cylinder compression

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woogie_man

New Member
Aug 9, 2013
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Fargo, ND
What numbers should you be getting from these motors? I can't find a solid answer, and am curious. You always hear about high compression heads and what not.

So what would be the break down?

Stock
Medium
High
Stupid high

I am currently getting 85-90 psi on my cylinder now. So curious if this is more, or just right.

Thanks guys
 

Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
That's about right for a stock engine, are you holding the throttle wide open during the testing? and did you run the test on a warmed up engine, these 2 factors can cause a considerably lower reading so try to take your readings on a warmed up engine and with the throttle held wide open.
High compression is like 130 to 140 psi, and stupid high is above 160, this is where ya need to use race gas in your mix to prevent detonation, you can usually get away with 87 octane pump gas with compression up to around 110 or even up to 120. Mine runs just fine using 93 octane premium with 145 compression, but I could probably get away with the 89 octane mid grade fuel in the wintertime.
 

woogie_man

New Member
Aug 9, 2013
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Fargo, ND
Sounds good. Just realized that I was using gas from last summer as well. Plan on getting new fuel tonight and a new plug to try again.

Hoping for good things.
 

Robbie

New Member
May 21, 2015
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Calgary canada
That seems 2 be right on. Stock 90 to 110 psi. I've Shaved my stock head down to 6cc (165psi) with a head gasket. Without one it pings or knocks. (Detanation) running 94 Octane.With stp octane booster.
I have found the stock heads have poor casting tolerances. For example 1 engine I got would only go 40km /hour when Useally my stock engines would go 55km/ hour. So i took off head & noticed that it was way deeper than my others. So i sanded it down with sandpaper & wow it whent 58-60km/hour & that's when I realized how significant compression was to the proformance of our motors. A LOT! The most free power I've got!
Also you can measure The cc 's of your head by using water 1 ml. = 1 cc. I found that 5.8 to 6cc was the smallest I could go without pinging on pump gas (premium) 94 octane.
Just the info. I've got from trial & errors.
 

Davezilla

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Mar 15, 2014
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San Antonio Texas
Since the subject of octane came up, there are cheaper and more effective ways to boost it to meet your requirements... do a Google search on using toluene to boost octane... I've used it before to keep my 300zx happy and it boosts the octane considerably more than off the shelf octane boosters. I used to use VP's C5 octane booster because it boosted octane by 8 points, then I found out that all the octane boosters boost by points, not full numbers, therefor 8 points boosts 93 octane to 93.8 octane, not 101 like they want you to believe... somebody in a 300zx forum made that very clear to me and when I researched it I found out he was right. He later on told me about using toluene and showed me a ratio chart so I could make 100 octane with like a gallon of toluene and about 10 gallons of 93 octane premium gasoline.... the toluene costs under $20 per gallon so using it for a small engine like this, a gallon would last a really long time...
it's actually pretty rare to need more than 93 octane in these engines since the effective compression ratio is a lot less than actual, a good cooling head like the fred, puch, diamond, and a few others will keep things plenty cool enough for pump gas, and a good tight squish band also helps prevent detonation even more... my last engine had a 12.7:1 compression ratio and about 150 psi static cylinder pressure and it ran just fine on 93 octane pump gas, my new engine should run on 89 octane mid grade since I didn't need to ramp the piston to get the desired port timing, and the 66cc fred head with a .6mm squish clearance will keep all the fire away from the edge of the piston where detonation is most likely to occur....
Anyway, back on the octane booster subject, there's a lot of interesting ways to boost your octane without paying too much for it and getting full number increments instead of "points" which are tenths of an octane like most companies advertise...
 

