I have a 96 Chevy truck with TPI 7730 ECM, 35" Tires, 3:42 Gears, the motor is pretty much stock but dome top pistons that bring the compression to approx 9.5 to 1 maybe less, I am getting a lot of spark knock at low RPM (1100-2000 RPM), i am still fine tuning, but if i modify the stock table by decreasing the advance about approx 7 degrees all across the table in that RPM range, it is gone, does this seem like too much to decrease the timing?
Also is the MAP pressure in Kpa almost constant for a certain elevation or does it vary with vacuum? Any tips for this?

 

It sounds reasonable... How did you come up with 7 deg? Can you try to isolate some cells by getting more datalog? I would try to work on the cells just before the pinging starts. You may have taken the easy road by simply changing all the lower values isntead of narrowing it down.

Map is just a value associated with vaccum. The higher the map the lower the vaccum. WOT should be fairly close to map 100. Map value should move around with the amount of foot you put on the peddle (unless my grandma is driving).

Good luck

 

Ive been thinking about it and i think i still got a TBI cam in this TPI motor, would that explain knock? what would the MAP approx be at 1000-2000 RPM Im looking at the ZZ4 Cam but i dont know if ill have low RPM issues
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Its actually approx 10 degress retard, i kept retarding 1 degree at a time till it was gone

 

We got two different topics to discuss here.

First Map (vaccum) is not a constant at any particular rpm. if you are going down a steep hill, with your foot off the gas... using the engine to slow you down you can hit 2000 rpm and a map of say... 20. On the other hand, if you're just getting started off a stop light and you want to show a Honda what real power sounds like... stomp on the go-peddle and hit 2000 rpm with a map of 100.

Engine knock can be caused by several things. You should start with the simple and inexpensive first. Fuel! go to a different gas station and full it up with premium (OK so this isn't inexpensive, but it is easy). Next check all your timing on the engine. Go down the list...

I have heard stories about loose, worn out receiver hitches rattling and knocking on the frame enough to cause the ecm to think there was an engine knock. Can't verify the story but the point is to make sure everything on the frame is bolted down tight.

One cam or another will definitely change the tune on an engine. Instead of reducing timing try adding a little fuel or a combination of the two. And you should definitely try to narrow down the cells and conditions that cause the knock.

I think you just need to keep tuning. Yes, it can be a long process... Keep at it.

 

Post your whole combo.
Post your base bin and main SA table.

 

Truck = heavy

F-body = lite comparitively speaking

35" tires + 3:42 = no gear

Both of these would tend to increase knock occurences as well compared to an f-body.

I know F's had the garbage 2 series gears in some, but it wont hurt it as bad as the weight increase would going from car to truck.

Also the domed pistons and 9.5 compression seems odd to me, I could always get to 10(+ or - a bit actually) using flat tops and 64 cc heads?

What kind of mish mash are u running? Give us all the details of combo?

later
Jeremy

 

Technically, an engine shouldn't care too much what it's attached to. I.e. it should perform the same on a dyno or truck as in a car. But, chamber and piston temps change depending on the time at each load. A truck will increase the time at load, thus bringing it to worst case scenario sooner than a car engine. But, it's also a good idea to have the car engine tuned to worst case as well. This is part of the reason many people knock during shifting, because after 2 gears of load, and then another gear drops the RPM, the piston and chamber temps are at worst case, but the tune was at best case of short bursts.

Also, I'm putting together an engine (0 decked 0.039 gasket) with flat tops and -6cc reliefs, and 74cc!! chambers and still will get like 9.2:1. You're probably at 64cc chambers and 10:1 or more depending on the dome and deck height and gasket. That's a lot for 87 octane in a truck.

High load = high MAP = low Vacuum.
Low load = low MAP = high Vacuum.

Are you starting from a stock spark table??

ZZ4 should be fine in a truck. Make sure your valve springs and heads can accept that lift.

I'm in a similar boat of wondering if the 9.2:1 and LT4 hotcam and AFR 195s and Cyclone Plus intake and 670cfm carb will make the engine I'm building too soggy for a truck with a th350 and new 2000 rpm converter... Don't really care though. The engine isn't going to stay in the truck for long after I get back to the US for good.