355 Compression Check

A good 355 will have 200-215lbs of compression.

 

Your experience is greater than mine but haven't most of the hundreds of threads on this subject concluded that as the test procedures and the atmospherics are not consistent nor will the readings be (hence most tests reported here seem to have no figures above 200) however that the spread is more important and the consensus seem to be +/- about 10 % is normal which is why the report here was 'no issues'

 

Most tests reported here have been above 200 and ALL tests from reliable sources have been above 200.

 

A reading of 110 would be of concern in a lawnmower. A reading below 185 is of concern in a 355.

 

I'd take what rifle driver says as gospel on matters Ferrari.

A compression test rotates the cylinder and measures psi within.
It doesn't give us any information on how long that pressure is held or where it goes.
A leakdown test is designed to do that extra exploring. If the compression psi is low another reading can often be taken after the insertion of a small volume of oil. The oil will temporarily seal leakage past the piston rings and thus any increase or not of psi can indicate wear.
Another test will listen with mechanics' stethoscope for the sound of air leaking past the valve seats during the leakdown test.

A mechanic with expertise on a motor can discover quite a lot about its condition through this testing. That information is in direct proportion to his skill and experience however as judgement plays a large part in interpretation.

 

Nope. Those are figures we see at 5500 ft elevation with a cold engine, Brian and you are near the same elevation. I assume that a mechanics compression tester is in good working order and accurate to plus or minus 2% which I find rarely to be the case. I would insist on a proper leakdown test.

 

Common concensus among shops is for comp testing to be done with hot engine, throttle WFO. Mentioned and debated many threads on this forum. Obviously, as for maybe a 'non start' issue this is not always possible, but for consistency of available data, the test should be carried out per the "normal" criteria.

Similarly, cylinder leakage tests seem to vary due to the different interpretations of the procedure. I have always carried this out at TDC for each cylinder either hot or cold. Any readings out of range, tests are repeated to eliminate possible errors.

The interpretation of any of these numbers does however open a very big can of worms, but I would consider no more than a 5% variation acceptable in comp testing and leakage figures.

 

Graham - this seems to have opened (again) a can of worms and is being fuelled here it would appear

Your car works fine & your mechanic is happy - focus on that

 

First, you need to find a more qualified mechanic. No one in their right mind would buy, own or fly a cheap POS Cessna 150 worth a measily $12K without running a leakdown test, why do people who can afford something as valuable as a 355 skirt around this?

Secondly, it seems amazing to be almost a decade into the 21st century, and the vast majority still have no clue how to use or understand a compression tester. Stop putting oil in the cylinder to test the rings, all its going to do is raise the CR and give you phony readings. Anytime you have a low compression test, it should be followed up with a leakage test. Use the tried and true, easy and simple compression test to check basic health, and use the leakage test to analyse suspicious compression readings.

Up to 2000 ft of altitude, there is not enough drop in atmospheric pressure to bother calulating any drops in test pressures. An engine that can pump out 200 psi at sea level should still pump up pretty close to 200 at 2000 ft. From there you need to look at the differences cylinder to cylinder, and the cylinder reading 143 should be of particular concern.

Unless the engine will not run, all tests should be carried out on a warm engine. Leakage testers should always be calibrated to a baseline pressure, and cylinders should always be at TDC while testing. On aircraft we have a prop to hang onto that gives us a lot of leverage. We drop the test pressure to 80 psi, simply because a big cylinder like on a Bonanza, the prop can just about lift you off the floor at 80 psi, 100 would be impossible. In any case, 5% loss would be the maximum I would accept.

Some of you really need to stop question people like Brian and Dave about stuff like this. Its all they do all day, and they have been doing it for years. They are both very honest individuals and will not steer you wrong. If Brian says it should be 200 plus, you can take it to the bank. Instead of questioning him, try asking him why its so low?

 

Any insertion of oil is, of course, a secondary test following an unsatisfactory initial compression test. The purpose being to attempt some differentiation between pressure loss at valve or ring seal.
I wasn't advocating as much as attempting to provide the OP with the overview he lacked and asked for specificly, as no-one else had.
Don't really understand why I'm accused of undermining either pro here but as the only poster mentioning oil it must be me you refer to.
I thought posting "I'd take what rifle driver says as gospel on matters Ferrari." left little room for misunderstanding unless willful.

 

How much would compression readings be affected by having spark plugs in the remaining 7 cylinders? Can a valid test be done that way?

 

Not really. The motor cranks too slow. As with so many things consistancy is vital. Plugs all out, same number of rotations, good battery etc. On the 12's the battery can get a little low by the time you have done all 12 so I hook up a charger at the start.

I have done many compression tests on stone cold 355's. In general people are correct, it should be done warm but when you are diagnosing a non running motor or one not even in the car it just isn't possible. The material being used in new motors just does not change much in respect to how well the cylinders seal on new motors. A good 355 motor will produce within just a couple of pounds and within 1 or 2 percent done hot to cold.

If the test is showing those compression numbers my guess is either the test was not done correctly in which case it is meaningless and you should not try to extrapolate conclusions from it or the tester is faulty.

Either way it should be redone.


Someone said if it runs OK just drive it. I concur...........if you already own it. Drive it until it becomes symptomatic. At least that is what I tell my clients. If you are buying it why on earth would you want to pay market price for a car which shows such strong signs of advanced wear?

 

warm engine, all plugs out, fully charged battery, reliable compression gauge, same number of compression strokes on each cylinder tested, then you should have an accurate compression test, i have seen good running 355's run any where from 140psi to 190psi
keep in mind, all cars are different and all compression gauges are different and different people have their own method of performing compression test, as long as it is running smooth and has good power, i would not worry about it!

have a good one!!!

 

best reply yet,and from mayberry no less

 

No offense, but 140 is just not acceptable. To get that low something is drastically wrong and should have been investigated. Ive gotten over 150 PSI out of lawnmowers and VW Beetles, engines that have only 7:1 compression.

Folks, there are not two or more sets of standards in the world. An engine is a mechanical device, and each particular engine, if assembled correctly, and if all its parts are in good order, will perform more or less equally. If Brian Crall and Dave Helms are in agreement, this is ALL the verification you need, IMHO.

If you choose to buy these cars with compression numbers below those figures, thats your business. I have seen aircraft engines losing 10% on leakdowns across the entire engine that will still pull full RPM, indicating they are still developing normal power, and some didnt even use much oil, and they could still be signed off as airworthy. But in absolutely every case, these were engines with a lot of hours that were reaching the end of thier life. The numbers were never going to improve, but would continue to deteriorate at an ever more accelerated rate. In a car like a Ferrari, a tired engine with lower compressions could last a very long time, especially with some of the light use they recieve. But its still no reason to pay the same kind of money. Anyone looking at aircraft would judge an engine with 10% loss as run out, and deduct the full cost of an overhaul against the planes value. No matter how good it ran.