01 busa....mecahnic says the pressure is low on every head....120s and down so question is motor pretty much done...only had it for 3 years only been riding for 4 years so dont know much about busas yet and not sure if Im just getting tricked into a complete tear down /motor swap
Low compressions
- Thread starter Rck
- Start date
instigator1225
Registered
why was it being compression tested in the first place?
c10
Registered
suzuki part #'s 09915-63311 ( comp gauge attachment ) 09915-64512 ( actual gauge )
1974 lol
pistons are still round , with rings ( except honda NS something ) only real change is NSC coatings doing away with cast iron liner . Compression is easier to do as no air supply needed , and it tells you if ya need to go in . Below is a link on the two methods . I'm a dam good mechanic , and use a compression tester .
1st rule on unknown engines test , and see if there is 10% or more difference between cylinders on a street engine . The engine in my Avatar has 6 wallys now as it won Bowling green last weekend in Sportsman motorcycle . Guess who built it .
Now on topic the difference per Suzuki is no more than 28 PSI from cylinder to cylinder , Standard PSI is 199 to 256 , and service limit is 142 PSI .
So if you are truly 120 something PSI then its a issue .
Lets read together
https://www.lcengineering.com/LCTechPages/gentech47.html
1974 lol
pistons are still round , with rings ( except honda NS something ) only real change is NSC coatings doing away with cast iron liner . Compression is easier to do as no air supply needed , and it tells you if ya need to go in . Below is a link on the two methods . I'm a dam good mechanic , and use a compression tester .1st rule on unknown engines test , and see if there is 10% or more difference between cylinders on a street engine . The engine in my Avatar has 6 wallys now as it won Bowling green last weekend in Sportsman motorcycle . Guess who built it .
Now on topic the difference per Suzuki is no more than 28 PSI from cylinder to cylinder , Standard PSI is 199 to 256 , and service limit is 142 PSI .
So if you are truly 120 something PSI then its a issue .
Lets read together
https://www.lcengineering.com/LCTechPages/gentech47.html
c10
Registered
FYI you can get a low reading if battery is low or intake is blocked ( closed throttle ) as you need air in to pump air out ( pressure build ) It takes you several breaths to blow up a balloon . Think of your piston stroke as a breath , and the gauge is the balloon . No breath = small balloon while many breaths = big balloon .
Mythos
Registered
If it feels week, I guess I'd change engines or find out about boring bigger. My 14 had compression barely in spec but it feels strong to me so I'm not worried about it yet.
Leakdown tests are for valves leaking aren't they? That's important to know too but I would think the cylinder compression is the main indication of engine wear.
Leakdown tests are for valves leaking aren't they? That's important to know too but I would think the cylinder compression is the main indication of engine wear.
ItsDooomz
Registered
Negative, A leak down puts pressure in the cylinder and measures the percentage of leakage. So you put 100 psi in the cylinder and it's leaking say 20%.(no bueno) Listen for the leak. If the intake valve is leaking you'll hear it from the throttle body. Exhaust valve leak? Listen for that in the pipe. Break a ring? Listen for the leak in the crankcase through the oil fill hole. Blown head gasket? Open the radiator and look for bubbles or put your ear by the fill cap.
Or you can dust off the old school compression guage, crank the hell out of the motor with no oil pressure and guess at the where the leak is occurring.
They're $100 guys, buck up and do it right the 1st time. http://www.longacreracing.com/produ...prodid=7635&pagetitle=Engine+Leak+Down+Tester
Mythos
Registered
Sounds like a plan but is there a spec for leak down? Is it ok if it leaks down 10%? My 14 had 150~160 psi on every cylinder. Couldn't I just watch the gauge of the compression tester I used and determine how much it leaks down over a period of time? How long do you wait? My cylinders did not seem to be leaking at all--not in the 20~30 seconds I took to look at the gauge.
ItsDooomz
Registered
10% leak down is good, 15-20% might still run ok but you need to keep an eye on it, 20+% and it needs to come apart. I've seen lower numbers (3-4%) with zero gap rings but they have their own issues.
150-160 psi really doesn't mean anything. You could move the cams around and get 200 psi or 120 psi from the exact same engine.
The comp. tester has a shrader valve (just like a tire valve core) installed and that will NOT allow the pressure to bleed off, even if there is comp. leakage there.
Mythos
Registered
Thanks, guys. Sorry for not following up. I was blocked out for a few days.
What do you mean by "move the cams around?" Change cams? or let them turn with the starter during a compression test? I tested some cylinders more than once and I got the same reading each time. The compression must mean something. If the compression tester is accurate, doesn't in spec compression indicate a healthy motor?
What do you mean by "move the cams around?" Change cams? or let them turn with the starter during a compression test? I tested some cylinders more than once and I got the same reading each time. The compression must mean something. If the compression tester is accurate, doesn't in spec compression indicate a healthy motor?
ali123
Banned
I would also think a leak down tells a story also... IE.. if u have a head gasket that is just starting to give up.... a compression test wouldn't show this... it would have good compression.....
ItsDooomz
Registered
As you advance the cam timing (move the cams around) on the intake side you create overlap. Overlap is when both valves are open as the piston pushes exhaust out and starts to pull the air/fuel mixture into the cylinder. When both valves are open, reversion gets pushed up the intake track which slows down the velocity of the air/fuel mixture coming into the cylinder when exhaust gas is still trying to escape. This overlap in valve timing is what gives racing engines the rump/rump/rump sound everybody loves. Overlap gives you a nice scavenging effect that increases power at higher rpms. If you increase overlap, your 150 psi number may become 120, retard the intake cam and you could see 190 psi. Your compression ratio hasn't changed, just the number on a "compression test".
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