mk3cn4

iTrader: (3)

I am tuning 10psi and 87 octane, and am seeing something strange I could use some help with, have 3 questions if someone could chime in.

Between say 5200rpm and say 5700rpm (changes a bit at times) at 10psi it feels like "sometimes" there's some slight missing that clears up above, say, 5800 rpm. Maybe one or two cylinders...? Hard to tell, I have exhaust dumping under driver's feet and it sounds mostly normal, not popping or anything. I've pulled plugs and wires, even regapped to .025 and made sure they're all good and still happens.

This is apparent in the HPTuners graph, you can see the RPM at WOT during this stretch absolutely flatten and get jagged, the RPMs even kinda drop very slightly and bounce between two close RPM numbers. It seems to clear up and then pulls harder after 5700. Doesn't do it all the time, just sometimes. Am trying to find an example to post screenshot. I am absolutely not hitting rev limiter, nowhere near it.

One big point here is that I'm using a Yank PT4000 from my last build which I'm somewhat blowing through. Makes the car sound a bit like a golfcart, so those low shift extensions and maybe just the physics of that are creating this scenario, and what I am seeing is just the stall hitting the wall and flattening as the invested power is leaving the converter...?

The MPH graph seems to rise steadily throughout as though there was no problem.

During this "blowout" (or whatever) duration, the timing mostly stays around 14 degrees and the AFR is at or under 11:1. Some of the runs there is small KR in this "missing" range as well, but not sure if that's a result of the anomaly or a cause of it, or related at all.

I am currently running .025 spark gap. The blowout, if that's what this is, is somewhat subtle, not like hitting a rev limiter as some describe. And when it goes beyond the range I can noticeably feel it pulling stronger.

So my questions to the pros are:

1) Can spark blowout occur during a stretch at a certain RPM and clear up at a higher RPM above that?

2) I have a new set of plugs and wires I'm going to throw in, should I do .020 gap this time?

3) Does spark blowout EVER create KR? Tuning 87 Octane, that's mission #1 right now (and it's going well mostly).

GMCGreg

Again , no expert here. KR pulling timing with definitely make it feel flat , also I've found that richer than 11:1 definitely costs some power ( track time prove that) 10.8 -10.9 doesn't "feel" bad but definitely faster at 11.5 . If it were me , I'd take a couple degrees out in that area , try and dial in low 11s afr and then see how much timing it can take. IIRC you are L33 ? 10:1 compression , 87 octane , no W/M , it probably won't tolerate much timing. Not likely blow out at 10 psi / .025 gap unless you have a bad coil or wires . Most will say read a plug strap , to see how the timing is- definitely worth the effort. Do as many as you can easily or concentrate on #7 if you first determine that they are all similar . Guessing you didn't get to the track ?

mk3cn4

iTrader: (3)

I think that's a good plan. Maybe I'll gap these new plugs at .023 (just a little tighter) but I'll pull more timing in those areas. The KR doesn't happen every time, that's why I thought maybe blowout was happening and occasionally causing the KR...? Not sure if that's a thing. I also suspect my exhaust hitting, it seems when the motor mounts warm up I guess it lets it hit from the looks of the body/pipe. I'll try to fix that too. The car's so darn loud I can't hear what's going on under there LOL. That'll be fixed soon too.

Yep I got to the track, just been a little ill with a stomach bug, I was going to make that a new post LOL. It ran 11.48 at 123 ON 87 OCTANE with 1200 DA on a full weight car with a/c but did have some KR at the top of the run. I was amazed! I see now how everyone says it's easy to accidentally make 500HP with this combo, because apparently I just did.

Yes stock L33 with springs/pushrods, AFR I think was in the 10's during the whole run, maybe low 11's at points. Timing was 15 or slightly lower (at the points of KR and spots where I pulled out before). My test pulls before the track seemed mostly clean so I went for it on the 87. The track pull was actually a bit higher PSI it appears, I guess when I can WOT over a longer haul (on a sticky track) it does build better PSI, went over 11psi I believe. My goal was to keep it in single digits, but it seemed OK. Time will tell in hot weather if that still holds. I may do a boost controller so I have the ability to pull that down a bit. I was actually very happy with the run. A newer corvette 2 cars ahead of me ran an 11.50 and I was able to pull out an 11.48 with my beat up red bird. If I knew that was gonna happen I certainly would have gotten video of that. :-) Note that I have the stock magnesium 5-spoke wheels and dents/chips all over it with cracked windshield and faded paint on the headlight panels... and it ran an 11.48!!! I am very very happy with that if I can smooth it out and make it safe.

Once I am feeling better I'll post the log from that run in my other thread.

