82SC Compression 5 in the 180s #1 @150

JMH82SC · 07-14-2008, 04:03 PM

I have had my SC going on 5 months now and have had a blast. Today I was replacing the plugs and figured I would do a compression test and found all but one cylinder to be in the 180s. The #1 will only do 150. I put a little oil in #1 and it did not change the compression (150). As for tonight I plan to leave the plugs out and pick up a leak down tester tomorrow. I have searched and searched and have not found a thread that gives info on what it could be ( USER error ) so if some one could point me in the right direction I would appreciate it.

Information
82 SC (CIS) 101K, runs on all 6, no noticeable vibration and all 6 plugs look good including #1.

2.7RACER · 07-14-2008, 06:27 PM

I would put it back together and drive it for a day or so, then retest.
If number one continues to test low compared to the other cylinders, then do a leakdown to determine where the compressed air is leaking, (valves, rings, leaking head).
I wouldn't get too nervous over one test.

Keith Perry · 07-14-2008, 06:33 PM

Lower head studs. A little burned oil on the heat exchanger below #1 might be an indicator. So maybe search headstuds for some background. Then pop that valve cover and see if a couple of stud nuts are rolling around.

samsnead · 07-14-2008, 06:38 PM

whats oil consumption like

JMH82SC · 07-14-2008, 07:41 PM

I checked studs last week and all is fine none broken I pulled lower covers ( could not stand the suspense) even after the last owner had the valves adjusted at the local Porsche dealer 1500 miles ago and documentation shows a clean bill of health. I have no oil leaks on floor or exchanger. Oil consumption is low but have not had a long term way to measure because of service related issues for example I did a oil change and had the correct amount of oil in it for 2 weeks then had to pull lower valve covers because of stud curiosity. If I pull 5k in second and let off the gas I do not see smoke. No smoke on shifts or hard acceleration.

Flieger · Flieger's Garage

Could have been a valve was stuck open a little because of some carbon. Repeat the test and do leakdown numbers. Listen for where the air comes out.

May require an "Italian tune-up" to clean off the carbon deposits.

JMH82SC · 07-16-2008, 01:52 PM

As for a update I did a leak down test and the troubled cylinder (#1 150) leaked 5% @100 PSI. I went ahead and replaced the plugs with some NGKs and went for a Italian tune-up. The first thing I did on the return was a compression test of all driver side cylinders.
Before ITU After a light ITU
#1. 150 #1. 165
#2. 180 #2. 170
#3. 185 #3. 174

I like the post Italian tune-up numbers much better.

This brings me to the next question regarding detonation. I have put about 2k on the clock in the last 4 months. In the last week I have started to notice detonation under load WOT. I first want to state I do not drive like a gram paw but I try to be easy on the throttle. I shift between 2500 and 3500 for the most part. Today is the first time I have pulled it up to 6000RPM (ITU) and did that in second getting on the freeway?

Detonation helpful information
CO 1.6 Open Loop
Timing 5 BTDC vacuum unplugged and distributor cleaned and lubed (plates were frozen before this weekend)
93 Octane gas

Now for the question! Could the fact that the distributor was frozen so timing was not correct till mechanical curve came in and the fact I have been easy on the car caused a carbon build up? Carbon could explain compression numbers in the mid to high 180s and could also explain detonation starting.

Superman · 07-16-2008, 02:14 PM

Compression numbers in the 180's is not unusual for your engine. Remember, your engine has a CR of 9.3:1. Numbers between 175 and 190 should be common.

There is no need to shift at low engine speeds. Your Porsche engine was designed to be spanked hard.

Back timing off until it stops detonating. Drive it hard. Then perhaps see if advancing the timing can be done without detonation.

JMH82SC · 07-16-2008, 02:41 PM

Timing was backed down to Z1 mark and still could hear detonation and temps were up a bit. I set timing back to stock before drive today and temps went back to normal. FL gas is now 10% Ethanol not sure of the effect that has but I can hear detonation. when temp is on first hash mark (180) no detonation. When temp is in the middle (210 - 220) detonation. When I pulled the new plugs the electrode was white on all 3 plugs not sure how it could be lean with 1.6 CO except for a air leak making it go lean.

Superman · 07-16-2008, 03:27 PM

Yes, air leaks can lean it out.

Yes, carbon buildup can cause detonation. Not only by increasing CR, but also by glowing hot and starting ignition that way.

My '83 SC could handle 35 degrees full advance. That is seven degrees advanced over factory spec. It is very nice, performance-wise, if your engine can handle it. In your situation, I might try to remove some of that carbon, or make sure it is not there. In your situation, I would drive the car much harder. Some of us redline our engines pretty much every single time we take a drive. A few do this pretty much every time they shift. This is not a Chevy 350 V8. Baby the transmission and spank the motor.

JMH82SC · 07-16-2008, 06:29 PM

I think I've found the answer to the detonation problem that has started all the testing that led to this thread. I started the engine and shot a bit of brake cleaner around the injectors and bring on the smoke as well when I shot some brake clean around intake boots. All and all I think the problem is old CIS rubber is porous and letting false air in to cause the detonation I have been tracking down (I Hope) and the plan is to stock up some parts and have some fun. Thank you for the help and I will keep you posted on the outcome.