Robbie

New Member
May 21, 2015
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Calgary canada
Thanks davezilla 4 the info. I could use in the summer but my bike is running good with the 94 Oct. With no knocking on my 6cc head. What head are you using now??? I ask because iam thinking about getting a fredhead 5.5cc or 6cc??? Do you or anyone have any experience with one? Witch would you recommend?
Also lastnight I tightened my flower nut on my clutch & got another 6km topspeed out of my bike 81 kmph(it takes a long stretch 2 get up thair) it must of been slipping alot in top gear 7th 14t sprocket on cassette & 11t on the jackshaft. Are you using a shift kit? & should i be able to get more? Iam intrested to know what topspeed other shift kitters are getting? Anyone let me Know? I would love to here It.
.flg.
 

Davezilla

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Mar 15, 2014
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San Antonio Texas
I'm not using a shift kit on any of mine as of yet, but I may add in a 3 speed hub for my stingray project...
I get all my speed from making the engine capable of higher rpm's and my best speed so far with the stock 44 tooth rear sprocket was 43mph which takes around 9600 rpm with 26"wheels. Getting these rpm's is possible by doing extensive port work and correcting the port timing so it can make most of its power at higher rpm but still have enough down low to be able to accelerate on its own. The other thing it needed was a free flowing exhaust, the stock exhaust is very restrictive on these engines and this can be fixed by drilling 3 to 5 extra 5/16" holes into the end cap which will help the stock pipe, but for the most power possible a good expansion chamber pipe with a decent size stinger will make the best power increase once tuned to match the engine. I used a 2013 KTM 50 dirt bike pipe and added an extra 4" to the belly section to bring the power down the rpm range enough to perform on these engines. For the carb I used the stock nt carb which is good for up to about 45mph or so, and the nt speed carb will give about 2 more mph.
For the head I use the 66cc fred head or that other one that looks like a rounded off fred head, they both perform similarly and the Puch hi hi head also works really well since it can keep the engine nice and cool and also has a 66cc combustion chamber.
My combo with the 66cc fred head and a .75mm squish clearance gave me 12.7:1 compression ratio and it ran fine on 93 octane pump gas. I also shaved down 1mm off the top of the jug and 1mm off the bottom as well by putting the jug on a lathe, I shaved the top for a tight squish band and the bottom was shaved to true it up. I had to make a 1mm thick spacer for the base and used a .016 " copper gasket on the base then used a .016 " copper and a .025 " thick aluminum gasket at the head to get the desired squish clearance.
Other things that helped contribute to making lots of rpm was the shorty intake and the port work.
I built another engine for a road bike I'm building now and it's going to have more compression, an 18mm mikuni carb, a kx65 expansion chamber pipe and it has wider ports and slightly more intake transfer and exhaust duration so this engine will have more power and should be able to make power at above 10,000 rpm so I'm shooting for over 45mph with a 44 tooth rear sprocket and a 700c wheel, the 700c wheels are about 27.5" tall so they will go slightly faster than a 26 with the same sprocket size.
I would have to write a 100 page or more book to explain the porting, but there are some simple things to do that'll get your engine to rev really high and make good power at higher rpm.
 

Davezilla

New Member
Mar 15, 2014
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San Antonio Texas
As far as recommending a head, the 66cc fred , the diamond head from jake's, or the puch hi hi head all work very well on pump gas. You can get plenty enough compression from the 66cc heads by setting a tight squish clearance which will also help prevent detonation. The 5 or 5.5cc heads will work and give slightly more compression but this is usually where higher octane fuels are needed, they can still work with 93 or 94 octane depending on what type of pipe is going to be used... with a stock pipe it should be ok, but with a good expansion chamber pipe the cylinder pressure may get too high for pump gas.
 

