dished pistons make more hp than flat top?

[big block fiero]

Stan, this last curve represents 150% volumetric efficiency, Hummmmm.

Everybody, if you noticed the thread where I ask darin morgan and joe mondello about horsepower with dished pistons vs flat. They dont believe there is any horsepower to be gained by going dished but that you can go dished to a point but must stop short of exceeding the limitation where horsepower will be lost. Also, to those of you who have seen a dished piston gain, it is because your motor had wet flow problems to begin with. I can't remember the exact words but this is the content that I interpreted. If this is all true (and it could be because how do you know if you have wet flow problems or if you checked it properly) it kind of oversimplifies everything back to old school engine building techniques. Go flat top, K.I.S.S. -------keep it simple stupid. Same with squish clearence, keep it tight (old school), K.I.S.S.

Matt.

[Stan Weiss]

Stan, this last curve represents 150% volumetric efficiency, Hummmmm.
Matt

Hey Matt,
Can you tell me how you got VE from these graphs?

[big block fiero]

Originally Posted by Stan Weiss

Hey Matt,
Can you tell me how you got VE from these graphs?

I compared the swept volume to the traped volume but I now realized that I hadent subtracted the clearence volume.

So what we could do is take the swept volume and static compression ratio and work the numbers backward to get the clearence volume then subtract that from the traped volume. now just figure the percentage difference of the swept volume to the "traped volume minus the clearence volume".

I think this would all be a little easyer to follow for perspective with the VE numbers plugged in. I dont have time to do it right now but will do it later if someone hasent done it allready.

[Stan Weiss]

We have a difference in the meaning of terms. The swept/cylinder volume is in ci the effective cylinder volume is in ci the trapped volume is in cc's. The trapped volume is the volume measured with the piston ATDC with the head installed with a head gasket and the valves. The effective cylinder volume is the same as swept volume calculated using the dynamic stroke or stated another way the cylinder volume @ intake valve close.

[automotivebreath]

Stan, I happen to think your software is the best thing since the invention
of the internal combustion engine. If you would like to play along using your
program we could answer some of the questions sounding flat top vs. dish
vs. reverse dome pistons.

In the picture below on the left is a typical heart shaped combustion
chamber. Using the flat top or reverse dome we would use all of the available
squish area (let's say 35%). Replacing it with a dish piston drastically
changes the picture. One distinct difference being the squish to bore ratio
is now lower (let's say 25%).

If you would calculate MSV for both.

Bore 4.030"
Stroke 3.48"
RPM 8500

[Stan Weiss]

Using Bore = 4.03, Stroke = 3.48, Rod Length = 5.7, CR = 12:1, RPM = 8500,
Intake Close = 74.125 BTDC, Squish Clearance = 0.04
Squish Ratio = 35% MSV 56.428 m/s @ 9.035 BTDC
Squish Ratio = 25% MSV 47.690 m/s @ 9.035 BTDC

[automotivebreath]

Now if you will Stan, redo the calculation for 35% squish area to
obtain a result of MSV 47.690 m/s by opening the squish clearance.

The picture that emerges is the engine builder is able to control MSV
to the levels that suit the need by using squish area and clearance. In
addition with the dish piston the bottom of the chamber is opened allowing
more area for flame growth. I would think this has some positive influence
on the early stages of flame development before MSV @ 9.035 BTDC.

Comments?

Here's a quote that fits right in, I can't remember where this is from
perhaps Professor Blair.

"For high-performance engines, such as those used for racing, the design of
squish action must be carried out by a judicious combination of theory and
experimentation. If the MSV is too high the mass trapped in the end zones
of the squish band may be sufficiently large and, with the faster flame front
velocities engendered by a too-rapid squish action, may still induce
detonation in a worst-case scenario or slow end-zone burning at best.


Stan, thanks for sharing you software with us!

[Stan Weiss]

Stan, thanks for sharing you software with us!

You are welcome

Using Bore = 4.03, Stroke = 3.48, Rod Length = 5.7, CR = 12:1, RPM = 8500,
Intake Close = 74.125 BTDC, Squish Clearance = 0.04993
Squish Ratio = 35% MSV 47.689 m/s @ 9.805 BTDC

Note that it has moved almost .8 degrees away from TDC

Note you can see this trend in my previous post which has the graph where squish clearance starts at 1.2 mm with each lower line increased by 0.1 mm.

[big block fiero]

Stan, The term trapped volume is something I thought was trapped dynamicly but I now see that this is a static number because a 327 with 12 to 1 static compression ratio would have 3.71 cubic inches (or 60.84 cc's) left in the chamber at top dead center.

I am more versed with the term chamber volume or clearence volume to describe this TDC position. I have seen trapped volume refered to dynamicly but in this case its not so you are correct that we cannot establish volumetric efficiency from this chart.

[Stan Weiss]

Matt the reason I ask how you were calculating VE was your 150%.
The effective cylinder volume is the same as swept volume only calculated using the dynamic stroke or stated another way the cylinder volume @ intake valve close.
I use that term on the 2 stroke page because somewhere I found that is what they (2 strokers) use. On the CR page for 4 strokes I call it "Total Volume".