Thread: getting some knock starting around 4800 rpm.. any suggestions?

I'll post the log if needed, but wondering what may be causing this...

I see some knock starting around 4800/4900rpm. usually only a degree or two, but then goes up to 4 +/-1* before I shift around 5.5k.. not quite sure what would be causing that. E85 for a few days now with a 3.5 pulley. AFRs are plenty conservative. I can back the timing down a bit (currently 17* at WOT), but don't want to if I can fix this easy..

Let me know of any suggestions! I'll probably start tweaking the tune again tonight.

Just throwing this out there but could it be false knock from the downpipe? Or did you flip the sway?

Originally Posted by Dschroeter63

Just throwing this out there but could it be false knock from the downpipe? Or did you flip the sway?

Didn't flip the sway, but last time I checked the DP looked like it had decent clearance. I should check again though.

Are you still running a genIII blower? My car had a 3.4 on it when I bought it back when everything was bone stock. It did blow up after 6k miles but that was on 93oct too. I ran a 3.1 @ 22* when I first went to E85 with plenty of headroom for KR. That was before the IC, genV and all that stuff. I did have the headers and cam though.

It it seems like you should be able to dial in 0kr fairly easy.

Originally Posted by Anthony Smith

Are you still running a genIII blower? My car had a 3.4 on it when I bought it back when everything was bone stock. It did blow up after 6k miles but that was on 93oct too. I ran a 3.1 @ 22* when I first went to E85 with plenty of headroom for KR. That was before the IC, genV and all that stuff. I did have the headers and cam though.

It it seems like you should be able to dial in 0kr fairly easy.

Thats what I would think as well.. Not quite sure what is causing that KR to creep in. I'll try making a few small changes tomorrow and see what it gets me. And yes, stock Gen3 minus the 3.5 MPS. I could try backing timing off a bit more but I think 17* should be plenty safe with E85 at WOT on a 3.5 pulley

Originally Posted by 97AutoXVette

Thats what I would think as well.. Not quite sure what is causing that KR to creep in. I'll try making a few small changes tomorrow and see what it gets me. And yes, stock Gen3 minus the 3.5 MPS. I could try backing timing off a bit more but I think 17* should be plenty safe with E85 at WOT on a 3.5 pulley

It's real knock. What's your A/F ?
I personally was not that impressed with the E-85 for a non intercooled setup. It seemed to vary too much tank to tank. I always had KR issue on and off because I had no IC.
That is the biggest cause of the KR plus the exhaust backpressure. I had better results with methanol injection and AV gas. AV gas is pretty cheap, just a little more expensive than 94 octane for us.
Could run a 2.6 pulley all day with no knock, and it was a monster for a M90 !

The E-85 could not run the 2.6 pulley with no IC and no headers and no Cams. But AV gas plus Meth... no problems..
.

Originally Posted by TwinV6GTP

It's real knock. What's your A/F ?
I personally was not that impressed with the E-85 for a non intercooled setup. It seemed to vary too much tank to tank. I always had KR issue on and off because I had no IC.
That is the biggest cause of the KR plus the exhaust backpressure. I had better results with methanol injection and AV gas. AV gas is pretty cheap, just a little more expensive than 94 octane for us.
Could run a 2.6 pulley all day with no knock, and it was a monster for a M90 !

The E-85 could not run the 2.6 pulley with no IC and no headers and no Cams. But AV gas plus Meth... no problems..
.

Hmm, that's interesting.. How does the IC play an effect into the equation outside of normal inlet air temp cooling? I was under the impression that I wouldn't need an intercooler as soon with running E85 since the added fuel can take excess heat out of the air. Appreciate any more info you have on that end.

AFR is pretty conservative and around 7/1 to 8/1 anytime the engine is under load/boost (down to around 7/1 @ WOT).

I've never read much about the difference between headers and a plog setup but for what you have into your plog and dp you could already have headers. If you plan on making more power yet in the future I would suggest getting a set sooner than later. A lot of people have had great results with E85 myself included. I wouldn't dismiss it quite yet with your setup. I've maxed a genV out to a 2.50" on straight E85 with no meth Like I said, the first year of my build the only differences between our setups would have been I had a small cam, headers and a self ported blower. I ran that down to a 3.1 and 22* and the car ran 12.80@107 on DRs. My AFR was showing around 11.3 on a gasoline calibrated WB on that tune.

With a few small flow mods you could probably do the same. Make sure your exhaust isn't partially restricted or anything. Look into headers. look into doing a cam. If your going to continue on with the gen3 there is a couple easy cuts you can make on it to get the most out of it. If you want to keep the stock cam forever the only way to make decent power is with a turbo

Originally Posted by 97AutoXVette

Hmm, that's interesting.. How does the IC play an effect into the equation outside of normal inlet air temp cooling? I was under the impression that I wouldn't need an intercooler as soon with running E85 since the added fuel can take excess heat out of the air. Appreciate any more info you have on that end.

AFR is pretty conservative and around 7/1 to 8/1 anytime the engine is under load/boost (down to around 7/1 @ WOT).

