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Tech Q&A - April 2014

Kevin McClelland Feb 16, 2014
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"FLO" Knows

Ok, by know you have all seen the Progressive Insurance commercials where you can get deep discounts by installing their "SnapShot" data logger into your OBD II diagnostic port and recording your driving habits for a 30-day period. They claim that they monitor how often you slam on the brakes, miles driven, and when you drive. They are looking for high-risk times that you drive like between midnight and 4 am. With this data they can assess the proper insurance rates. Along with the data they list above, they could also record vehicle speed, throttle opening, engine speed, etc. With this tool, the insurance companies could rule our lives and our enjoyable driving habits.

This brings me to a little story that happened this week, and the resulting work that I'm off to the garage to do. Coming home from work three days ago I noticed a small puddle of something under the rearend of Daniel's 2003 Z06 Corvette. When I got out of the car and laid down to check it out there were two puddles, one from the day before, and a fresh one from today. It appeared that it was gear oil coming from the differential. My heart dropped as I looked at the factory rearend attached to the back of the 6-speed manual.

Well, I rolled into the house and told Daniel that there was a puddle of oil under his rearend. After dinner we went out to check out what we could find. As we were all standing around after checking for the source, I asked him "Did you wheel hop the thing before you had the new tires put on the car, or after checking out the traction of the new tires?" The look on his face was priceless. The day before, I found the oil leak before he installed four new tires, as the original Goodyear F1's were dead. They were great, they would spin, leave no evidence, smoke, or noise to draw attention. The new tires are a type of very aggressive performance tire that really gives the car grip. I figured either he had burned the tires down before the new install, or the new grip shocked the differential and possibly caused the oil leak.

Kevin 2/2

The point I'm trying to make here is, you young hot rodders out there; if you're raised in a gearhead house, I doubt there is anything you can do with a car that we haven't already done! I would never put a tire on my big-block Chevelle until I could see the air through the tire. I couldn't afford killer tires so I would put the best front tires I could afford for handling, and Caldwell re-treads on the rear. I couldn't keep a set on the rear for longer than six months. That would bring me to a citation for "Breaking Traction," but that's another story.

Well, I'm glad that I'm finished with this month's edit so I can put some real tools in my hands and help Daniel fix his car. Hopefully, it's something simple, and not what I'm reading online about the left side oil leak, which is common with these diff's. The warranty time to fix this issue is only 2.2 hours. What that means is that you have to about double that for an at home garage repair. Oh, bother! Wish me luck.

Ford on the Mind!

Q. I wanted to make a correction to Kevin McClelland's article in which he was talking about the 2-6-6-6 Allegheny Steam locomotive. His statement of pulling fully loaded coal cars at 60 mph is false. It could in fact pull passenger cars that fast, but was only ran at 18 mph pulling loaded coal cars through the mountains. The museum web site states this fact about its top speed. I know it's not car facts but I wanted to ensure the true facts were printed. Thanks!

Chad Kollars
Council Bluffs, IA

A. Chad, we're with you and this is why we're printing your correction. Yes, we always want to print the facts, as that is the purpose of this exercise. Now, for an excuse! How about a Chevy guy that has lived, eaten, and breathed General Motors all his life roaming the Henry Ford Museum. What do you expect? I'm surprised that I haven't started painting engines Ford Blue, and thanks again for the correction!


Rare as Hen's Teeth

Q. I am a long time subscriber to your excellent magazine, and I have a question. I have a 350 engine in my hot rod and I found that I have a cracked head, casting number 3973487X. These castings are hard to find, and expensive, and was wondering what other more available/less costly casting that will work next to the 487X so I don't have to change both heads as I am on a tight budget. Thanks for any help.

