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Author Topic: Whats the compresion on your C5 / G5  (Read 765 times)

chinoy

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Whats the compresion on your C5 / G5
« on: February 09, 2010, 01:05:34 PM »
Just a generic poll.
What compresion readings have you seen on your C5 / G5.
What does the service manuall say it should be ?

Also how do you get a reading with the auto de-comp in place ?

BTW I got 120 PSI on my C5.
Another owner says he is getting 140.

So is mine low or his high ?

ace.cafe

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Re: Whats the compresion on your C5 / G5
« Reply #1 on: February 09, 2010, 01:29:30 PM »
You'd have to spin the motor above the auto-decom rpm, or else you'll have to pull the auto decomp to get a correct  reading.

With the chamber shape in the UCE, it should support higher compression levels than the old Bullet, because of squish, IF the deck height is set at the right distance to get it.
There should be about .050" distance between the top of the piston, to the flat squish platform in the chamber, to get best squish there. The key reference is the area on the piston that corresponds to the flat squish area on the chamber.
Ideally, more than .040" and less than .060".
If it's more than .060" it may not be productive for squish.
If it's less than .040" there may be a possibility that the piston could touch the head at high revs.
If you get it right, it should provide an extra margin of compression before detonation occurs. You can actually exceed the fuel limits to a certain extent by having this squish working properly.
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singhg5

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Re: Whats the compresion on your C5 / G5
« Reply #2 on: February 10, 2010, 05:57:20 AM »
Just a generic poll.
What compresion readings have you seen on your C5 / G5.
What does the service manuall say it should be ?

Also how do you get a reading with the auto de-comp in place ?

BTW I got 120 PSI on my C5.
Another owner says he is getting 140.

So is mine low or his high ?

Ron Ji:

The service manual for C5 and G5 states that the correct compression reading is 100 +/- 10 psi (average of 3 readings).  If compression is less than 80 psi, check for valve damage, or timing, or blown out cylinder head gasket etc.  If compression reading is more than 110 psi, then the engine requires decarbonisation of cylinder head / piston.  

Regards.
Singh

PS - Dont you have twinspark C5 in India ?   May be there is slight difference in readings as we get single spark plug C5 and G5 here in US. 
« Last Edit: February 10, 2010, 06:31:49 AM by singhg5 »
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chinoy

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Re: Whats the compresion on your C5 / G5
« Reply #3 on: February 10, 2010, 07:35:33 PM »
We get negative deck deight in India.
What is the deck height on your modells ?


ace.cafe

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Re: Whats the compresion on your C5 / G5
« Reply #4 on: February 10, 2010, 07:40:28 PM »
We get negative deck deight in India.
What is the deck height on your modells ?



There is commonly a negative deck height on the piston in a closed chamber, because part of the chamber is flush with the top of the barrel liner.
If the piston came all the way to the top, it could hit the head.
There should be a minimum of .040"(about 1mm) clearance there. No less.
This can be made up of the head gasket thickness, if the compressed thickness of the gasket if at least .040"
If the compressed thickness of the gasket is less than .040", then you need to have some negative deck height on the piston, to ensure that it's at least .040" below the head, at TDC.
The .040" is a minimum, but you may have up to .060" clearance there for extra safety margin. No more than .060", or the squish won't work right.
« Last Edit: February 10, 2010, 07:44:53 PM by ace.cafe »
Home of the ACE Fireball 535 Bullet,  Ace GP Hi-Lift Roller Rocker Head . Pistons, cams, etc. Highest performance Bullet engine mods available .  AVL mods. Redditch 700/750 Twin mods. UCE kit soon.

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motomataya

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Re: Whats the compresion on your C5 / G5
« Reply #5 on: February 10, 2010, 09:03:51 PM »
With an auto decompression the only way to check how well your engine is sealing is by leakdown. You are wasting your time with a compression gauge. You also need to be carefull cranking the motor with the sparkplug out, If your sloppy you can damage the ECU.

chinoy

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Re: Whats the compresion on your C5 / G5
« Reply #6 on: February 11, 2010, 04:35:12 AM »
Well as we get two plugs and are only opening 1 to check the risk to the ecu is less.
I would not advise anybody in India to try and dial in the squish as per ace recommendations for various reasons. The first being we are already borderline with the gas we get here. I know in theory it should help you run cooler / less deto etc.
But trust me wait untill you can buy a replacement head. As things stand there are no spares.

The question was more generic i.e. how does checking the comp work ? With auto de-comp.
Because it does work. Even with the auto decomp in there. Maybe they decomp after TDC.



motomataya

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Re: Whats the compresion on your C5 / G5
« Reply #7 on: February 11, 2010, 01:11:53 PM »
The auto decompresion lifts the exhaust valve for a moment as the piston is comming up on the compression stroke. This makes it easier to turn the motor at low RPMs when you dont have the help of the flywheels. Using a compression gauge is not a good way check the condition of any motor, its just fast and easy. A leakdown test tells you how bad your problem is, and what that problem is without taking the motor apart

chinoy

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Re: Whats the compresion on your C5 / G5
« Reply #8 on: February 12, 2010, 06:29:28 PM »
Thanks man I totally agree.

BTW figured something out.
a. If you do it with a cold engine the de-comp will kick in.
b. If you do it with a weak bat the de-comp will kick in.

The best way to take a compression reading is.
Do a light warm up.
Get it to a point where its idling but the auto de-comp is not kicking.
Then hook up a comp gauge.
Take a reading I got 90.
Then kick it over I got 110. Which is what they claim is the factory stock.

This indicates my bat is probably weak. Or the auto decomp is not kicking in.
This limits spin up speed.

And its simple and easy.
Leak down is typically only done after we rebuild the motor. Or open the heads.
Compression checks are done on a periodic basis.

One funny thing I did notice. Comp on my engine is falling the higest was the day I picked it up. I have been checking it at regular intervals and its falling.
Most engines we work on. Comp is weak when new. And as you run the engine in the comp starts to climb.


« Last Edit: February 13, 2010, 04:25:05 AM by chinoy »