I have the ARC2 w/87mm MAS, 720cc injectors and advanced base timing. I can run 17psi on 93 octane all day long with 0 knock. I keep the O2's in the .94 - 96 range with the EGT's peaking at a very safe 900c.

The only time I see any knock is if the engine is heatsoaked from stop n go traffic etc.. I don't see any knock until the coolant temps get above ~194deg. When I do get knock from heatsoak, I don't get more than 12-14.

Granted, the Stage II heads help quite a bit. However, I don't see why you shouldn't be able to run 15psi with no issue.

 

you've actually ADVANCED the base timing? Or is it at 5deg BTDC??

Do you think it's the ARCII allowing this?

Any idea why??

 

Posted this on your other thread..

http://www.3si.org/portal/forums/showthread.php?s=&threadid=142457&highlight=timing

 

you've actually ADVANCED the base timing? Or is it at 5deg BTDC??

Do you think it's the ARCII allowing this?

Any idea why??

It's set to 10deg BTDC.

 

The ECU CANNOT tell what airflow signal conditioner is being used. It makes no difference if it is the ARC2, VPC, AFR, AFC, or GM-T. When it sees lower air flow values it assumes less load and advances timing.

Now knock is a complex issue. There are several "knock links" on the Tech Page at my web site. For Jeremy I suspect that there is a condition in his engine or head that could promote knock. When he added the larger injectors and had to decrease the air flow signal, the ECU advanced timing and this brought out the problem. It is not an AFC vs. ARC2 issue.

 

Ok, very interesting.

And if Jeff is correct, that sucks.

The weird thing is I could run 15psi on stock turbos and fuel with no problems at all, never saw knock.... even tried 16psi once and no knock... then immediately after installing the injectors BOOM... knock.

I'll have to read up on your knock links and may have a few more questions.

 

The weird thing is I could run 15psi on stock turbos and fuel with no problems at all, never saw knock.... even tried 16psi once and no knock... then immediately after installing the injectors BOOM... knock.

If nothing else changed and you set it at an even % across the board (to keep it at stock levels) the only thing changing would be a jump in timing...

 

Though larger injectors put you in a more agressive timing map, the ECU still has control over the final amount of timing. It's still listening to the knock sensor and adjusting timing as far as possible before knock.

To me this fact may make the whole "advanced timing from larger injectors" a moot point. Yes it's advanced initially, but does it stay that way once all the feedback is applied?

 

Though larger injectors put you in a more agressive timing map, the ECU still has control over the final amount of timing. It's still listening to the knock sensor and adjusting timing as far as possible before knock.

To me this fact may make the whole "advanced timing from larger injectors" a moot point. Yes it's advanced initially, but does it stay that way once all the feedback is applied?

I haven't read all of Jeremy's issues with timing, but is the computer even pulling the timing? He's getting the steady knock sum, but I hadn't seen him say the ECU was pulling timing.

 

I haven't read all of Jeremy's issues with timing, but is the computer even pulling the timing? He's getting the steady knock sum, but I hadn't seen him say the ECU was pulling timing.

From what I've seen, at WOT, the ECU gradually increases timing. The only time timing is pulled is if knock sums are greater than either 12 or 14 (can't remember which).

 

that I'm aware of the ECU pulls timing only when it sees knock and does NOT learn that it needs to apply lessend timing across the board (ie- the way that the Supra guys unplug the ECU to reset timing higher). I've not done the research with the datalogger to be sure- but that is a point that HellBringer should be able to shed light on.

Did you notice knock at first, and then the knock tuned itself out?

I don't think so- we've ran 450's with a 550 VPC chip for a bit now- and always found that timing was insanely high, even after ecu pulled timing- next run it resulted in the same timing and same knock if we changed nothing.

I've not tried to run 720's and enough runs to actually kill a motor- but I think we've seen too many people who's motor's have blown at 16-17 psi to accept such a thought without some serious investment into research.

 

As for the ECU pulling timing... yeah it's pulling it but as we know, once knock starts, it's difficult to get rid of.

Last night I did a run, was getting only 4-5 knocks, but at around 5500 rpms in 2nd, it shot up to 18. Timing did pull but it didn't help the knock (well, I got off the gas pretty quick so I don't know if it would have sooner or later). o2's stayed in the .94-.96 range and EGTs were still in the 850 range (front).

I do see my timing slowly advancing as the RPMs go up.. which is great as long as knock doesnt' happen...

 

I'll have to check my logs when I get home. I'm fairly certain that in most cases when I've seen knock it was subdued rather quickly by the ECU pulling timing.

Unfortunately for the cause, I don't have many logs with significant knock (sums more than 12).

 

This is where I def do not understand these cars. MSD has a boost retard function on its ignition boxes to retard timing when you get into boost--I guess to keep detonation down while under a lot of boost. Meanwhile, the way all the quicker cars are set-up, they are running lots of timing. Most stock cars are knocking with timing set from the factory at only 15-16psi. How come others are able to stop detonation with increased timing and the same boost. Is it because the turbos are more efficient also? Very odd. What do the AEM ECU guys run for timing and whats the highest boost they can run with pump gas before they get knock?

Sam

 

Increased timing should yield cooler temps, maybe that's why?

