TINS

[sidecar]

Sidecar, Hi

Rover heads are not 'squish' heads.

The chambers are open unlike Chevy.

Hi Rossco,

The rover heads are not totally open hemi heads therefore the must be some squish effect around the edge of the chamber where the raised portion of the piston is. I agree that the effect is not that great due to the fact that the pistons are not flat top, they are dished.

Really I supose what I ment was that getting the piston that close to the heads (20 thou) is getting a bit dodgy when you take rod stretch and bearing clearances into account.

Maybe its OK if the engine is VERY carefully put together!


Anyway, I'm usually wrong and probably am this time too!

Cheers,

Pete

[ian.stewart]

Im not changing anything over what was in the engine before, I agree there has to be some squish, as I am using flat tops, but the later open chamber 4.6 heads, and I am thinking of reducing the chamber CC down from 36cc to 30cc by skimming the heads, that would take my static somewhere above 14.1

[sidecar]

Im not changing anything over what was in the engine before, I agree there has to be some squish, as I am using flat tops, but the later open chamber 4.6 heads, and I am thinking of reducing the chamber CC down from 36cc to 30cc by skimming the heads, that would take my static somewhere above 14.1

If your chambers are open and the pistons don't get anywhere near the heads then there won't be a problem with the pistons hitting the heads!

That is some compression ratio that your running though

Pete

[Darkspeed]

I cant agree with that explanation Rossco as everything I have read and what I understand about mass flow says otherwise. In my day to day job I come across velocity over pressure in gas pipework systems regularly and the operation of the ICE is no different.

My understanding is that maximum torque will only happen at maximum dynamic compression ratio and volumetric efficiency hence why cam choice has a key effect on this as does compression ratio.

This is a real loose explanation and Vizaerd and Bell do a far better job giving the relationships between ECR TCR and port velocity.

Port scavenging is mainly due to a good exhaust port velocity sucking in the charge whilst the piston is still on the way up during the exhaust stroke

Cheers

Andrew

Don't forget that you make no compression until the inlet valve is closed.

So if you run a wild cam you can have a static comp of 13:1 but a dynamic comp of 9.5:1

I thought that the static would be the lower figure - and the dynamic getting higher as the effect of ram charging due to gas mass and velocity filling chamber when piston rising and inlet still open.

Hence with wild cam you raise the compression ratio.

Andrew

Andrew, Hi

No, all that has little effect on the compression ratio.

Mainly it serves to scavenge the cylinder and ensuring a good fresh charge in the cylinder.

Until the inlet valve is closed the engine makes no compression at all as despite what you say all that the piston does is push the charge back out of the inlet valve.

[sidecar]

A very simplistic way of looking at things would be why do cam manufacturers leave the inlet valve open for part of the compression stroke. The anwser is because the gas in the inlet track as weight and velocity which gives it momentum. This momentum forces more gas into the cylinder even though the piston is going up on the compression stroke. The result of this is that even at the start of the compression stoke the gas is already slighly compressed.

Of course this only happens when the RPM is reasonably high which one reason by long duration cams don't work well at low RPM. (The other reasons being gas reversion back up the inlet and back from the exhaust port).

Pete

[Rossco]

You can argue all you want about port velocities but until the inlet is shut you ain't gonna compress anything.

This is the over V8 we're talking about here.....it's 40 years old and far from a race engine.

If you can find anyone who will back your argument that dynamic is greater than static you'll be acknowledge as the engine building guru of the century.

If you want to keep it simple then look at it this way.


Compression ratio is simply head chamber volume divided by cylinder swept volume. But if it were as simple as that then you may have a valid point.

Truth is though that if you have a cam which closes the inlet 70 ABDC then you are only using about 70% of the total swept volume. So your dynamic compression ratio is only 70% of your static at best.

yes there will be some charging effect from port velocities etc but it aint ever going to make up for the fact that until the inlet is shut there is no compression.

If what you said was true then we wouldn't need forced induction.

Try this http://www.not2fast.com/turbo/compressi ... sion.shtml one of the better calculators on the web

Also put 'Miller Cycle' into Google.

[Darkspeed]

Perhaps its terminology.

If by Static you mean the TCR (Theoretical Compression ratio) then no I am not saying the Effective CR will be higher - not unless the engine acheives a VE of more than 100% -unlikely- but I am saying that the ECR at Maximum torque RPM will be higher than the ECR at a cranking speed. And this is down to gas mass velocity.

I agree that compression will not effectively start until the inlet is closed BUT - because of gas mass velocity there will be more gas in the port to compress and therefore a higher combustion pressure.

The TCR only really serves as a datum.

As I understand it.

Andrew

[Rossco]

Ah......now we have it

Semantics is a great thing

Static compression is the basic calculation of head volume/swept volume.

I agree totally about 'Theoretical Compression'