dished pistons make more hp than flat top?

[DavidHarsay]

The total volume at BDC will be the same, if the combustion chamber adjusts the compression to keep it the same... by definition of what the CR is. It's just in a slightly different location.

Originally Posted by HPaddict

... once the piston is moving down the bore the air is really rushing in the cylinder and once the piston is at bdc the dish part is lower than a flat top so there is actually more room to cram in a bit more air for good measure, then when the piston comes back up it still compresses the mixture in the cylinder the same amount as the flat top because you have used a slightly smaller combustion chamber to up the comp ratio to equal the flat tops comp ratio, which will give the same static compression but because more mixture got trapped earlier on the dynamic comp is actually higher....
at bdc there is a bit more area to fill still with a dished piston because the dish sits lower than a flat top for the same stroke.

[Stan Weiss]

Here are a couple of graphs which show Max. squish velocity.
In this one the green line has a squish ratio of 0.5 (50%) each lower line has had this decreased by 0.05 (5%) but the point (degrees BTDC) does not change

In this one the green line has a squish clearance of 1.2 mm each lower line has had this increased by 0.1 mm but the point (degrees BTDC) moves away from TDC

[automotivebreath]

Very good information Stan, please explain what is meant by:

"a squish ratio of 1.2 mm"

[Stan Weiss]

That should have been 0.5 (50%) I have changed the captions for the pictures.
Other items used were Bore = 4.0, Stroke = 3.25, Rod length = 5.7, RPM = 6500, CR = 13.594, Intake Close = 74.125 BTDC

[big block fiero]

Originally Posted by HPaddict

at bdc there is a bit more area to fill still with a dished piston because the dish sits lower than a flat top for the same stroke.

Look at it this way, At bottom dead center "engine A" is dished (say an 8 cc, dish) but has a smaller chamber head ( -8 cc,s) so it all breaks even. "engine B" has flattops and has a larger chamber head. Both would contain the same volume at B,D,C (chamber volume + gasket volume + cylinder volume + piston volume).

now say on 400 c.i. engines the cylinders in each have 50 cc,s of swept volume. At top dead center they both have the same clearance volume (chamber volume + piston volume + gasket volume) and so have the same compression ratio.

[Stan Weiss]

While we changed different factors in the above graphs to see how they changed MSV these factors will remain pretty much unchanged in a running engine (piston to head will reduce a little as RPM's increase). That brings up the major factor that will change in a running engine RPM. In this graph we have in green are baseline (6500 RPM) we than reduce RPM by 1000 for the next two line and the last line is at 3250 RPM or 1/2 of are baseline and also 1/2 of its velocity. Remember while MSV raise with RPM time /duration of each cycle is reduced so at 6500 RPM you have 1/2 the time as at 3250 RPM.

[automotivebreath]

Stan, thanks for this interesting information. The data used for the grafts,
is this calculated mathematically?

[Stan Weiss]

Yes. The 2 stroke people use this information all of the time (MVS). To calculate MSV and the degrees at which it happens you need to calculate the whole curve. I am just adding the graphing function to the program now. this shows the two stroke screen of my program.

[automotivebreath]

I have read about this sort of calculation mostly with the two stroke crowd.
I did read where professor Blair explained the calculations needed to use this
with four strokes. I believe this will become common in the years to come.

With one of the iron head SBC engines I'm using for combustion testing, the
squish to bore ratio is ~36%. The engine RPM is between 5000 - 7500. I run
the piston down in the hole 0.025", this gives me the option of adjusting
squish clearance with a head gasket change. Some of the common thickness
are 0.015", 0.026" and 0.040". I'm currently running the 0.015" gasket for a
squish clearance of 0.040" at assembly or ~0.010" at max RPM. Unfortunately
the compression ratio changes along with each head gasket reducing the
benefits of the test.

This is a combination I have been working on for several years, my goal is
> 12:1 compression (iron head 23 degree SBC) on 93 octane premium fuel,
actually I think I'm there now. I'm thinking the MSV is relatively high with
this combination, so high that can causes combustion problems. My first
attempt with this engine was poor combustion and detonation limitation with
100 octane requiring ignition retard. Eventually I modified the engine and
opened the squish clearance and eliminated the problem. After several
variations of engine modifications, I'm now able to test different ignition
advance curves with out detonation on 100 octane, progress. Soon I will
be weening it off the 100 octane to 93.

Knowing MSV would be a big advantage with this testing. I have read of
maximum levels of squish velocity before problems arise, your software
would eliminate some of the guesswork. Please tell us more.

[Stan Weiss]

Calculation of these different numbers just gives the builder more information. The build has to decide which pieces of information he finds important and needs to monitor from build to build. The green line is 7500 RPM and 0.01 clearance, the red line is 7500 RPM and 0.04 clearance while the blue line is 5000 RPM and 0.04 clearance. Just as a side note. People talk about how dynamic CR changes as an engine is running verses static CR. In your above example when the piston to head is reduced by 0.03 at 7500 RPM your static went from 12:1 to 13.24:1