Flash IV

[evilone]

push the solder to the wall of the cylinder to get the squish measurement. If its between 1mm to .6mm your good.don't run to high of a compression if using pump gas.engine knock is going to be hard to hear with the carb noise and exhaust noise.you will hear it better around 1/4 throttle if there is engine knock going on.for the mini stroke 135psi is about max for 92 octane pump gas.

Just wanted to add my 2 cents. Too high compression not only will cause engine knock. It will give you more low end torque but less top end.

[Wheelman-111]

Greetings:

Thanks, EvilOne. So do you say 150PSI is "too high" for premium pump gas? Remember Flash II was too high at 175, but settled down after I added the base gasket back in. I regret I never re-checked the compression afterwards.

What underlies your assertion that higher compression hurts on top? I've always believed it increases output at all ranges ... until it blows up.

[evilone]

Too high compression slows the rpms down at higher speeds since motor is working harder to compress the fuel mixture in a smaller area. And now a quote out of Jennings book

"Maximum power will be obtained at cranking pressures somewhere between 135 and 165 psi. Going higher with compression, in a conventional motorcycle engine, can give a neat boost in low speed torque, but the thermal repercussions of higher cranking pressures will surely limit maximum output"

and

"One of the most undesirable side-effects that comes with too-high compression ratios is an enormous difficulty in getting an engine to "carburet" cleanly."

So you got a harder working motor and more heat and too much heat also robs power. But, looks like your 150 psi is dead center between the 135 and 165 psi.

[Wheelman-111]

Greetings:

Thanks for the citation. I agree with everything except:

you got a harder working motor and more heat and too much heat also robs power.

I believe the reason it "robs power" is that the piston is eventually launched into one's Nether Regions.

But, looks like your 150 psi is dead center between the 135 and 165 psi.

I really like the sound of that! I'll resist the temptation to get it closer to 165 on this Daily Driver. Still considering whether or not to wear Kevlar BVDs.

[evilone]

[bakaracer]

Too high compression slows the rpms down at higher speeds since motor is working harder to compress the fuel mixture in a smaller area. And now a quote out of Jennings book

"Maximum power will be obtained at cranking pressures somewhere between 135 and 165 psi. Going higher with compression, in a conventional motorcycle engine, can give a neat boost in low speed torque, but the thermal repercussions of higher cranking pressures will surely limit maximum output"

and

"One of the most undesirable side-effects that comes with too-high compression ratios is an enormous difficulty in getting an engine to "carburet" cleanly."

So you got a harder working motor and more heat and too much heat also robs power. But, looks like your 150 psi is dead center between the 135 and 165 psi.

thats before breakin.on the mini stroke I would lower it just a little to 135-140 before breakin.you can also run 2 head gaskets too.

[Wheelman-111]

Greetings:

Ah, 2 head gaskets - great idea. But where to find a 51mm gasket? Remember this is JW, and shipping takes 2 weeks.

I suppose I could make one, but with my skills that would also take 2 weeks...

I'm thinkin' Baka you have some specific inside scoop on this particular crankshaft:

"On the ministroke I would lower it to 135..."

You're not holdin' out on me, are you brotha? Do you know of any other Ministroker Tuners who tried to fly too close to the sun?

[bakaracer]

the guys in hawaii that ran to high a compression blew up.plus alot of them are young and don't know that you can't run high compression on pump gas. As for making a 51mm head gasket, get a stock head gasket and trace your 51mm one out on it and grind it out with the dremel and grinding stone.

[Wheelman-111]

Greetings:

I don't have a stock head gasket, but still have a Malossi 47. Unfortunately checking it against Flash IV's it appears to be from an AF05 - the stud centers are off quite a bit. I may still be able to make it work, but don't like this additional variable on this critical sealing function.

Did some final pre-assembly work on the cylinder tonight. The Ruima cylinder features a Logo:

Now my Chinese is a little rusty, but I think it says "Port this Mutha!". And so I did. First the exhaust outlet was black, rough and full of wavy casting swirls. Cleaned up a bit:

Before final polish to "Satin" finish. Raised the top 0.5mm and added chamfer back. I decided not to enlarge the outlet much. Too much work, and unsure how the shape changes would affect flow. No flow bench in the Boudoir. Crudely knife-edged the transfer port "splitter".

This pic gives a peek at the final exhaust port finish better than the one above. I called it "Done" and bolted it up with some Honda 98mm flange bolts PN 90157-ZA8-730, which neatly make up for the 2.25mm added base gasket thickness. A little extra thread length sure can't hurt. 12 Ft.-Lbs.

[Wheelman-111]

Greetings:

The bits & pieces from Far & Wide are nearly all there. Some commercial vendor scores, but mostly bits from the Good Forum Folk. Thanks to all I'm forgetting/omitting.
JC of Z50R-Blank has been a huge help (pump, starter, fan, manifold barb and great advice)

as has DaKine -

kickstarter parts and more great advice. Also the Fastest Shipper ever, period.
Burnt_Toast has been more help - both directly in terms of parts/advice and in terms of inspiration - than he knows.
Many others contributing half-shroud here, a fender skirt there and a few hundred slow-trickling dollars out of Wheel-budget later... You know who you are. Thanks.

I'll document final assembly as it proceeds.

[Saved]

Is the cat Metric or SAE?

[z50r-ghost]

Thanks for the appreciations, Mr. Wheelman

You are too kind....

later, jc

[z50r-ghost]

i was blushing so much i forgot to mention - NICE LOOKING MOTOR and CAT!!! So squeaky clean!!

-jc

[Wheelman-111]

Greetings:

Saved Axt:

Is the cat Metric or SAE?

I dunno, dude. Z50R-Blank sent me the cat in the box with the starter and stuff. I think it's part Siamese. I plan to use it as Auxiliary Power. Rigging up a treadmill under the saddle... Catnip throttle control.

[noiseguy]

My 2 cents. I think at 150 PSI you're fine. Running premium will lower the combustion temps a bit and might be advisable. It's also cheap to do on a scooter.

You had problems at 175 PSI? We should probably get a chart started with a PSI --> Octane chart for these engines and throw it in the Wiki.