Hi, I'm new to the forum but I looked through past posts to see if my questions had been answered already. I just had a 383 stroker LT1 put together with ported heads and a (relatively) mild street cam. I'm just starting the process of tuning it. To get a baseline, I got a tune from Ion at MadZ28.com. Looking through it using TunerPro, I saw the timing advanced in the low RPM/high MAP conditions quite a bit more than stock, presumably to gain torque. My motor is relatively high compression (11-11.5:1 static, mild cam/low overlap). I loaded the tune and it pinged like hell, audibly, under acceleration at low RPMs.

Reloading the stock tune (with cylinder volume modified although that doesn't seem to have changed anything), I datalogged a highway run. I'm using a stock knock module, LT4 module hasn't come in the mail yet. The O2s at WOT look good but I'm seeing up to 12 degrees of knock retard at high MAP/low RPM, even on the stock tune. Seems like it's keeping about 7 degrees of retard in at WOT at 5000RPM although I didn't see the knock sensor increasing while at WOT. Pinging only occasionally audible, and pretty much only for a fraction of a second when the A4 is shifting. No knock at cruise, only when accelerating.

So my question: on a 383 with a ported head, mild street cam, and higher compression ratio, on 91 octane, should I expect to have to pull timing from the stock map to keep the knock sensor from pulling it for me? And how bad is it to have the knock sensor pulling timing every time I accelerate? Is there something wrong with my motor causing it to ping more than it should?

 

are you sure it is actually spark knock and not a loose component

just an idea, i dont know how to tune

 

Send me your tune, I can look at it for you. meanlt1fbody@aol.com

 

What's your Compression ratio? Is it considerably higher than a stock motor/

Also, dvancing the timing does not always mean more power.

 

Okay, just an update, I put in about 7 gal of 100 octane and took it out again. With the stock timing map, I got just a little knock retard under heavy load and low RPM, and the knock count would jump 50-100 at a time. With the MadZ28 tune, there was knock retard any time I gave it gas under 3K RPM, and the knock sensor would go up 200-500 at a time. With a relaxed timing map that takes out 2-3 degrees from the stock map below 2400, I saw virtually no knock except when the A4 shifted under heavy acceleration. Looks like the knock sensor is working with no false knock. Sounds like my motor is pretty high compression, then, after all. I'll get the specs from the builder but he seemed to think it would weigh in around 11:1 static.

Would you expect to have to take timing out of a 383 motor when tuning it, or would you expect to add timing down low (<3000RPM)?

 

That's 7 gal of 100 in addition to the rest of the tank of 91, btw, making it around 95 octane.

 

Its dynamic compression ratio (DCR) that determines the tendancy to knock, not static compression ratio (SCR). Have you calculated the DCR? How big is the cam?

What is the coolant temp running at? What does the A/F ratio look like when it is knocking? Both those can cause knock.

 

For what it's worth, my experience is that generally 383s require less timing, which I suspect is due to the larger air/fuel charge combusting at a faster rate. We can usually get 35-36 degrees of timing into a 350" LT1 with aluminum heads, whereas we tend to top out at around 31 degrees with a 383" LT1.

However, as Injuneer says there may be many factors that can affect the "spark knock threshold" of the engine and all need to be evaluated.

 

I'm going to agree that the timing should be backed off.

I noticed the same problem with my Madz28 tune for my h/c 350 w/ 8.6:1 DCR: The timing was WAY too advanced at high MAP/low rpm causing lots of audible pinging. Reducing timing to slightly less than stock levels fixed the situation. My dynos dont show any abnormal power loss in those low rpm ranges, so i think the timing reduction was appropriate.


VinceTrifecta: How do you ramp in the timing vs rpm in those high MAP ranges?

 

Looking at a 383 LT1 tune I did on the dyno recently, I started at 22 degrees from 400-800 RPM. At 1000 it goes up to 23, at 1600, up to 25, 28 at 1800, 30 at 2000, 32 at 2200 and 33 at 2400, and up.

 

So, on the street you dont see pinging problems with that? Say you lugged the engine a bit - like you were in 4th at 1600 rpm and floored it?

Are you relying on the KS to back off the timing in those types of situations where the real-world load is probably alot more than what you see on the dyno?

 

We didn't have any pinging problems on the street with that timing curve. Every motor is different, though, of course.

When I do a hands on tune, I prefer to tune both on the dyno and the street, because the dyno cannot always recreate situations you run into on the street. At WOT I don't mind seeing 0-1 degree of knock retard if that makes the most power, but I do not like seeing KR during cruise conditions at all.

 

You can see my setup and sounds similar to you. my engine takes 32* up to about 2800 RPM at WOT. From there, I graduate up to 35* at 4K RPM and beyond. This is as read by DataMaster.

NO issues with predenotation.

 

I'm really amazed that y'all run so much timing at WOT and <= 2800 rpm, it makes me wonder if there is something wrong with my setup (and maybe the OP's too).


Maybe it's the AFR? I'm running a flat 12.8:1 at WOT according to my wideband. Are y'all running any richer than that?

 

You might try 12.5 and see if that helps curb spark knock. I've found the LSx engines tend to like to run a bit richer then the LTx engines because it helps keep spark knock away.