Chassis dyno results, tuning advice

[chilly460]

Ran the car today on a DynoJet just to get a baseline.  Basically, I'm not really happy with the way the car pulls but I've been playing with the tune and can't really seem to get it much better.  Last night I pulled 2* of timing out of it to be safe and hit the dyno this morning. 

390ci
Holley 750dp
Performer RPM
Edelbrock heads with light bowl cleanup
Comp 270H 224*/.519" on a 110LSA, installed at 106
9.5:1
FPA 1 3/4" TriYs



So, it peaked at 302hp at 5400, 369ftlbs at 3500.  As a reference, made 280hp with 3* less timing in it, and sounded and drove a ton lazier so big difference.

Numbers back up how it feels in the car, it's a torquer around town, kind of surprised it made peak power at 5400 as it feels flat after 5000rpm. 

Flywheel numbers, which I'll estimate since I think guys are more familiar, would be 380hp/460ft lbs using 20% loss. I figure my setup, optimally, would make 400hp on an engine dyno, throw exhaust on it I guess this isn't too bad.  Not sure I'd believe 460ftlbs as I don't think any similar builds come close to that number. 

If you read the AFR chart though (the dotted red line is 12.5:1), car is tuned fairly well on the hit, but then goes dead fat until 4200rpm or so.  That's about how it feels if you roll into it in say 3rd gear and it has some gear to work against....very soggy.  The peak A/F ratio at peak is perfect so not sure I'd mess with the jets, just not sure how to lean it out before that point?  I don't think a lower power valve would make much difference as this is WOT and I'd think vacuum drops immediately.  Any ideas?

[My427stang]

I answered on the FE Forum, but I think you have WAY too much pump shot.  I think the sensor was out of limits and you bathed it hard and it took a lot to clean up.

Which pump cam and shooters, and is it a 50 cc rear pump?

[ScotiaFE]

Have you thought about switching to a VS carb?
Rolls in to the throttle better and all around better carb for "street use".
I think the VS with the modest 390 may be the way to go.

[chilly460]

Just 30cc front and rear, pink cams.  Believe .031 shooters front and rear.  Think I'll try a .028 front?

If punched in first or second it'll take off clean with the deep gear in the TKO, but feels lazy in third. 

[ScotiaFE]

So I looked up the TKO.
It list's the 1.34 and 1.28 3rd.
The 1.34 would be the best split but that only comes in the 500?
By the time you have shifted to 3rd you have pumped "double pumped actually that's what a DP is " the
carb three times in about 4 seconds.
That's a lot of fuel not burning.
The VS will "roll into" 3rd with less fuel but more likely a better burn.
You can slow the DP down quite a bit, but really is that what you want.
A lazy DP?
Me think's it's just a tad too much carb for the cam and purpose. 

Not mention, you have overdrive. The only purpose of that is to save fuel.
And the VS will save way more fuel than the overdrive.

[chilly460]

Well, the 5spd was really to allow me to cruise at 70mph without going deaf, but I know what you're saying.  I think my course, right now, is lean out the pump shot and see what happens.  If I can't get it tuned, I'd look into a VS. 

My rationale is 1. Laziness 2. Expense.  If I go VS, I'll have to tune it all over again, and drop additional cash on it when I have a pretty good working carb as is. 

[Autoholic]

What gear were you in for this dyno info? I think if you were to record the AFR from a pull in 1st, 2nd, and 3rd... that would give you some useful info to look at. If 1st and 2nd are pulling really strong, but in 3rd it feels like it just falls on its face, maybe it's a bad gear combination with the final gear ratio. An engine doesn't completely change behavior just because you shifted into another gear. If you're seeing hesitation in 3rd gear and it has to do with your carb setup, I'd be looking for that hesitation in every gear. If the carb is flooding the engine above 3,000 RPM, that means the high speed circuit is too rich, which would impact how the engine operates regardless of gear. Same if the carb is flooding the engine below 3,000 RPM, except that this would be the transition circuit.

If you post what TKO you have, your final drive gear ratio and the exact size of your rear tires (###/## R 1#) , I can put that info into an excel and share the results. It might reveal something about your shift changes that could be useful. If the shift dropped you into a bad spot on the power curve, that would make the vehicle feel sluggish. And looking at your torque curve, you do have a not-so-ideal area where it drops off a cliff. It looks like your transition range goes dead rich, which makes me think your low speed or transition circuit isn't setup well.

[chilly460]

It was in 4th for the pull, I think 1:1 is pretty standard for chassis pulls.  It has a short 3:27 first gear, with second also being fairly short, so I think its a matter of it pulling quickly through the rpm band so I don't feel the laziness in that 2500-4200rpm band where it's so rich.  Carb isn't rich above 4200, it pretty much falls into perfect AFR. 

TKO 500 is 3.27/1.98/1.34 with 4.10 gear and 28" tires, bit of a jump to third but with the torque curve being so low, I don't think it's any kind of gear issue.  It actually feels pretty good if banging through the gears as it doesn't dip into that rich region, more rolling into the throttle at say 2700rpm.  I think the way I stated it above it made it seem like I was saying it was lazy when I was shifting up through the gears, when I meant pulling from lower rpm in third. 

I do want to get that big dip in the torque curve out, it's losing a ton down low obviously.  Right now even with a peg leg rear it's not too badly "traction limited" unless you wind it up a bit and drop the clutch, I'm thinking it should be able to spin the tires from a roll if the carb was tuned better. 

[Autoholic]

Your main circuit looks to be fine, it's the transition circuit that's killing your engine. It shows in the AFR table, as it levels out to a good AFR around 3,000 RPM which is when the main circuit takes over. Which could be the accelerator pump, but it could also be the size of the idle jet.

I can't generate the transmission data without the exact size of the tire. It's the way I've designed the excel sheet, it takes all guess work out of the setup. It's very useful to compare different tire sizes or different final drives. I shared it on here...

http://fepower.net/simplemachinesforum/index.php?topic=3411.msg34983#msg34983

[chilly460]

275/60/15"

[Autoholic]

Attached is what your tires and gears say about your setup...

It shows that after you shift from 1st to 2nd, your RPM's drop from 5300 to 3210. And from 2nd to 3rd, 3590. And from 3rd to 4th, 3950. So not terrible spots on the power curve after the shift, if your shift point is 5300 RPM's. Cruising at 75 might still be a little loud.

It won't help you understand the car's acceleration, as that is a matter of torque. But it can help with understanding the gears and tires.

[Barry_R]

If that is one of those "clip it into the tailpipe" O2 sensors I would not give it much credence.
They are pretty inaccurate.