rithsleeper

I have a J&S electronics knock module. I have never heard reference until the other day just reading up on the unit and people's experience. They were talking about Forged pistons have piston slap and it creating false positive knock on the sensor. I've never used the unit but I intended to use it on my new engine but the engine uses forged pistons... Is this a problem that I need to look into? Don't LS7's and new LT1's have forged pistons and still use a knock sensors? The Le mans cars have 4 knock sensors iirc.

Just any ideas you can give me or insight before I go incorperating this into my setup.

johnny c

yep forged pistons can make a little extra noise at idle. i noticed this with my build. I have a Holley ECU so i switched off the Knock retard, at anything under 1500rpm. anything above that rpm the pistons shouldn't register a false knock.

briannutter1

Well designed forged pistons should be quiet when warm. A couple minutes of warm up is good. Skirt length and taper are considerations and you would be amazed the companies who doesn't get it right who otherwise have a well known name. Message me if you like and I can assist.

Kevova

If piston slap is causing knock counts sensor is too sensitive or there is something else going on.

rithsleeper

ok, that makes sense. I'm sure there is a setting to make it stop listening for knock at low rpm.

Kevova

Once engine is at operation temp. there should be no piston slap. At that point pistons should has expanded where the excessive clearance is gone. So if it was set up by piston slap knock count should fade as engine heats up. Piston slap will sound similar to a noisy lifter.

briannutter1

Some good info here. To add definition to this: Valve train is constantly sewing machine sound where piston slap is more guttural and entirely more annoying. Sewing machine valve train is caused by fast closing exhaust lobe ramps slamming exhaust valve against seat and ringing through thin wall headers. Piston slap occurs most when engine is loaded less than half throttle and often transition between...think mild accelerate/decel part throttle. . Wot loaded, the piston it's not rattling about. If you are still hearing something at anything near wot, pistons are likely hitting the heads. Generally .035-.040" piston to head static cold depending on stroke and resulting piston height.

SouthernSon

Torque value while installing the sensors is very important. You have to get it dead on. Just another link in the possible.

John Anthony

John at J&S here:

We recommend 0.006" or less piston to wall clearance.

Torque the sensor to 15 ft lbs.

The Universal version for distributor ignition has a FI/NA mode switch. In NA mode, I set the software to disable knock retard if the onboard MAP sensor sees more than 15" vacuum. For FI, it disables knock retard if more than 5" vacuum.

In either mode, engine must be above 1250 RPM.

The Interceptor and Vampire versions for coil on plug do not have a NA/F1 mode switch, so I have to program it before shipping for FI or NA. On these versions, knock retard is disabled if the engine is below 1750 RPM.

These trip points can be custom programmed for a slight charge, but you will have to send the unit in.