A technician might be better able to explain. Here goes. There are certain factors which cause bad compression and certain ones that cause bad leakage. If the leakage #s are bad, there could be a leak coming from the valves (intake or exhaust), the crank case, etc. Let's take the example of cylinder leakage. If there is bad leakage from a cylinder, then the compression achieved by the movement of the piston will not have maximum pressure because air is "leaking" out. Now let's take the case of the valves being sealed up very nicely -- low leakage #s. In this situation, there still could be a problem with the piston itself, the rings, etc. If the piston has a problem and does not compress properly, then the compression pressure will be lower than optimal. A variety of factors can cause bad compression. Cylinder leakage (or more specifically, the scary scary oh-so-scary valve guide problem) is one of them.
For compression test to have good numbers, everything has to be working well. The valves have to let in a full charge of air, teh cams need to be within a couple of degrees of spot on; durring the compression process (up stroke) the air cannot leak out of the cylinder by the rings, valves, or other means. Thus at the top of the stroke, air pressure will be as high as possible, and this pressure is let into the compression reading gauge. In a leak down test, compressed air is allowed to pressurize a cylinder with the piston at TDC. If air leaks out of the cylinder, then somethign is amiss, and this is read out on the gauges. Air leaking out by way of the intake valves will produce a hissing noise in the intake velocity stack. Air leaking out of the exhaust valves can be heard as a hiss in the headers/exhaust system. Air leaking out by way of the rings will produce a hissing noise that can be heard with a stethescope on the bottom of the dry sump. The compression test is dependent on the speed at which the starter motor can crank the engine, so make sure the battery is at full charge.
Just for "sh^ts and grins" I would like to see an fchat member with bad numbers run some seafoam through the motor to see how that changes the numbers. There is no doubt in my mind that a valve guide issue exists but also these garage queens built in 1995 with 20k miles and putted around town by chicken owners who think the cars will break are contributing to significant carbon buildup and part stickage.
Hate to disagree...but well...I disagree. Valves can be sealed tighter than a ducks ass, but if the rings are worn or damaged, your leakdown numbers will be high as the pressure is just bypassing the rings. Thats why when you do a leakdown, you determine if the air is leaking into the intake, the exhaust, or the crankcase. In my opinion, compression and leak down tests are quite similar in purpose, but the leakdown is far more useful in pinpointing problems.
While I'm not one to argue that ferraris are not worth the expense of maintenance and repairs, I find this statement silly. A corvette is no less of a "real race car" than a 355 is. A lotus exige S is arguably more of a real race car than the 355. But yet neither need new valve guides or other major engine repairs before 50k. I love ferrari and the mystique they create, but quite frankly they produce cars that are not fully sorted from the factory due to both the small production numbers, and the arrogance they display. just my .02
I think this is somewhat of an unfair statement. While I have no compunction in slamming Ferrari when they are wrong, you can't possible compare the aforementioned cars to an F355. The F355 engine, in 1994, was a true "race engine" - 8500 RPM red line, 5 valves per cylinder, titanium con rods, the first use of hydraulic lifters with such a high red line, cam geometry to allow the central intake valve to open 10 degrees earlier than the other two, etc. Yes, Ferrari did experience problems with the valve guides, but I don't think this was caused by Ferrari cutting corners, but rather through a combination of factors (e.g., head design, faster than expected wear due to high RPMs, valve geometry, etc.) A number of cars had the valve guides replaced under warranty. Starting with Assembly No. 27,689 they (purportedly) began using an upgraded version.
I appreciate the appeal the such an engine. I also tend to think that the car would have been better with half a liter more displacement and a redline that was 500rpm lower. While its all very exotic in there, it creates a lack of torque and reliability concerns. I guess I just view it from a more functional point of view.
A real race car, by the definition of Carroll Smith, has its engine in the back ahead of the transmission, and radiators on both sides between the driver and the engine (bay). But I digress. If you frequent the Corvette forum(s) you will see all sorts of Z06s (their race car model) having all sorts of engine related issues (blown up). So while the valve guides get a pass, there are still a lot of outright engine failures (rods through the side of the block) stuff. Much of this is being traced to the inadequate dry sump and oil starvation issues. I will give the Exige a pass as a race car.
Well you just scratched the surface on the vettes. C5Z06's are the most bullit proof LS engine on the linage and do not blow up. C6Z06 LS7 engine blew only because the oil tank was not up to the task under race conditions from those few driver who could drive 10/10ths in those cars on sticky race tires. People who race know the dynamics of tires can change everything about how a car works. The aftermarket quickly figured out what to do and that was change oil tank designs which are now part of GM 2010 C6Z design. Problem solved! Where the problem has not been solved is the LS3 C6 base models being raced in SCCA T1 which will blow up when driven 10/10ths also solved with dry sump retrofit or the GM solution of the dry sump as part of the 2010 LS3 C6 "grand sport"...again problem solved for a heck of alot less than a 348/355 major service in the worst case of doing a dry sump retrofit for racing. Let it be known that HPDE'ers and lower level racers are not having these cars blow up. Me for example racing a C5Z took a year of racing before I aquired the skill to put my oil temps in the 300's and now I have to add an oil cooler. So the issue you bring up is really a minor one that 99% of vette owners will never see unlike the ever prevalant 355 guide issue regardless of mileage.
Roughly the same argument we face when building race engines, how long is it designed to last and at what power output during that duration. Personally I think the 'lack of torque' is a personal preference issue and I do not see it as a fault in the design as this engine was designed to rev and it's power output between 4K and 8K was a new high water mark for the era. Much the same as the Ferrari vs Cobra vs Vette battles of the 60's....each was genius although completely different in design and function. The guide issue is real and I do agree with what has been said in this thread regarding material and fitment. I also believe that available lubricants played an issue as well as the usage. Valve stem size and the resulting bearing area proved the early guide material far under designed for the use and conditions, change either the use or the conditions and the results change. Personally I like the steel material used in the upgrade and believe it will become the std in the future when a high RPM, hot running engine is designed. With the advent of variable cam timing, variable intake and exhaust lengths....the RPM's required for extreme outputs can be lowered as computers will determine and set the exact conditions required for any given RPM and with that will come better reliability. There will always be something magical to me about extreme RPM engines and the sound they generate. I grew up in the muscle car era and the wonderful sound's associated with those cars but nothing will snap heads around like the scream of a flat plane small displacement engine at 8K, it's part of the allure to me. Form over function? Was function ever a consideration (albeit reliability would have been a nice bonus)? The 355 will always be one of my all time favorites as that sound will never be matched with the current design direction. The need to make sure we are not just moving the problems around but eliminating them all together, which is paramount. Testing for this guide / liner situation can be difficult and requires every test to be done and closely critiqued in my opinion. Recently we saw a situation where a fairly well running car had carbon on the intake vales built up to the extent where there was no airflow until just short of full lift. The carbon had packed itself onto the valve to the point where an exact port shape was reveled when the valve was removed. This valve showed no leakage but very poor compression even at cranking speeds. One cant test enough, this is not an issue of one test or another when we are talking about the costs required to correct the condition.
Thank you Dave for your usual Sunday morning posts....I set my alarm clock early just so I can be one of the first to read it. Very nicely stated.....
Ahh gotta love Daves posts. I woulnt give up the sound of my 355 for all the vettes in China. Spent a few bucks this past year on her including valve guides and headers. Add in Daves precious metals connector kit and now she runs the same every day.........AWESOME.
