Ask Ray - letters from August 2012

Ask Ray - letters from August 2012

Tips from Hemmings Muscle Machines

August, 2012 - Ray Bohacz

Oh No!
I have a 1972 El Camino SS396/402 that I have owned for 10 years. It now has 56,000 miles on it. I purchased the car with 48,000 miles. I always wanted a muscle car to go to show and cruise nights.

Last year, I was on my way to a cruise and all of a sudden, the oil pressure started to fluctuate, but at no time dropped completely. I was about five miles from home, so I drove it back easy. It seems that when you are not driving, the oil pressure is between 40 to 50 PSI and does not fluctuate. Only when it is driven does the pressure change. I am hoping it is an easy fix and not a costly one. Someone said I should have the engine rebuilt. Thanks.
Richard Betro
Via email

The fact that the oil pressure appears to be good and stable when the vehicle is not being driven tends to make me think that the oil pump pickup has dislodged from the pump or is loose. That is, as long as your gauge is accurate and does not have a loose electrical connection that is apparent when driving. You might want to confirm this first. As the vehicle moves and the oil sloshes around in the pan, the pressure drops, since it is sucking in air.

The first step with any oiling problem would be to drop the pan and look at the pump and pickup, and if there is any baffling in the pan, check that it is not loose. If nothing is found there (which I doubt), then you should pull the oil pump out and take it apart, paying attention to the relief spring and bypass, and for excessive wear in the gears and cover. Inspect the oil pump driveshaft from the distributor as well. I feel quite confident that the cause for the low oil pressure is in the delivery system and not a failure in the engine that would require a rebuild. But if you do not fix the problem and keep running it with huge swings in the pressure, then you'll have to save your money for a major repair job. Good luck, and for your sake, I hope that I am correct.

Rough-Running Rocket
The problem that I am having is with my 1971 Oldsmobile Cutlass S with a Rocket 350 and a two-barrel carburetor. The engine starts and runs at idle without any indications of any issue. However, when you start to drive and reach 35 MPH, the problem appears. The engine begins to run rough. If you continue accelerating to 40 or 45 MPH, the engine still runs rough. It feels like it will stall or stop running. However, if you accelerate to go faster, the engine does not hesitate or stumble. The faster you go, the less you feel the roughness. I believe you do not feel it because of the speed you are now going, which is between 50 to 60 MPH. Also, at idle you can see blue smoke coming out of the passenger-side exhaust pipe; you can't see it while driving. It is using a little oil, too.

Here is what has been done: new Mallory distributor, rebuilt carburetor, new fuel pump, plugs and wires, distributor vacuum control switch and lines. I have checked the timing, carburetor adjustment, modulator valve, intake manifold for leaks, compression (about 150 PSI in each cylinder), fuel pump pressure, moved the coil from the block to the firewall, checked wires for cracks or burnt areas, drained the gas tank (nothing in it), flushed the fuel line and checked engine vacuum (20 to 21 inches at idle). Finally, there are no strange noises coming from inside the engine.

This all started--except the blue smoke--about two months ago, after I got some 93-octane gas from a Texaco station. When it started, I immediately drained the gas tank and put in a bottle of HEET. That seemed to help, but the problem returned.

Any and all help will be appreciated.
Chuck Comstock
Via email

First, I doubt very much if the tank of gas you purchased has anything to do with causing the problem--it just happened afterwards. Though you have performed many tests, it is hard to believe that in a simple engine such as yours, everything checks out, yet it still has a problem. Are you sure that it is in the engine and not the driveline? It appears that the only valid thing you have been able to discover so far is the oil smoke from one side of the engine.

If your numbers are accurate, with 20 inches of mercury at idle and 150 PSI of compression and all of the new parts, the most likely possibility is a broken valve spring. If the spring broke in a location that allows the valve to close under slow-speed operation, then it would idle fine, as you state, but as the engine speed increases, it will hang the valve open.

The good thing about this is that the spring will not know if the car is moving or not. With the vacuum gauge still connected, slowly raise the engine speed to what it would be when the issue occurs going down the road. If the engine starts to run rough and the vacuum gauge needle starts to shake rapidly, then there is a valve train issue. My money would be on either a weak or broken valve spring. Good luck, and let me know what you find.

Clipping Along
I am having an issue that I hope you can help me with. I have a 1955 Packard Clipper that will not run over 50 MPH. Here is the story.

The Clipper had sat with an ailing Ultramatic transmission since the mid-1970s in indoor dry storage. Last winter, I removed the transmission and had it rebuilt and put it back in. I then did a complete tune-up and rebuilt the brake system. The engine starts right up and idles at 700 RPM with 18 inches of vacuum.

