Mobile One consumption question

[75Carrera]

My motor is fresh. The leakdown was almost perfect. I have Ross forged 9.5x1 pistons and 964 cams (see details below). The motor has about 1500 miles on it now. I'm noticing smoke on start up once in a while (no biggie) But... I'm burning about a quart every tank. On decelleration it throws some smoke (unexceptable).

My wrench thinks going to a semi like a Castrol 20x50 will help this. What do you think? Semi or back to regular motor oil?

Someone else mentioned something called Royal Purple but I never heard of it. Sounds like STP... ugh!

[Early_S_Man]

What oil did you use during break-in?

What are the hot compression test numbers, now?

My suggestion is to switch to Valvoline Racing 20W-50, or Kendall GT-1 20W-50.

[Jack Olsen]

But jeez, a quart of oil per tank is pretty outrageous. How does your wrench explain that in a freshly-rebuilt motor?

[88-diamondblue]

I am having similiar problem with my rebuild. Using about a qt of oil every 500 to 600 miles. My suspicion is I have a pinched valve stem seal or a broken oil ring. I used Camgrinders DC-22 cam which is a high lift cam and may have pinched a seal. The compression numbers warm were between 195 and 205. Looks like I will be dropping the engine again this fall to determine the problem. Car runs really strong and no smoke on start up or when running. I am using Vavoline VR1 20-50 as per Steve Weiner.

[aigel]

I also would ask what you used to break in the engine. The rings may not be seated well if you used synthetic from the get go.

But that said, the decel. smoke and smoke at startup point towards valves.

Sounds like you had this engine rebuilt? I hope it comes with a warranty. Time to claim it.

George

[stlrj]

the decel. smoke and smoke at startup point towards valves.

Can't be valves, they're too fresh so it must be rings. They must not be seating.

Joe

[WERK I]

Originally posted by 75Carrera
My motor is fresh. The leakdown was almost perfect. I have Ross forged 9.5x1 pistons and 964 cams (see details below)..........

Are the aftermarket pistons "slap-happy" at cold startups? Maybe the expansion rates on these pistons allow for higher consumption when the engine is cold?

[JeremyD]

Valve guides were replaced, right? the smoke in decel is creating a vacuum - oil is getting either sucked in through the guides or around the pistons. If it's coming around the pistons - are you getting any blow by (as in having any trouble keeping the crankcase breather hose on?) - by having the leakdown #'s ok, that would point more to valve guides again.

964 cams are not high lift so you should not get stems pinched which can happen on some of the higher lift DC-22 or 993ss cams like I have.

You have some issues with one qt per fill up/300 miles.

I am using Mobil 1 in mine - and no issues, no measurable oil consumption at this point (also around 1500 miles on the rebuild)

[aigel]

Originally posted by stlrj
Can't be valves, they're too fresh so it must be rings. They must not be seating.

Joe

There would be blowby the other way as well. The crankcase would pressurize and oil would leak out of every conceivable place. It certainly would be intersting to pull the breather hose off and see if you get a bunch of exhaust and smoke your way.

George

[schoward]

Another data point

fresh top end rebuild on my 81 SC. Motor has about 3500 miles since rebuild.

Running Mobil 1 15/50 synthetic and I was down about a quart after about 2000 miles following the break in oil change (assuming my tech filled it to the max on the dipstick).

Now at 2400 miles since oil change and probably need to add some soon as I have not added any yet but i am still in the 'range' on the stick.

Before the rebuild, i used a quart every 600 miles and I had 30% leak downs going on.

Your consumption sounds signficantly high.

[Porsche_monkey]

What were the cylinders like on disassembly, or did you buy new?

How did you break the engine in?

Is the leakdown still perfect, or was that 1,500 miles ago? You could have a cracked ring (or two) now.

[wholberg]

I have always heard that a new motor should be run on dino oil for 5000 miles before being switched to synthetic, to ensure full break-in, ring sealing, etc. Running Mobil 1 with only 1500 miles on it may be the problem.