Lou Lopardi
Tolland, CT

A. The 3973487X heads were early castings that were produced from 1968-73. These were some of the first castings with the accessory boltholes in the end faces of the cylinder heads. They were especially sought for their slightly larger intake runners than the standard cores that casted the 487 cylinder heads. Both of these casting numbers were either equipped with 1.94/1.50 and 2.02/1.60 inch valve packages. They are within 0.1cc difference in combustion chamber volume coming in at 75.4cc. The standard 3973487 casting was used from 1968 through 1979.

If the "X" heads actually had slightly larger intake runners you wouldn't know it from the engines they were originally installed on. They were used on 350 cid displacement engines producing anemic power levels of 165, 175, 200, and 255 horsepower levels. I believe that all of these engine combinations came with the Rochester 2-barrel carburetor. As for the standard 487 castings, they were also used only on the 350 displacement engines, but with much higher power levels. They were also used on the pedestrian engine listed above, but were also on a 330 hp 350-cid small-block.

The bottom line is the 3973487 casting will work just fine next to your "X" head. Make sure that you run matching valve sizes and the very slight combustion chamber difference won't make a hill of beans. Check around your local area to see if you can pick up one of these castings. If you can't find one, give Cylinder Head Exchange a call in So. Cal. My brother-in-law Tony Knight has been there for over 30 years and their inventory of current model, and vintage cylinder heads will blow your mind. I'm sure that he could scrounge you up a 487 casting and prep it to whatever configuration you need.


Silent Running

Q. I was wondering when GM quit using the fiber timing gear. I have a 1994 GMC 1500 with a 350 eng. I have put new plugs and new wires, new rotor, new cap, new fuel pump, new fuel filter, new EGR regulator, and the timing is right. It will start right up and pretty much idle perfect, but as soon as you put it in gear, it will hardly pull itself. So was wondering if this still had the nylon timing gear and thinking that it could be worn down. If you have any help at all on what I need to do, would be greatly helpful.

Mike Bergen
Via email

A. Back in the day, as a line mechanic at Muller Chevrolet, replacing timing sets was a regular occurrence. Why this is titled "Silent Running" is because these nylon/aluminum camshaft gears would leave you stranded. You couldn't hear them run because they weren't! If the owner was lucky, the engine would diesel on key off and the chain would jump a tooth or two. With the very mild camshafts in the production small blocks you could get away without bending the valves. Now, if the vehicle owner's son was driving the car, the valves were always bent. Not only did you have to tear the front of the engine off and replace the timing set, off came the heads for a valve job and several replaced valves. It was not only the small-block Chevys that had the benefit from the silent running timing chain sets; the Pontiac V-8's of this era were equipped with the same technology. For as many jumped timing sets we replaced on small-block Chevys, we probably replaced twice as many Pontiac sets. Great design!

From all indications the composite timing sets (Nylon and they also used a Bakelite material) ended in the mid 1980s and they went with straight steel camshaft sprockets. The Nylon was used for quiet operation and I think that the marketing name was something like "Silent link." Your 1994 vintage small-block most certainly has a steel timing set installed. I have a couple of questions. You have definitely replaced a few parts chasing down your problem. Any or all of them could create the condition of poor performance. You said that you had checked the timing and it was correct. Your truck is equipped with electronic spark control. To check the timing correctly you must disconnect the distributor from the computer to set the initial or base timing. I believe that your base timing is zero degrees in '94. There is a tan/black wire that has a weather pack connector in it to disconnect the ESC and allow the engine to run on the distributor ignition module. This will allow you to set your base timing. The timing connector has been moved from the engine compartment to inside the cab. This new location is not currently reflected in the Service or Owner's Manual. The new location is under the dash directly below the glove compartment. The tan/black wire is routed for a short distance outside of a large wiring harness. This large harness is routed between the heater blower motor and evaporator case. The timing connector is taped to the wire bundle. Once the base timing is set correctly, re-connect the tan/black wire and the timing should jump up into the 20-degree range at idle.

From your description of the lack of performance and that when you put the truck into gear it falls on its face, sounds like a timing issue. If you were setting the initial timing with the computer connected, it would only retard the timing more when you drop it into gear. Hopefully, this gives you a direction to find your problem. Good luck.

