Is it possible to get knock retard only in 4th gear.
With my latest tune I gained 2 mph in the 1/8th mile, but nothing in the 1/4th.
There seems to be a slight hesitation when I shift into 4th.
What else could it be?
I only notice it at the track, of course I don't normally redline 3rd on the street.

 

What type of intake do you have? Any Cold air intake that has any RAM effect at speed could be causing a lean condition that you don't see at lower (less than 4th gear) speeds.

I have a Vararam.. And it was very pronounced that it was happening to me..
And when I was tuning for the additional air over 60mph I had a similar problem.. I'd do testing on a deserted road and when I hit 4th I was usually well over 80 to 90 and I was getting lean. I added 1% more fuel and extinguished the K/R..

Monitor your o2's they may show you what is happening.

 

I too have the vararam, I was just puzzled why I only noticed it in 4th so I guess its possible. Is it the added air or the fact that going through 1,2 and 3 at WOT there is alot of heat buildup? I guess either way I need less timing or more fuel...HMMMM I wonder if that is the reason I only gained 5HP with the new headers

 

KR is ussually more pronounced in the higher gears.It is good to tune a car in 4th on the road to scan for knock.A very common mistake is to Dyno tune a car without checking it on the street for KR with real world load.

 

Unless there is something else going on, the VR will NOT cause a lean condition in 4th, the MAF will compensate for the additional air. 4th gear loads the engine the hardest and that is what is causing KR, just log the gms/cyl at which the KR is occurring and remove the appropriate timing.

 

You are partially correct.. The MAF DOES compensate.. But remember. When in closed loop the PCM will keep adding fuel till its perfect 14.7 Stoich...

At WOT (Open Loop) the MAF is still reading the additional air but....The PCM just refers to what ever setting the enrichment is at for that MAF reading. If it was perfect before the addition of the Vararam it will be too lean after. With the Vararam it's not enough fuel at that point. It is certainly a repeatable condition when logging. Your O2's get real lean and there is Knock.

 

Then it's the the design of the VR that causes the KR, only because of the turbulence is causes as the air enters the MAF. It has nothing to do with open/closed loop. That is oxygen sensor feedback based, wide open, or open loop, is MAF fueled only (well, mainly). Trust me, I've seen many, many, MANY MAF's compensate completely for fueling on 5psi ATI and Vortech setups for fuel, I highly doubt the VR is pushing more air than those. I'd bet a few bucks it's the design of the VR's plenum where the two meet causes turbulence and MAF readings are thrown off because of it.

 

This has everyything to do with Open/Closed Loop. It is impossible to run lean at closed loop on a motor. The PCM will adjust injector pulses based upon the O2 sensor input.

At WOT (Which this conversation is about) Fueling is determined ONLY by what the Enrichment settings are in the PCM. If you are on the edge of leaness before the addition of the Vararam then you most certainly will be lean after it.

You are still Correct but you are missing the point... you are suggesting that the MAF can compensate for the 5PSI of an S/C.. True.. ONLY after the Enrichment MAPs have been set properly in the 1st place. If you just stuck that S/C on with no tuning and went WOT you'd Knock and melt the motor into slag.

On to the Vararam.
The Vararam has no real ability to introduce turbulence..even with the dividers. It is a sealed plenum and at "speed" there is a slight positive pressure so Turbulence is not the factor.

One the enrichment MAPs are set to handle the more positive airflow at speed you will be all set for good.

The MAF is ALWAYS doing it's job.. And at WOT is is the tables that determine enrichment.

 

PE vs. RPM is only an adjustment above and beyond what the MAF says to add fuel for. If a stock car pulls 285gms/sec, and a VR car pulls 305gms/sec in 4th gear, the MAF WILL COMPENSATE. The PE vs. RPM table is there to compensate for misreadings in the MAF (until the MAF is maxed anyway), not as the sole proprietor of fueling. If that's the case, just take your whole MAF table and scale it up by 120%, see what happens to your WOT A/F. Both the PE and the MAF control open loop fueling. And the comment on the S/C cars, I've tuned more of them than you've probably ever seen, trust me, the MAF will compensate a LOT until it's maxed out.

