Engine Oil for an Early C4: SM or CJ

[Joe C.]

For normal use, stock L98, driven occasionally. Normal oil consumption has always been about 1 quart/1200 miles.

[Duke W.]

Re: Engine Oil for an Early C4: SM or CJ

If it has roller lifters and roller trunnion rocker arms SM is probably okay. If not - C-category. I'm using CJ-4 or CI-4 in my "modern" cars - an '88 and a '91, both of which are OHC and have sliding surfaces. Given their low mileage, low rate of mileage accumulation, and low oil consumption, I'm not worried about catalyst life with the higher P concentration of C-category oil.

I'm counting down the number of remaining emission tests before they qualify for California Historical Vehicle license plates and no longer require bienniel emission tests - one more for the '88 Merc 190E 2.6 and two more for the '91 MR2.

Neither has ever failed a tailpipe test, and both still have good to excellent passing margins for all measured tailpipe gases. The Merc in the past has been close to the HC limit on the 15 MPH ASM test, but by changing a resistor and plugging the vacuum advance, the spark advance is considerably retarded from OE at the 1500 RPM test speed. It won't get out of it's own way below 2000 in this configuration, but the higher EGT keeps the catalyst hotter and cut the HC readings by about half, and it only takes a few minutes to swap the resistor and short piece of hose (plugged with a slug of silicone sealer) for the vacuum advance.

With the normal resistor I use for the most aggressive spark advance map (equivalent to light springs in a Delco single point distributor), it will pull from 1000 revs is fifth and gets 24 MPG in my mix of driving, most of which is surface streets. The EPA mileage ratings were 18/26.

The spark advance map has a major impact on emissions, fuel economy, and torque, and low emissions conflict with good fuel economy and torque.

Duke

[Jim T.]

Re: Engine Oil for an Early C4: SM or CJ

Joe does your 85 C4 still have the original exhaust system?

My 85 was purchased in 92 from the original owner. It still has the original exhaust and I think it runs very strong and has accumulated 105K miles on original unmodified engine.

Have used Pennzoil oil in it all these years. Last month changed to CI-4 10-30. I want the 85's engine to continue to last.

I would "think" that if the converter would suffer damage from ZDDP additive in oil it would of happened by now.

I saw no change in the driven performance of my 85 when I stopped using premium fuel and started using what the owners manual and GM Motor Vehicle Specifications for 1985 MVMA-C-85 information indicated.

[Duke W.]

Re: Engine Oil for an Early C4: SM or CJ

Joe does your 85 C4 still have the original exhaust system?

I would "think" that if the converter would suffer damage from ZDDP additive in oil it would of happened by now.

I saw no change in the driven performance of my 85 when I stopped using premium fuel and started using what the owners manual and GM specification information indicated.

If the '85 has to be emission tested and the numbers are okay, then your catalysts should be in good shape. The reduction in ZDDP was a "hedge" by the auto industry to not get into trouble with 100K emission performance warranties, and modern "roller everything" engines don't need the higher ZDDP concentration that is best for vintage engines with sliding surface valvetrains.

Any "premium fuel" engine that has a detonation sensor will run okay on regular unleaded, and it may be completely transparent. Some modern engines are pushing CRs (up to 11:1) to the point where running regular may force such a retarded spark advance map that a degradation in performance or fuel economy may be noticeable.

Eighties vintage premium fuel engines typically had CRs in the range of 9-10:1, so most will run transparently on regular unleaded, even if premium was "required" by the manufacturer.

My '88 190E 2.6 has an advertised CR of 9.2:1 and premium is "required". However, it runs okay on regular even though it has no detonation sensor. It will exhibit some transient detonation if coolant temperature is over 90C or ambient temperature is over about 70F, but I can negate it by shifting at higher revs or feeding the throttle in a little slower, and since I only drive it from November to May, it doesn't see hot weather. My suspicion is the the vacuum advance has a bit of lag, which is what generates the transient detonation on low rev upshifts because if I feed in the throttle a little slower, no detonation is evident.

Duke

[Joe C.]

Re: Engine Oil for an Early C4: SM or CJ

If it has roller lifters and roller trunnion rocker arms SM is probably okay. If not - C-category. I'm using CJ-4 or CI-4 in my "modern" cars - an '88 and a '91, both of which are OHC and have sliding surfaces. Given their low mileage, low rate of mileage accumulation, and low oil consumption, I'm not worried about catalyst life with the higher P concentration of C-category oil.

I'm counting down the number of remaining emission tests before they qualify for California Historical Vehicle license plates and no longer require bienniel emission tests - one more for the '88 Merc 190E 2.6 and two more for the '91 MR2.

