I have a 1967 Gran Prix 400 HO Motor that I have had rebuilt by a machine shop and putting in my 1970 GTO and was wondering (rebuilding motor changing cam does it affect timing ? )
By the book it says to set the timing at 6 above TDC with the Vacuum advance unhooked. Thanks in advance
Nice looking car & pics.
It can affect timing if you advanced or retarded the cam - but several factors need to be done to get this number. Many cam grinders build in a 4 degree advance and will be found on your cam card. So the best way to set-up your cam is to dial it in with a cam degree wheel. Of course, many of us don't and just install and line up the cam/crank sprocket dot's and go with it - me included.
Some of the advance ground in the cams are to take into account the eventual stretch of the factory type timing chain. However, if you use a double roller chain, they don't stretch very much and maintain a more accurate timing. The built in advance may even take into account that some engines need to be slightly align honed which can change the distance between the crank and cam gear axis. Align boring can affect this more and should be measured and if too much change, they sell timing chains specifically for this which are a hair smaller to compensate.
Using a degree wheel will confirm the cam position as per your cam manufacturer's recommendations as found on your cam card. This ensures your cam has been ground correctly and there are no parts, like chain & gears which could be off a tad.
So, advancing or retarding the cam more than what the cam grinder has built in is done with either a multi-slotted crank gear which has built in advance or retards and is typically marked on the crank gear. The other option is to use an off-set keyway specific to the amount of degrees you want to advance or retard.
I found this on another website which sums it up perfectly: "Retarding the cam typically decreases cylinder pressure and therefore more timing MAY be needed. Advancing the cam typically increases cylinder pressure and therefore less timing MAY be needed. The advancing or retarding of a cam usually moves the power band up or down in the RPM range as well. Moving the cam will not affect TDC as that is fixed - piston at the top of the cylinder is 0 degrees, but if you advance or retard the cam on an engine with a distributor based timing system the ignition timing will change as the cam drives the distributor and this will affect the reference point of the distributor (and cam) in relation to the crankshaft - so you must recheck timing after the cam is moved - ie advanced or retarded."
That said, the technical answer is "yes." The advance or retard, either added in by the cam manufacturer or by the engine builder will have an affect on your timing at the harmonic balancer. Timing is something you will find that you will have to play with as each engine build is different and many factors will affect what your engine needs with regards to initial timing, timing curve, and full timing. You did not say if your compression is stock or has been lowered to run on pump gas? 7 degrees is probably going to be a little low in general IF you lowered your compression (10 - 14 seems to be the average I see on this forum). If you still have the factory 10.5 compression, then 7 degrees may be right or even too much depending on the gas/octane you use. Listen to the engine as it will "tell you" what it likes. Too much retard and the engine will run hot. Too much timing and you will hear that engine destroying "pinging" sound. Today's gas can also have a big impact on timing.
Keep in mind that if the original balancer is used, the rubber ring that bonds the inner and outer inertia ring deteriorates with age and can move around quite a bit and be slightly off or even be way off - it is wise to replace them for safety, and to get the correct reading when timing the engine.