Robbie

New Member
May 21, 2015
50
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Calgary canada
I noticed you don't have a spring loaded chain tentioner on your single speed bike i have one leftover from my old bike. The one that attaches to your existing chain tentioner. Thay work absolutely awesome never adjust your chain ever again!
I have done a fare amount of Dremel porting too. I use a lot of the info i got from dragonfly's (jaguar) website. The only thing i haven't got into is reed valves so i dont have to go through the exparimenting with boost port holes & the jetting that goes with it. Along with the head shaving i widend & lowerd intake port 1.5mm & exhaust port raised 2.2mm & port matched evreything as 4 the transfers i cleand them up really well. On this cylinder i bought 3 week ago the transfers were allready corrected. It must be new the chinesse must have figured it out. When you turn cylinder on side & look through bottom at transfers thair pointed up towards ex. Port not leaval like thay useto be. Also when you look at the transfer exits thair triangle shaped not rectangle. Not new some are some are not. all the porting got me was 8800-9200rmp under load 10000 no load (i have the tach/hour meter from bikeberry) but since i put on the SBP ex. Chamber w/15inch header length & hd lightning cdi at the same time. it only revs to 7800-8000rpm under load.
Also do you have the fredhead brand (from fred) or the gasbike knock-off? P.S.: god bless TEXAS.flg.
 

Robbie

New Member
May 21, 2015
50
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Calgary canada
Oh! And piston work, ramped on 3 sides not intake side just the edges like in the cr Machine pictures.
put piston two top of stroke (TDC) and marked the skirt that was blocking the top of Intake port and cut off with my rototool & sanded it nice.
I did this after shaving head & before the porting & noticed a definite power and sound increase !!!
Also using the Runtong Dellorto clone carb. It was 4 - 6km faster than the speed carb.
Have you tried the better clutch pads from CR Machine (fred)????
 

Davezilla

New Member
Mar 15, 2014
2,705
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San Antonio Texas
That's pretty typical of the newer cylinders with the slanted transfers, they make a lot of torque but they also fall short on rpm in my experience... I have found a few sources where I can still get the flat transfer jug's and these are best for making high rpm power, they usually sell these on ebay as high performance cylinders for around $25... finding a U.S. seller that has them can be quite a challenge tho so I usually order 2 or 3 at a time. Even the ones with the 40mm intake spacing have the smaller cat eye transfers, but some of the older ones had the flat top tansfers and I'm still looking for a source for these, the one I got from boy goes fast had the big intake, small slanted transfers, and it was pretty much junk as the fins are cast really thin not to mention it came to me with bent fins so that one was written off as a paper weight...
anyway, there are a few other sellers I will try and see if there's are any good or not.
I agree on the Dellorto clone carbs as they flow a lot better then the nt or the speed carb just out of better design, I got a genuine Dellorto SHA 14.14 at the shop but decided on the 18mm mikuni for the new engine since the carb became the limiting factor in my last engine, I still got free revs well past 10k rpm and it would hit about 9600 or so on the road, getting the rpm wasn't a problem but acceleration would be better with that restriction removed by using a bigger carb and free flowing air filter. I had a cns on it briefly but got tired of constantly fixing air leaks, it did make strong power with it but it was always some issue with air leaks or tuning so I put the nt back on since it ran better and still had good power and speed.
 

Davezilla

New Member
Mar 15, 2014
2,705
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San Antonio Texas
For the head, I'm using a genuine fred head, I got a knock off version of the rounded head and it looks good from a distance but the machine work looks pretty sloppy with lots of sharp edges that need to be filed down, that thing sliced me at least once while installing a spark plug and another time when setting the engine in a frame for a trial fit. The combustion chamber is the same 6cc but slightly different from fred's.
This one will be ok after I file down the sharp edges but it also looks like a cheaper alloy so it mat have corrosion issues later on, but I'll keep an eye on that and may need to clear coat it if that problem comes up.
Pretty much all the aftermarket heads like the Fred head, Jake's diamond head, the puch hi hi, and the knock offs will do a better job keeping things cool as well as having much better compression. The knock offs are definitely of lesser quality as far as the machine work goes and most likely a cheaper alloy is going to be used, but they will perform well... for the most part.
I also know Venice motor bikes is developing a new head that should also do very well so I'm keeping my eyes out for when this one comes out.
 

exokinetic

New Member
Mar 18, 2016
108
4
0
Lake Forest, CA
What reading on a compression gauge is correct for our 66/80cc China Girl Engine?

Long Answer?


