Check your engines cranking pressure

[Deleted member 12676]

In Blairs research paper about his dyne design and a karting engine there is this horsepower graph for the same engine with the same expansion chamber but with two different baffle cones (the last cone). One was normal and the other one had holes drilled in it to lessen its effectiveness at returning a pressure wave back to the engine to boost dynamic compression. The one with less dynamic compression gives less peak horsepower boost but gives a longer powerband. The one with a normal baffle cone (12.2 degrees from center) has an awful power dip right where it should show the highest power boost. Why? Air cooled 2 strokes shouldn’t have more than 135 psi cranking pressure which translates to around a 7:1 compression ratio at sea level. This engine had a 10:1 ratio and so when the dynamic compression was made too excessive then something bad happened (overheating or detonation) which reduced the power. I wouldn’t doubt that most of the aftermarket heads sold for these chinese engines increase the compression ratio to at least 9:1. Also the stock slant head greatly increases the cranking pressure. Be warned. Everyone with a 2 stroke should have a compression tester. Professional engine builders know that as you increase the compression you also have to retard the ignition timing. But these engines aren’t designed to adjust the timing like you can with a motorcycle engine. You really need to know what your engines cranking pressure is. Here is the link to the research paper: http://www.dragonfly75.com/motorbike/dyno-blair.pdf

 

[Deleted member 12676]

135psi :
1) keeps the head temp from going too high which can :
A) warp the head if too hot
B) reduce power from the piston getting too hot
C) increase possibility of seizing from excess heat
2) keeps bearing load from being too much which can :
A) shorten bearing life
3) keeps the peak cylinder pressure from being too much for those with standard CDI's* which can :
A) keep the bearing load from being too much
4) Keep the cylinder pressure from increasing to the point that it contributes to flaking off of the cylinder plating.

* standard CDI's advance the ignition timing too much for any cranking pressure more than 90psi (see my page on the subject)

This advice is for those who live where the summers are often over 95 degrees F, and who ride at full throttle for prolonged periods of time.

 

[Steve Best]

Jag, I can testify from personal experience, you are right.
135psi is not a distinct line in the sand, but it is a darned good indicator of where the problems begin. If you run at 135 you can build a darned peppy engine that will last well for thousands of kms (if everything else is in line). If you raise that pressure to 150psi you will gain a bit more low end power and a very slight gain through the rest of the power band (if you don't get detonation) but you will suffer accelerated chrome stripping off the cylinder wall, higher temperatures and I can tell you from experience the rod bearing wears faster.

Nikasil cylinders are a bit more forgiving. We look at 165psi being "that point" with 185 being the crazy point where you need a witches brew for fuel. he MX crowd looks at 100-120psi as being worn out. I have run enduro harescrambles with as low as 90psi and still start on first kick and competitive (in the "old man" class!).

 

[butre]

I've pegged out compression gauges on some of my racing motors, and get a full season out of them. I estimate my current racing motor is turning 230 PSI. nothing particularly magical about the fuel, just regular old 100ll. that said, compression doesn't matter near as much as people think. I've built some seriously fast endurance racing motors that barely cranked 80 PSI. lower compression ratio = less work wasted compressing the charge = more power to go to the wheel. you make less power to start with, but also waste less between power strokes. as with everything, there's a good balance somewhere. unfortunately where that balance is isn't something you can just tell people online, it depends entirely on what the engine is being used for. lots of putting around at 3000 RPM? crank it up, you'll make more power and use less fuel, barely noticing a difference in longevity. drag racing? crank it up, the runs are so short the hit doesn't matter. endurance racing? feather touch, you need something that can handle high RPM for extended periods without beating up the motor or the rider. daily driver trying to keep up with traffic? that's the point where you have to find a good balance. normally I'd say 135 for the cheap chrome plating they use, but really the prices for a new jug are so low I say somewhere in the 175 PSI range is perfectly fine. just keep a fresh jug and rings on hand. I run 180 PSI in my daily driver and get enough mileage out of a jug that I lose track. by the time I have to spend another $25 on a jug, I'd have had to do an oil change in a 4 stroke anyway.

 

[Deleted member 12676]

Looking at a % thermal efficiency gain to compression ratio chart the gain from something like a compression ratio of 6 to 9 is only 8%.
8% of 3 horsepower is only .16 which is nearly nothing (and this only counts if the thermal efficiency % translates to horsepower %).
People sacrifice their engines reliability for .16 horsepower? really?
You can increase power without sacrificing reliability.
by doing things such as;
better porting
better carburetor
better air filter
aftermarket CDI
better head gasket
longer intake tract
iridium spark plug
a motorcycle piston/rings that don't have the crazy wide ring end gap that these chinese engines have
putting disulfide powder on the cylinder to reduce friction
jetting your carburetor to near perfection

 

[gary55]