Rock On

Q. Your column is the best. It's the first thing I look at when my magazine comes in the mail. I'm always learning something new and interesting. I was reading a past (August 2009) column about GM Performance Parts 1.6 ratio rockers (PN 12370839 for kit) and was wondering if these would fit my truck? I have a 2003 S10 with a 4.3L Vortec V6. Would they fit under my stock rocker covers? If they fit, do you have a part number for a single rocker since I don't need 16 of them? Thanks for your time.

Todd Yoshioka
Honolulu, HI

A. Todd, thanks for the props. We try to help as many folks as we can with our answers. If we spread the info around and make it different every month, it will stay interesting. The 1.6 ratio rockers that you listed from the August 2009 column have been discontinued as the original supplier went under. These rockers were exact replicas of the 1996 LT4 aluminum roller rockers with 3/8 inch trunnions in place of the metric version that were in the production rockers. Not only did these rockers have a specific 3/8-inch trunnions, they were supplied with extremely short Posi-lock adjusters which would clear the factory valve covers on small-blocks, and your 4.3L V-6.

As you listed, the set of 16 were sold under PN 12370839, and have been superseded by PN 19210729. Our pal Ken Casey at Elway Chevrolet in Englewood CO. has the new rockers in stock, but they have changed the body from the original LT4 design. We would be concerned that they may not be a direct swap.

The single units were sold under PN 12367346. The interesting thing is that the single unit service part number is still active and GM has stock. You can order these directly from Ken at Elway. We've also found a limited stock of the original 16 set part number LT4 design in stock at Summit Racing. You may want to check the pricing as the set of 16 may actually cost you less than purchasing 12 of the service rockers. You may want to scoop these up as soon as you can because when they are out of stock, they will be gone.


Octane Requirements

Q. I have a 355-cid small-block with iron heads (882), a moderate cam (COMP Magnum 280), and 35 degrees mechanical advance, 10 deg vacuum and approximately 9.3:1 compression ratio. I assumed it should need 89 octane, several tankful's later I tried 87 and there was no pinging to be found at any load. A local engine builder has an aluminum headed Pontiac 11:1 compression ratio, moderate cam with a wide lobe separation angle, 180 thermostat and runs 87 in it. Could you give some rules of thumb and some variables for judging octane requirements? Is it even feasible to hear spark knock in a hotrod? If I understand fuel correctly, an 8.5:1 motor running 87 octane should dyno higher than running it on 93? Thanks.

Michael Roshon
Pataskala, Ohio

A. Octane requirements for a specific engine will vary greatly dependent on many factors of the component makeup. As you have listed, you're running production iron heads with approx. 9.3:1 compression. This alone would lead you to believe that you couldn't run 87-octane fuel. That would be true if you were running a stock production camshaft that produces high cylinder pressures at slow engine speeds (torque). When you throw in the COMP Cams Magnum 280 camshaft, its aggressive profile changes the engines personality completely. This relatively aggressive street camshaft has 230 degrees of duration at 0.050 inches tappet lift, and is ground on 110-lobe separation angle. With these lobes you have bled off all the slow speed cylinder pressure with its late closing intake event. Slow engine speeds are right where engines love to knock when you put them under load. You didn't give us what the engine was installed in, stall speed or manual trans, gearing, vehicle weight, etc. All of these factors apply the load on the engine, which induce knock.

