C4ProjectCar

Last month, I took my '90 Corvette to the drag strip for the first time (if pertinent, it has a 1227727 ECM and runs $8D). I've been swapping parts bit-by-bit, and I've reached what I think is a pretty respectable (albeit a bit mismatched) combo:

• 383ci
• 9.3:1 SCR (8.4:1 DCR)
• AFR 190 heads (the old-school ones from before the Eliminators)
• Lunati 211/219 @ .050" cam (small, I know, but I didn't want to risk failing smog)
• 1.6:1 roller rockers
• Super-Ram plenum and runners
• Edelbrock High-Flo intake base, port-matched (as well as I could) on the head side
• Hooker Elite 1 5/8" long-tube headers
• Accel 33.3 lb/hr (advertised as 36) injectors
• 58mm Holley throttle body
• 3" front Y-pipe
• Corsa cat-back

While it was a blast at the strip, my highest trap speed was just 106.23 MPH (at density altitude of 1,697 feet). According to an online calc, that corresponds to 322.38HP. Even taking into account a 20% drivetrain loss (which I've heard is accurate for the ZF6 transmission), I'm barely breaking 400HP at the crank. That seems pretty weak for a SuperRam 383. Granted, my static compression ratio is a little low, but with such a small cam my DCR is already at the limit for pump gas.

Here are my relevant tables: spark advance, VE lower, VE upper, and PE enrichment and spark advance. I've also attached a datalog (not from the strip, but it has some WOT).






As many hours as I've put into tuning, I'm still lacking the intuition that you guys have. Do my tables look about right, or do you notice anything off with them?

ULTM8Z

dont have my computer in front of me, but is there WB O2 data in the log? without a WB its nearly impossible to do a good tune....

C4ProjectCar

I do have a wideband, but I haven't had time to set up datalogging with it. It was showing lambdas of around 0.85-0.88 (12.5-13:1 AFR) at WOT.

C4ProjectCar

Here's a dyno graph I calculated from a datalog. It's not super precise (estimates peak HP as 270ish), but it gives a rough idea of the shape of the curve. Seems like HP is falling off quite a bit before it should be.

ULTM8Z

the spark map is a little too lazy I think... SBC like yours is probably going to like something a lot more aggressive.

you're not quite to the short runners of a Miniram, but I might suggest using the LT1 corvette spark map and zeroing out your PE spark. I would have said LT4, but the dynamic compression you're probably building with that much displacement and that small a cam will likely result in crazy knock counts. Lt4 spark map is quite a bit more aggressive than the LT1, but the LT1 map is better than the TPI map for what you're running IMO.

speaking of that cam, in a 383 is really short changing the horsepower potential of a 383 with those AFR heads. on a 383, I probably wouldn't have gone with anything under 218-220 deg on the intake (and I'm someone who prefers a smooth idle over max power).

C4ProjectCar

Lazy as in too little peak timing, or as in too slow of a ramp up to peak?

Huh, it never occurred to me to try the LT1 timing curve. Although isn't timing more a function of turbulence and combustion chamber geometry than intake manifold style? And why do you suggest zeroing out PE spark? I've read people suggesting that, and I've read people arguing against it, but I haven't really seen any good reasons either way.

Yeah, my DCR is 8.9:1. What did the LT4 run?

I know, but unfortunately it's registered in CA. The engine actually came with a CC XR282HR (230/336), but I had no confidence I'd be able to tune it to pass smog. I was hoping to be able to swap that cam in with some Rhoads/similar lifters, but I ran out of time before I drove out to VA for school (and out here I have neither time nor tools). I also wasn't sure if the 282HR might be too big for the rest of my build. How much do you think a larger cam would wake it up?

ULTM8Z

at least the peak needs to be in the 32-34 deg range IMO. currently you're not even getting to 30 with PE spark.

8.9 or 9.9? you said 9.9 in the original post. typo?

LT4 was 11:1 and LT1 was 10.5, but they had the reverse flow cooling.

C4ProjectCar

What RPM are you talking about? By 4000 I'm at 30°, and at 5200 I'm at 34.5°.

9.9 static; 8.9 dynamic.

Wow, and I can't imagine the LT4's cam was all that big, so its DCR must have been way up there. Is there any way to rig up reverse cooling on an L98?