Neither has ever failed a tailpipe test, and both still have good to excellent passing margins for all measured tailpipe gases. The Merc in the past has been close to the HC limit on the 15 MPH ASM test, but by changing a resistor and plugging the vacuum advance, the spark advance is considerably retarded from OE at the 1500 RPM test speed. It won't get out of it's own way below 2000 in this configuration, but the higher EGT keeps the catalyst hotter and cut the HC readings by about half, and it only takes a few minutes to swap the resistor and short piece of hose (plugged with a slug of silicone sealer) for the vacuum advance.

With the normal resistor I use for the most aggressive spark advance map (equivalent to light springs in a Delco single point distributor), it will pull from 1000 revs is fifth and gets 24 MPG in my mix of driving, most of which is surface streets. The EPA mileage ratings were 18/26.

The spark advance map has a major impact on emissions, fuel economy, and torque, and low emissions conflict with good fuel economy and torque.

Duke

It's an '85 L98 with 38K miles. The engine is all original which, in its 1985 config, was built with iron heads and the FIRST (of three) version of the L98 camshaft, and so has flat tappets and non-roller rocker arms.

The oil consumption has always been a tad higher than I would have liked it to have been..........@ about 1 quart/1200 mi (given its "modern" design, ringing and cast pistons). I would have liked it to have been closer to a quart/3000 miles. My Honda Accord uses less than 1 quart per oil change, which occurs every 6 months, or roughly 7000 miles! Because of the slightly high oil consumption (which, I understand is well within normal limits for this engine, albeit probably skewed toward the high side of the acceptable range), I believe that I'm trading converter longevity for camshaft longevity.........or AM I?

Joe

[Joe C.]

Re: Engine Oil for an Early C4: SM or CJ

Joe does your 85 C4 still have the original exhaust system?

My 85 was purchased in 92 from the original owner. It still has the original exhaust and I think it runs very strong and has accumulated 105K miles on original unmodified engine.

Have used Pennzoil oil in it all these years. Last month changed to CI-4 10-30. I want the 85's engine to continue to last.

I would "think" that if the converter would suffer damage from ZDDP additive in oil it would of happened by now.

I saw no change in the driven performance of my 85 when I stopped using premium fuel and started using what the owners manual and GM specification information indicated.

Jim,

The car is a low mileage, all original car, so the cat is in place. The L98 coupled to the 700R4 with its 3.06 first gear and 3.08 axle makes for a VERY quick Corvette. I feel like a teenager because it's so easy to light up the original tires! I'll let you know how different it will be with the new 275-40ZR-17 Goodyear Eagle F1 GS-D3's after I install them (today, tomorrow, or Monday). The motor begins to lay down after about 4500 RPM, but there's enough horsepower there to enable the car to reach 148 MPH.

Regarding the fuel, as Duke mentioned, the ECM will adjust spark advance to eliminate detonation if it receives a signal from the knock sensor.

When the car was about a year old, I tried an experiment. I already knew that the engine would not detonate with regular gas, so I computed the cost per mile with regular versus premium. The cost per mile was actually very slightly better with the high test. There was no noticeable difference in power (SOTP evaluation). I had been filling with high test before the test, and have continued to do so to this day.

Joe

[Bill B.]

Re: Engine Oil for an Early C4: SM or CJ

My wifes 1989 with 167,000 original miles L98 uses (0 to .5) quarts of oil between oil changes @ 5000 miles. We have always used 5W-30 Mobil 1.

I guess we just got lucky then!

Bill

1961 Black/Silver/Red Int. 283/315 FI
power windows. Frame off 65% completed.
1989 White/hard top/Grey Int. daily driver 167,000 miles

"The problem with Liberalism is that eventually you run out of other people's money" - Margaret Thatcher

[Duke W.]

Re: Engine Oil for an Early C4: SM or CJ

It's an '85 L98 with 38K miles. The engine is all original which, in its 1985 config, was built with iron heads and the FIRST (of three) version of the L98 camshaft, and so has flat tappets and non-roller rocker arms.

The oil consumption has always been a tad higher than I would have liked it to have been..........@ about 1 quart/1200 mi (given its "modern" design, ringing and cast pistons). I would have liked it to have been closer to a quart/3000 miles. My Honda Accord uses less than 1 quart per oil change, which occurs every 6 months, or roughly 7000 miles! Because of the slightly high oil consumption (which, I understand is well within normal limits for this engine, albeit probably skewed toward the high side of the acceptable range), I believe that I'm trading converter longevity for camshaft longevity.........or AM I?

