327_TPI_77_Maro

iTrader: (8)

Hi guys, I have been tuning for a while on my 327 HSR'd w/ 58cc 305 HO heads. I just found out that my driveline lash (slightly sloppy 10 bolt rear) has been triggering the knock sensor when I shift gears really hard, and that I have been running much less timing WOT than the engine will tolerate. I had the max WOT timing at only 30 degrees because I thought I had actual detonation. It looks like the factory still ran around 36 degrees WOT with 305s and more in the aluminum 58cc headed 350 vettes. A 305 TPI was around what, 9:1 compression? Is 36 degrees still a safe # for my 9.6:1 static compression w/ the small 58cc chambers? If anyone would like to show me their MAF timing tables with 58cc iron heads I'd be very interested. I have to play with the knock sensor sensitivity and slew rate in the narrow load/rpm range where my false "knock" is occuring due to the sensor picking up driveline noise.

Denis.V

I'm testing my FWD V6 with alum. heads and flat top pistons (compression ratio around 12:1) with 12º - 13º retard in the main spark advance table.

Previous, I set the coolant temperature a 91º C +- 3ºC (between 88º to 94ºC. Stat open @ 88ºC = 190º F) setting fan On @ 94º and Fan Off @ 89ºC, because @ 95ºC new knocks starts caused by the high temperature.

Denis V.

MonteCarSlow

No engine is the same (darn it all). I run the same heads on a TPI 350 with an edelbrock performer cam. My compression is too high (I have no idea what it really is, I measure ~220psi cranking pressure). My cruise spark is 32-34 and WOT in the very low 20's. I'm still learning to tune, so don't read to much into my #'s. Generally speaking, these heads run WOT #'s in the 20's and cruise in the 40's (but not my engine for some reason).

Fast355

iTrader: (2)

I ran 52 cc 601 HO 305 heads on a 355 TBI engine and a Mellings MTC1 cam, with 10.3:1 static compression and 93 octane fuel. Cranking compression was 220 PSI. The bottem end torque was awesome. I was running 25* total advance at 2,800 and cruise in the high 30s and low 40s. With a functioning EGR I feel cruise would have been about 4* more.

I will state however that this engine was detonation prone in HOT weather with the A/C on. So much so that I burned a hole in a piston, climbing a long grade, pulling a 5,000 lbs trailer.

Denis.V

FYI:

I use 97 octane fuel and my cranking compression is 242 PSI.

My main knock control is via spark retard & temperature. For my motor 97 and 93 octane fuel is the same.


Denis V.

327_TPI_77_Maro

iTrader: (8)

Does 1.0 increase in compression really make an iron headed engine take 10 degrees less total timing? It really seems like I should be able to run 34-36 degrees with 9.6:1 static, large overlap cam, on an iron headed SBC. The reason I suspect my knock retard is false is that it showed up in the same rev/load range on my datalogs even when I only had 8:1 compression on this engine. The KR then was significantly less though (although I can't imagine that any detonation was occurring with so little compression and such a large overlap cam). I'll have to start datalogging and get under the car and rock the driveshaft to see if I can trigger the knock sensor. Otherwise I'll just burn a chip with lots more WOT timing and see if that makes any change as to how much KR occurs during my WOT shifts. I had 150 psi cranking compression cold with the old 76cc heads (8:1 compression) and this 280 HL Isky cam, 232 @0.05. I imagine I have around 170-180 cranking compression now. I have my gauge with me, so I suppose I should start by checking cranking psi.

Denis.V

What about the knock connector?. My knock wire from ECM is welded to the knock sensor's top. False reading for a bad connection is impossible.

327_TPI_77_Maro

iTrader: (8)

I doubt I have a problem with the sensor itself, it is working properly. I just have to figure out if the knock counts it is sensing (which is only 1 or 2 when I shift hard and trigger knock retard) are actually a detonation event or other mechanical noise fooling the sensor. Only being able to run 26-28 degrees WOT with 9.6:1 compression and lot of overlap to bleed it off seems unusual to me.

