwill112x]

my m42 is bullet proof, it has 220k, and still see's 7000rpm's daily.

[nando]

wait until the timing chain grenades. you wouldn't be the first.

[Lurker27]

It's like this: 4v heads and variable valve timing allow the engine to run most efficiently over a wide range of rpms and loads - at a given time, you need the combustion chamber to fill back up totally with air at atmospheric pressure. So, at very high rpms, you have a small amount of time to fill the chamber back up, with a pretty small pressure difference (just the vacuum pumping of the engine).

When you add a turbo, you increase the pressure head at the valve, so for a given time and valve Cv (think of this as resistance to electrical wire, 4v heads are like bigger wires, total airflow is amperage, and boost is voltage) you will flow much better with a turbo. The difference is that 4v heads will still tend to work the turbo less hard for a given horsepower rating - the difference is in pressure drop across the valvetrain itself, so for a given psi at the turbo compressor wheel, your engine will see less boost with a 2v head than a 4v head. This is why you get the slower spool - you're essentially going to a higher psi at the turbine to make the same power at the engine. You also tend to have to worry about heat soak less at the very high specfic output ranges.

So, while 4v heads can ultimately main more power per liter than 2v heads, because under boost the turbo fills the entire cylinder with air molecules at a specified flowrate, you tend to see more IMPROVEMENT under the torque curve in 2v engines - you're getting an optimization effect that the 4v heads essentially already have. When you're using oversized turbos anyway instead of one very specific to the engine, there won't be much difference.

[LowR3V'in]

Two exact same engines, except one having 4 valves.
The 4 valve engine would own.

[nando]

Two exact same engines, except one having 4 valves.
The 4 valve engine would own.

they aren't the same basic engines though, completely different.

also, there are good 4v designs, poor 4v designs, and exceptional 4v designs, so that still doesn't hold. the M50 is a good design, the honda S2000 would be exceptional..

I still think the M42 is the worst $/hp of any BMW engine.. even worse than the M20. I would own an M42 car now if the cost to do the work it needed wasn't several times more than what the car was worth. :p

[NC325iC]

hes speaking theoretically

i think the OP was also just using the m10 and m42 as examples.

i think better example would be, an m10 with an m10 head and an m10 with an s14 head

[shiboujin]

Simply put, more valves allow for more air to get in and faster. Head design is also a big factor but in this case not as much as 2v vs 4v.

It all comes down to volumetric efficiency. If an NA 2v puts down x power, I bet you a 4v with the exact same specs will put down more power. Why would boost change that?

[nando]

Simply put, more valves allow for more air to get in and faster. Head design is also a big factor but in this case not as much as 2v vs 4v.

It all comes down to volumetric efficiency. If an NA 2v puts down x power, I bet you a 4v with the exact same specs will put down more power. Why would boost change that?

because with lots of boost, it doesn't matter as much how well the head flows. It still matters, just not as much.

I realize now that he was using the m10/M42 as an example. Regardless, I would build the motor that was more stout/reliable, rather than go for the "better" one just because it has more valves.

[LowR3V'in]

I agree with nando.
That should defiantly be
taking into consideration when choosing an engine, and for
what purpose. For example: m30 vs m50 :) j/k

Either way, you're able to make 400hp pretty easy with
any of the 6cyls (m20 included). Just figure out what you want out of the engine.
Quiet, mpg, torque, power band style.

[Pinepig]

2 or 4 valves doesn't matter, volumetric efficiency does. If the 2 valve motor has a greater VE than the same sized 4 valve motor the 2v will win ( highly unlikely to find a 2v with more VE than a 4v but I'm sure there are examples )

You guys are asking the right questions, your just not using the proper terms.

[xwill112x]

wait until the timing chain grenades. you wouldn't be the first.

i've dropped the oil pan, found no bits of chain rollers or guide rails so i think i'm good :up:

[nando]

i've dropped the oil pan, found no bits of chain rollers or guide rails so i think i'm good :up:

it just hasn't happened yet. it's about as inevitable as breaking a rocker on a 200,000 mile M20.