EDIT: Just realized I pulled the wrong numbers for that. It's actually 9.3 static and 8.4 dynamic.

ULTM8Z

LT1 is pretty much all in at 2800 rpm at 33°. On yours, at 2800 you're only at 24 (main) + 1 (PE) = 25°. That's kinda what I meant by "lazy"... It's coming in too slowly. With the bigger cam than stock (it's a small cam, but still a lot more than stock), you'll want your timing in sooner.

Probably won't do anything for your top end (I think you're just choking off the 383 with that cam), but it'll probably feel quicker on the street.

ULTM8Z

LT1 map is the first picture. LT4 is the 2nd.

ULTM8Z

Here's a comparison between the LT1 and LT4 spark maps...

And then a comparison between ANHT (which is very close to the AXCN that I think is you're running) and LT1. Note that the LT1 spark is more where it counts... mid-upper rpm range and high map, where the engine is making its power. My comparison matrix only goes up to 4800 (just looks at the lower spark table), but the upper continues in the same vein... Only thing is the ANHT doesn't include the PE spark, so a figure a couple extra deg for that.

I'm running the LT4 map on mine (350, Miniram, AFR 180 heads, 9.9:1 compression, 212° intake duration) and it hauls a$$ with that spark map. The only thing I did was fill in a little more timing at high map and low rpm.

C4ProjectCar

I had no idea their maps were so different. I always imagined the LT4 was just a polished version of the LT1. Interesting how the LT4 gets into timing so much faster, while the LT1's peak is so much higher. I thought 38° was pretty much unheard-of for a later SBC.

What are ANHT and AXCN? I'm guessing designations for factory tunes? Regardless, I recall the acronym "AUJP" being associated with my ECM - is that the same thing?

Hmm, your DCR must be very close to mine. And you don't have any problems with knock? What's your quench distance? Unfortunately mine is pretty huge, which does me no favors.

I think I'll give the LT4 map a shot. Although it seems like my engine's real weak point is the top end, where I've already got a tad more timing than the LT4 map. Knock really makes me nervous though since I have no experience identifying it by ear. Should I work my way up to that map while keeping an eye out for knock counts, or with forged pistons am I safe risking some knock?

EDIT: Just browsed through that SuperChevy article. That's a nice looking Camaro! Did you lower it a little?

ULTM8Z

I'll send you the LT4 and LT1 spark maps a little later in excel format.

ANHT and AXCN are the Corvette tunes for $8D. automatic and 6speed, respectively.

ULTM8Z

Here's the excel file associated with those pictures.

For each tab, the two compared maps are there. The extended spark table values are the same values as the 4800 rpm row in the main spark table.

C4ProjectCar

Wow, there's a lot of info in that spreadsheet. Thanks! I guess AUJP must be for third gen L98s?

I'm a little mixed up into how specialized the spark maps are. So ANHT is for auto L98 Corvettes, and AXCN is for ZF6 L98 Corvettes, right? It looks like you just specify "LT4" without going into further detail, so I'm assuming that's the only calibration for the LT4. But for the LT1, you specify year, transmission, and model. Did the LT1 have a lot of different factory calibrations, depending on those three things? If so, I'm guessing I'd want to try one from a manual, so the '93 M6 Z28?

EDIT: This post over on Gearhead-EFI breaks it down, and it looks like BINs differentiate year, platform, transmission, and even rear-end (though I'm guessing rear-end ratio wouldn't affect the spark maps). My car ('90 ZF6 w/ 3.33) isn't listed there, but it looks like AXCN would be a '91, whereas the closest thing to me is ARFP. Not sure the spark maps would be any different.

