Donzi502

Guys how far should one go with a 9.8 inch bolck and 6.385 rods. What i mean is at what point in the rpm and HP scale is it wise to go to a 10.2 block and 6.7-6.8 inch rods. Was thinking of building a 540 with real nice heads and a efi tunnel ram but want to keep the rod ratio under control for good BDC breathing and some consistant volocity through the intake runners. I want to try and make 750-800 on 100 octane gas and 6800rpm. Hope some of the BB wizzards can chime in here.
Thanks
Jason

Young Performance

As long as you keep the rod ratio at or above 1.5, you will be fine (6.385/4.25). I also don't like to run a piston with a compression height of less than 1.25. Doing a 540 with a 9.8 block and a 6.385 rod will work just fine.

9.8-6.385-2.125(half of stroke)=1.29
If you use a piston with a 1.25 or 1.27 comp. height, you will still have to deck the block to get the compression up to make the power that you want. Hope this helps, Eddie.

Donzi502

The question is, will a rod ratio of 1.6 (6.8 & 4.250) breath much better than a ratio of 1.51. (6.385 & 4.250) I am led to beleive that a bigger rod ratio breathes much better at the bottom of the stroke and there for you can make much better use of a short runner tunnel ram with all the top end they make and you dont need such big ports in the heads and ram so torque in the midrange jumps up as well. ????? Thanks. Jason

Young Performance

I would think that whatever you gained, if any, would be more than offset by the loss of torque and acceleration with the long rod. That motor will really suffer in the low and mid range, especially if it is a heavy boat. Personally, I would stick with the 9.8 block and the the 6.385 rod.