Ok, here's a challenge for you. I have a '91 speed density setup TPI 383 stroker. I am running 10 degrees base timing. 91 Octane fuel. 45psi on the fuel rail. Manual tranny. Problem: When shifting gears, or when cruising through town, and go to accelerate, until I hit full throttle I get a pretty severe ping that lasts for several seconds. Also in overdrive, it stays with me pretty constantly, especially under load or uphill. The guys at street & performance said I need a bigger injector, to go from the 22# to a 24#. I am now running 24# injectors, and have much more power, however still have the bucket of bolts rattling around. I unhooked the knock sensor and drove around the block, and the knocking was eliminated, however so was alot of my power. Also, driving at a steady speed, I can occasionally feel a miss. I am also running a pete jackson "quiet" gear drive, however I don't feel that this is where my problem lies. The engine and the knock sensor are only 6 months old, and it is supposed to be the 91 speed density knock sensor, although packaging mistakes have been known to happen. But with the ping going away with the sensor unhooked, I don't get it at all. Any ideas? Thanks

 

Are you using a stock PROM? If so, what is the four letter code on the silver sticker under the MEMCAL cover?

10° base is four degrees more than the factory programming expects so it's getting added to you total.

With a manual tranny it's going to be harder to control the spark knock than would be with an automatic and a stall converter.

I can't run anything less than 93 and that's with a very modified PROM and a 3200 Vigilante stall converter.

No point in asking more questions until we know more about your total package. Compression, iron/aluminum heads/ type of programming that's been done.

Also, you really shouldn't be in 5th gear under a load. Your getting the ECM into some of the highest timing advance regions in the prom when you do that. You're going to have to keep the RPM's up on that and quit lugging it! Spark knock will destroy an engine!

BTW, that pinging quits and the power is reduced when you unhook the knock sensor because the ECM is keeping the timing severly retarted.

____________
SMaster

SBC 383 w/TFS 23° aluminum heads, full roller, 11:1 SRP's.
CC306 camshaft w/1.6 ProMagnum RR
730SD w/30# injectors, 58mm Holley, Holley Stealth Ram Intake
700r4, 3200 Vigilante converter, 3.73 gears w/BFG drag radials.
4040# Step Side Chevy pickup truck
8.34/83.80mph w/1.88 60' w/stock suspension

 

Stock L98 Cast 68cc heads, stock 1.5 rockers, comp cam and springs, flattop hyper pistons, should be 10.5:1 compression. I am running a custom chip that S&P burned with all my specs: cam specs, cubic inches, tranny type, vehicle weight, tire diameter. Question on the chip. Does the chip have a section for base timing programmed into it? If so, I bet they left it at 6deg like stock. Now, i'm not running in 5th under a load type load, just refering to going uphill, etc. My most common driving is a cruise around 30-45mph in town in 3rd or 4th gear, and when I step on the gas to accelerate, that's when it happens. If I step lightly, it rattles until I quit...If I hammer it, it rattles for just a second until the injector is wide open. I know it's running rich, wot O2 reading is .94v, but my regulator is not wanting to adjust right now below 48psi. TPS havent checked in awhile, but initially set it I think at .48v last fall. Cant think of anyhting else to say.

 

Stock L98 Cast 68cc heads, stock 1.5 rockers, comp cam and springs, flattop hyper pistons, should be 10.5:1 compression




Thats way to much compression for iron heads!! The only fix for you is evens coolent. Its 20.00 a gallon and you run only the coolant with NO water, this way there is no water and no pressure! You can get away with a 10.5 iron headed monster

 

That's going to be tough to tune with 91 octane gasoline for sure. Back it down to 6° and see what it does. You have nothing to lose if it works. Also, the engine temp will need to be down to about 180. You HAVE to get the VE tables pretty close to get a speed density setup to run correctly. There's no other way for the ECM to get the fuel curve correct once the BLM's and INT's (GM's fuel correction methods) go too low or too high.

