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| cams, valve timing and ignition timing | Rating: 
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| Posted: 10-12-2016 07:55 am |
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101st Post |
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Esprit2 Member
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Tim Murphy wrote:Hopefully, I will then keep quiet and let the other, more knowledgeable guys offer help :<)Discussion is good, don't go silent on us. Regards, Tim Engel
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| Posted: 10-12-2016 08:00 am |
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102nd Post |
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Esprit2 Member
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qwerty wrote:Just checked the back of the cam gears and there's a black mark....Keep that up and we'll make you go sit in the corner. ;-)
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| Posted: 10-12-2016 10:15 am |
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103rd Post |
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qwerty Member 
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You may as well, it's like 90 degrees over here.... ...sorry I couldn't resist. :)
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| Posted: 04-11-2017 02:01 am |
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104th Post |
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allstateguy Member 
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Just starting cam belt/tensioner replacement on B74 2 5757 When camwheel marks face each other, intake notch is 110 and exhaust notch is 115 I've heard the engine run, and it sounded fine, but haven't driven the car (15615) yet Should I time it back to 110/110 when I put the belt on?
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| Posted: 04-11-2017 05:17 am |
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105th Post |
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Esprit2 Member
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Does your car have to pass a strict emissions test. If so, it will have a better chance with 115 In / 115 Ex cam timing. If the local emissions standard is a little more lax, or if your car, being rather old, doesn't have to be tested, then the engine will run better if the standard cams are timed to 110 IN / 110 Ex. It also helps to dial-in a little more static ignition advance at around 12* BTDC, and a richer mixture will also help performance.
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| Posted: 04-11-2017 12:59 pm |
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106th Post |
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allstateguy Member
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Thanks! No emission test for this car, classic plates and insurance obviates that in AZ. Will time back to 110/110. I was just checking in here to be sure that the 110/115 timing wasn't some "hot tip" that was somehow "better" than the stock setting.
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| Posted: 04-11-2017 06:25 pm |
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107th Post |
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Esprit2 Member
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If you want to play with cam timing, give 104 In / 110 Ex a try. But for that, you'll need to buy a 104 pulley. If you just want to use the best option available on your stock pulleys, then it's 110 In / 110 Ex. Regards, Tim Engel
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| Posted: 04-14-2025 01:44 am |
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108th Post |
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vnavaret Member 
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Lads: Having read this VERY old thread, I wanted to clear up some misconceptions that are woven into the discussion. First and foremost, lobe centers (that is, the timing of the intake cam relative to the exhaust cam) affect three things; Power, idle, and emissions. With tight lobe centers (e.g. both cams set to 110 degrees) the following occurs: 1. HC emissions go up. The amount of time BOTH valves are open is relatively high. This means that while the exhaust gasses are exiting, the fresh intake charge is trying to move in. Remember that air has inertia, so as the exhaust gases continue out they tend to pull the intake charge along behind them. Some of the intake charge goes out the exhaust, giving rise to unburned hydrocarbons in the exhaust. This is pollution. It also causes fuel economy to go down (unburned gas is wasted gas). 