Big dip in airflow from 6200-6800 and same for power band

[Turbo 707]

My ITR has a huge dip in the power band from 6200-6800 and on the Hondata same for air flow. Dip was not there before . Anybody else have this same problem or can give there thoughts to why this has happened?

parts that were changed
Went from a ITR head with skunk valves and springs to a decked and ported Head with supertech valves , valve springs , and I went from a DC 4-1 Header to a HItech style Tri-Y-header, and went from a 2 1/2 header back exhaust to a full 3inch . I used the Same cams but the cams were taken out and put into the new head. They were not degreed since the motor stayed in the vehicle but they were optimized on the dyno . With these changes I went from 176 peak whp to 205-208whp both on 91octane . Peak gain of 32whp but I gained a Hp and torque dip from 6200-6800 also loss of air flow coincides at the same RPMs on the Hondata airflow map.


Engine components 98 ITR
CTR Pistons
ITR Rods
ITR crank
Head is decked & ported and polished
Supertech valves , springs and retainers
Skunk 2 tuner stage 2 camshaft
Cam gears both at +1.5 degrees

Bolt on components
Skunk2 74mm throttle body
Skunk2 ultra street manifold
Alpha 550 Injectors fuel
DTR tri-Y- header
3inch header back Skunk2 mega RR exhaust
3inch inside diameter cold air intake
Skunk2 cam gears
Walbro fuel pump
Aem rail and regulator

[coolhandluke]

Can you further describe the the dip?

Isolating out randomness in my question.

...and I went from a DC 4-1 Header to a HItech style Tri-Y-header, and went from a 2 1/2 header back exhaust to a full 3inch...

New or used header? I ask to make sure there aren't any dents, leaks, etc. I've seen all sorts of failures in new and used, high end and low end, etc.

...I used the Same cams but the cams were taken out and put into the new head. They were not degreed since the motor stayed in the vehicle but they were optimized on the dyno. With these changes I went from 176 peak whp to 205-208whp both on 91octane . Peak gain of 32whp but I gained a Hp and torque dip from 6200-6800 also loss of air flow coincides at the same RPMs on the Hondata airflow map...

What was the feedback from the tuner? I have a question in my head but I'm curious what a subject matter expert shared first. What was the final compression rato?


I'll see if I can tap some more intelligent folks to chime in with tuning experience from our community.

[Turbo 707]

So the dip was never there before but I changed 3 things at once so I don’t know what is causing the dip. So before when the vtec hit it hit harder and pulled hard to redline but the DC header and 2 1/2 exhaust was choking it and was limiting peak power potential . Now when the vtec hits the power comes a little more gradual and then for a split second or 800-1000 it has a dead spot then it pulls like normal. It is something you wouldn’t notice if you wouldn’t have drove it before the parts changes. The cams are same as before but when they were put in the different head they were not degreed with a wheel . The tuner has both cam gears at +1.5 which is +3 degrees on crank timing.

So the header is new in perfect condition no dents and so is the 3inch header back exhaust.

To me it’s got to be either the came timing is a little off . Or the head may have a velocity issue from being ported and polished cause some times flowing a bunch can loose speed . I know that decking the head can change what the right cam timing is by a little also!

And last but not least maybe after the vtec crakes open all exhaust gases increase and then bottle neck at the collector causing a wave of back pressure for a .100 of second and changes the volumetric efficiency, intern, causing less airflow into the engine until the exhaust gases are over come with enough pressure and wave reversion is gone Letting everything continue to flow like normal . The thing is It is a TRI y header that scavenges and out flows most headers out there so that’s why it’s last on my list.

Maybe it’s something else??? I was hoping someone in the Tuning community would have seen this before when tuning with Skunk 2 Stage 2 tuners since there a popular cam and be like it’s your cam gear timing or something.

Thanks in advance
For any help or input

[not_a_k]

What RPM is crossover set currently? Since you describe it as happening just after the crossover, it would be interesting to see overlaid graphs of the low cam stretched out past the “dip” RPM and the high cam engaged well before the dip.
My line of thinking is that a crossover slightly too low would result in a flat feeling before the motor could utilize the big lobes, but testing the point moved as I described would also isolate out your theory about crossover itself causing some reversion effect in the header.