Z69

Do a timed test.
Slight knock may not slow you down.
But if you add timing and go faster.....

I ran 10.9 cr with vortecs and a similar cam to yours that closed 4 degrees later in a 383 that only needed 28* on the dyno.
The old hyd with very similar specs took 34*.0

When you get tired of that mushy cam you might try this.
It has 4* less overlap and will close about 8* earlier than your current cam.

http://www.holley.com/products.asp?product=401A6LUN

vernw

iTrader: (2)

Only problem with that cam is it's a solid instead of a hyd. roller LOL (I hate adjusting valve lash with all the emissions cr@p on my motor)

Bet the idle sounds wicked though. Betcha it won't pass a sniffer though!

funstick

Forged Pistons ??? could be pistons slap ?

327_TPI_77_Maro

iTrader: (8)

Yes they are forged Speed Pro (Federal Mogul) flat tops. The motor has 35k miles on it, hone is still visible on cylinders. I never get audible piston slap with this motor. Do you think it is possible that I would get piston slap only under that instantaneous high load situation? The knock sensor is a great tool but its tricky trying to second guess it!

RBob

iTrader: (1)

Pull the plugs and look for signs of detonation. If there, put some new plugs in for a short while and look again. If signs of detonation show up again, then you know there is an issue.

RBob.

327_TPI_77_Maro

iTrader: (8)

Rbob, will I really be able to see detonation on the plugs if my only instances are split second high load scenarios? I'm looking for aluminum on the insulator, right?

RBob

iTrader: (1)

Little black specks about the size you would expect fly poop to be.

RBob.

funstick

Is the knock audiable ?? you could definately get some piston slap under instantaneous high load situations. Pistons are always rocking in the bore. Smack it with a high load quickly and it could produce some nosie for a split second. Im with rbob check plugs. if no signs of knock then move forward into mechanicl noises. Ive also seen headers touch the frame and ring enough to cuase false knock. Drive won't here it over the exhuast either.

Denis.V

Fyi:

My knock is audible. Its main range is between 70-80 MAP & 2200-2600 RPMs.

My old spark plugs had strange kind of points attached to the central insulator.

327_TPI_77_Maro

iTrader: (8)

I have never had audible detonation, or even missing to suggest preignition, on this engine. I am not running anywhere near enough timing with 93 octane and lots of cam overlap to ping with 26-30 degrees WOT. My pass side header is very close to the floor, unfortunately -- that is also suspect. I will take a look at the plugs, but I'm pretty sure 9.6:1 compression with lots of overlap can handle 26 degrees of timing. Thanks for the ideas guys! I'll let you know what I find

dimented24x7

iTrader: (2)

Your in pretty deep **** IMO if there is melted aluminum on the insulator. That implies detonation bad enough to knock chunks off the piston faces.

327,

The detonation may not show up on the plugs (I for one would hope not!) so that probably isnt the best indicator, but always good to check for to be safe. I had problems with the stock tune and detonation. Never showed up on the plugs, or the piston faces, but was audable. Ive also had lots of issues with mechanical noise setting off the sensor.

Grumpy

Black specs are carbon off the piston dome. Silver specs are AL from the piston crown.

Preigntion, may or maynot leave specs, and it can be bad enough left unchecked to ruin a piston in just a few combustion events.

Also, detonation can lead to pre-ignition. This transition of going from detonation to pre-igntion can lead to a very short piston life (or rod bearing).

While pistons can sustain some detonation before failure, the rod bearings, will always suffer. Not to mention that the oil temps can skyrocket from detonation (not that you'd see a dramatic rise on a guage, but rather at the lubrication point).

There used to be an excellent article on the Net about Abnormal Combustion, by one of the Northstar engineers, but sadly it was taken down. If anyone finds it it would be a neat URL to somehow save here.

