My truck is dyno tuned with 91oct. (highest pump around). If i were to drain my tank and put in 5 gallons (cans) of 76 race fuel 104 oct would it make a diiference on the 1/4??

i think you would have to turn the timing up to really gain from the extra octane in the tank. typically that is what high octane is for. help reduce knock with higher timing

Right, but the PCM will still adapt some, wont it? Or, is it a waste of $$

The PCM will pull timing if it detects knock, but will not add timing when there isn't any. If you're not tuned for high octane it's a waste of $$.

the only thing i think would be the benefit would be: if your tune was running a lil on the rich side, it would lean it out. i might be wrong though.

Not only is it a waste of money, you may lose performance. It is a falacy that higher octane will give you better performance, it only holds true if your tune requires it. As goshawk23 has meantioned, to get additional performance from higher octane you would need to bump up the timing. But at some point there is a dimenishing return.

Higher octane slows down the burn speed, which in turn decreases cylinder pressure. As meantioned, there is a point of dimenishing returns where the octane value exceeds the amount required to keep pre-detonation from occuring, when this occurs the potential in the reduction of burn speed decreases the cyinder pressure and hence performane is impacted. So finding jsut the right octane for the tune is important for maximum performance, but it comes at a cost, especially since fuel prices are going up.

Unless you have a force induction adder or high compression pistons, anything above 91-93 octane is probably a waste of money in your case I believe, but only a dyno and tuning software can tell.

I'll save the $$. Thanks for the responses.