It depends on a few factors.

1. The Combustion Chamber Volume. Usually measured in CC's (I. E. Fred's 5.0, 5.5, 6.0, 7.0 CC Heads). The Stock China Girls heads, as far as I have seen, come in two flavors. An angle fire spark plug, with low (relatively) combustion chamber volume (I haven't measured it). A straight fire spark plug, with high (relatively) combustion chamber volume.

If everything else was kept the same, the reduced combustion chamber volume of the angle fire head would result in a higher compression ratio, and thus, a higher reading on the compression gauge.

Conversely, on the same engine, the increased combustion chamber volume of the straight fire head would result in a lower compression ratio, and thus, a lower reading on the compression gauge.


2. The Piston Dome Volume. This is difficult to measure, and is usually provided by the piston manufacturer (balls!). If the piston had a flat top this measurement would be 0. A dome on the top of the piston (like our lovable China Girl) results in a positive measurement, in CC's, equal to the volume of the Dome Area above the piston crown. Due to loose manufacturing tolerances there are minor variations in this volume, witch would result in minor variations in the reading of a compression gauge if all that was changed was a piston of the same type.


3. The Head Gasket Thickness. The head gasket lifts the head higher relative to the piston crown at top dead center. This means a thicker gasket will result in increased combustion chamber volume, reduced compression ratio, and thus, a lower reading on the compression gauge.

3a. Special Tip: Different thickness head gaskets are often used to adjust compression ratio. This has the unfortunate effect of changing the squish gap, google "squish band" if you want to learn more.


4. The Deck Height. This, in the most basic sense, is a measurement (mm or in) of the distance between the piston crown (the topmost edge of the piston) at top dead center, and the cylinder deck. If the piston crown is below the cylinder deck at top dead center, the measurement is positive and represents "positive deck" (because the deck of the cylinder is above the the piston crown. If the piston crown is above the cylinder deck at top dead center, the measurement is negative and represents "negative deck".

The greater the Deck Height (or the higher the cylinder deck is above the piston crown at top dead center), the more area is left between the dome of the piston at top dead center and the head. This results in higher (relatively) combustion chamber volume, lower compression ratio, and thus, a lower reading on the compression gauge.

Factors that effect Deck Height:

A. Crank Stroke: If using the same piston and cylinder without modification, reducing crank stroke from say 40mm (long stroke) to 38mm (short stroke) would increase deck height by 1mm. 40mm - 30mm = 2mm. 1mm reduced piston travel at the top and bottom of its stroke. This would increase combustion chamber volume, reduce compression ratio, and thus, result in a lower (relatively) reading on the compression gauge.

B. Connecting Rod Length: Even if the actual swept stroke measurement of the piston remains the same (same crank) if a 1mm shorter connecting rod is used, the Deck Height is now increased by 1mm. This results again, in increased combustion chamber volume, reduced compression ratio, and a lower (relatively) reading on the gauge.

B1. A little secret. No two connecting rods (even if marked the same) are EXACTLY the same length. Even these seemingly tiny variances in length will have an effect on the reading on the compression gauge. If there is a tolerance variance large enough to change the position of the piston by 0.010" up or down, it will result in about a 10psi change in the reading on the compression gauge (for example, changing out a head gasket for one that is 0.010" thicker or thinner)

C. Cylinder Height. The height of the cylinder, measured from the bottom face that seals to the crankcase, to the top face that seals to the head. A 1mm taller cylinder than your current one (I have seen MANY different heights in the wild) will result in a "taller" deck height. This will again result in increased combustion chamber volume, lower compression ratio, and thus, a lower (relatively) reading on the compression gauge.

D. Cylinder Base Gasket. The cylinder base gasket (obviously) lifts the cylinder by an amount equal to the thickness of the gasket. A thicker cylinder base gasket will lift the cylinder, resulting in a taller deck height, increased combustion chamber volume, lower compression ratio, and thus, a lower (relatively) reading on the compression gauge.