[Carrera Kid]

Someone else mentioned something called Royal Purple but I never heard of it. Sounds like STP... ugh!

I don't use Royal Purple in my car now, but have in the past. It is a very good, full synthetic that is used by a lot of top engine builders. OK, I'm familiar with the American engine builders, as in the Engine Masters Competition, and most of them use it. I'm sure they have a website with info. My brother just put Royal Purple gear oil in his Lotus Esprit transmission and said it really smoothed out the shifting.

It may have a funny name, but it is highly regarded by racers.

Rick

[schoward]

to wholberg's point above (another data point)

yes - my tech had me on dino 10w40 for break-in and then over to 15/50 mobil 1 following break-in service which included a valve adjustment.

[75Carrera]

Thank you Mr. Wolberg and others: The pistons are Ross (forged) and we only ran about 800-1000 miles on dino before going to the Mobil-1 15/50. I agree the likelyhood is that the rings did not have a chance to fully seat. The heads, valves and guides are all new (Ollies). Today I decided (with my trusted wrench) to go back to dino and see what effect that produces. The forged pistons are a factor but I can't say I completely understand exactly what it is. They do rattle a bit untill they heat up a little.
The car (even under the strain of speed and working the twisties) never gets much past 180 degrees. About 220 even on a hot day. The guys behind me on our last outting said they could smell my exhaust and that along with the visible discharge of smoke occassionally has me looking to Pelican and my wrench for answers.
Carrera Kid: What does the Royal Purple stuff actually do or is it just another synthetic? Ever heard of it being used for something like this in a Porsche?

[Halm]

When I bought my '70 E, the PO had Ross pistons in it. By reputation (and on their website, as I recall) Ross pistons are known to smoke more and burn more oil. That is by design. Why, I have no idea.

[75Carrera]

It would be hard to see a logical reason for "wanting" a piston to allow any waste or smoke? The point about seeing it on their website is interesting though. I'm going there now and see what I can find. Perhaps it is the piston (by design) I should give them a call. The Ross pistons were the only "what if" we dealt with during the build. WE chose to go with them over the price of all new barrels and pistons $$$. And the leak down was excellent. I'm gonna be pissed (maybe susicidal) if this can't be remedied. The 3.2, upgrades, build and labor twernt cheap... Thanks man...

[aigel]

Generally forged pistons have a higher coefficient of thermal expansion. Therefore the forged cold piston will have a larger tolerance in the bore than the cast piston will. However, as soon as the piston warms up, it should exhibit the same tolerances in the bore than a cast piston. The only thing that can happen with forged pistons is a light "piston slap" from the enlarged tolerance when the engine is cold.

I know that porsche engines may consume more oil than others, but I have not owned one to date that would drink noticeable amounts at all between oil changes.

George

[75Carrera]

aigel,

Yep. I just got off the phone with Chris at Ross Racing Pistons. According to him Ross never did or would design a piston intended to be characteristic for oil burning or consumption. The forged pistong does have a rattle when cold but should fade as the car temp comes up. But it doesn't go away. My car seldom gets over the 180 mark even pushing it pretty hard through the mountain roads. I wonder if thicker oil and somehow raising the operating temperature a bit would help. A cold piston would have more gap until it expands. Ever think you'd hear a Porsche owner wonder if his car was running hot enough? LOL

[aigel]

The oil temp is much less than the piston temperature. If you are at 180F oil temp your engine will be sufficiently warmed up and if there is still a noise, it means that there is something wrong! The piston expands pretty much linear with temperature, so an extra 20 degrees won't buy you anything, especially in an aluminum cylinder bore. It is possible that the pistons are just plain too small for the bore. Maybe some rings are broken?

If your engine builder is a stand up guy, he should be on this, not you. He should diagnose (leakdown, compression) and pull the engine to go through it and correct the problem. Make sure you do not let him string you along until the warranty period is over and act now.

Good Luck, George