EFI Tuning Question (Big Blocks, Blowers, Flat Spots at ~3K RPM)

[RSX302]

I have tuned a couple of blown big block chevy's (GM572 w/whipple and a BBC w/Procharger) and both had mild flat spots around 3-3.5k rpm while cruising part throttle. Idle to 3k rpm perfect..mild accel thru 3-3.5K 'bobble/flat spot' and then will smooth back out the rest of the way up. Land on it and it will power right thru without notice. I've tried cuising A/F ratios between 12:1 & 14:1 along with timing from 20-40 degs and tip in fuel +/-. All other drivability and power is great except for the lousy mild bobble/flat spot.

Nothing seems to affect and we are thinking its the big block cams and blower combo? The 572 cam is factory and I'm sure if milder cam was installed, that would help it out? I need to look again at the manifold pressure to see exactly where it starts. 5"hg, 0psi? We are good on the boost side.

One system was a Fast XFI and the other Accel Gen 7.

Any input is greatly appriciated.

[RSX302]

Found it...

I could swear I tried lower timing. Guess not on the hemi. DOH..

3500 rpm off load. Full vac..wanted 30deg not 38 like I normally use to starting with. Cleaned up nicely at 30. Who knew. 450ci hemi head like much less timing. Live and learn. My first hemi tune.

Now I need to try on the blown GM572. Maybe that's the one I tried lowering to 30 with no response.

[Derekk]

Less timing equates to a more efficient chamber/cylinder head. I've seen mod motors, and LS engines make peak power with as little as 26* in N/A form.

[RSX302]

Less timing equates to a more efficient chamber/cylinder head. I've seen mod motors, and LS engines make peak power with as little as 26* in N/A form.

Wow..26 on a n/a? Higher compression 12:1?

My previous twin turbo destroked Ford 308 made peak power at 24deg. Which is where I start all my blown tunes at WOT and work from there. I was running 38 deg at cruise with 15:1 a/f getting close to 30 mpg. That was sweet.

I've tuned a few dozen engines now and all liked mid to upper 30's during full vacuum cruise. Since this was my first hemi tune, it threw me a little with the 30 deg cruise. Unless the initial timing is off on the hemi by 5 degs from the engine builder? Always a possibility to check. Dutweiler built so I didn't check and sure it's ok. Kenny tuned WOT, I tuned everywhere else. Heard back from the owner and he said that it's never run better, so I guess I did something right.

My current twin turbo 370 LQ9 is running 35deg during cruise and 25degs WOT. I've run up to 27deg on the Dyno and picked up another 20hp, but I chose to leave at 25 for safety since running 87oct. Still tuning during cruise and testing gas mileage. Started New engine during breaking around 10-15 mpg. Now that it's got some miles and more tuning, I think I'm around 15-20.

[Derekk]

The engine I was referring to was a Stock LS1 Short block, cam only, shaved heads. It made more peak power @ 26*. The previous tuner had 38* in it, and at the 1/8 mile the car went lazy as hell. Pulled the timing out, and it picked up 2 tenths. In my position with Holley, I get to speak with several tuners, and engine builders. I've learned a lot over the last 15 months.

[RSX302]

Hmmm..interesting. I'll need to remember that if and when I start getting into N/A LS tunes. A friend I know has been playing with HPTuner on an aftermarket N/A LS6/AFR setup. He swears 37deg gives him more power. I keep telling him he needs to get it on the dyno. One thing I've learned is every motor is different and you need to give it what it wants.

Thx for the chat.

[Derekk]

One thing I've learned is every motor is different and you need to give it what it wants.

This is very true. Your welcome on the chat