You're going to have to scan it as you drive it to really figure out what's going on. Check out the DIY PROM board here for a similar setup and some more educated suggestions.

https://www.thirdgen.org/techbb2/for...?s=&forumid=16

A cheap laptop and cable and free scanning software will answer a lot of questions and provide information on what really needs changed. Tuning the chip yourself becomes a hobby all in itself! I can be frustrating but rewarding.

BTW, I've seen $400.00 custom chips that were tuned by engine size (stroked 305), camshaft, tires, tranny, gears, injectors and all that and the car wouldn't even hardly run. One scan and it was obvious the injectors were going static at 3500RPM under WOT!!! I looked at the chip and it was tuned strong enough for a built 406! Way to much everything! 1.5 days and 12 burns later the guy thought he had a new engine!

Also, have you contacted your tuner with these questions?

 

Was the CR figured with 68cc head volume?

L98 heads are 64cc raising your CR more, unless you opened them up...only adding to the problem.

 

Yeah, the heads actually cc'd out at 66. I haven't done a compression test on it, but I went with the tech info for the pistons I bought, they actually said 10.3:1 compression, however I zero decked the block, so i'm sure that upped it alittle, so i'm figuring a good 10.5:1. I have tried the timing at 6 degrees already, and it didn't make much difference, except the power was gone totally. It still had it's knock. I have talked with my programmer about the problems, and he swore up and down that it was too small of an injector. I'm going to get ahold of them again tomorrow though. The guy that did my machining is an engineering guru. He builds race engines, as well as designs parts for chevy. He feels that it's a chip problem also.
If it is a chip problem, where could I take it and get it hooked up and fixed correctly? I'm really good with computers and all, but i've spent so much money on the motor already, that the wife is about to kick me out, ha. So I can't really afford to go buy a bunch of tuning equipment right now. Thanks for the help guys, keep the ideas coming.

 

Ok, well here's an update. After researching for a few days, and talking to the programmers, I figured out part of my problem. When I installed my knock sensor, I used teflon tape around the threads, and aparently it broke the ground connection to the sensor, so it wasn't working the way it was designed. I fixed the rookie error, and took it for a drive. It doesn't do an unassisted burnout in 1st and 2nd like it did with the 22# injectors, 24's now, however I can step on the gas pedal and almost all the knock is gone. I am thinking now possibly a vacuum leak somewhere, or just the chip needing to be reprogrammed for the bigger injectors. I'm supposed to get it dyno'd next week and scanned at the same time, so hopefully that will tell us something new. Thanks for the help!!!

 

Good luck with it. I don't think the injector would make much difference. The 22# injectors will supply plenty of fuel at the lower RPM that the knock is apparent. I think that your CR is too high for the stock heads. You need a lot of overlap to bleed down cylinder pressure to help it. Have you tried octane boost?

 

Ok, I did a compression check on my motor the other day, and did some research on it. I am cranking 245-250psi on each cylinder. I read that I don't want over 160 for pump gas. Possibly a problem? I figured my compression at 10.56:1 So I don't understand why my pressure is so high, I guess it just breathes real good. Let me know what you guys think. Thanks.

 

My problem was the same in a Chevrolet 2.8 V6 engine.

In a rebuild process the mechanic replaced the original dished pistons for flat top pistons. Problem was a hard knock during acceleration. I did a compression test and it was 248 per cylinder!

I fixed it at follows:

1.- I retarded the main spark table in 13º into the EPROM.
2.- Since problem continued when engine temp was up, I did a modification in fan ON & OFF temperature in order to run into a small and "in line" temperature range. Thus, my fan is ON when the temp is 93ºC and OFF in 89ºC (thermostat is 88ºC = 190ºF).
3.- I swapped to 97 octane gas, replaced EGR and coolant without improvements in the knocks.

The main solutions were points 1.- & 2.-

The only unknown for me is the actual HP for this stock 135 HP motor with these modifications.

(you can, you can)

Denis V.

 

How much cam?

I think your CR is higher, and maybe not enough cam duartion to bleed off cylinder pressure.