I have a 1992 Z28 with the L98 in it and had to do a rebuild on it due to a spun bearing. I haven't looked into it too much, but I believe that the car has the G92 option as it had the oil cooler on it. Anyway, during the rebuild, a hotter cam was installed along with Edelbrock heads, intake manifold and headers. The plenum is unchanged with a BBK 52mm throttle body on it.

My question is whether this setup would cause problems with the knock sensor as the car is a torque monster off the line, but seems to bog down once you reach higher rpms (basically above 4000). Any help would be appreciated.

 

It bogs above 4k rpm because of the TPI. They do not breath well at all. Your only true cure for this would be a stealth ram or the like.

 

That's over-simplified and probaby not accurate. I know someone with SD TPI who spins to 6000 RPMs (although he's probably losing power above 5500). Simply being TPI isn't the answer.

"Bogging" at a specific RPM could be a number of things. Fuel delivery comes to mind, have you verified fuel pressure under the bog conditions? Ignition could also be dying at that RPM.

No PROM tuning was mentioned. SD assumes certain conditions, which will be altered with things like cam and exhaust changes. The PROM should be modified to accomodate those changes.

It could be hurting for air through an unported plenum and stock runners, but it shouldn't just nose over. Not continue to build power, perhaps, but not "bog".

 

"Simply being TPI isn't the answer."

dont hear that often.

 

Refreshing, huh?

 

Maybe bog is the wrong word to use, chug may be better. Before the rebuild, the car ran 14.1 in the 1/4 and after the rebuild, was running 14.8-14.9. I had a trip down to Florida earlier this year, and I think I got a tankful of bad gas as I had no power at all and the car chugged under moderate to heavy acceleration (1500-). Several tankfuls of permium later, the chug is still present, but not as bad (slight at 3000, worse above 4000). I'm sure one problem is that the guy who did the rebuild for me used Champion plugs as he couldn't get a cross for them to another type. I have picked up a set of Bosch Platinums that I am planning on putting into it when I have time to do the work. Unfortunately I don't have the specs on the new cam other than 'it's three stages hotter than stock'. PROM tuning was mentioned, and I have yet to get to that, I installed the BBK recently and wanted to make sure the timing was set right before I got into the PROM. I also figure that I may need to get larger injectors at some point. Fuel pressure has not been checked as I don't have the capability to do that right now. Stealth ram is also an option for the future, but it doesn't seem that the car would get that much slower just by adding some higher performance parts, but i will admit to not knowing much about breathing when it comes to engines.
Kind of a side question here, is there a difference between an OEM harmonic balancer and an aftermarket one? It was mentioned to me that using the stock timing cover with an aftermarket balancer could cause the timing marks to be off. I'm told the timing was originally set at 18 degrees after the work was done, but since the ecu adds timing, I figured that it needed to be retarded or it'd be advancing too far. When the timing was checked, it was around 38 degrees, figuring something was wrong, I lowered this to 30, but the engine became very rough with no acceleration so I put it back. Is it possible that the balancer 'slipped' or is the OEM/aftermarket more likely?
Thanks for all the suggestions so far.

 

Time to get a fuel pressure gage. A must-have. If you suspect you got a tank of bad gas, you may have plugged up your fuel filter. You need a fuel pressure gage.

The damper and timing tab must be meant for each other. That is something that should have been checked upon assembly. You can do it not, but it will be a little more difficult. You need a piston stop that screws into the #1 spark plug hole (available from the dealer and speed shops) - bring the engine over (by hand) until the piston comes against the stop, mark the damper at the "zero" spot on the timing tab; turn the engine back over the other direction until it hits the stop, mark the damper again, the midway point between the two marks is top dead center.

How are you checking the timing? If the ECM wire is still connected, that's the wrong way. You set the "base" timing with the ECM disconnected, then when you reconnect the ECM it adjusts timing for the operating conditions.

You really need to know your cam specs. '...three stages hotter than stock' is completely meaningless, and you could have a cam that's driving your ECM bonkers.

 

I'll pick up a pressure gage as soon as possible then.

The timing was checked with the ecu disconnected as per the book. Before it was disconnected the timing was roughly 10 degrees higher and unsteady (part of why I was wondering if the changes affected the KS). With the ecu disconnected, the timing was fairly smooth, with a slight jump every 5-8 seconds.

 

Base timing should be in the 6-10 degree range.

Maybe your knock sensor is saving you and producing the noted effect.

 

is your timing chain new?
Eric

 

yea,

usually TPI is first to blame. not in my case, I have nothing but praise.

 

If I remember correctly, he said that he used timing gears instead of a timing chain.

 

Timing gears instead of chain can cause the knock sensor to give false readings to the ecm.

 

Do I need to replace the gears with a new chain then? Or is there some way to bypass the effects of the knock sensor?

 

There has been discussion on the boards about de-sensitizing knock sensors. I do not know how to go about it. Type knock sensor in a search.

As for the timing gears and a knock sensor, IMO, and from what I have read, I think that one or the other, may have to go.

For testing purposes, I have wanted to take the knock sensor out of the block and replace it with a plug, then ground the sensor to the frame, so that it would not detect anything. I have not tried this yet.

 

Thanks for all the information. It looks like I will have a bit more research to do and probably some trial and error to get this taken care of.