ULTM8Z

May want to get hold of Chris90Formula. He's running a 383 with a cam at 218° intake duration. He has a Miniram on his car, but his signature indicates he's running a 12.6 in the 1/4 at nearly 117 mph.

https://www.thirdgen.org/forums/diy-...s_aujp-v5.html

Orr89RocZ

iTrader: (20)

Chris90Formula

iTrader: (1)

I'm running the Comp Cams XFI 268 (218/224 @ .050). Drive-abilty is great. Idle is decent; I think I should be able to make it idle perfect once I work out some bugs on my system. Power is good to 6,000 rpm. I trapped 116.8 mph and I was on the rev-limiter (6400 rpm). I think with a change in gearing or tires I could add a few more mph. With 4.10s and street tires I was only pulling 2.10s 60' times. I think I'm my headers are probably costing me some power. They are 1-5/8" MAC shorties. I think I am running the LT1 spark tables; I'll have to check on that. I don't have to pass emissions, but once I fix the idle I suspect that I could get it to pass. The super-ram should idle even better than the miniram. I passed emissions back when I had my TPI 305 and a Lingenfelter 213/219 cam.

C4ProjectCar

That's really helpful, thanks. Without access to a dyno (or time for more than one or two trips to the strip this semester), for now I might just add a degree or two at a time till I get knock, then pull 2-3°. If I have time, I might play around with richening AFR a bit.

I haven't measured cranking compression, but my calculated DCR is 8.9, which on the top end of what you can get away with on pump gas (or so I read). And yeah, I was hoping for 110. My car is 3400ish with me in it, and I'm brand new to drag racing, so that didn't do me any favors.

Hmm, seems like the 282HR is probably too big for good streetability. Maybe I'll sell it and go for something like that 268. Wow, 117 is great, especially considering that's not a huge cam and you're using a factory spark table. I know the SuperRam doesn't have quite the HP potential that the miniram does, but I'd love to get a little closer to that.

C4ProjectCar

Alright, I added a single degree at 100kPa and 4000 RPM and below, and I'm getting knock retard from 1400 up. I'll run a couple more datalogs to make sure it wasn't a fluke, but in the meantime here's the datalog I got today. I'm seeing that the actual knock count occurred at 2550 RPM, but from what I've read there was probably fast knock occurring elsewhere? So, as I understand it, that knock retard could have been residual from a previous knock event, or it could have been from fast knock counts at that particular moment?

ULTM8Z

all of your recorded knocking is occurring below 3600, where the ECM is pulling at least 1 deg out, up to a max of about 4 deg. Of those instances, the vast majority is actually below 2000 rpm. It's also occuring at map values far below where I'd expect knocking to occur... as low as 25 kPa (and 0% TPS).

It may be the case you have some sort of oversensitive knock sensor situation, or something generating false knocking noise, or something like that... from the data, the knocking doesn't appear to be real.

I'm skeptical adding 1 deg at WOT is going to cause all this. Have you datalogged prior to adding the 1 deg? Does it do the same thing?

Worst case if the knock is real, ECM is only pulling 4 deg tops even under worst case load, so it's not horrendous. I'm assuming that the bin has the stock value of allowable knock retard. That means the it's probably safe to disable the knock retard entirely since there appears to be margin to being maxed out on spark retard.

In which case, you can ease into throttle to see if you can hear audible knock (don't just lay into WOT right off the bat). Drive with the windows closed so you don't get a lot wind and road noise. If you can't hear audible knock it most likely would be some sort of false knock condition.

C4ProjectCar

Thanks for looking at my log.

I've wondered about false knock for years, because I've been getting a ridiculous amount of knock counts at low RPMs and any load. I've even seen knock counts ticking upward as I sat idling in the driveway after a datalogging run. I do have roller rockers and headers, which both I think can be contributing factors to false knock counts.

Looking back to a datalog I took about a month ago (attached), I still saw a bit of KR at WOT. Doesn't seem to be as much though. I wonder if it would be worth adding in some PE fuel to see if that helps?


I haven't touched knock retard settings in the BIN, so I'm sure everything is still stock. The thought of disabling knock retard makes me reaaaally nervous though. What if a particularly hot day rolls around, I step on it, and I get bad knock?
Like I said, I've never definitively heard knock, so I'm not even sure what to listen for. Would I definitely know it when I hear it, even over fairly loud exhaust?

If it's false knock at WOT, adding timing shouldn't change the amount of knock retard being applied, right? So what if I add a couple degrees, go for a drive, and compare knock retard values to a previous datalog?

Edit: Just realized I put the wrong numbers down for my compression ratio. It's really 9.3:1 static and 8.4:1 dynamic. So it really shouldn't have trouble on 93 octane.