Dio, budget 72cc kit has 215 psi compression

MediumKahuna · Post by **MediumKahuna** » Thu May 21, 2009 4:28 am

Trying a budget build of a 94-01 SA50P with 72cc kit from Taiwan, a used Malossi head and a Typhoon pipe.
I used the thin paper base gasket and fairly thin aluminum head gasket that came with the kit (seems to be holding).
I'm getting 215 psi compression which seems high to me.
That's 14.6 to 1 compression ratio, I think (215 / 14.7 I live at sea level).
I have to put on shoes to kick start it (I don't like to wear shoes).
I just started breaking it in (with stock carb) so I don't know yet how its going to perform but it sure seems like it wants to go like *.

This is what I'm guessing I should do about it (please give me your opinion / suggestions):
I think I should shoot for 11 to 1 compression ratio (I think that's 162 psi compression).
I'm guessing a thicker base gasket would be better than a thicker head gasket or grinding out the head.
The stock crank in it has a 41.4 mm stroke so:
(41.4 / 11) - (41.4 / 14.6) = 0.93mm thicker gasket (to get 11 to 1).
I'm thinking about making a base gasket out of 1.0 mm thick copper.
Will it blow up if I leave the compression at 215?
I have a 24mm carb and new intake that I want to put on it when its broken in.

Also I just want to say that the guys at VT are great.
They've been very supportive of my project even though it is obviously a little stupid, I'm obviously a noob, and I didn't even drop that much cash on them.
Its been fun so far.

wiguy05 · Post by **wiguy05** » Thu May 21, 2009 9:14 am

Please tell me your using race gas with that much compression?

Otherwise I don't think that engine is going to hold together very long, if at all.

MediumKahuna · Post by **MediumKahuna** » Thu May 21, 2009 9:42 am

wiguy05 wrote:...tell me [you're] using race gas...

I want to run 94 octane so, tell me, what's the best way to lower my compression?
And what compression do I want?

Also, can anyone verify that:
Compression Ratio = Compression divided by 14.7 (at sea level)
or tell me what the formula should be?

Also I'm running a PC, Typhoon pipe which seems really sweet so far

Post by **noiseguy** » Thu May 21, 2009 10:22 am

Woah. Don't run it like that; you'll burn up a piston or bearing, esp. on an air-cooled bike.

Get a more traditional aluminum head gasket (or two) and stuff them in there to raise the head and lower compression.

I'm assuming that you're running the stock base gasket. Whether you raise the head or jug depends on how you're trying to set the porting. As a start, the piston should reach the top of the jug (and no farther) at TDC and at BDC the exhaust port should be fully open. If that's happening, then leave the jug alone. Otherwise make the appropriate adjustments. As to which is better: Start with this and then experiment with jug height.

I'm not up on what these bikes can handle, but I think that 162 PSI and premium fuel will be a safe combo; 4 cyl Hondas run about that compression, and on regular fuel.

MediumKahuna · Post by **MediumKahuna** » Thu May 21, 2009 10:38 am

noiseguy wrote:...Get a more traditional aluminum head gasket...

OK, Thanks.
This is the kind of input I'm looking for.
But, I can't find a traditional head gasket for 72cc kit.
Any suggestions on where to look or how to make one?

The service manual says, "114 to 171" psi compression and the Polini kit specs say, "11.7 to 1" compression ratio so, I'm pretty sure the bottom end can take 162.

Arnadanoob · Post by **Arnadanoob** » Thu May 21, 2009 8:25 pm

Definitely don't run that kind of compression unless it's a race only, built and reinforced to be that way and a limited distance per run bike.

Having super high compression doesn't mean it's good, in fact having just high compression alone doesn't necessarily mean it's going to be good either. At some point having all that extra compression won't yield the kind of power that justifies all the extra heat and metal fatigue it's going to cause in the end.

Tuning it to be around 150ish is a safe starting point since it's the midpoint of what the service manuals will tell you, which will usually be between 110 to about 170ish I believe.

maddog · Post by **maddog** » Thu May 21, 2009 10:07 pm

get a new psi guage, cuz you are way off- lucky if you get 143 psi, my polini evolution 70cc gets 165 psi. yes u-r a noobie

Arnadanoob · Post by **Arnadanoob** » Thu May 21, 2009 10:42 pm

Well, I'd agree with you if he was using a complete kit from 1 brand. From what I read he's mixing and matching parts and that can yield odd results. I've seen over 200psi on a few bikes before that involved wrong parts being matched up.

A Polini Evo is far different from a Taiwan bore + Malossi head combo. There's more than 1 Taiwan brand out there.

Post by **noiseguy** » Thu May 21, 2009 11:54 pm

The ultra-high compression is unusual. What head are you running?

Ultimately, you haven't moved the 4 head bolts, right? So start with a stock Honda head gasket and file out the center to the right diameter. At least 4 of the holes will be in the right place and size. These are 0.020" thick. At about $5 each, buy a couple.