In Blairs research paper about his dyne design and a karting engine there is this horsepower graph for the same engine with the same expansion chamber but with two different baffle cones (the last cone). One was normal and the other one had holes drilled in it to lessen its effectiveness at returning a pressure wave back to the engine to boost dynamic compression. The one with less dynamic compression gives less peak horsepower boost but gives a longer powerband. The one with a normal baffle cone (12.2 degrees from center) has an awful power dip right where it should show the highest power boost. Why? Air cooled 2 strokes shouldn’t have more than 135 psi cranking pressure which translates to around a 7:1 compression ratio at sea level. This engine had a 10:1 ratio and so when the dynamic compression was made too excessive then something bad happened (overheating or detonation) which reduced the power. I wouldn’t doubt that most of the aftermarket heads sold for these chinese engines increase the compression ratio to at least 9:1. Also the stock slant head greatly increases the cranking pressure. Be warned. Everyone with a 2 stroke should have a compression tester. Professional engine builders know that as you increase the compression you also have to retard the ignition timing. But these engines aren’t designed to adjust the timing like you can with a motorcycle engine. You really need to know what your engines cranking pressure is. Here is the link to the research paper: http://www.dragonfly75.com/motorbike/dyno-blair.pdf

If we are talking the chinese knock offs Most I have encountered actually lower the compression as they are 8cc heads and the stock slant plug head is 6cc. I have received one knock off that is a 6 cc head and it only equals the stock head. It's my experience that cylinder work is required to maintain stock comp. with most most of these heads and the larger chamber allows to adjust the quench gap without getting comp. levels that eat bearings. With this I think their cooling effects and lack of need for constant re torquing make them a good upgrade.

 

[Street Ryderz]

Looking at a % thermal efficiency gain to compression ratio chart the gain from something like a compression ratio of 6 to 9 is only 8%.
8% of 3 horsepower is only .16 which is nearly nothing (and this only counts if the thermal efficiency % translates to horsepower %).
People sacrifice their engines reliability for .16 horsepower? really?
You can increase power without sacrificing reliability.
by doing things such as;
better porting
better carburetor
better air filter
aftermarket CDI
better head gasket
longer intake tract
iridium spark plug
a motorcycle piston/rings that don't have the crazy wide ring end gap that these chinese engines have
putting disulfide powder on the cylinder to reduce friction
jetting your carburetor to near perfection

Does this look like a crazy wide end gap thats after at least a 1000 miles!And run at 180 psi do you see any chrome flaking or missing you need to buy a late model engine or replacement parts things have changed in the past few years!

 

[gary55]

Looking at a % thermal efficiency gain to compression ratio chart the gain from something like a compression ratio of 6 to 9 is only 8%.
8% of 3 horsepower is only .16 which is nearly nothing (and this only counts if the thermal efficiency % translates to horsepower %).
People sacrifice their engines reliability for .16 horsepower? really?
You can increase power without sacrificing reliability.
by doing things such as;
better porting
better carburetor
better air filter
aftermarket CDI
better head gasket
longer intake tract
iridium spark plug
a motorcycle piston/rings that don't have the crazy wide ring end gap that these chinese engines have
putting disulfide powder on the cylinder to reduce friction
jetting your carburetor to near perfection

I think all of these suggestions are good, well accept for the longer intake, and would be even better with a big ol fat after market head.

 

[Deleted member 12676]

the longer intake gives a low RPM boost. so does a longer header pipe before the stock muffler.
the "fat" heads take longer to heat up but I don't consider them much of a performance boosting product. If they have a squish band and you don't shave the head till you have about .8mm head/piston clearance then they may even lower power due to excess air/fuel mixture hiding out in the squish area while the rest of the mixture is being ignited near the center of the combustion chamber.

Even more telling than the cranking psi is the head temperature at the base of the spark plug. Some engines, including my Suzuki 100, don't like anything over 300 degrees farenheit. (ie: Rotax 4 stroke aircraft engines, VW 2 strokes). Over that and the engine power suffers. Everyone should have a temp gauge and keep track of the temp while riding and then if the engine starts losing power to take notice of the temp. Then add a head gasket to lower compression and see if the temp remains lower than that limit.
http://www.jnmotorsbikes.com/product_p/jnm1183.htm
this one is a little more reliable although $10 more:
http://www.trailtech.net/digital-gauges/tto/surface-mount/temperature/742-et3-5294
. . . .

 

[gary55]

the longer intake gives a low RPM boost. so does a longer header pipe before the stock muffler.
the "fat" heads take longer to heat up but I don't consider them much of a performance boosting product. If they have a squish band and you don't shave the head till you have about .8mm head/piston clearance then they may even lower power due to excess air/fuel mixture hiding out in the squish area while the rest of the mixture is being ignited near the center of the combustion chamber.

Even more telling than the cranking psi is the head temperature at the base of the spark plug. Some engines, including my Suzuki 100, don't like anything over 300 degrees farenheit. (ie: Rotax 4 stroke aircraft engines, VW 2 strokes). Over that and the engine power suffers. Everyone should have a temp gauge and keep track of the temp while riding and then if the engine starts losing power to take notice of the temp. Then add a head gasket to lower compression and see if the temp remains lower than that limit.
http://www.jnmotorsbikes.com/product_p/jnm1183.htm
http://www.trailtech.net/digital-gauges/tto/surface-mount/temperature/742-et3-5294

On these engines you don't want to shave the head to achieve proper squish gap. You do this work on the cylinder. Shaving the head reduces the diameter of the chamber which you want to be as close as possible to that of the cylinder . To suggest doing so is thinking backwards for both performance and monetary reasons as the after market head costs 2 to 3 times the cylinder.