Question about compression test

fmxona500 · January 29th, 2011, 03:35 PM

Changed out my spark plugs, oil and uninstalled my vortech today (blew an oil seal), and noticed that I had a slight black crust (looks like carbon build up) on the base of my spark plugs and the ground strap. The plugs were dry past that.

I figure it's time to do a compression and leak down test since this motor gets driven pretty hard. I have 18CC Dished Manley pistons that have been bored over .020

What should I expect to see for numbers in the cylinders?

fmxona500 · January 29th, 2011, 03:42 PM

Or should I call the engine builder on Monday and see what they say since some decking has been done.

ytngt · January 29th, 2011, 03:48 PM

Depends on many factors including cams, heads, static CR...

fmxona500 · January 29th, 2011, 04:16 PM

Originally Posted by ytngt

Depends on many factors including cams, heads, static CR...

Yeah I thought about that immediately after I posted. Heads had port work and over sized valves done as well as the block being punched to .020

I'm just going to call my builder in the morning and talk to him and see what he recommends.

ndfrspd · January 30th, 2011, 05:15 AM

if it was bored over that would raise compression to say 11.5:1?

stal94gt · January 30th, 2011, 05:23 AM

consistant compression is the easy way to see if your motor is good. if all your cylanders are +-5 of each other i would call it good, unless your cr is under 140 than you may want to see if all your rings are smoked.

stal94gt · January 30th, 2011, 05:25 AM

Originally Posted by ndfrspd

if it was bored over that would raise compression to say 11.5:1?

bore has little to do with cr. the compression ratio is affected by stroke, piston cc's, combustion chamber size, and head gasket thickness. decked heads/block also effect cr as well.

jivepepper · January 30th, 2011, 08:36 AM

Consistencey is your main concern. Anything around 140+ is decent on average

mach1soldier · January 30th, 2011, 09:14 AM

Originally Posted by stal94gt

bore has little to do with cr. the compression ratio is affected by stroke, piston cc's, combustion chamber size, and head gasket thickness. decked heads/block also effect cr as well.

It might not be the biggest concern but bore size will affect compression. The bigger the bore, the higher the compression.

To the OP. Consistency is what you are looking for more than anything. 5% is the standard accepted deviance.

January 29th, 2011, 03:35 PM	#1
fmxona500 Enthusiast 1996 Mustang I take myself way too seriously. Join Date: Apr 2010 Posts: 545 iTrader: 0 reviews	Question about compression test Changed out my spark plugs, oil and uninstalled my vortech today (blew an oil seal), and noticed that I had a slight black crust (looks like carbon build up) on the base of my spark plugs and the ground strap. The plugs were dry past that. I figure it's time to do a compression and leak down test since this motor gets driven pretty hard. I have 18CC Dished Manley pistons that have been bored over .020 What should I expect to see for numbers in the cylinders?

January 29th, 2011, 03:48 PM	#3
ytngt Enthusiast 2002 Mustang GT Join Date: Apr 2010 Location: LA Posts: 953 iTrader: 0 reviews	Depends on many factors including cams, heads, static CR... __________________

January 30th, 2011, 05:15 AM	#5
ndfrspd Enthusiast 1998 Mustang GT 1:34.43@13.86 Join Date: Sep 2008 Location: Detroit Posts: 679 iTrader: 0 reviews	if it was bored over that would raise compression to say 11.5:1? __________________ "The only way to learn about a car truly is to break something and fix it yourself..."

January 30th, 2011, 08:36 AM	#8
jivepepper I Post Entirely Way Too Much Join Date: Mar 2006 Location: Under a sombrero Posts: 5,025 iTrader: 1 reviews	Consistencey is your main concern. Anything around 140+ is decent on average __________________ 665/541 WHP/WTQ 4.6L SOHC 2V ON 93 OCTANE PUMP GAS. NO METH INJ., NO E85, NO RACE GAS, N2O BABY BOTTLES! TRAP=POWER: 11.01 @135, IN SUMMER HEAT, 1350' TRACK, 315/35/17 DR'S

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January 29th, 2011, 03:42 PM	#2
fmxona500 Enthusiast 1996 Mustang I take myself way too seriously. Join Date: Apr 2010 Posts: 545 iTrader: 0 reviews	Or should I call the engine builder on Monday and see what they say since some decking has been done.

January 30th, 2011, 05:23 AM	#6
stal94gt I Post Entirely Way Too Much 2003 mustang v6 ;) slow Join Date: May 2010 Location: eglin afb florida Posts: 10,197 iTrader: 2 reviews	consistant compression is the easy way to see if your motor is good. if all your cylanders are +-5 of each other i would call it good, unless your cr is under 140 than you may want to see if all your rings are smoked.

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