Grumpy

Preignition is most often found in tip-in. ie not enough AE, and or too much timing. The reason for *quickly* ramping the timing down from cruise to WOT, is to keep the motpr out of tip-in preigntion/ detonation. It's also why there's *Burst Knock Retard*. BKR is like timing but kinda the opposite Instead of running the least amount of timing before losing performance, with BKR, you want to run as much retard as possible without losing performance. There's also a decay rate the helps.

Being sure has cost many a guy an engine, you need to know for certain.

Grumpy

BTW, the new Car Craft has an interesting dyno flog comparing iron to AL heads. Too bas all they looked at was WOT, and tuning for threshold detonation, ie the same ole 34d max performance. And the most HP was at the lower CT, but no other data about oil temp, EGT, or AFRs was given....

dimented24x7

iTrader: (2)

How many of the ecms have the knock prevention stratagy in place? I saw it in the pcm.

Grumpy

Off hand I don't recall any that don't. There's probably some thou, about the time, you state something as a *truth* someone pops up with a little known mask, that makes ya a liar.

91GTABird

That was a VERY interesting article.

Dave_Jones

This one?

http://www.contactmagazine.com/Issue...ineBasics.html

Grumpy

Yes, that one!.

327_TPI_77_Maro

iTrader: (8)

Hi guys. update! I got rid of my timing map, which had 30 degrees at WOT, and replaced it with a 305 1989 5 spd map and also the PE table. This has 36 WOT. I am running the same headsa as a 305 TPI, same 5 spd trans, and only a couple tenths more compression, so I figured this is a good start. The car is a lot more responsive, especially WOT from lower rpms, and guess what -- I have the same # of knock counts (2-3) and same knock retard (4 degrees) when shifting hard at WOT. So I added a lot more timing and the amount of KR and knock counts stayed the same. Plus the car picked up quite a bit of power. Looks like mechanical noise is triggering the sensor. I have a mechanical clutch, slightly sloppy 3.73 aftermarket gears, and full length headers that are quite close to the floor. I also have the parking brake cable hitting the exhaust to the point that it chatters and makes noise with the parking brake on and the engine running-- I should try to re route this (I will have to cut and weld one side of the exhaust so it is up tighter to the floor). If I can't find the source of the noise it is not a big deal, my 36 WOT dropping to 32 for only 1 second right as I am letting the clutch out shifting is probably going to help the T5 last a little longer behind my combo.

Z69

I put 1/4" vacuum line on mine to get rid of that.
I'd guess yours is no where near as bad as mine.
I had to fix mine after one drive.


The PE and other tables are a set.

For an example, look at a few 8D 350 PE tables.
Not the same yet the motor was very similar.

91GTABird

Same problem here. Ive been datalogging using 89 octane and 93 octane and get around the same knock counts and deg. retard with both grades of gas. I wanted to try that before i started fooling with my timing tables. My knock counts only come in at upper RPM's with WOT. Low to Mid range in PE i dont get any counts.

RBob

iTrader: (1)

Has anyone pulled the plugs to see what they say?

RBob.

JP86SS

iTrader: (1)

This could also be that the noise signature is not in the tuned range of the knock sensor you are using.
The 305 has a different range than he 350 sensor. Seeing that you are running a 327 I would think the 305 sensor would work better than the 350 but with higher comp and cam, the level of picked up noise may be in the range of the sensor.
I know the 350 sensor (GM # 10456549, 92' maro) has 3.9K to ground resistance. Don't know the 305 value.

Not sure if the frequency pickup range shifts with the sensor or by just changing the resistance of them that the output is dampened to not indicate the noise from a different displacement combo. Maybe somebody has a better handle on it and will chime in.
There have been several posts on knock counts generated by shifting hard. Never saw a difinitive answer on how they corrected it (except slowing down, which is out of the question)

Anyway, I'm not sure if your noise signature is compatible with either sensor completly and what could be done to taylor the sensor to properly respond.
Just thinking out loud today

327_TPI_77_Maro

iTrader: (8)

Thanks JP. The 327 is the same block as the 350, just a shorter stroke, so I figured the 350 sensor/filter, which is what I am using, would be the best bet.