Crankcase compression for experts

[Nosubforcid]

More reversion than you might think upstream of the reeds. Once that column of air (or air and fuel) is moving towards the reeds it too has momentum so when the reeds slam shut the energy in the column is reflected. The reflected wave, depending on: rpm, engine design and intake tract length, may make it all the way back out the carburetor(s) or throttle body(ies). On most carbureted or non-crankcase efi outboards you can observe this phenomenon quite easily with a bright flashlight.

[Mark75H]

Are you sure? Does it have to be dark?

I don't think it is significant enough on most motors to even consider......I'm going to check it out at my next oppertunity.

Back to the animation. Watch very closely......there are 2 separate bursts in the intake track animation....one exactly at BDC and the second distinctly different after BDC while the piston is rising bringing in the fresh charge. I wish I knew how to slow down a gif animation.

I've changed my mind about what he is showing at BDC. Now I agree with CaptCarb. He is showing the expansion chamber pulling so hard it lowers the crankcase pressure enough to open the reeds at BDC even before the piston starts going up.

An intake length tuned to have a resonant wave headed in at the same time is a winner (at that rpm).

[captcarb]

I Don't know much about this subject. I put the GIF there to stimulate a discussion because I do not understand the last phrase in NOSUB's explanation.

It came from this web site which does not help much:

http://w3.one.net/~jschust/animations.html

At BDC all the ports are open and I assume the low pressure from the supercharge effect is pulling mixture by the reads if the GIF is correct. Look at which way the fuel is going at BDC, it looks to me like it is going towards the crankcase. The pictorial of the air is confusing, but it looks the same to me during the intake stroke. I think it is a pretty good picture of his explantaion, but does not demonstrate what crankcase volume has to do with it.

NOSUB,

I am sure you have tried this and dynoed it, so you know what works.

[BRENT GILL]

this was a very good exchange of information something i almost never find in any magizines. this is a great web site . thanks to all that had imput. brent

[Mark75H]

With a new version of Apple's Quicktime software you can stop the action or move it along to any point by manually moving the slider. Stopping at BDC you can see the artist depicts the fuel air charge bypassing the crankcase and going strongly and directly into the transfer ports to the cylinder.

[Mark75H]

The supercharge effect Nosub refers to is this:

Let's say that just after the transfer port is closed by the piston going up......and the exhaust port is still open.......first that the cylinder pressure is the same as atmospheric......and second that bit of green stuff that has snuck all the way out into the exhaust header pipe is then pushed back into the cylinder by the returning pressure wave from the end of the expansion chamber. At this point the exhaust port closes before the burst can effectively reflect out of the cylinder. A high pressure gulp has been captured in the cylinder......several psi above atmospheric.


Actually it can be even more complicated than that. The resonant effects can allow the pressure to be above atmospheric when the transfer port is closed and become even higher when the returning wave works its way back into the cylinder.

If you want to see expansion chambers in use on racing outboards check the PRO racing schedule on APBA's website.

You can see that an expansion chamber works best in a narrow RPM range maybe 2,000 or 3,000 rpm around the tuned length. A longer pipe works for lower rpm because it gives more time for the sonic waves to travel to the far end and reflect back to meet the piston closing the exhaust port. A shorter pipe works for higher rpm so the reflected wave can meet the piston sooner. There are several design things that can allow expansion chambers to be more effective across a wider rpm range. The first expansion chambers used on racing outboards had very short cones on the end where the stinger outlet pipe is. This was 1962. These gave best max rpm power, but a very very narrow powerband. I have driven a race boat with a pipe that worked like that......at the end of the straight it leaped forward with a sudden burst of speed. Giving away a little bit of the very top end power by using a longer tapered cone before the stinger makes the power come sooner (and stay a little longer above the peak power rpm). In racing, accelleration is as important as top speed....we are only at top speed for a fraction of a second before we are turning again and need to accellerate again.

[Mark75H]

The next trick racers came up with was a variable length pipe. For part of the header close to the motor there is a section where 2 pieces overlap, one inside the other, fitting very tightly but not so tight that they don't allow the outer pipe to slide back and forth.

Coming out of a turn the pipe is used extended and near the end of the straight it is shortened.....tuning the pipe to the motor's speed while underway!

