Tech Q&A - April 2014

Kevin McClelland Feb 16, 2014 0 Comment(s)
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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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