2. Peak power goes up. In an ideal engine, when the exhaust valve closes there will still be exhaust gasses present in the combustion chamber. Then the intake valve opens and a fresh charge enters the cylinder, DILUTED BY THE REMAINING EXHAUST GASSES. In a 10:1 compression engine, this means a 10 % reduction in power because there will be 10% less room for the intake charge. By holding both valves open longer, the exiting exhaust will pull intake charge behind it, "flushing" the remaining exhaust gasses out of the cylinder. Theoretically, it is possible to flush ALL remaining exhaust out, such that there is no dilution of the intake charge and more fuel is burned for each power stroke. A larger amount of fuel burned means more power. The driver may notice the engine feels "peaky" as the power comes on suddenly which is more exciting, but there is a commensurate loss of torque (see number 3, following) because combustion is compromised at low RPM. 3. Idle gets rougher. At low RPMs, there is sufficient time for the for the exhaust to actually reverse direction and push some of the intake charge back out the intake valve. This causes worse conditions for combustion, and a rougher idle. As the RPMs increase there is not enough time for the exhaust to reverse direction, and so the engine smooths out. This is why aggressive cams give a choppy idle, but make more power as RPMs increase. Depending on the engine, as lobe centers get tighter power goes up until the lobe centers get to about 106 degrees (e.g. 106/106 cam timing) after that the power begins to fall and the engine begins to run ever more roughly with decreasing power. When lobe centers are wider (e.g. 115/115 cam timing) the following occurs: 1. Emissions go down. There is less time for the intake charge to follow the exhaust charge out through the exhaust valve. That means less unburned fuel in the exhaust, and lower HC emissions. Fuel economy also improves since there is less wasted fuel going out the tailpipe. 2. Peak power goes down slightly. The intake charge will be more diluted by remaining exhaust gases since little "flushing" of the exhaust gasses occurs. Coincidentally, NOx emissions go down, since peak combustion temperatures go down by virtue of inert gasses being present which reduces peak combustion temperature. Low end torque improves, so the engine feels less "peaky". 3. Idle smooths out, since the exhaust has little or no chance to reverse direction and push fuel out the intake valve. So, what is "best" - 110/110 cam timing or 115/115 cam timing? Well, it depends what you want. Good fuel economy, good torque, smooth idle and good emissions or more peak power, less torque, less fuel economy, higher emissions and rougher idle? You can't have it all, it is the nature of the beast. You pay your money and make your choice. Vance Last edited on 04-14-2025 01:48 am by vnavaret |
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| Posted: 04-19-2025 07:14 pm |
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109th Post |
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vnavaret Member
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Barthol wrote: sorry Old thread, but I thought I would answer this question for any future readers out there. There are two ways duration is rated. The industry standard (in the US) is 0.050: lobe lift is where the duration is measured. I believe in metric terms, 1mm is the lobe lift point most commonly used (0.040"). There is another measurement called "advertised duration" which has no standard, but is typically quoted when the VALVE lift (not lobe) is 0.005" off the valve seat. This number depends on the rocker ratio in overhead valve designs (1.5:1 in a small block Chevy for example). HOWEVER it could be measured "seat to seat" or 0.001" valve lift to as much as 0.010" valve lift. This means that comparisons of advertised duration can be meaningless. For our engines, there is no rocker ratio to confuse things and the numbers quoted are advertised durations. The lift and duration are measured AFTER the valve lash has been taken up by the cam. So valve lift will be 0.010" less than the lobe lift because there is a 0.010" clearance between the cam and the valve bucket that needs to be taken up before the valve starts to move. The most meaningful measurement is taken at 0.050" lobe lift because the valve is not really flowing much until the valve is significantly off its seat. Also, the difference between advertised duration and 0.050" lobe duration tells you something about how modern the cam is. Modern designs will have about 60 degrees difference (or less!), but old school cams can have up to 100 degrees, meaning the cam is "lazy" and will not perform as well as a cam that opens and closes the valves more rapidly. You can infer how modern the design is by looking at the lift for a given duration. If two cams have the same advertised duration, and the 0.050" duration is unknown (as with our cams) then the cam with more lift is the more modern design. For a street driven car, 270 degrees advertised or 210 degrees 0.050" duration is about the limit. More than this idle suffers, low end torque diminishes, and fuel economy and emissions degrade. How much things degrade depends on how tight the lobe centers are, the design of the engine, number of valves, etc. David Vizard has observed that the optimum lobe center spacing (for power only) depends solely on the overall engine design. Optimum power may not be acceptable for the other considerations (driveablility, economy, etc). All of the above explains why modern engines go to the trouble of having variable valve timing and/or phasing. If you want good economy, emissions, driveability, and power (the whole enchilada) the cam timing at 6,000 RPM will need to be much different than at 700 RPM. Incidentally, this is how a modern car can get 100HP/liter with excellent economy and emissions. The ECU adjusts the cam parameters depending on temperature, throttle setting, and RPM. Vance Last edited on 04-19-2025 07:15 pm by vnavaret |
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