L6 squish discussion... (the battle against detonation)

[Daeron]

DIRECT flat top pistons, with .022" Piston to Head clearence

 

Thats just a hair over half a millimeter. Less than half of the 0.050" that you had stated OTM, and that is a COUNTRY MILE when you are thinking of flame kernels.

 

You could probably slip a fingernail clipping in between the piston at TDC and cylinder head on 1fastZ's engine... but I doubt you could fit a big toenail clipping in there. (If that is a little gross, forgive me, but it was the simplest real-world grounding I could think of )

 

Obviously I am no Quench Sage, nor am I the omniscient Master of Combustion... but I have gathered from discussion here that 0.050" of clearance is not squish, its more like squash.

[jc052685]

Was doing some resurch and figured I would dig this up to seee if there were any new ideas or findings.

[HowlerMonkey]

At Z-shop of miami we used to get cars that were never driven hard which would come in sounding like they had serious bottom end problems.

 

We would take them out and run them hard with high revs and the noise went away.

 

Apparently the very small space between the piston and the head would stack solid with crud at TDC and running them up and slightly beyond redline would liberate said crud.

 

RE: Squish.....

 

I'll bet a lot of big block chevy guys look at the L88 closed chamber heads a lot differently than they did way back in the day when everybody said open chamber was the way to go.

 

There was a good article about at least a decade if not 20 years ago where Jim Feuling (olds aerotek engine builder....1000hp quad4) did some big block chevy heads that could run something like 11 to one compression on 89 octane making like 700hp......Wish I could find it as it had lots of discussion of "squish".

[KillerBjt]

would having the combustion in the head on one side create more side loading problems on the side skirts of the piston?

[rejracer]

Some things to think about...

 

Decreasing the deck to piston clearance increases turbulence inside the chamber. This does a few things.

 

1. reduces the amount of time the mixture stays near hot components in the chamber, by swirling it around no single molecules stay near them as long when compared to a low quench chamber.

 

2. Turbulence increases the burn rate of the combustion process with a given fuel. This requires less timing to "prevent detonation", in reality you are only delaying peak combustion pressures in the cycle.

 

3. Requiring less timing reduces the amount of combustion time in the chamber. Less combustion time means less time exposed to combustion temps, which means lower chamber temps.

 

4. Less timing means that the crankshaft is further down in it's rotation, trigonometry tells us that there will be be a greater Y component for a given force the closer the crank gets to 90* ATDC. This means more torque.

 

5. Reducing the amount of timing also reduces the pressure the piston encounters on it's upward travel.

 

6. Faster burn rates in the chamber result in snappier throttle response.

 

Personally I would take a different approach to the "reducing detonation" topic. Ultimately we are not after reduced detonation, we are after more torque. The best way to get more from a given intake charge is to gain more mechanical advantage. To me the question becomes how do I gain more mechanical advantage from given chamber pressure? Remember that it's pressure and temperature that cause detonation.

 

I have come up with the following:

A. Increase the fuel burn rate

B. Delay the combustion event, to delaying peak cylinder pressure

C. Getting a better rod ratio also helps, but that's another thread...

 

 

For a good read on the matter, I suggest the following:

How to Hotrod Small-Block Chevys

By Bill Fisher, Bob Waar

 

I know, it's chevys... but the chamber design is similar between the SBC's and closed chamber datsun heads. The physics is the same regardless of the engine, and the SBC heads are very similar, and more importantly, very well researched, tested, and tested and tested....

 

This post is not addressing the octane of fuel, boost level's, fuel mixture or the cylinder head temp issues our engines have. All of those wonderful things are relevant, just not focused on for what I am trying to convey.

 

Brian,

I think your conclusion is spot on for the quench issue. I am very surprised at the amount of timing you are able to run with that compression ratio. I am curious if timing is delayed, will power go up? If timing can be pulled, can the mixture be leaned out? All of these things affect the bottom line. So I am curious what kind of tune is on the car.

 

Me personally I am into the whole package deal and am into fuel economy, as well as emissions. I would tend to go for the leaner mixture, and less timing. Would you be willing to disclose your approach to tuning?

 

Thanks,

Robert

[WizardBlack]

Bumpity. Ktm, is there any way you can relink the pics of your head?

[Tony D]

RE: Squish.....

 

I'll bet a lot of big block chevy guys look at the L88 closed chamber heads a lot differently than they did way back in the day when everybody said open chamber was the way to go.

 

There was a good article about at least a decade if not 20 years ago where Jim Feuling (olds aerotek engine builder....1000hp quad4) did some big block chevy heads that could run something like 11 to one compression on 89 octane making like 700hp......Wish I could find it as it had lots of discussion of "squish".

 

There is nothing really, truly 'new' in this area, we are all just rediscovering that which has been learned previously. There is common misconcpetion, and the odd few who test, try, and learn.

 

Each generation has it's oddballs who took the time to try something 'different' and get results. But when you start really investigating, you find...for the most part...it's all been done by someone before.