High compression with large cams?? 2

[PowerDubs]

Maybe its just me, but I see an awful lot of people build a engine with lets say 11-1 pistons, but also put in large cams without considering that the effective CR is actually lower now.

How do you calculate how much different cams will lower your CR so that you can raise the static CR up to compensate??

Thanks- Josh

 

[patprimmer]

I have heard this argument many times, but I ask the question, why run the bigger cam.

Answer, to make more power.

How.

By increasing volumetric efficiency in the working rev range.

How does increasing VE drop effective compression at working speeds.

It will drop the compression at low speeds, and depending on a lot of variables, might decrease the the cylinder pressure on the compression stroke by a small amount, but gain power by moving the power band up the rev range.

I never increased comp to allow for cam duration, as I always considered the result to be at best marginal, and at worst destructive.



Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patprimmer]

Sorry, I forgot to mention, I am only considering normaly aspirated engines here. It might need a rethink for a turbo or directly driven supercharger.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[sitbc]

if a "large" cam is used on a low compression engine, a lot of the cylinder pressure will be lost on overlap which is the period where both intake and exhaust valves are open. This is especially true at low rpms. That is why the cranking compression lowers when you install the bigger cam in an engine which doesn't have its cam timing and compression ratio properly matched.giving it more compression minimises these losses.Of course at higher rpms there is less time for the pressure to escape so it does regain some of its efficiency but remember that every engine becomes less efficient past its torque peak and raising the compression ratio restores some of the lost efficiency.It is possible for an 8:1 engine to have the same cranking compression as a 11:1 engine just because of different cam timing.
sitbc

 

[SMOKEY44211]

I think what you are considering are the dynamics of entrapment. Entrapment being the amount of air in the cylinder after the intake valve closes. Long overlap and/or late intake closing events will pump some of the air in the cylinder back into the intake at low cycling speeds. If the intake and exhaust conduits are sized properly and in conjunction with relatively long valve open duration, it is possible to achieve greater then 100% cylinder filling at certain engine speeds (normally aspirated) by taking advantage of the inertia of the working fluids to acomplish this. What a lot of people overlook is that an increase in compression ratio also increases the expansion ratio. It is the increase in expansion ratio where the gains in power and fuel economy are derived from.--------Phil

 

[evelrod]

Too many variables in just raising the CR without taking into consideration cam timing/overlap. I use fairly mild
CR's (by todays standards) 13.37 on the mini and 13.68 on the twincam with a fairly open design combustion chambers but they still require very high octane racing fuel even with a big overlap. Just bumping the CR up without changing a fairly mild cam can be disastrous.
My Model A engine is now 7.0:1 CR with a mild "B" cam and it is right on the ragged edge of detonation with 91 octane fuel if I advance the timeing beyond 17 degrees total or when the engine temp gets a bit high!
VE is what your after if it's horsepower you need. Static pressure is a fair way of judging an engines relative condition but not much else. My 1380cc mini pumps up to 260 psi per cylinder at cranking speed and the 1594cc Lotus 200 but, the Lotus makes substantially more hp per cc than the mini ! The real reason I use a high CR is that it gives me a power increase throughout the entire rpm range without the need to go to outrageous rpm ranges (both engines make power in the low 7000 rpm range). Camshaft timing and fuel choices are made with these goals in mind.
My cars don't make the big hp numbers that others do but, they are pretty easy to drive and have kept me in the winners circle more often than not.

Rod

 

[Marquis]

I Actually Do increase the compression ratio when I go longer on duration, but to be more precise, I actually increase the CR when going later on the IVC.
I'm sure we've all heard of Dynamic compression ratio versus static or geometric CR terms.
If I have a configuration that I KNOW will NOT knock on a certain octane of fuel, I will generally equalise the dynamic compression ratio (which is a ratio of volumes, but that of the volume in the cylinder at IVC point versus TDC) of this config to the newer configuration. This is only a starting point ,before I go away and do my analysis work and iterative testing.

The later IVC of the new wilder cam will reduce Volumetric efficiency at lower engine speeds. Now Pat primmer is quite correct, but this is the clever bit: When the engine gets into its working range- the volumetric efficiency will be just as high if not higher then the older less "tuned" configuration and hence the cylinder pressure may be high also, however, because it is taking place later in the rev range it is less likely to be as knock limited- due to less real time being availiable. Engines are typically much more knock limited at lower speeds.
The other point of note in favour of the High-CR-late IVC cam is that, much like the Atkinson cycle, because the compression stroke is occuring much later, initial temperatures will be lower- and this will tend to offset the knock condition too.
To illustrate the Knock/high-speed point, I know of a supercharged engine running 95 octane RON fuel, that is running 0.9 bar of boost, that is not knock limited above 4700 rpm upwards and this is an engine that is producing 130-140 % VE!

To illustrate the late IVC point, during some Miller cycle work investigations I found that with IVC occuring at 51 deg ABDC the output torque was limited by knock at a cylinder pressure of 1.35 MPa, when the cam timing was retarded the onset of knock didn't occur until a cylinder pressure of 1.55 MPa!

 

[Artsi]

Does anyone have even a few IVC figures ABDC which are known to work with certain static compression ratios?
Naturally combustion chamber shape, and quench parameters affect this equation (i.e. make engine knock or not).
Oh yes, data on tuned engines alone please.

MARQUIS - did you bother to tweak with ignition timing as you ventured to chart out knocking habit?

 

[Marquis]

"MARQUIS - did you bother to tweak with ignition timing as you ventured to chart out knocking habit?"

Did I BOTHER?
The testing would be invalid without it. I carried out a full ignition loop- however because it was knock limited, MBT wasn't reached.

 

Marquis,

How was ignition timing affected during these tests?

That's great information.