Dan87IROC-Z

I've heard of all these various types, cam, ingition, base, all sorts, can someone please just give me a run-down.

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1987 Chevy Camaro IROC-Z
L98 TPI 350 (5.7L)
TH 700R-4 Transmission with 2.77:1 Rear End

Current Mods: Edelbrock TES 1 5/8" Headers, Hooker 3" Aerochamber Cat-Back System, Performance Resource Chip with 160* Thermostat, Accel Ignition Coil, Cap, Rotor, 8.8mm Wires, K&N Filters, JET TPI Air Foil, All Free Mods, Falken ZIEX Z-Rated Tires.

Best ET : 14.32 @ 97.7mph
(corrected for elevation)

Soon to come: LT4 HOT Cam, Air Flow Research 190cc Heads, 2800 RPM Stall Converter and SLP 3.70 Rear End Gears.

Vader

Dan,

Cam timing doesn't change. Once the cam is ground and installed in the engine, there's no way to change the cam/valve timing in relation to the crank without removing the chain and sprockets. When you're ready to install that LT4 camshaft and want to know a little more about cam timing, give a yell and you'll get lots of responses.

Ignition timing is quite different, and quite variable, on the other hand. Ignition timing is just that - the point in the engine's crankshaft rotation that the ignition spark occurs. This timing will be changed up or down several degrees by the ECM as the engine speed and load changes.

The ECM can control the timing within a given range. The starting point of this range is determined by where the distributor is rotated and clamped in place. This is appropriately called "Base Timing". Setting this base timing is important to proper operation, maximum power, and controlling destructive detonation. This setting can vary from engine to engine, depending upon options, modifications, fuel used, climate, elevation, and other variables.

The only other automotive "timing" issue I can recall is the anticipation of a green light on the tree at the track. This one is entirely up to you.

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Later,
Vader
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"No matter how hard you try you can't stop us now"
Adobe Acrobat Reader 4.0

AlkyIROC

Actually ignition timing can be broken down a little. Non ECM distributors have 3 timing steps.

Base timing is what you set with a timing light. It can vary from TDC to 15 degrees before TDC depending on the engine and modifications done to it.

Next is mechanical advance. As the engine increases in rpm weights inside the distributor spin out and advance the timing. Typical mechanical advance will bring the timing up to about 38 degrees BTDC by 3000 rpm.

Vaccum advance. This allows for even more advance during part throttle operation. Engine vaccum pulls the advance a few more degrees. Usually 10-15 depending on demand.

Dropping in any old distributor into your car and expecting it to run is hit or miss. GM produced hundreds of timing curve combinations in the HEI distributor. You should have a distributor recurved for your particular engine.

As Vader said, cam timing can't be changed since it's ground into the cam from the factory. It can be modified a bit though. The majority of cams are installed "straight up" This means that all the numbers on the cam card will be exactly that if you check the cam with a degree wheel. An example of some cam timing is:

Intake valve opens at 41 BTDC and closes at 73 ABDC when installed straight up.

Some of the better timing chain sets have 3 keyway positions. straight up, advanced and retarded. Some are 2 degrees others are 4 degrees. To get more power it's best to advance the cam timing. Most CompCams are already advanced 4 degrees. This would change the example to:

Intake opens at 45 BTDC and closes at 69 degrees ADBC.

Retarding the cam timing is popular in oval track racing because of where it moves the power curve.

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Stephen's racing page

87 IROC-Z Pro ET Bracket Race Car
383 stroker (carbed) with double hump cast iron heads and pump gas
454 Big Block buildup now in progress for the 2001 racing season

Best results before the 383 blew up
Best ET on a time slip: 11.857 altitude corrected to 11.163
Best MPH on a time slip: 117.87 altitude corrected to 126.10
Altitude corrected rear wheel HP based on power to weight ratio: 476.5
Best 60 foot: 1.662

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

Damon

Timing, in general is just a measure of WHEN some event happens in relation to the position of the engine's crankshaft.

Ignition timing is how many degrees of crankshaft rotation before the piston reaches Top Dead Center on the compression stroke the spark plug is fired. 4* of crankshaft rotation before top dead center, 8*, 32*, whatever. It's all measured it crankshaft degrees.

Base timing is simply the factory specified timing spec that the engine is suppsoed to be set to at idle speed (there are special procedures to go through before setting timing in most cases). Typically a factory spec will be about 4 to 10 degrees Before Top Dead Center. As engine speed increases, and depending on load the timing will increase from that spec by quite a bit in an operating engine. That's known as the ignition's timing CURVE. And many people will play with "re-curving" the ignition timing for more performance- not just setting their base timing to factory spec.

Cam timing is the same basic idea- but it's when the intake or exhaust valve opens or closes RELATIVE TO CRANKSHAFT POSTION. THis is a huge topic but it all boils down to a measurement of degrees of crankshaft rotation and when each valve opens and closes that give the timing specs for a cam. That's a bit of an over-simplification, but you get the idea.

Here's important thing to remember- you can't easily alter cam timing. First off it's buried deep in the engine and has no obvious adjustment point. Second- you can only advance or retard the entire cam relative, again, to..... you guessed it- CRANKSAFT POSITION! Most engines are built with the cam installed "straight up" - not advanced OR retareded from spec. To make any serious changes in cam timing you have to replace the whole cam with one that is built to the spec you need or want.

I'm just hitting the tips of the mountans here but I hope it helps, and doesn't confuse further.


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94 Firebird Formula M6- No options but Z rated tires. No mods over $10 and no more planned for now. 13.5 @ 105.
79 Malibu "beater" w/junkyard 400 SBC, tweaked QJet 4bbl, a few other tricks. A maddening 13.000001 @ 108 on the motor with little traction. All it's go-fast parts just yanked off to go on the OTHER Malibu...
Grandma's old 78 Malibu (33K miles!). Just built a fresh 383 short block, topping it off with the go-fast stuff from the other Malibu right now!

"One of the last remaining QJet tuners on Planet Earth!"

Dan87IROC-Z

So, then too much base timing will cause detonation, right? And, any suggestions as to where to set my base timing, I live in Colorado at about 5200 feet, and my mods are listed below.

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1987 Chevy Camaro IROC-Z
L98 TPI 350 (5.7L)
TH 700R-4 Transmission with 2.77:1 Rear End

Current Mods: Edelbrock TES 1 5/8" Headers, Hooker 3" Aerochamber Cat-Back System, Performance Resource Chip with 160* Thermostat, Accel Ignition Coil, Cap, Rotor, 8.8mm Wires, K&N Filters, JET TPI Air Foil, All Free Mods, Falken ZIEX Z-Rated Tires.

Best ET : 14.32 @ 97.7mph
(corrected for elevation)

Next on the list: LT4 HOT Cam