The open/closed loop thing has nothing to do with it..AT ALL. During closed loop the PCM uses the MAF reading AND the 02 feedback AND MAP AND IAT AND etc. etc. to reach what it knows is 14.68-14.73. This is all in the Fuel/Air Multiplier table. It's a calculation, nothing more. 4000hz does not mean XXX amount of fuel, it just helps in the calculation. The overall calculation is based on about 10 or 11 tables/readings. When in open loop, the F/A Multiplier table is ignored and the VE, MAF, MAP, IAT, PE, etc. determine fueling. There are much, much better ways to tune for a knock in 4th gear only than just adding to the PE table. Use a wideband if you're going to do that.

And no, it's not impossible to run lean in closed loop. An intake leak around only a couple of ports will cause it to run lean on those cylinders even if the PCM is compensating by adding to the LTFT's, that's just an average. The only way a motor would NEVER run lean is to have EGT's/02's for each cylinder.

And again, no, the MAF is the primary fueler when used, although it can be tuned out ie speed density. I've set cars PE tables all to 1.00, so no additional fuel anywhere based on PE, and been able to easily manipulate the MAF table to get the A/F perfect all the way across.

Also, by using the PE table to add fuel you're going to be running rich in the slower gears if what you're suggesting is true since you won't get that "ram air" the the VR supposedly causes. So, run rich in all your gears just to compensate for a supposed condition caused by the positive pressure in 4th. Well, it's not my car..........

 

All points addressed above.

Back to the original point...
He can fix the K/R easily with a PE change..

 

Power Enrichment can be set at any RPM or RPM range. That's why it's called the "PE table" and not the "PE setting."

 

Exactly. So if you change the PE for the RPM range for where the VR is supposedly making you run lean in 4th, the PE is not gear specific. ALL of the other gears will now be running rich in that RPM range that was change until you are going fast enough to get the ram effect that it causing the lean situation. That is why I said the PE table is the easy way to fix it, not the right way.

 

Maybe it's just me but air induction and exhaust mods have actually fattened me up. They have leaned me out then the pcm has corrected for it which made me run rich. At least that's my theory as to why I'm rich.

 

Could someone please explain to me how a device that increases air mass flow to the engine results in that air mass not getting measured by the Mass Airflow Sensor? The only explanation have heard is turbulence, which was quickly discounted by some people here.

Michael

 

Get back to the the basics before looking at the unlikely. Verify your tune by logging all the appropriate params. Then , if needed, retune part throttle fueling by getting your load cells in check. Next move on to PE and get your WOT AFR in line ...most like about 12:9 for an NA set up (on the street).Then work on the timing (timing can effect AFR so just be aware that more timing equals a slighly leaner AFR). Once your tune is in line rule out false KR. For example: Do you have a pulley? If so, that could be knocking at the higher speeds. Maybe something else is banging around in the engine compartment? Adjust your KR settings in the PCM so you can identify if the KR is load KR or PE mode KR by using 2* for load and 3* for PE mode KR. Also change the decay rate to 250% so you can see if the KR is continuous (indicates real KR) or is pulled quickly (indicates false KR). KR can be stubborn to run down....good luck.

Greg

 

in my experience. In all gears in the 1/4 mile I see the same cylinder airmasses. Before tune, the vararam and headers made my car very rich (11.0-11.8AFR) at WOT in 4th. I see the SAME AFR in all gears 1-4 as logged at the track. I have the LM-1 logs if anyone is interested.

Dave

 

Obviously it's ideal to eliminate KR in all gears, but getting it only in fourth means it's not going to hurt your quarter mile performance very much since you'll spend most of the time in the first three gears anyhow. I bet a lot of C5 owners end up getting KR in 2nd, 3rd and 4th gear and don't even know it. When I had my car dyno tuned last week it was showing 7 degrees of KR in 3rd (equivalent to 4th since mine is automatic and the ratio in 3rd in an auto is the same as 4th in a manual) Turned out my plugs were in bad shape, even with only 33k on the car. Changing them to fresh ones (in my case I went with NGK TR6s) dropped my KR down to zero.