High Compression Ignition

High Compression Problems

I'm new to the Forum, and there seems to be a ton of knowledge here, so any input would be greatly appreciated. I just had the top end of my 97 Heritage rebuilt with Wiseco 10.5:1 pistons, and I am experiencing some drivability issues that I was told by the mechanic are normal with those pistons.

The specs on my engine are: mildly ported heads, Mikuni 42, Crane 316-2B cam, SuperTrapp Mean Mothers exhaust w/ baffles, and .40 over Wiseco 10.5:1 pistons.

The bike ran beautiful before, with no shakes, knocks or rattles. All modifications (except the high compression pistons) were done 10,000 miles before the top end was rebuilt. The only thing that changed were the pistons.

Now, anything under 2400 rpm, either cruising or rolling on the throttle, the engine sounds like it's knocking and the whole bike shakes. I use premium gas and even threw some NOS octane boost in the last tank. After the engine was broken in, it was dyno tuned and the a/f ratio is as close as it gets to perfect.

Any ideas? The only thing I can think of is to upgrade the ignition, but the mechanic said the stock ignition if fine, and an upgrade will only help over 4000 rpm. Sorry for the long post, just frustrated.

Thanks for the input. I put in some NOS octane boost that is supposed to raise the octane 7 points from 93 for premium to 100. That didn't do anything. I've heard it ping when it was hot before, but this is more of a knock, which had me concerned. Thanks again.

To the best of my knowledge it was. I had a Harley certified mechanic do the work, I'm still nervous working on the internals myself.
I have about 1000 miles on the new top end. Just a side note, the engine feels real good between 3500 to 4500, but that seems a little high for cruising.

Try dialing back the timing(retard)a bit, you're kinda pushing the envelope at 10.5:1 with the crap being sold as gas now.
Check your comp sprocket nut to see if it loosened and retorque it.

Wisecos rattle like a bucket of bolts cold. They're forged pistons with a different alloy than stock & they grow as they heat up. Consequently, the engine has to be set up "loose" to allow for the expansion - the typical piston-to-wall clearance for Wisecos is .0025". And if the cylinders weren't bored/honed on torque plates, then the PTW clearance is wrong.

I've seen cylinders change shape as much as .004" from measuring on the workbench to having torque plates installed & measuring again - that might as well be a mile. You don't care if the cylinders are round just sitting on the bench (they won't be) but you do care about the precise diameter when they're installed on the engine & the only way to check that is by bolting torque plates on, which duplicate the stresses put on the cyls. when the heads are torqued down. If that was not done, there's your problem.

Thanks for everyone's input, it is very much appreciated. I'm going to bump back the timing on my factory ignition to see if that does anything, and talk to the mechanic to verify the cylinders were machined properly. If that all checks out, I think I will try the aftermarket ignition. Anyone have any preferences on the best one to use? Thanks again.

A factory box with 10.5:1 is a seriously bad idea. I would suggest going to a box that allows full control over the curve so you can back out some advance on the lower end. V-Thunder and Compu-fire both had good boxes when I closed shop. Both had software that allowed you to tailor the advance curve to your needs.