PackardV8 wrote:Yes, many downsides, as related above.
No, don't do it on anything other than a pure race application.
Maybe, you should read up on all the reasons why the OEMs went to so much trouble and expense to add centrifugal and vacuum advance curves. Bottom line: ease of starting, off-idle transition, part throttle response, fuel economy, top end power. Care to live without all of these?
thnx, jack vines
99% of the OEM motors do not require locked out timing cause the OEM factory cams don't have the duration or overlap that would require it.
A 1969 427 L88 corvette motor (264°-273° duration @.050" cam) with automatic trans
is a example of a OEM (one of the exceptions) that definatly can and will benefit from locked out timing.
With big cams like this one manifold vacuum drops when idleing in gear which take the idle spark advance with it if/when you depend on vac advance and quick mech advance curve to get the required idle timing.
The (manifold vacuum dependant) timing drops as soon as you put it in gear. The timing drops off as soon as you punch the throttle, right when you need the advance. The engine loads up from lack of idle timing and flairs up when you go from in gear to neutral cause the vac advance fluctuates wildy.
The engine will heat up from lack of idle timing. The power valve fluctuates at idle creating a over rich idle and killing the plugs.
This was one of the few cars factory offered with suck a radical cam and auto trans.
There is a few reasons why the OEMs didn't use the type of racey high overlap/long duration cams that require this much base idle timing.
( long duration and tight LSA )
1. 99.5% of the new car buyers would bitch about the idle quality.
2. Poor gas mileage.
3. High exhaust emissions.
4. Restrictive untuned exhaust systems with log style manifolds. Theses exhausts systems don't work with Tight LSA cams. The exhaust scavenging effect you can get from a tuned exhaust (long tube headers) is all but supressed making a tight LSA cam useless.
5. Air conditioning vacuum control and power brakes dependant on high manifold vacuum.
6. 99% of new buyers would bitch if they had to use two hands to start the car. (spark interupt switch)