|
Post by robb on Jan 3, 2011 9:36:46 GMT -5
When it comes to compression ratio's; the volume to be compressed is the negligble factor compared to the the volume it's being compressed into. The fact that Environmoto is really going against his best interest, by saying you don't need an aftermarket head for a 72cc kit tells me something. He does sell heads after all. The dome and dish on the pistons may look small but are huge in relation to the tiny combustion chamber. 1cc added to a 5cc combustion chamber is a huge difference and I can see, in my humble opinion, about that much in the pictures above. The people with short life 72cc engines ? Probably riding them hard. Just my opinion. Robb
|
|
|
Post by qwertydude on Jan 16, 2011 1:10:06 GMT -5
Usually the 72cc kits come with a thicker head gasket too, this reduces the compression ratio and helps compensate for the added volume going into the same combustion chamber. Ultimately if you take a compression test of the engine before and after the conversion you'll find both are pretty close to each other. Mine at 50cc was pretty good at 140 psi at 1000 miles, after my BBK conversion and a 100 mile "break in" it measured 160 psi. That's pretty normal. It's still a tad bit on the higher side but that just means it gets to leak down a little more later on in its life before the engine falls flat.
|
|
|
Post by Bluefront on Jan 16, 2011 7:25:17 GMT -5
My own 72cc kits (two) did not come with thicker head gaskets. I had to take the head back off the first install (leaks), so I measured the thickness of the compressed gasket.....it was the same thickness as the OEM gasket.
Measuring the compression with a compression gauge is not super accurate......the readings will differ depending on the gauge you use, the engine temperature, the throttle position while you're cranking, etc.
A before/after reading under the exact same conditions may give some indication of a compression increase.....but with not enough accuracy to reach a reliable conclusion. Going from 140 to 160lbs on the gauge shows a compression increase of about 15%.....still too high if you started out at 10:1. IMO......
|
|
|
Post by qwertydude on Jan 16, 2011 10:11:16 GMT -5
Well my original gasket was an enamel coated piece of sheet metal. My 72cc gasket was like 1.5 mm thick with a metal "crush" combustion chamber seal. I did the tests as similar as I could get. Both engines broken in and cold. If it's a tad higher than normal I'd consider it better than it being too low.
|
|
|
Post by takotruckin on Jan 16, 2011 11:58:58 GMT -5
A before/after reading under the exact same conditions may give some indication of a compression increase.....but with not enough accuracy to reach a reliable conclusion. Going from 140 to 160lbs on the gauge shows a compression increase of about 15%.....still too high if you started out at 10:1. IMO...... 140psi cranking compression is NOT 10:1. Depending on cam profile cranking speed etc it could be anywhere from 7..6-9.0, check out this link, I'm looking for one that goes higher as well. I know I've seen it somewhere. www.type2.com/library/engineg/comrat.htm
|
|
|
Post by Bluefront on Jan 16, 2011 12:34:14 GMT -5
Again......So you got a reading of 140lbs. Using a different gauge for instance, may give you a reading of 170lbs.
About the only thing these compression gauges are good for......to measure compression differences between all the cylinders on a multi-cylinder engine.
|
|
|
Post by qwertydude on Jan 16, 2011 12:51:12 GMT -5
You can't just go by that fixed list. There are more variables that can affect your absolute pressure. For example, the actual gauge itself adds to the volume of the compressed area. On a large cylinder the 1cc or so the gauge adds doesn't affect the reading so much. But if you've got a 50cc engine with 10 to one compression it means you've got a 5cc combustion chamber volume. Add the 1cc from the gauge and you've got 6cc's now. This drops the measurable compression ratio from 10:1 down to 8.33.
Anyways the point to take is you can't just rely on the number alone to determine your compression ratio. There are too many variables. What you want is consistency. If you take into account the volume of the tester being 1cc or so which is entirely reasonable I'm actually within the specs of that chart. But what I'm saying is that the big block kit does obviously increase your compression ratio, but the thicker head gasket keeps it from going through the roof. 72cc's going down to 5cc's stock head can be 14.4:1 compression ratio. That's just insane.
The 1.5mm gasket adds about 1.5cc's of volume compared to the sheet metal gasket. Doesn't sound like much but that's now a 6.5cc combustion chamber. Compression ratio is now 11.07 to one, this explains my slight rise in compression test, even though it is nowhere near where an 11:1 compression ratio should be according to the gauge and chart. But compared to the 10:1 original the added 20 psi is indicative of a slightly raised compression ratio. Calculations prove it.
|
|
|
Post by Enviromoto on Jan 16, 2011 12:53:59 GMT -5
If you can show me a kit that has anything over 12:1 or higher let me know so I can buy it.
