Variable Cam Control Tuning: Aftermarket Cams
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Aftermarket Cams
04.54
| 00:00 | - One of the common concerns I hear from novice tuners when it comes to tuning variable cam control systems is that they'll go too far and result in contact between the valves and the pistons. |
| 00:10 | While that definitely would be cause for concern, fortunately with a standard cam control engine, it's not a possibility. |
| 00:17 | Understandably the OE designers go to significant lengths to ensure that the amount of cam movement can't result in any valve to piston or valve to valve contact regardless which extreme of travel the cam is at. |
| 00:30 | The same however can't be said when the stock cams are replaced with larger aftermarket cams and some consideration is necessary when choosing and installing them. |
| 00:39 | Fortunately for popular cam control engines, there are numerous manufacturers offering cam options that have been developed and tested to ensure that sufficient clearance is still retained. |
| 00:52 | Even with a reputable cam profile though there are other factors that can affect the final clearance such as the piston design and valve pocket depth if you're also switching to an aftermarket piston, the head gasket thickness or deck height of the block if it's been machined and the head itself if that's also been surfaced during machining. |
| 01:11 | All of these factors can reduce the available clearance to a point that it could become dangerous. |
| 01:17 | My advice here is that the valve to piston clearance should be checked and confirmed during the engine assembly process to ensure that sufficient clearance remains. |
| 01:27 | This gets a little trickier with variable cam control systems because we need to know when we could expect the clearance to be at the minimum and check under these conditions. |
| 01:38 | If we take another look at the valve timing events as the engine moves through the cycle, it should be obvious that the potential danger zone is when the piston is near TDC and the inlet valves and exhaust valves are both open. |
| 01:50 | Usually for the exhaust valves, the danger area is approximately 10° before top dead centre as the piston is chasing the valves closed. |
| 01:59 | Anything that results in the valves being open further at this point in the engine cycle will reduce that clearance. |
| 02:06 | It hopefully should make sense that retarding the exhaust cam will reduce this clearance and therefore we want to check the clearance with the cam in its most retarded position. |
| 02:16 | The inverse is true for the intake valves though and the danger area here occurs around about 10° after top dead centre. |
| 02:23 | Here the valves are opening and now they're chasing the piston down the bore. |
| 02:27 | As with the exhaust valves, anything that results in more valve opening at this point will reduce the clearance and advancing the intake cam will have this result. |
| 02:37 | For this reason, we want to have the intake cam at full advance while confirming clearance. |
| 02:42 | While it's hard to give absolutes on what is safe when it comes to valve to piston clearance, it's generally accepted that a minimum clearance of 2 mm is more than sufficient. |
| 02:52 | We do need to factor aspects such as the valve train design and RPM ceiling into this though and if you need further guidance, then the cam manufacturer should be able to help you with a recommendation. |
| 03:04 | If you're dealing with a VTEC style switch cam lobe design then it's still advisable to check your valve to piston clearance and make sure it's sufficient and also give you an indication of how much you're going to be able to advance or retard the cam with vernier adjustable cam gears if you intend to try optimising the timing on the dyno. |
| 03:23 | The trick here though is that we're going to need to lock the lobe switching mechanism into the high lobe operation during this process. |
| 03:31 | If you don't do this then the valves will be actuated by the small cam lobe that provides less lift and duration and understandably will give you a false sense of what the actual clearance may be when switched to the high lobe. |
| 03:44 | The rocker mechanism can either be mechanically locked into the high position for the purposes of testing or in some instances you may be able to use air pressure to actuate the mechanism instead of oil. |
| 03:56 | If you want more specific information on checking the valve to piston clearance then we do cover this in depth in our cam degreeing course and there's also a webinar covering the process in our webinar archive that we'll link to at the bottom of this module. |
| 04:10 | For those chasing serious power numbers, the cam profile required will probably render a cam control system useless as the amount of cam movement that's tolerable with a very large cam is usually so small that the advantages are lost. |
| 04:25 | This usually becomes a bit of a moot point anyway since this sort of engine will normally be designed and optimised for performance across a relatively narrow rev range. |
| 04:35 | In between these extremes you may find that it's necessary to mechanically limit the amount of movement the cam pulley allows by shimming or pegging the internal mechanism. |
| 04:44 | The process of doing this will be completely dependent on the style of cam pulley and is beyond the scope of this course. |