Usually a spring pulls the pipe to the long length and a cable operated by a foot pedal or hand lever in front of the throttle is used to pull it shorter for high rpm.

Most alcohol burning APBA PRO racers use something like this.

Another variation of underway tuning is done by taking advantage of the speed of the sound wave in the pipe. Sound travels faster in hotter air than cooler air. By spraying water into the expansion chamber the sound wave can be slowed down enough to compensate for lower motor rpm. This can be used on open stacks as well. Mercury used it on the famous open stack 1250 and 135 BP engines just before the Twister era. Some PWC's use computer controlled variable water injection to cool their expansion chambers to tune the pipe to the motor rpm.

I have considered automating the underway tuning by having a water pressure controlled valving arrangement....maybe within 10 or 15 mph of max speed water pressure on a regular speedometer pitot could shift a controller to change the pipe length or water injection.

[Terry Jackson]

What are the power gaines by increasing crankcase pressure & what recommendations would you have for someone building a v6? For example, I have heard of milling the case half .040" to reduce volume, is the gain noticable?
How do crankcase gaines compair to reeds, shaved heads, etc?
I have a 1989 2.4 200 project motor, and want to do all I can without getting to a radical race engine.

[Mark75H]

Milling your crankcase won't tighten it up.....it will make it out of round.

Crankcase milling (and align boring back to round) was a trick that was done on the old inline Mercs that didn't have removable cylinder heads. By milling the block you moved the crankshaft (and pistons) closer to the head and increased the compression.

You guys have basically never seen a "loose" crankcase, all the modern stuff is as tight as it needs to be.

Back in the 1950's little Merc inline twins and 4's had their crankcases stuffed when they were upgraded to run in the alcohol classes, Merc stuffed the Mark 20H compared to the Mark20 and Mark25, O.F. Christner sold stuffers to tighten up the split case 4 cylinder Mercs and also made super tight split crankcases to replace the Mark20 single piece crankcase. Some of the 1960's drag racers stuffed their 89, 93 and early 99ci inline Mercs for drag racing, but I don't think any racing outboards ever benefited from it after that.

[Tony Brucato]

Sam,
The case half is milled at the intake side of the front half to sink the reed cages in towards the crank. No line boring required.
I've often thought it would be beneficial to be able to adjust the intake runners on my SVS while under way just like you've described about the expansion chambers.
I've run engines on the dyno with my SVS set at different runner lengths and witnessed the effects of the expansion and compression waves by using a phased strobe light. By adjusting runner length you can induce air flow through the reeds past TDC for a super charging effect. Because the tuned length is RPM specific, the synchronized RPM/length adjustment would be very attractive.

Tony

[espen]

Hi Tony !
Maybe you would like to buy one idee I have, that is a trottle
plate for merc with varying runner lenght. You have probably greater possibility to make buissness out of it than I am.
Be shure to have one extra channel on your ecu to control it !

Have a 280 hp powerhead on hand and need to raise some money fast :-)

Thanks
Espen

[Tony Brucato]

Espen,
Thanks for the offer, but I'm working on a couple of my own ideas right now.
Good luck with the powerhead.

Tony

[Laker]

What would be more benifit in general on a modified motor.
Lightened pistons with the undersides cut away = less crank case volume or stock weight pistons with more crank case volume? Is the lighter rotational weight worth more than increased crank volume?

[Bill Rogers]

Sam, why do you say the "Two Stroke Tuner's Handbook" is highly criticized? I'm new to this board and I've apparently missed some lively discussions!

I've used Gordon Jennings' formulas and methodologies to build some pretty stout OMC crossflows. I've carefully surveyed and calculated time-areas for several engines and the numbers on the exhaust side always fall neatly into place. The numbers on the transfer side always come up short, but they're consistent and there's a useful pattern that I've been able to put to good use.

I believe the discrepancy on the transfer side is due to the suboptimal conditions in the exhaust system. Jennings' numbers assume a properly designed expansion chamber exhaust system.

[Mark75H]

Some people like to point out how old the book is, but I don't know of any errors in the book. There are 2 newer books, with references to newer motorcycle models, but I don't see much really new in either of them other than explaination of how a variable exhaust port works.

Laker, I think balanced pistons and blue printed ports would be of more benefit than either mod.