I am just lining up the ducks because I'm putting new valvecover gaskets on and decided to do new plugs/wires again while everything's opened up and easier to do, so wondered if others thought I should close that gap a bit more with this symptom.

Also, the mark on the plug band does seem to be near the bend the one time I checked, but not sure if I need to check it right after a run...? I need to google that and see. Someone mentioned that any drive time after a run will alter results.

I appreciate your input!!!!!

Pro Stock John

iTrader: (34)

That's cool congrats!

Do others think you should pull out some timing?

jon1440

I was having similar issues once upon a time and found out my coils was the issue. It would miss, logs would show knock retard. My timing was conservative, running 91 or better fuel around 10 psi. At low rpm it’s hard to notice but at high rpm when coils need to be spot on the miss was extremely noticeable. Mine was miss firing around 5300rpms usually. Anything above 4K was free range. Went to junk yard and grabbed a different set, installed and problem disappeared. No more knock retard logged and pulls rpm’s smoothly. My current setup im runnning 17 degrees above 20psi with ethanol.

Also you should run no lower than 91 for safety measures. Cost more but replacing an engine is costly also.

GMCGreg

Read your last post again- definitely deal with the exhaust hitting anything first . Also not hard to try some jy coils , a cheap experiment

dawgs74

Boostane ……and 91 octane

LetsTurboSomething

I personally never saw blow out until 19psi and that was gapped at what ever NGK shipped it out with. So I would probably rule that out if it were me.

I run my build on 90 octane because that's the best i can get here, but I run water/meth washer fluid too.

Mine runs well at 10.8 AFR but that's not the same for every car. My guess is you are just hitting the octane wall. I can't really go past 10psi without the meth spray running. But with it I run 28psi on 90octane and ET a 10.8@ 126 in a long bed square body truck on factory suspension, foot braking. Full exhaust with dead quiet factory style muffler and everything.

If you throw money at anything from this point on to go faster, the water/meth is going to give you the most bang for your buck when you don't have all that corn juice to feed it. But you're at the limit of what I found regular octane would do for me...and about a half second faster than I was when I was at 10psi and on radial tires.

mk3cn4

iTrader: (3)

I guess this is the "bug" that everyone says I'd get when I started down the path of higher power LOL. The meth stuff isn't hugely expensive, possibly the biggest HP bang for the buck there is in my specific case... just something I didn't want to babysit.

It's so tempting to just start running 93 octane since it's everywhere here, but my goal was to "build a very fast DD car keeping 87 octane". Before covid, I drove this car as much as 40k miles/year, rough math that's 2000 gals per year costing .75 more per gallon, like $1500 EXTRA per year every year I have the car if I go back to pre-covid driving levels running 93 octane. As it sits now, a DD that runs say mid 11's is still PLEEEENTY fast.

I am going to think about it. Right now I am just considering getting adjustable boost regulator, and tuning for colder days and hotter days with lower boost and just dial back the boost on hotter days. Then I'm done. I'll have a 20MPG DD running on cheapo gas that runs mid 11's on a cool night at the turn of a dial.

Is there a vaccination I can get to protect me against this "go faster" bug?

Full Power

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Any UPDATES ? This was a GREAT build thread. How's the TUNE ?

mk3cn4

iTrader: (3)

As far as my original problem in this thread, it was NOT blowing out spark, pretty sure I remember that it was just a plug wire at high RPM if I recall. It took a bit to figure out because the plug wires were new and assumed they "could not be bad" but one was.

Just like much of the country, I did not go back to the office full time post-covid, and instead of driving the thousands of miles per month for work I'm down to 2-3 on-site days per month, so I just went to the 93 octane.

To this day I have been running it as a daily (even during the winter!) with single digit PSI on an adjustable regulator. Very happy with it. It has a "very slight" knock at startup that I don't think is piston slap even though it typically gets quieter as it warms up (the opposite of what rod knock should do), so haven't cranked up the boost on it past, say, 10PSI-ish because, well, it's plenty fast just the way it is and I want to just get some miles on this build before I blow it up LOL. This is the 4th engine for this car.

Kfxguy

iTrader: (1)

I see the original post is from a couple years ago but.....

in my experience:

Ls1 with 243 heads/cam , 93 octane, 10-11psi of boost, 11-12 degrees of timing..... I broke the top ring land of a piston. In my opinion, you are running too much timing for 87. When i broke the piston, it would rattle when cold and then quieten up as it warmed up. Pulled it apart and 1/4 or the top ring land was gone.

Full Power

Best "Road Fuel" here in Alaska is 90 Octane.
Many out of the way small stations only sell 87 Unleaded.
Doing my tuning on pump 90, 11 degrees in the higher boost cells, okay so far at 140kPa ( stock 03 suburban fuel pump, can't push much further... )