E. Piston Wrist-pin Location.The distance between the piston wrist-pin hole and the crown of the piston. There are obviously two standard piston wrist-pin locations we have in our China Girl engine, the type A and the type B. If everything else stays the same, and you switch to a piston with a higher wrist-pin location (relative to the piston crown) the piston will be lower in the cylinder at top dead center resulting in taller deck height, increased combustion chamber volume, reduced compression ratio, and thus, a lower reading (relatively) on the compression gauge.

E1. Just like the connecting rod, small tolerance variations in individual pistons means, generally, no two pistons will ever achieve the EXACT same compression reading, in the same engine, at the same time. (Again this relates to manufacturing tolerances. In precision manufactured modern engines from major manufacturers this is not an issue. In our "loose tolerance" machined China Girls, careful selection out of a batch of the "same" part will result in the ability to assemble engines with much finer tolerances from parts that are otherwise not very consistent from part to part. This is what seperates a skilled engine builder -among many things- from someone who just knows how to assemble all the parts together in the right order.)

D. Other Variances. There are other variances that play a role, such as the free-play up and down in the big and small rod end bearings, and their pins, as well as free-play in the crank bearings in the case. The case casting itself, and how low or high the cylinder mounting face is to the crank axle center-line. The higher (relatively) this face is in relation the crank axle center-line, the higher the cylinder will then be. The higher (relatively) cylinder now results in a taller deck height, increased combustion chamber volume, lower compression ration, and thus, a lower (relatively) reading on the compression gauge.


5. The Condition of the Rings. Brand new rings will not give as high a reading (relatively) on the compression gauge as rings that have been properly worn in. Once the rings are sufficiently worn in they will provide their maximum available compression reading. After this point the compression reading will steadily drop as the rings wear, and more and more blow bye is allowed past the rings, reducing compression. As a VERY GENERAL rule of thumb: Once the reading you get on the compression gauge is 20-25% smaller than the reading you got when you installed brand new rings, it is time to freshen up the top end. For us that means new cylinder and rings, I will usually re-ring the piston once before I replace the cylinder though.


6. The Pre-MiX Ratio used for the Oil/Gas Mix. Simply put: The more oil you mix in your gas, for example: 20:1 vs 50:1, the higher the reading you will see on the compression gauge.


Short Answer:


As you can see, due to the different types of parts that are used to assemble our lovable China Girl engine, as well as the various "loose tolerance" variances involved in its manufacture it is literally IMPOSSIBLE to tell you what your engine "should" achieve in a compression test.


The best I can do is give you general guidelines.


If it reads below 80psi on a brand new engine, don't expect amazing performance, but it should totally start as long as it has somewhere around 40-50psi... and there isn't something drastically wrong inside the cylinder.

Anything from 90-120psi is smiles all day long, good stuff for a stock engine, and not too shabby for a mildly upgraded engine either.

120-150psi is performance engine territory, definitely have to keep an eye on engine temps, and it would generally be recommended to keep the oil mix at 32:1 or richer (i.e. 24:1, 20:1, 16:1).

150psi+ is extreme performance engine territory, and you had better know what you are doing.

I run my engines anywhere from 230-250psi, but they are strictly race engines, they are not run on the street, only at race events. I run a big Jake Head, 16:1 oil mix, 0.025" squish gap. I tune the ignition advance and the carburetors judiciously, run oxygenated race gas (oxygenates help cool the cylinder) and I keep a SHARP eye on my spark plugs.

After all of this, I get roughly 10-20 hours of run time before I have to rebuild the top-end... If it makes it that long.

Luckily I always have a new porting idea I want to try. When I finally tackle exactly what I want out of the port arrangement I am going to pony up for some Nikasil. Gotta love that stuff.


.... I could go on for a few more pages on the subject, but I think I'll leave it here for now. If anyone has anymore question don't hesitate to ask... I like typing.




Sean Davis
Apollo Moto Racing
https://www.facebook.com/ApolloMotoRacing/