MediumKahuna · Post by **MediumKahuna** » Fri May 22, 2009 3:27 am

I made two head gaskets out of 0.025" copper for a total of 0.050" and replaced the head gasket that came with the kit with them.
Now I get 180 psi which is 12.25 compresion ratio.
This is real close to what my formula predicts so I think I got the formula for calculating what compression results from a given gasket thickness right.

maddog wrote:...my polini evolution 70cc gets 165 psi. ...

I calculate 11.2 compression ratio for 165 psi compression which also sounds about right for a top of the line Polini kit. Thanks for sharing, Maddog.

I agree with you guys that I should be down around 150-160 psi.
Also my piston doesn't quit uncover my exhaust port at BDC (thanks noiseguy for that insight).
So If I use the head gasket that came with the kit (about .027 thick) and add 0.040" - 0.050" of base gasket I should be golden.

What I need to know now is a better way to make gaskets from copper or aluminum. I used a zip bit in a drill press and a grinding bit in a Dremel tool which was a huge pain. My friend has a plasma cutter but I'm afraid it will be like trying to crack an egg with a machine gun.
Maybe I'll just buy a whole stack of base gaskets like another guy I saw in here somewhere.

Anyway thanks a lot for the help. I learned a lot.

Here's the formulas I verified during this exercise:
Compression Ratio = Compression (psi) divided by 14.7 (atmospheric pressure).
Don't forget to hold the throttle wide open when checking compression.

The thickness of gasket you need to add to lower your compression ratio by a certain amount equals:
(stroke / CR you want) minus (stroke / CR you got)

For example if you got 180 psi and you want 155 psi compression then:
the compression ratio you got is 180 / 14.7 = 12.25
the compression ratio you want is 10.5 = 155 / 14.5
The stroke on a 94 up Dio is 41.4mm
(41.4 / 10.5) - (41.4 / 12.25) = 0.56mm or 0.022inches of gasket to add.

Also I have two 0.025" copper head gaskets available that cost me 2.5 hours labor each.

tru72 · Post by **tru72** » Fri May 22, 2009 3:36 am

Get that compression down to like 135/150 and it will run way better. Sometimes more power isnt the key in building these motors.
longevity is the key.

scooterwerx · Post by **scooterwerx** » Fri May 22, 2009 10:32 am

for the head gasket use either copper or 1100 series aluminum in the dead soft condition. you can cut that stuff with shears, or a stout pair of scissors even. leather punches work great for smaller holes.

Wheelman-111 · Post by **Wheelman-111** » Fri May 22, 2009 10:38 am

Greetings:

MK Submits:

My friend has a plasma cutter but I'm afraid it will be like trying to crack an egg with a machine gun.

Isn't that the Breakfast of Champions? Very funny!

My Polini kit with unmodded stock head yielded just over 135 last time I checked. It's clocked about 900 miles.

Building my Malossi 47/65cc kit with Scooterwerx's help on o-ringing and chamber mods, I was able to eliminate gaskets altogether. My piston easily clears the exhaust port and comes within a whisker of the cylinder rim at TDC. (~0.75mm) However Werx enlarged the CC enough that it yielded 143 (cold, just a squirt of GN-2, and only my POS Harbor Freight drill to spin... it might be 150)

I'll be installing and trying it out next Monday.

I'm with Noisy: Making your own gaskets has a rewarding flavor, but at $5.00 per hour it may not be the best use of your precious ScooterWrenching time. I've never tried a plasma cutter, but I saw a pro cut a hole in our stainless-steel countertop with remarkable precision.

I've enjoyed reading and learning from your experience and good humor. Keep it coming!

MediumKahuna · Post by **MediumKahuna** » Fri May 22, 2009 10:57 am

tru72 wrote:...longevity is the key.

I hear you man.
This bike is going to a kid that will probably run it pretty hard and, of course, he doesn't have cash for another rebuild (hence the low cost kit and used parts in the first place).
It doesn't have to go that fast.
I only promised him that it would beat his buddy's stock Eaton Beamer (which does 41 mph).

I'm really liking this forum. You guys are great.

MediumKahuna · Post by **MediumKahuna** » Fri May 22, 2009 11:09 am

scooterwerx wrote:...in the dead soft condition. you can cut that stuff with shears...

I did use dead soft copper but, at 0.025" thick its hard to cut and doesn't grind well. I was afraid shears would bend it up and stretch it too much.

From your suggestion, I'm thinking I could use more layers of thinner stuff that cuts easier and come out with less total work.
Thanks

Honda Spree and Elite 50 Forums

Dio, budget 72cc kit has 215 psi compression

Dio, budget 72cc kit has 215 psi compression

I made two head gaskets out of 0.025" copper

The Breakfast of Champions