Compression? How much?

[Jeffrey gagnon]

Ok so i fabricated a copper/aluminum head gasket. I have a iridium tip spark plug bpr8hix with a 16 mm Keihin Carburetor Clone with a dual boost bottle kit i already know alot of people think its a waste of money but i can feel the performance from it anyways so after i fabricated my head gasket took for compression reading was about 150 psi is this an ok number for the mods i have? Also grinded off the little part of the piston skirt on the intake side. I also want to add that i also made a spacer out of an aluminium pop can dropped the compression to 130 psi

 

[Steve Best]

Jeffery, more important than the compression pressure is the squish gap distance between piston and cylinder head.

There are two ways to measure this,
1) With the head off, tape a 0.060" (1.5mm) piece of solder to the piston or head.
Then bolt the head back on.

2) Or drop a curved doubled piece of solder down the sparkplug hole and feel for it to touch the cylinder wall.
(some only use one piece. The piston will cock and give a false reading)

Turn the engine over slowly to feel it compress the lead, then pull it out and measure it.

The goal is to get a gap of 0.5mm to 1.5mm or 0.020" to 0.060" with 1mm or 0.030" about right. (I know they are not the same but it doesn't matter)

This gap is quite critical to keep in this range, if it is and your combustion chamber is a decent shape, you can run a lot of compression (150-160psi or so)
Steve

 

[Jeffrey gagnon]

When you say 1mm or 0.030 your talking about the thickness of the solder right

 

[Steve Best]

When you say 1mm or 0.030 your talking about the thickness of the solder right

When compressed, which will equal the gap. 1mm = 0.039" or round it off to 0.040"
Ideal gap on this engine is 0.030" to 0.040" from my testing. No gain or loss outside these limits.

(this is a stock photo to demonstrate principle, not a China Girl Happy Time engine)
Steve

 

[Jeffrey gagnon]

One last question on this if i raise the compression I know it gives you low end power what will this do as far as my top end power goes

 

[Steve Best]

Very little effect on top end power if you have a tuned pipe.
My KTM 125 will still do 140kph even if the rings are worn to only 90psi.
I drove it home one time with a broken ring and zero compression.
I didn't push it hard but would do 80kph with the broken ring.
Had to keep it reved. Would not idle but it would run at speed.

Did you see my thread where I installed a high compression aftermarket head, gained low rpm torque and lost 4kph?
It was due to the shape, not the compression, but understand compression pressure alone does not relate to power or detonation.

Steve

 

[Jeffrey gagnon]

So it wont do anything to my top end but i can barely hit 30 mph after raising compression was able to do solid 32 mph before? Could my loss be that i regapped the plug to .030 because i was reading some people have it gapped just a little higher then spec i had it at .028 when i was able to hit 32 mph

 

[butre]

bigger gap improves low end power, it can hurt top end if you go too far or if the ignition is on the weak side. I use a very large gap (usually .040) because bottom end is what these engines lack.

if it's a shifter, gap it tiny. single speed needs the broader power band.

 

[Deleted member 12676]

increase compression over 135 psi and it is sure the chrome plating will be coming off due to the excessive pressure outward of the rings against the cylinder walls.
Years ago HS proved you can't have a squish band head on these small engines and not have too high an engine compression.
Shame on all retailers that sell these heads to these completely non-informed buyers. (and shame on buyers for not investigating a topic before buying)
Yes you can increase power by having higher compression but all you need is around 135psi, nothing more. Just use sandpaper and glass to take almost a millimeter off of the stock head to get the right psi.
On my Suzuki 100 it was running too hot in the summer and so I reduced its compression from 145 to 135 and that lowered the heat enough without losing any noticeable amount of power. (actually now it doesn't lose power after running for 15 minutes like before due to the excess heat)
Squish bands were invented to reduce piston heat. Research has already proved that there is no power benefit other than not having power loss due to excessive piston heat.
The research paper is "Squish Velocity in the Combustion Chamber of a 2-Stroke Cycle Engine" by Avraham Ziv of Engineering Div., McCulloch Corp.
Here is my infor page about squish bands: http://www.dragonfly75.com/moto/SquishVelocity.html

 

[Deleted member 12676]

Yeah, people buy high compression heads with squish bands and then add head gaskets to lower the excessive pressure. But that also has the negative effect of lowering engine power because the fuel mixture between the piston and the squish band is basically isolated from the main combustion area, the piston dome. The more space that is there the less the engine power will be. It's a no win situation unless you are handy with a dremel to enlarge the dome area and you also insure the squish clearance is around .85 to 1mm so you don't lose power by having that area too large.