When we talk about intake closing events, this is where the intake closes the cylinder to the atmosphere. If you close the valve earlier, you have more stroke to build compression. This is why we have measured static compression. This is done with a cc burette and you measure and mathematically come to a compression ratio. Then you have dynamic compression ratio and this is when the intake valve actually closes and you have the rest of the compression stroke to build cylinder pressure. Let's say just for numbers, if you put a stock camshaft in your small block with a 240 degree advertised intake lobe. This is 40 degrees less duration than your 280H. The 280H has in intake closing event of 66 degrees After Bottom Dead Center. The 240H's closing event is a scant 50 degrees ABDC. This 16-degree difference has the cylinder closed at 130 degrees Before Top Dead Center, which gives it the ability to produce compression 14 percent more time. Please remember that all of these events are from 0.050 inches tappet lift specs. With a standard 1.5 ratio rocker, the valve is open 0.075 inches. We're assuming the closing ramp of the intake lobe is the same between the two listed camshafts. If you truly mock up an engine and check the actual seat timing you will find that the valve doesn't fully seat until sometime after 90 degrees Before TDC on many racing engines.

Another great tool to measure the actual compression of an engine is with a compression gauge. This is a dynamic reading of the engines ability to capture air and convert to pressure. With the above 280H camshaft and 9.3:1 compression you should have a cranking compression of around 150 to 160 psi. If you were to install the much shorter 240H with no other changes you would probably see the pressures jump into the 200 range. Compression gauges are sometimes a good indicator of what type of fuel you will need. If you're above 200 psi, cranking the engine could be knock sensitive and require higher-octane fuel. We've pushed the limit with aluminum headed small block with a little over 10 to 1 compression and a short 208-degree intake lobe. This engine produced 230 psi of cranking compression and would only run on 92 and was timing limited at around 30 degrees total before it would knock its brains out.

Yes, it can be a challenge hearing spark knock and detonation in high performance engines with loud exhaust. When we were doing dyno calibrations for GM we had a headset arrangement, which used the knock sensors in the side of the block through an amplifier and into headphones. You could hear every mechanical noise of the engine. Everything that you heard was completely repetitious, except for engine knock. This is totally random and has no pattern. This is how you knew the engine was in knock. As for our performance street and racing engines running on pump gas you must be very careful. You must look for detonation on the spark plug. Pre-ignition and detonation create very high temperatures that blister porcelain, and you can see on the ground strap of the plug. In extreme conditions when you melt little particles of aluminum from the pistons they will be deposited on the center porcelain electrode of the spark plug. This will look like very fine black specs on the white center electrode and we call it "peppering."

Also, on the engine dyno you could watch the torque. You wouldn't hear the engine knock, but you would get to a timing setting that the torque wouldn't be steady or repeatable. As soon as you backed the timing back a couple of degrees, the torque would come right back in line and repeat. You should also be able to see this on the "black dyno" drag strip if you have good weather conditions and no wind.

Finally, the reason that engines will make better power on lower octane fuel is because of the burn rate. The lower the octane, the faster the fuel will usually burn. With the same ignition timing, the cylinder pressure will rise faster, which will give you more measured torque. Now, if you advance the timing a few degrees you will be very close to the same torque with the higher-octane fuel.

Now for the disclaimer: As stated earlier, the component mix of an engine build would affect the octane requirement. We only went into the compression ratio and the camshaft events. Cylinder head design and material, piston dome/dish configuration, intake runner length and size, fuel injection vs. carbureted, header design and exhaust system performance, bore size, stroke and rod length all effect the octane requirements of an engine. Hope this has giving you a little look into the running engine and the way it works. It's a magical thing!

Arm Swapping

Q. I would like to change my front control arms on my 1968 Chevy Nova. Can I change just the top ones, or do I have to change both top and bottoms? Thanks!

Orville Homburg
Via email

A. Orville, great question. You know I've never thought of this upgrade in this way. Usually, you go all in and rebuild the complete front end at one time. If budget only allows component upgrades one at a time, you do what you can.

Unless the aftermarket control arms are unique to a specific spindle you can swap out aftermarket control arms, as they are replicas of the factory pick-up points. It would be a good idea to ask your manufacturer of choice if they are a direct swap. We do know of specific control arms on the market that allow you to run different spindles from later model vehicles to improve the "camber gain" as the suspension travels throughout its range. Again, your supplier can answer this specific question. Enjoy your upgrades!



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