 

most people haven't played with this enough...

We're running the MSD- mostly because it allows you to keep really high timing (as the ECU likes) off boost- then once you get to spooled range- your timing leans- the issue- the MSD is a straight RPM style- so if you like to take it to 5000RPM off WOT (like me) then you'll want the extra boost...

MSD offers a BOOST related timing retard- but it isn't incorporated in the DIS4

 

So you're saying the MSD could fix my problem?

Basically I want to run 8deg BTDC or higher until around 3000 rpms, then once boost kicks in, the timing can get pulled.

I don't know squat about the MSD, so edumacate me... you adjust it by RPMs?? Is there a good range of control??

And how much does it cost? You guys sell it?

And can you better explain what you mean by: "so if you like to take it to 5000RPM off WOT (like me) then you'll want the extra boost..." in a real-life situation, how would my car feel?

 

Jeremy- lemme get back into the manual so I don't tell you wrongly-

simply put on the 5000rpm thing- if you've driven around at TDC and it wasn't horrible- you'll be a happy man

 

Also remember that The heads play a major role in Knock/Detonation.

Hellbringer has DR heads which get rid of the hotspots that the factory castings cause. So Matt is able to bump his timing 5 degrees and still see no knock. This is also one of the reasons I can run 22psi on pump and a little bit of Propane. My heads are also ported, polished and all the casting burrs are gone.

 

Bringing this one back up...

I got an email from Matt Saskin, who I talked about this problem for a while when he stayed here last week. He thought about it and had an idea..

He says the DSM guys have a problem when upgrading turbos that the MAS cannot read all the air coming in. Basically, it could be possible to over-run the stock MAS's ability to read the airflow.

Also in Turbo mag (I think) this month, there's an article on step-by-step DSM tuning... one thing is once you upgrade a DSM turbo, you have to replace the MAS for this exact reason.

Of course this could lead to a lot of problems... one being more air getting to the engine than is being read... but it seems like this could also effect timing.

If that's the case, and the ARC actually replaces the MAS, and the ARC can read that much air... maybe that's one reason the ARC is better!??!


So I'm thinking that the new MAFT (LS1 MAS and Translator) could possibly solve the problem?

It's less than $300, probably worth a try. As a bonus, if it does work then I'd no longer be limited to the famed 500hp limit of the stock MAS.

But the MSD also sounds like it would work.. plus add the ability to build boost with the "Stutter box" thing... plus I believe the MSD actually increases spark, too. I found a used one for $250, so that's pretty cheap.


What do you guys think?? Of course BOTH would be best but let's say I have $300 to spend, which one should I try first??

 

I forgot you are using the stock MAS. Yes ,it does have a maximum volume flow rate it can measure and then it starts dropping counts (DSM and 3S basically use the same MAS). This is, of course, unrelated to any timing issues (except that missed counts will lower timing because ECU sees less air flowing).

Regardless, get rid of the stock MAS. It is choking your intake (causing too much pressure drop at high volume flow levels). Mike Licht measured the stock MAS to have 0.8 psi pressure drop at 400 cfm. Pressure loss increases with the square of the volume flow. So at 600 cfm (300 cfm to each turbo), pressure loss increases to 1.5^2 or 2.25 times 0.8 or 1.8 psi. At 800 cfm pressure loss across the MAS has increased to 3.2 psi.

The GM MASs that are used with the DSM translator and the Ford/Granatelli MASs used with the ARC2 have much less pressure loss - like about 0.2 psi at at 400 cfm (or about 0.8 at 800 cfm). Less pressure loss (that is higher pressure) before the turbo is a very good thing.

 

JeremyG: you've ran it after the new intercooler install and it still knocks?

 

Thanks Jeff, I am gonna scrounge up the $$ for the MAFT.


billvp: Yes but there may have been some improvement. When I first installed the 680's, I would get knock immediately when boost came on. Now it's later on, around 5000-5500rpms.

I'm sure the problem is NOT the relative hotness/coolness of the air, I'm positive that I could be putting super-cold air in there and it would still knock (actually I have been.. when I ran at the track it was 50 degrees outside )

The problem is most definately TIMING... with my base timing at 0 degrees, I don't get any knock. At 3 deg BTDC it's a very small amount in the upper RPMs, and at 5deg BTDC it's a larger amount and it comes on sooner.

I can get rid of the knock, obviously, with 0deg timing, but the car feels like a SOHC on the launch, and wants to buck and complain during low-speed cruising.

 

jeremy, if you still check this thread i am having the same issuse with 680cc and s-afc2. Timing is set to 5 degrees and I can get knock sums in the mid 20's which yanks timing into the teens. The kicker is, I have stage 3 heads!

 

jeremy, if you still check this thread i am having the same issuse with 680cc and s-afc2. Timing is set to 5 degrees and I can get knock sums in the mid 20's which yanks timing into the teens. The kicker is, I have stage 3 heads!

Who's stage III haads? Just becuase someone calls them stage III don't mean all stage III are equal. Bowl contouring and shaping is an art and takes research and many trials and errors....

If they are DR III heads then I would be very surprised. If they are from somewhere else I would not be too surprised. Not knocking other heads but you can't just port away and get great results....don't work that easy