Bad tune, or engine at its limit?

**90Vette** · 09-22-2018

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 a pretty respectable 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, gasket-matched 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

I've never had it on a dyno, and I've done 100% of the tuning. It would have been cheaper and easier to pay to have it dyno-tuned, but I wanted to learn to do it myself. I finished the header install just hours before I went to the strip, so I didn't have as long as I would have liked to nail down the tune. That said, the timing shouldn't have needed much tweaking, and by the end of my tweaking my wideband was showing lambdas of around 0.85-0.88 (12.5-13:1 AFR) at WOT.

While it was a blast at the strip, my highest trap speed was just 106.23 MPH. 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. Based on what I've read, that seems pretty weak for a built 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.

Do you think my combo just at its limit, or does my tune need work?

Here are my relevant tables: spark advance, VE lower, VE upper, and PE enrichment and spark advance (if you're not familiar, it adds in the fuel and timing in those tables when I step on it). I've also attached a datalog (not from the strip, but it has some WOT) if you feel so inclined to look at that.
SR34 Timing.PNG SR34 VE Lower.PNG SR34 VE Upper.PNG SR34 PE.PNG

**woody80z28** · 09-22-2018

I had a Vortec-headed 355 in my Camaro that trapped the same. Started at 101 and I tuned it to 106. 3300lbs race weight and I figured about 400hp on it. I was less cubes with more compression and more cam (10.55:1 and 230@.050). Seems possible that the extra cam & compression on mine offset the cubes and heads on yours.

**Dirtybob** · 09-22-2018

do you know what the density altitude was? (it can be approximated from the time/location of your pass)
could you post the timeslip info?
400 hp isn't necessarily weak considering that mild cam....
assuming you were into the low/mid 12s with that mph?
how many rpm at the shift (you need to rev to make big hp #s)?

**90Vette** · 09-22-2018

Looks like the density altitude was 1,697 feet. I'm new to drag racing, but that doesn't sound too bad. According to another calculator I found, with that DA my HP should be 100.5% of rated HP.
I was actually at 13.4. Granted, that was my first time going to the strip, but on that run I felt like I had a good launch and fast shifts. I'm running street tires and wasn't launching too hard, so that might contribute to the slow ET. Honestly, I was mostly shifting just by ear and feel, but I think I was shifting around 5200 rpm. It's hard to say because my tach is a bit off.

By the time I figured out the basics, bracket racing had started, so I didn't want to mess with my launch RPM or shift point (no regrets, I somehow ended up second). From what I've read though, 2000 rpm is a decent launch for street tires, and 5200 is right about where a Super-Ram 383 with a small cam should be shifting. Am I off on that?

R/T	.064
60'	2.104
330	5.792
660	8.758
MPH	83.66
1000	11.314
E.T.	13.436
MPH	106.23

**Dirtybob** · 09-22-2018

I don't think a 1700' DA equates to "100.5% of rated HP". I would expect it to calculate out at around 96%. Are you sure which barometric reading you used (corrected vs. uncorrected)?
Cant speak to the power curve of your motor but you may be able to squeeze more rpm out of it (good heads & intake might carry the hp past peak for a while), try raising the shift points and see what happens.
60' times are abysmal unless you're running hockey puck (aka all season) street radials. Get some better rubber and/or practice your launch. You will probably find that your mph will drop (but ET will be much better) with improved 60' times. I would be looking for 1.8 or better 60'.
When using ET/mph to calc hp, just remember that the numbers are really only good for comparison. Say you put gears/stall converter/drag shocks/slicks on your car and it picks up 7 tenths and loses 2 mph? HP did not change but your numbers did...

**90Vette** · 09-22-2018

I pulled that number from this calculator, which appears to use an SAE standard. Corrected for what? Temperature? The linked calculator takes temperature, altitude, barometric pressure, and humidity into account, so I assume it's making corrections.

I was under the impression that even the old AFR heads are pretty good? And unfortunately the Super-Ram is my only option for a smog-legal intake. Next time I hit the strip I'll try some runs with a bit higher shift points.

I know I suck at launching, I guess I was just hoping to have a higher trap speed with all the work I've done. But my spark and fuel curves look decent to you?