|
|
|
Post by takotruckin on Jan 16, 2011 13:21:31 GMT -5
On a large cylinder the 1cc or so the gauge adds doesn't affect the reading so much. But if you've got a 50cc engine with 10 to one compression it means you've got a 5cc combustion chamber volume. Add the 1cc from the gauge and you've got 6cc's now. This drops the measurable compression ratio from 10:1 down to 8.33. Anyways the point to take is you can't just rely on the number alone to determine your compression ratio. There are too many variables. What you want is consistency. If you take into account the volume of the tester being 1cc or so which is entirely reasonable I'm actually within the specs of that chart. But what I'm saying is that the big block kit does obviously increase your compression ratio, but the thicker head gasket keeps it from going through the roof. 72cc's going down to 5cc's stock head can be 14.4:1 compression ratio. That's just insane. The 1.5mm gasket adds about 1.5cc's of volume compared to the sheet metal gasket. Doesn't sound like much but that's now a 6.5cc combustion chamber. Compression ratio is now 11.07 to one, this explains my slight rise in compression test, even though it is nowhere near where an 11:1 compression ratio should be according to the gauge and chart. But compared to the 10:1 original the added 20 psi is indicative of a slightly raised compression ratio. Calculations prove it. The gauge has a check valve in it so that hose/gauge volume does not effect the readings. When you get into higher end gauges they are more accurate, my Snap-On compression gauge is dead on with every other high end gauge i have checked it against, as well as digital pressure transducers. I find that chart to be pretty accurate. The wide range is to take in account the different variables, and it agrees with the measurements taken that a stock scoot is not at 10:1 compression.
|
|
|
Post by tomcas on Jan 16, 2011 14:13:01 GMT -5
You can't just go by that fixed list. There are more variables that can affect your absolute pressure. For example, the actual gauge itself adds to the volume of the compressed area. Exactly, P 1V 1= P 2V 2
|
|
|
Post by qwertydude on Jan 16, 2011 14:26:33 GMT -5
The check valve only keeps the needle from dropping. The volume of the compressed area is still the same and still affects it the pressure measured.
The only way to get an absolute reading is if there is no hose and no hole. If the pressure measurement device was completely flush with the tip. Like you put a solid bolt in it. Even digital ones don't have this feature they're just a digital version of the analog ones. They'd all read about the same pressure because you probably end up using the same hose and adapter. But gauge itself along with the hose still throws off the real world value, hence you can only use it relative other measurements and cannot accurately determine compression ratio by the compression pressure, especially with engines this small.
|
|
|
Post by mainepeace on Jan 16, 2011 14:42:42 GMT -5
>> no hose and no hole.
Ideally, it would also simulate the volume of the spark plug and dead space around the tip that is inserted into the chamber. That will make a small difference depending upon the electrode design.
Greg
|
|
|
Post by Bluefront on Jan 16, 2011 16:04:24 GMT -5
I happen to own Snap-On high-end compression adapters and the gauge. The set comes with a bunch of adapter hoses that screw into different size plug holes. They're about 12" long.
The hoses themselves are straight-through.....no check valve (as has been stated). The check valve is in the gauge itself which connects to the hoses....this means the volume in the adapter hose is added to the combustion chamber volume. The end result (particularly on a small engine) is a reading much lower than a cheap-gauge that you just push against the spark-plug hole.
You just can't compare readings from all the different gauges out there.
|
|
|
Post by takotruckin on Jan 16, 2011 20:26:45 GMT -5
I happen to own Snap-On high-end compression adapters and the gauge. The set comes with a bunch of adapter hoses that screw into different size plug holes. They're about 12" long. The hoses themselves are straight-through.....no check valve (as has been stated). The check valve is in the gauge itself which connects to the hoses....this means the volume in the adapter hose is added to the combustion chamber volume.. You must have an older unit then. Mine has the check valve in the adapter hoses.
|
|
|
Post by timelancer on Jan 28, 2011 21:14:16 GMT -5
then in your theory the compression would be higher than that without the compression gauge? if that's true then my compression was sitting closer to 190PSI and I ran an 82cc (50mm) piston and the valves it came with, the gauge read 180 PSI and I got 3000 miles out of it, running synthetic oils and 91 octane gas, in the desert with 120F+ days WOT the majority of the time, I think a lot of you don't maintain your scoots right, the faster they go the hotter they get, the hotter they get the quicker the oil degrades. Granted I was changing my oil every 500 miles as well.
|
|
|
Post by Bluefront on Jan 29, 2011 5:08:45 GMT -5
Again.....it's problematic to try to compare compression readings using different gauges (even the same brand/model). But I will say this.....I used my Snap-on gauge with the adapter hoses numerous times on countless cars, even high performance cars like Nissan Z cars. I never saw a reading on that particular gauge as high as 180lbs.
A car mechanic uses these gauges to compare readings from different cylinders on the same engine. Even then readings between cylinders will differ somewhat, and still be in a normal range. However....if five cylinders read around 150lbs (for instance), and one cylinder reads 100lbs, you've got a problem.
