Atkinson/Miller confusion...

[Shortie771]

I recently read a thread about somebody wanting to increase their compression ratio for improved FE and in it another member suggested something about doing an Atkinson regrind on the cam. I have tried reading up on it, but it left me with nothing but a headache and ran into a lot of debates on Atkinson vs Miller.

I am also interested in increasing the compression ratio on my D16Z6 for better throttle response in the low end and increased FE. I just don't want to increase compression to the point of having to run higher octane fuel, because that kind of defeats the point of getting better FE if I will just have to pay more at the pump. Anybody know what the limit might be on that, without engine knock or retarding the timing? I was thinking about 9.5:1 or 10.0:1 vs my stock 9.2:1.

1) How high of CR can I go without having to run higher octane or retard timing?
2) Could somebody dumb down how Atkinson/Miller setups work and the advantages or disadvantages to them?
3) Would the Atkinson/Miller setup work well with increased compression?
4) Would I even be able to do this with my engine? (pretty sure I have VVT)

Sorry, I know I am kind of all over the place here. Just a lot going on in my head.

[cRiPpLe_rOoStEr]

The so-called Atkinson setup usually features a higher compression ratio than its regular equivalent. That might be not required with the Miller setup because it uses a supercharger. Without a timing retard I'd probably not want to run more than 10:1 CR in any engine which didn't feature direct injection, altough if you go with the cam retard you SHOULD go to 13:1.

Miller-cycle engines were relatively popular during the 40s for marine and stationary applications such as gensets. Basically, they had the intake cam timing retard and a supercharger, quite a cheap mod. In the 90s it was used by Mazda in at least one version of the 929, and currently Nissan uses it in some versions of the Micra.

[Occasionally6]

Just a quick comment:

If you are increasing the compression ratio +/or altering valve timing, check the valve to piston clearance.

If the compression increase comes from machining the cylinder head, the camshaft timing will be altered (retarded - with a belt driven camshaft).

[Shortie771]

Thanks for the info. So, what are the advantages and disadvantages of the Atkinson setup? What exactly has to be done to the cam?

[JETZcorp]

People complicate the Atkinson thing far too much. All it is, is an intake lobe with a later-than-normal valve closure point. That's it. It is accompanied by an increase in compression because of the resulting reduced cylinder filling will prevent detonation. This is well known in the hotrodding community, where a fat nasty cam allows higher compression. Fat nasty cams also, all else being equal, reduce low-rpm power for the same reason an "atkinson" cam does; the reduced cylinder filling. At higher rpm, this tendency is reduced as the intake charge rams its way into the cylinder more violently. A cam that allows 13:1 is traditionally huge, but in an economy application, we'd forgo the other "fat nasty" aspects of a performance cam (big overlap and early exhaust opening) and keep the rest normal.

[cRiPpLe_rOoStEr]

Originally Posted by JETZcorp

People complicate the Atkinson thing far too much.

The real Atkinson cycle is more complicated than what we know commercially as Atkinson cycle, since it requires an articulated crankshaft which leads to a power stroke really longer than the compression stroke, instead of emulating its effect just with a different valve timing.

[smokey442]

What you gain with both the miller or Atkinson cycle is an increase in the expansion ratio. Most engines specify compression ratio. Mathematically compression and expansion ratio are the same. You can manipulate them mechanically in them in the case of the miller cycle by delayed intake valve closing. This lowers the pre combustion pressure to lower the detonation threshold for a given octane. Small gain in reduced pumping losses at part throttle by reducing intake vacume. You do not need a supercharger to take advantage of this method.

[Shortie771]

Originally Posted by smokey442

What you gain with both the miller or Atkinson cycle is an increase in the expansion ratio. Most engines specify compression ratio. Mathematically compression and expansion ratio are the same. You can manipulate them mechanically in them in the case of the miller cycle by delayed intake valve closing. This lowers the pre combustion pressure to lower the detonation threshold for a given octane. Small gain in reduced pumping losses at part throttle by reducing intake vacume. You do not need a supercharger to take advantage of this method.

Can this be done to the stock camshaft? I wonder how that conversation would go at a machine shop... Me: "I would like to get an Atkinson regrind on my cam." Machinist: "You want to, huh?"

[smokey442]

The Atkinson cycle uses additional linkage between connecting rod and crankshaft to achieve the higher expansion ratio. Miller cycle is done by altering the intake closing event. The best way is a custom ground cam but you can just retard the stock cam to achieve almost as good results. 5-10 deg. should allow for a 2 point jump in compression ratio. I run 10deg. retarded in my own engine 12:1 cr on 87octane fuel. In the warmer its a little bit to much (ambient temperature over 70 deg) I just blend in about 50% 93 octane to keep it out of detonation. A dedicated water/methanol system will get the job done also. Low end torque is down at speeds below 2,000 rpm but over that it makes more power then running it straight up.

[freebeard]

Scuderi claim to be on the Miller Cycle, but I see it as more Brayton Cycle.