When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I had a local professional welder fix the custom distributor hold down bolt boss and angled adapter plate assembly. Way way better welding than what I did.
My little brother and I reset the base mechanical timing advance to 7 degrees advanced. The car immediately felt way better. Actually pulls now. In fact it pulls so much better, a new problem has arisen!! The clutch is officially slipping
was doing a 3rd gear pull, car pulled well up until 3.5k RPM and whooom the tach just let free and the engine spun up while the car stopped pulling and the unmistakable smell of clutch filled the cockpit. Good experience for my little bro and my buddy visiting from out of town.
I had already ordered a ZF6 clutch friction disc the other week, and was contemplating putting it in. I guess it is going in now!
But yeah, not sure how the base mechanical timing advance got so messed up, but in either case I take responsibility. Either I set it wrong or my sill custom distributor adapter didn't hold the distributor correctly. In either case, that is fixed now so on to the next challenge!
Got the car set back up with correct base mechanical timing.
Ended up downloading Datacat and Tunerpro and tried tuning a bit myself.
Car was showing steadily increasing knock counts at idle and tip in. Couldn't get the counts to stop.
Ended up tracking the car like that, by the end of the day oil pressure was at zero and car was audibly knocking and running dry up top, presumably spun main, rod, or cam bearing.
Got the car set back up with correct base mechanical timing.
Ended up downloading Datacat and Tunerpro and tried tuning a bit myself.
Car was showing steadily increasing knock counts at idle and tip in. Couldn't get the counts to stop.
Ended up tracking the car like that, by the end of the day oil pressure was at zero and car was audibly knocking and running dry up top, presumably spun main, rod, or cam bearing.
When I was using a converted LT1 intake on my 91 Corvette, I had a similar failure.
On mine, it ended up being the distributor depth wasn't right. I had bought an LT1 intake with the conversion already done, so the distributor hole was cut and a spacer had been machined and welded to the intake for the distributor to sit on.
I didn’t think anything of it and put it together.
On the dyno, at 6000-ish RPM, it started to clatter. When I looked down, no oil pressure.
Turns out the distributor spacer was a little too tall. The bottom of the distributor was only engaging the oil pump by about .010.
With the way the cam gear and distributor gear mesh, I think it might be trying to push the distributor up and out when it’s under load. Sort of like how a ring and pinion tries to push the pinion around when it’s under load.
If your hold down was suspect, or the distributor height wasn’t correct, that could be what caused your failure.
The knock count at idle and light throttle is suspect at best and more likely nearly meaningless. After you change the intake and exhaust manifolds, the resonance / frequency of the entire engine has changed. The knock sensors are designed to listen for a specific frequency; once the length of the intake or exhaust tract has changed the frequency of all the noises the engine makes has changed enough to render the stock
knock sensors suspect at best. It’s possible that actual knock may be loud enough / bad enough to still trip the stock knock sensors, but it’s likely the driver would hear or feel that level of knock anyway.
That it was reporting knock at idle demonstrates the lack of reliable reporting from the stock knock sensor in your application. If it were me, I would likely ignore the stock knock sensor data.
When you moved the base timing to 7, how much timing was in the chip? If your tuner put some timing into it and didn’t see results because it was 6 degrees retarded on the base timing, it’s possible he left the timing in it.
If I were in the tuners shoes, after a few dyno pulls with poor results, I might have put 32-35 degrees in the chip. I would do that because I want to make sure the changes I’m making in the chip are actually happening and verify that there isn’t a software or hardware issue that is preventing the tuning changes from being implemented in real life.
Had I put 35 in the chip with -6 base, you’re still only at 27. The engine will likely be lazy, not respond much, and it’s possible I’d have left the 35 degrees in it and sent the car on its way. Not the best thing to do from a tuning stand point, but not outside the realm of possibility either.
When you moved the base timing from -6 to 7, you added 13 (!) degrees of global timing to everything the tuner had done.
Now that you have the tuning equipment, can you check what sort of timing was in the chip when you went to your track day? Add 7 to those numbers and that should give you the actual timing the engine was experiencing.
Normally, I’d discount the stock knock sensor data entirely. However, with the distributor hold down being suspect, global timing swinging by more than 10 degrees, and timing setting in the tune being unknown…. I’m less likely to completely disregard the stock knock sensor input. There’s so much going on there that could be causing issues, it’s possible it’s out of whack enough that the stock knock sensors are reporting legitimate unstable / erratic combustion.
If that was the case, it would surprise me that you wouldn’t hear it or feel it, but I suppose it’s possible.
I own a dynojet 224XLC, and while I’m not an expert I’ve seen / generated more dyno charts than your average person. To me, your chart looks like the engine is misfiring on one or more cylinders.
Some rough math shows:
If it were down one hole, 225 HP / 7 cylinders = 32 HP per cylinder. If it were on all eight, 32 x 8 = 257 HP. Still not what you would expect to see.
If it were down two holes, 225 HP / 6 cylinders = 37.5 HP per cylinder. On all eight, 37.5 x 8 = 300 HP. That’s more in line with what you’d expect.
Obviously, this is just rough guesstimation. The numbers can swing wildly based on how healthy the engine is. But, based on the shape of the graph and the low power output, I’d guess you had two holes out while on the dyno.
The exhaust temps you mentioned support this theory as well. With the engine at hot idle, even with ceramic coated headers, 250 F or greater is what I’d expect to see on my temp gun if I point it at each primary near the head.
You mentioned one was at 300 and two were less than 150. It sounds to me like the two you checked that were cold were misfiring at best and not firing at all at worst.
The mention of cutting and building spark plug wires that had already melted on the header, the shape of the chart, the Chinesium distributor, and the math suggesting that two holes were out; all that leads me to guess ignition system problems.
Combine that with the distributor hold down allowing the distributor to move around and the base timing being kinda whacky; and I feel like the engine deserves credit for still making 225 HP.
In my opinion, if you slow down a little, I think you’ll experience better results.
Adding 13 degrees of timing to the engine without checking / knowing the total timing is extremely risky.
Doing that, seeing knock, and then deciding to flog the car at the track is also risky.
Assuming the poor dyno performance was entirely due to the base timing error and not investigating a potential ignition based misfire is also risky.
You might get lucky and duck a couple of those things; but if you keep adding risky moves to risky moves, it’s only a matter of time before you damage something.
Good luck with the car, let us know how it all shakes out!
Thanks a ton for your input and for getting back to my private message. I was searching around and found a decade old thread on your old miniram 383 build.
You bring up a really good point about the distributor mounting height/depth. I am building my 383 motor now and I will check this spec thoroughly before firing it up. My original distributor mounting angled adapter spacer was resulting in the distributor not fully seating. Probably a 0.150" gap. I ended up using some of those moroso distributor shims, a flepro distributor gasket, and some RTV in order to fill the gap and get it to seal and not leak oil, but I guess the chinesium distributor could have been out of spec plus my spacer shenanigans, all resulting in poor oil pump drive engagement?
I will check more thoroughly as I tear the old motor down, and I will carefully inspect this tolerance on the new motor I am building up..
I will have to check the map from the tuner to see what timing he might have thrown at it, but once I fixed the base mechanical timing issue it was running decent. For this upcoming build, I plan to use a better small cap distributor and coil for sure. Either a low mile parts yard GM one, or a Davis Unified Ignition one.
06-01-2023, 10:14 AM