I think what it boils down too is E-85 quality. It 87 mixed with ethanol. Perhaps other states use higher quality base fuel to mix with ethanol. I'm in Canada and the E-85 in my region wasn't impressive.
It was about equivalent to 100 octane gasoline at best. AV gas alone at 104-108 octane was far superior.

But I was running 2.6 pulley with the worst possible set up for KR. Just so you know Ethanol although very high octane, the threshold for knock resistance in some situations is much lower than Leaded race fuel. Leaded race fuel tolerates leaner mixtures a lot more than Ethanol. Also leaded fuel tolerate higher intake temps than E-85. E98 or pure Ethanol is another story. The little bit of cheap unleaded fuel is what causes the knock.

So you could be too lean at the moment. Way too rich can cause rich knock as well. You need to provide A/F ratio data.

In your case with the 3.5 and your mods you should never see any knock. What's your A/F ratio I don't think you answered this yet ?

Originally Posted by Anthony Smith

I've never read much about the difference between headers and a plog setup but for what you have into your plog and dp you could already have headers. If you plan on making more power yet in the future I would suggest getting a set sooner than later. A lot of people have had great results with E85 myself included. I wouldn't dismiss it quite yet with your setup. I've maxed a genV out to a 2.50" on straight E85 with no meth Like I said, the first year of my build the only differences between our setups would have been I had a small cam, headers and a self ported blower. I ran that down to a 3.1 and 22* and the car ran 12.80@107 on DRs. My AFR was showing around 11.3 on a gasoline calibrated WB on that tune.

With a few small flow mods you could probably do the same. Make sure your exhaust isn't partially restricted or anything. Look into headers. look into doing a cam. If your going to continue on with the gen3 there is a couple easy cuts you can make on it to get the most out of it. If you want to keep the stock cam forever the only way to make decent power is with a turbo

Sounds like I really should look into headers soon then (if I don't go turbo)... Do you think that headers and 1.9 rockers would be good enough for 12's on E85 considering the other mods I have in my signature?? I mainly just want to be done seeing knock for good! Lots of people looking for the P-LOGs and downpipe combos so I am sure I could recoupe some $$ soon after getting headers. Just was nervous originally about assembly issues with the headers, as I have heard guys running into problems... and it's currently my DD/only car.

Originally Posted by TwinV6GTP

In your case with the 3.5 and your mods you should never see any knock. What's your A/F ratio I don't think you answered this yet ?

A/F values (on my Wideband tuned for pump gas) are 14.7 @ cruise (target 9.85 on E85).

@ 28% throttle, it adds 1 to the A/F values, which yields between 12.5 and 13 on my WB.

after 45% throttle, full PE targets kick in which are set to 8/1. That yields around 10.8 - 11.0 AFR values on my wideband. I originally targeted 7.3 AFR values as base PE, but was seeing my wideband values of 10.0 - 10.3 during WOT, which seems too rich considering that is sub 7 AFRs on E85.

Let me know if all that made sense!

Originally Posted by darkhorizon

Knock during tip in can be related to the trans and whatnot... the biggest fear of kr you want to have is stuff popping up after you have been in the throttle for awhile.

Is "tip in" meaning right when I start getting into the throttle?

to your second point, I believe that is the KR I am currently experiencing. I can stay in WOT and not see KR, but around 4800rpm, it starts to set in and increases to 3-4* before I shift around 5500rpm.

Maybe I should get more flow mods??? I still think that 17* on E85 with the mods I have shouldn't see any.. but I have worked this tune multiple times and feel like it is safe...

1st thing you have to learn is never trust ONE wideband. It can easily lie to you. I always have a backup A/F gauge and sensor.
We dyno tested hundreds of machines and it's crazy how often the customer's wideband gauge OR sensor was lying to us. I've saved face many times in front of a customer when his wideband was saying LEAN but he was DEAD rich or the opposite. You have to be careful do not 100% trust a gauge. With E85 the smell is different but with gasoline. With gasoline, if you have a keen sense of smell you can easily tell rich from lean, it's a totally different smell (when following the car) or on a dyno, which is easier.

I would try richening up just to see if knock will go away. Monitor power loss or acceleration loss. I always use GPS performance meters which makes it easy to know gains or losses even tiny amounts. If knock gets worse, I would lean a little at a time and see if it improves. Do not let it knock too much or too long. #1 killer of engines.

Appreciate all the comments. I went ahead and uploaded the stock high octane timing tables and took it for a drive this evening. Successfully achieved 0* KR throughout my WOT pulls. With that, I will probably add a degree or two, and do the same thing. I may also leave the stock high octane timing table alone and add a smaller pulley and see what that gets me. @ WOT, I am currently between 11 and 13* of advance, so nothing crazy, but that was the stock table so can't be too bad I guess?

I am thinking it will handle 14* of WOT timing just fine. We'll see how it does over the next few days!

Either way, I was seeing up to 9.6* of KR before E85 on the stock timing tables, and now I am seeing 0* on the stock timing tables. So that right there was worth the fuel switch in my limited opinion