The car drives perfectly around town, and for a week or so, I didn't realize there was a problem. When you take the car out on the highway, it runs right up to about 50 MPH and then the vacuum falls to zero; the car loses power and will not accelerate any faster. Let off and the engine returns to its smooth in-town manners. Here is what has been done in a methodical troubleshooting way to try to rectify this issue:

Checked carburetor internally for issues (it was rebuilt when the transmission was out); changed the fuel pump; dropped the gas tank to check for a gummed-up pick-up tube sock; installed a known good carburetor from another Packard; installed a NAPA inline electric fuel pump for test purposes; checked for vacuum leaks around the carburetor and intake manifold with carburetor cleaner; unhooked each vacuum line from the carburetor and plugged off one at a time; removed valve covers to make sure all rockers were moving; changed ignition coil external resistor; bypassed ballast resistor; checked distributor advance function; removed distributor and rechecked TDC; unhooked exhaust at Y-pipe; and test-drove.

You can make the vacuum fall to zero while the car is sitting still by opening the throttle to the 50 MPH range, which is around 3,000 to 4,500 RPM. I have tried to follow your advice of testing and to methodically home in on the problem.

All of the settings have been made per the Packard service manual. My last two thoughts are either the distributor advance is going away under load or the intake ports have some kind of obstruction. I have not opened up the engine other than removing the valve covers to observe the valve train. Any insight you can give would be greatly appreciated. The car has 75,000 original miles.
Chad Riggleman
Via email

As I was reading through your letter, I said to myself, I bet the engine has an exhaust restriction, since even in neutral, the engine vacuum drops to zero at the RPM where the car hit a brick wall. But you stated that you tried it with the Y-pipe disconnected. That took my train off the track!

Your engine is a textbook example of a severely restricted exhaust. My question is, did you completely open up the exhaust, or only slightly separate the pipes? I learned many years back that on cars with catalytic converters (I know a 1955 Packard does not have one), you would have to completely open the exhaust system to see any improvement in performance. When I worked in the Buick agency, there was a rash of plugged converters from a defective purge valve. Eventually, the cars would not even run and had to be towed in. A trick we used to try to get the car into the shop was to pull out the oxygen sensor and thus create a good-sized hole in the exhaust. It usually made no difference. So if you did not fully separate the pipes, your test may not be valid.

Worn camshaft lobes can also create the same issue; so can a jumped timing chain. I have seen engines with a cam that is starting to go round, which would idle beautifully with good vacuum but make no power. Due to that, the engine vacuum would drop precipitously, because the throttle plates were excessively open. It is very possible that the cam has rounded lobes.

This would be easy enough to check with a dial indicator and a magnetic stand. Pull the valve covers off and disable the ignition so the engine will not run. Then, with the aid of a helper, measure the valve lift on each intake and exhaust valve while the friend bumps over the engine. Compare that value to the parts-book cam specifications. Remember, if you position the indicator on the pushrod side of the rocker, it will be the net cam or lobe lift. With the indicator on the valve side, the multiplicative effect of the rocker ratio will produce a higher number. For easy arithmetic: If the cam lobe lift is 0.350 and the engine has 1.5 ratio rockers, the valve lift would be 0.350 X 1.5 = 0.525 inch. Sadly, I think you will find your problem there.

Power Play
I am the owner of an original 1982 Corvette and a member of the American Car Club of France in Paris.

I would like to know if I can replace the original 350 engine with any big-block Chevy (like a 427-, 454-, 540-, 572- or 632-cu.in. engine) without changing the original THM 700-R4.

Are there any modifications to consider on the chassis or drivetrain other than an adequate torque converter? I need to know how much horsepower can be supported by the THM 700-R4. What kind of performance rebuild is then necessary inside the transmission?
Laurent Defrance
Paris, France

The early Turbo Hydra-Matic 700-R4 transmissions were not meant to hold much power, as GM didn't make any engines during the early 1980s that had any serious output. But before I assign a number for you, it is important to understand a few things.

When a car company rates a transmission for power input, they have a specific test that also includes a constant load, or what is known as duty-cycle. Torque is what breaks transmissions, not horsepower. Most gearboxes are rated for that metric. So if a Detroit manufacturer says a gearbox is good for 450-lbs.ft. of torque, that means the gearbox can handle that amount of load under constant output, not just an occasional burst of input to that level. So given this, if I remember correctly, an original THM 700-R4 should be rated to around 400-lbs.ft. of torque.

There are some caveats that need to be applied. Your car is 30 years old, so it would be impossible to determine the power your transmission could take at this point, due to its age. The value I stated was for a transmission that had no wear and tear and is not time-worn through the aging of its components. In addition, even if you bolted it to a 1,000hp engine, if you drove your Corvette gently, the transmission would see no more power than it does now. To move a mass at a speed takes a given amount of power, and that does not change with the capability of the engine. If it takes 30 horsepower to cruise your car at 65 MPH, then that need is constant, whether the engine produces 500 ponies or 150.

With regards to the transmission connecting to a big-block Chevy, all you need to do is confirm the bolt pattern on the bellhousing. To build a version with 540 cubic inches or more, you will need an aftermarket block, since a GM 454 or 502 cannot be made that large in most cases. Many racing blocks have multiple bellhousing bolt configurations.

I respectfully say that you are doing this backwards. You should first decide on the engine use and power; when that is finalized, you build a transmission that will work with it. You do not worry about the transmission first.


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