is my intake too small? jagged map signal

[ChevelleFan]

All,

When I look at the datalogs of my drag strip passes, the map signal at WOT is very jagged and seems to be lower than atmospheric pressure. On every single log I've looked at, map seems to peak around 4300-4400 rpm, then comes down a few points. Dragstrip elevation is ~ 1200ft, which is usually 96-97kpa (ignore the dips in TPS/RPM at the beginning, I had to pedal the wheelspin)

My guess it that my intake is undersized for this engine. It's a Holley Stealthram for Vortec heads (170cc intake runner), which I'm running on GM Fastburn heads (210cc intake runner). Just trying to figure a more certain diagnosis before I drop some $$$ on a new intake setup.

Attached a few logs and MSQ.

Thanks,

-Dave

[dontz125]

Why do you say it's below atmospheric? You said this track is normally 96-97 kPa, and your MAP signal is 99.5 kPa. You may actually be seeing a little resonant / velocity boosting.

[ChevelleFan]

I hear what you're saying, and at points, it does go up to 99, but at other points, its down at 90. I guess I'm trying to understand what it means and if it is hurting my WOT performance. It's a tunnel-ram style intake. with a plenum on top. I have the largest throttle-body that they make for it (twin 58mm), with a filter right on the end of the throttle body.

-Dave

[dontz125]

I think I see what you're saying - you should be seeing MORE ram effect?

Your comment about the filter being right on the TB is tickling at my brain; I know the older FZR motorcycles *hated* pod filters mounted right on the carbs. You had to space the filters off ~ 2" with PVC pipe to get proper part-throttle carburetion. I don't know if the same thing might be happening here.

[whittlebeast]

Having a couple of KPA drop at WOT and high RPM is very common. You may want to add a little filtering to the MAP signal. My VTec Honda drops about 2 KPA when the big cam timing comes on line.

Look just to the left of the blue center line in this screen shot.

http://www.nbs-stl.com/CrxTypeR/Throttl ... ration.png

Andy

[ChevelleFan]

Thanks Andy. Just looked at my MSQ and the MAP Sample Setting was set to Use Timed Min instead of Use Event Avg. I don't ever remember changing that. According to the help, Use Event Avg is recommended for most installs. I'm going to try that tonight and record a new datalog.

-Dave

[panel]

Dave , I asked a similar question here the other day. Not sure if the answers there might help you out but.....

http://www.msextra.com/forums/viewtopic ... 34&t=66176

[Six_Shooter]

If you see something similar after changing the MAP filtering, try a run without the filter and see what the signal looks like.

[ChevelleFan]

So I changed the setting to Use Event Avg and made a quick blast. My "MAP Averaging Lag Factor" under General Settings is set to 50%. (unchanged). Tonight's datalog looks slightly better (by better, I mean not quite as jagged), but it still isn't as smooth as I thought it would be. Atmosphere is 96kpa tonight, my WOT run varied from 91-94 kpa. Didn't see Six Shooter's note about removing the filter until after I got back. I'll try that tomorrow.

I can say this though, last year I tried to make a cold-air pipe, so that my intake wasn't drawing air off the radiator, but from off to the side. Made it out of 3.5" pipe. Tested it at the dragstrip back to back with the filter on the end of the TB that I have now, and the cold air pipe went 107.5 MPH in the quarter, the filter on the end of the TB went 109. I kept the TB filter.

The whole reason I'm looking at this is I don't think my car runs enough MPH. It's a ZZ383 crate engine from GM, otherwise stock aside from the injection. That 109 MPH pass I mentioned was in *perfect* air, -69ft density altitude that day. I would've though I could get 110-111. Yes, it's a small difference, I know.

Thanks,

-Dave