Joe

Given the low mileage, I expect your rate of mileage accumulation is low, so I don't see any catalyst issues even with the somewhat high oil consumption, and even if the car has to be emission tested forever.

If it were mine, I'd use C-category engine oil in order to provide maximum wear protection to the sliding surface valve train.

Duke

[Duke W.]

Re: Engine Oil for an Early C4: SM or CJ

My wifes 1989 with 167,000 original miles L98 uses (0 to .5) quarts of oil between oil changes @ 5000 miles. We have always used 5W-30 Mobil 1.

I guess we just got lucky then!

Bill

Today's 5W-30 SM-rated Mobil 1 is a very different formulation than Mobil 1 from 20 years ago - both the base stock blend and additive package. The same applies to any other brand S-category motor oil. It's not the same as 10 to 30 years ago, particularly the additive package.

In general, current non-synthetic S-category oils have a better base stock blends, but a less robust additive package, particularly the anti-wear additive.

Current "synthetic" oils have a poorer base stock blend than the early "synthetics", since most are now Group II and III, rather than the Group IV and V base stocks that were used in the early days. And the additive package of modern "synthetic" S-category oils is effectively the same as conventional base stock SM oil.

That's why "synthetic" oil is of no value in vintage engines. Any conventional oil you buy has a better base stock blend than even ten years ago and vastly superior to the base stocks that most of these engines lived on during their lives as daily drivers.

To paraphrase a famous political statement: It's the additive package, stupid!

I don't recall when Corvette started requiring Mobil 1, which corresponded to the addition of an oil monitoring system. And the other variable in the equation is when they went to an "all roller" valve train. If the requirement was Mobil 1 and the valve train was still conventional sliding surfaces, then I would switch to 5W-40 CJ-4, which has a "synthetic" base with about the same ZDDP concentration as pre-SL S-category oils.

Duke

[Bill M.]

Re: Engine Oil for an Early C4: SM or CJ

I don't recall when Corvette started requiring Mobil 1, which corresponded to the addition of an oil monitoring system. And the other variable in the equation is when they went to an "all roller" valve train. If the requirement was Mobil 1 and the valve train was still conventional sliding surfaces, then I would switch to 5W-40 CJ-4, which has a "synthetic" base with about the same ZDDP concentration as pre-SL S-category oils.

Duke

The '92 LT1 required Mobil 1. It is a roller cam. Chevy eliminated the oil cooler when they went to synthetic oil in 1992. (I've seen close to 300 degrees F. oil temp. when running my '92 in competition.)

[Jim T.]

Re: Engine Oil for an Early C4: SM or CJ

It's an '85 L98 with 38K miles. The engine is all original which, in its 1985 config, was built with iron heads and the FIRST (of three) version of the L98 camshaft, and so has flat tappets and non-roller rocker arms.

The oil consumption has always been a tad higher than I would have liked it to have been..........@ about 1 quart/1200 mi (given its "modern" design, ringing and cast pistons). I would have liked it to have been closer to a quart/3000 miles. My Honda Accord uses less than 1 quart per oil change, which occurs every 6 months, or roughly 7000 miles! Because of the slightly high oil consumption (which, I understand is well within normal limits for this engine, albeit probably skewed toward the high side of the acceptable range), I believe that I'm trading converter longevity for camshaft longevity.........or AM I?

Joe

Reference your post on having cast pistons. The 1985 Motor Vehicle Specifications MVMA-C-85 shows our 85's having forged pistons.
On page 5 it shows "Impacted forged aluminum. 579 (20.4)" for Engine-Pistons.
Crankshaft is "Nodular cast iron - 23.520 (51.85)"

[Terry M.]

Re: Engine Oil for an Early C4: SM or CJ

IIRC beginning in 1992 in the Y-body (Corvette) there was little room for an oil cooler and that drove the Mobil 1 requirement.
The nearly the same 1994 to 1996 LT1 in the B & D-body (Caprice, Impala SS & Fleetwood) used an oil cooler (of varying styles depending on vehicle line) and conventional oil.

[Joe C.]

Re: Engine Oil for an Early C4: SM or CJ

Reference your post on having cast pistons. The 1985 Motor Vehicle Specifications MVMA-C-85 shows our 85's having forged pistons.
On page 5 it shows "Impacted forged aluminum. 579 (20.4)" for Engine-Pistons.
Crankshaft is "Nodular cast iron - 23.520 (51.85)"

Well..........I'll be damned. That's what I get for ASSuming.