I've never put that gauge on my 72cc Roketa......might one day for the Hell-of-it. What will the reading reveal.....? I want my little 72cc GY6 to last longer than 3K miles. It probably wouldn't have with an excessively high compression ratio....
|
|
|
Post by timelancer on Jan 29, 2011 12:34:32 GMT -5
I admit that 3000 miles is fairly low, but I am comparing a 82cc BBK to a 72cc BBK there is a huge difference in power and compression(on my gauge most stock scoots read close to 120PSI, my girls 72 reads close to 150PSI, my 82 180PSI), my girls 72 I modded it is sitting close to 4000 miles on the new kit, with no signs of failure yet. I'm not however denying that what you are saying is true, higher compression= more power= more heat= more damage, so I can't argue that your scoot will probably outlast mine any day. I do however believe if I get an oil cooler and possibly use your computer fan idea, I believe I could get even longer mileage from the 90cc BBK (52 mm). Defiantly a good idea though.
|
|
|
Post by jerryd on Mar 16, 2011 20:55:22 GMT -5
ttt
|
|
|
Post by lhscr12 on Mar 19, 2011 19:46:22 GMT -5
This is my first post to this thread and I'll begin with saying I currently am running 730miles on my BBK and used royal purple synthetic for break-in. Just did an oil change today to 15w40 royal purple synthetic.(stuff works great). Anyways I am also having issues when starting the bike cranks real slow once then usually kicks on with a little throttle. Idles real high around 3000 rpm for 15-20 seconds then levels out around 1200 rpm. Is this a compression issue? Should I consider adjusting my valves?
|
|
|
Post by jerryd on Mar 20, 2011 10:52:43 GMT -5
I'll try to explain this, for people who might not understand the concept. Compression is based on two things (basically).....the capacity of the combustion chamber (the head/valve area) and the capacity of the cylinder (50cc to start with here). When you install a BBK (72cc in this case) without changing the capacity of the combustion chamber, your overall compression ratio will go up. If for instance you were running a 10/1 ratio to begin with, after the BBK you might be running 15/1 (I'm guessing). More power for sure, but you may suffer consequences. Gasoline is normally available only up to 93 octane. If you use an octane too low for your compression ratio, the engine can detonate (ping). It may also over-heat, damage pistons, damage bearings, etc. Obviously you don't want to risk that (unless you're racing)....I sure don't. So what I did here with this last head mod is to increase the capacity of the combustion chamber (by removing aluminum). This makes up for the increased cylinder capacity of the BBK......and if I removed enough aluminum, my overall compression ratio will be about what it was at the start (say 10/1). This is what I am doing here. I'll still get the benefit of the increased cylinder capacity (72cc), but I should not expose myself to the risks of a higher compression ratio. Now if I wanted to, I could be more accurate with all this by measuring the capacity of the combustion chamber before and after the mod (that's somewhat difficult. Racers do it). But for what I'm doing with this scoot, a practiced eyeball of the new combustion chamber tells me my new compression ratio will be only slightly higher than it was stock......but the scoot will be quicker because of the increased cylinder capacity. This may be the "dirty little secret" about these 72cc kits.....you may end up with an unsafely high compression ratio, unless you also change or modify your stock head. My 2c..... any idea what size the cc is on the stock heads .... instead of grinding on a set of heads why not use a thicker head gasket
|
|
|
Post by Bluefront on Mar 20, 2011 13:41:58 GMT -5
^^^^^Well slower starter cranking speeds could certainly be blamed on a too-high compression ratio. As far as other symptoms......hard to say. Pinging, knocking, high engine operating temperatures and even broken pistons and cranks would be symptoms. But idle problems and other carburation issues....can't say. And you could use two head gaskets and/or multiple base gaskets to lower the compression ratio......but you certainly risk leaks. Gaskets weren't meant to be layered on each other. Then there's the cam chain.....there's only so much free play in the thing. Raising the head with multiple gaskets might result in a too-tight chain (very bad). Grinding out the head was fairly easy....and didn't cost a thing.
|
|
|
Post by bullet on Mar 20, 2011 17:42:57 GMT -5
Bluefront, I hear what you are saying. It sounds like you know what you are doing. You are trying to get a balance of power and reliability and I think you are on the right track. You must stay within the engine design parameters to keep reliability. Do it right and you will be rewarded with a strong engine that will last longer than a stock one. I have always wanted to take a stock small engine and blueprint it to see what improvement there was, so please keep us informed.
|
|
|
Post by Bluefront on Mar 20, 2011 18:17:59 GMT -5
^^^^Thanks.....I like to think I know what I'm doing. But I will admit to neglecting this 50/72cc project since I got my Xingyue last year. I spent most of the summer modding the thing, and riding it the rest of the time.....put over 9K on it last summer. But I'll get back on this BBK project before too long. I still have some carb issues since I went to a 20mm carb. Mostly M/J experimenting.....
|
|