okay im not the smartest when it comes to asking this question , when i change the boost , the air fuel ratio goes up , right now on the dyno sheet it says 12 for air/fuel , whats the highest that number can go up before i need to do some work on the block (this is a shock block )

 

Bad news.....with a lean a/F ratio you can blow up a built motor just as quick as a stock block. It should be low 11's for good safety and power.

 

Nothing higher than 12.0 under boost. 11.0 to 11.5 is very safe, but as said, a lean mixture can blow any motor... built or not. You build to handle higher cylinder pressure (more boost). To do this, you need stronger pistons, rods, etc, etc, etc. There are alot of threads on what you need so I won't go into that.

 

Well, the fact is that 12:1 will NOT blow an engine if the timing is set correctly. In fact, that's the correct way to run an engine, and this is exactly what many, certainly I, will do in about a year, when a great tool will become available to ascertain peak pressure angles and knock. For the time being, the richer mixture yields a measure of safety by slowing the burn/flame advance rate. If you want to run as clean/strong/elegant as humanly possible, you will put the AFR at around 12 and retard timing to the correct point. The problem is of course that we simply don't have the tools (unless you are a formula 1 team) to do this with mathematical precision. So, for now, run it at 11.2-11.5:1 and still retard timing.

 

There are several factors you have to consider when determining a safe mixture. The most important of which are 1) Compression Ratio 2) Ignition Timing 3) Fuel Octane.

Now, if your motor is stock and your timing is retarded 1° for every # of boost, you should stay close to 11:1 mixtures on pump gas and can go a bit leaner on race gas. If you drop compression and or timing, you can run a bit leaner.

I'm assuming your running a stock bottom end and pump gas, so I would recommend a richer mixture than 12:1.