Compression Question

[Got Boooost]

Hey guys,
In a 2g, engine compression should be 178. Ive rebuilt my motor and bored it out 20 over, the valves got a 3 degree angle job and Im running Comp cams, does the bigger bore, valve job and cams effect what my engine compression should be? i check my compression tonight and its 130 across the board.

Thanks for the help

Vern

 

[toofast82]

What's your elevation? Are you still using 8.5:1 pistons?

 

[fwdmaster]

Did you use lower compression pistons? I just recently did a head job etc and compression is at 180 compression across the board, your numbers seems very low

 

[Red97Eclipseboy]

Cold, warm? How many cranks? Foot to the floor during cranking?

 

[Got Boooost]

I did the test on a warm engine, and the throttle to the floor. Pistons are 8.5:1

The reason i'm trying to figure this out is b/c ive got about 2000kms on the engine and i was coming home from the race track friday evening and my car started making a loud noise, that seemed to be coming from the dash, but the noise slowly went away. i started the car today to see if it was still there and im getting a really fast ticking noise when i rev the motor. The car idles fine and sounds normal idling. I checked my plugs and all of them are a little on the white side except for one that is really white. i checked timing and am right on the mark, so i dont think my timing jumped a tooth. Im trying to get to the root cause of whats causing this by eliminating all posibilities.

thanks for the help

Vern

 

[#40Fan]

Is the ticking noise coming from under the valve cover?

 

[Got Boooost]

Is the ticking noise coming from under the valve cover?[/QUOTE
sounds like the noise is coming from the top half of the motor. tonight i will check to see if theres spark in the cylinder that had the extremly white spark plug. I disconnected the fuel injectors one at a time and each one was working, it did make the engine run rough for each time i disconnected a fuel injector. although wouldnt my spark plug be wet with fuel if my fuel injecotrs working and there wasnt any spark?

thanks for the help

 

[96eclipzGsT]

How loud and consistent is the noise. Cross fingers its not a rod. Post a video that would help.

 

[Got Boooost]

I took a small vid of the noise

YouTube - 98 Talon Tsi engine noise

and here's a picture of what my spark plugs, one is really white not sure if that has something to do with my engine noise.

https://nhbpxa.bay.livefilestore.co...HBrA_MFhpigqOfqamfJWyFyr_gTM/all 2009 960.jpg

thank for the help

Vern

 

[notverygood]

until the motor breaks in compresion is going to be a little low-was the head machined? Did you mess with the valvetrain? sounds like you have too much clearance and you have a valve noise. Generally rods sound like a hollow knock, valvetrain noises generally are ticks. Whats your oil pressure? How long has it been driven? Im not sure if these cars have a bypass valve for the oil pump-ive seen a few get stuck open after rebuilds causing low or non existant oil pressure (metal from the machining gets into the valves and sticks them open.) cadillac has a quick fix for that-they overfill the motor with oil, causing it to have too much oil pressure which in turn opens the overflow valve causing oil to flow past. this generally flushes out contamination.

 

[notverygood]

it sonds like you have a vacumn leak-spray some starting fluid over the motor with it running (cold so you dont explode) and see if the idle rises-that would DEFINATELY explain the white spark plug. As for the noise, Again, Ill say you have low oil pressure and the hydraulic tappets arent filling causing a tap-take the vc off and see if you have excessive clearance between the followers/cam and valves. was the oil pump replaced? is there metal in the oil? what color/consistancy?

 

[Got Boooost]

The head was measured and in spec so it was never machined. the valves had a 3 degree angle job done to them, i also installed BC valve springs and Comp cams. The motor has about 2000kms on it since i rebuilt it, and Ive done oil changes every 500kms, how ever i never replaced the oil pump. the oil pressure guage does come up while the motor is idling, and the oil is clean and no signs of metal to the naked eye. I'll pull the Valve cover tonight and take some measurements and also see if i can get an exact oil pressure reading instead of looking at my indash gauge.

thanks for the help

Vern

 

[Got Boooost]

I think ive found the problem, something to do with the timing belt auto tensioner. I double checked the timing and when Im at TDC the 2 timing marks on the cam gears are off about 1/8 of an inch. this weekend providing i can get the proper parts today, ill replcae the auto tensioner and pulleys, and then do a compression test and fingers crossed everything will be good, and i wont have to rebuild the head.

Thanks for all the help

Vern

 

[TunaTalon]

Static compression is unaffected by cam timing or temperature. It is determined solely by the dimensions of the engine.

However; a compression test will be affected by cam timing because compression does not start until the intake valve is closed. With stock cams the intake valves close a few degrees after BDC. With more aggressive cams the intake valves close even later. I prefer the term "effective compression ratio" to account for the valve timing.

The effective compression ratio will always be lower than the static compression. A leak down test is more definitive for finding compression problems.

From page 31 of http://www.kidzuku.com/StrokeOrNot.pdf.

The compression ratio that is normally used discussing 4G63 specifications is the (30) Static Compression Ratio (SCR). SCR is simply the sum of displacement and head volume divided by head volume. SCR is a useful number because it doesn't change with the camshaft profile. However in any real 4G63 version, the intake valve doesn't close until several degrees after BDC. The delay in closing the intake valve allows the ram effect in the intake air stream to continue flowing air into the cylinder even while the piston is going up. (There's that velocity effect again). The delay allows more air in the cylinder but compression can't start until the intake valve closes. In any 4G63 version there will be some uncompressed volume because the piston is not at the bottom of the stroke when compression starts. The term Dynamic Compression Ratio (5) is often used for the compression that happens from the time the intake valve closes to piston TDC.
However piston position at the valve closing event does not change with engine operating conditions so this paper will use the less popular term of Effective Compression Ratio (ECR). ECR is affected by the cam profile and the rod ratio. At lower RPM's air is forced back up the intake tract through the open intake valve as the piston moves up. At higher RPM's the velocity of the intake stream still has air going into the cylinder as the intake valve closes. Peak torque occurs about the point where the air stream is just stopping as the intake valve closes. At higher RPM's the volumetric efficiency starts to drop off as the cylinder does not get the full charge of air.