Grant B

My recently-purchased race car:
1990 NA Miata
2001 F20C engine and transmission
4.3 rear end
shorty "header" with a full exhaust (no cat and a Magnaflow straight-through)
15x9 6ULs with 225/45/15 RA1s

There is definitely something odd going on around and before the VTec engagement point. Anyone have any ideas what that is? Looking at the AF and the weirdness around VTec, I'm guessing the car might have some electrical gremlins to sort out.

SAE correction:

The first (blue) run was done in 4th (to 100 mph), and the second (red) run was done in 5th (to 120 mph). The car wasn't cold for either pull, having just been driven to the dyno.

I tried connecting the VAFC that came with the car, but found it wouldn't start with the harness in there between the ECU and AFC. I didn't have time to figure out why so I just left it disconnected.

I'm hoping to be able to get the car into the 205whp range for running in NASA PTA/TTA. I'm going to try to adapt a S2000 header in it, with a low-profile Spintech race muffler and short side-exit exhaust. If I eventually decide to run ST2/TTS I'll shoot for 230whp (milled heads, adjustable cam gears, maybe cams, etc).

Thanks for any help.

starchland

weather temps?

zeroptzero

looks like it is switching into vtec too early at 5k, the power dies off and then picks up at the appropriate spot around 6k. Can you detect the vtec changing around 5k, just my guess

bgoetz

Where are you seeing that? I am clearly seeing it switching over right where it should be. It just does not seem like it is pulling hard afterwards, or before for that matter. That huge dip at 5k is kind of weird. I would start with trying to figure out what was going on with the VAFC and if one of he sensor readings are not comprimised without it. If everything seems right then maybe check the compression because something is not right. Did you guys use a wideban off of the back of the exhaust or the O2 on the car?

Grant B

The weather was 80F, not sure on the humidity or baro though I can look it up. Those numbers are SAE corrected, though.

The VAFC was completely disconnected. As I understand it, autocrossers shoot for a VTec engagement point of around 4k, which is what I was going to try to set mine to if my VAFC worked.

The wideband was a Dynojet piece off the back of the car.

I'll try and get a scan tool to see if the sensors are reporting correctly. When the VAFC harness was plugged in they seemed to show valid numbers, the car just wouldn't start.

The dip at 5k is weird, and the loss of <5k power on the second (red) pull is also weird. Compression loss would show low power across the board, not just at these specific points. Power was a bit low everywhere but the car was also lean everywhere. I'm not familiar with how Honda ECUs deal with knock, but I wouldn't be surprised if timing was being pulled. Still, I've never seen knock correction cause a dip like the above at 5k.

TerminatioN

Grant, its cool to finally seem some numbers from the car.

That is indeed a very odd dip though between 5k and 6k though, but I'm sure you'll have it sorted out in no time.

Was it noticeable when driving on the track? Or were the dyno readings the first sign of this?

-Will

zeroptzero

well I initially thought it was a mechanical system changing at 5200 rpms as it looks like someone flicked a switch at that rpm, sometimes if you have a big cam swtiching over too early it will kill power like that and then pick up at a higher rpm. But i f you have a stock pcm, stock cams, stock cam timing, and stock vtec engagement point on this dyno run, then I would suspect the header design, where you get a bad resonance in the system and it messes with the power over a certain rpm range. My other suggestion would be a poorly designed intake system, but I think it is more likely to be your header. You refer to it simply as a shorty header, any other information on it ?. I've seen this exact effect before when we installed a high rpm intake manifold and high rpm header on a b-series engine, it just messed with the airflow over a certain rpm range and then it picked up in the higher rpms.

9KRDLIN

get a competant tuner to set up vtec. he will make a pull on the small cam, then on the big, and where the big starts making power, he will set vtec here. i have seen some cars like it low, and similar cars like it high. just need a good tuner and a dyno. looks like you need the tune fast, as it doesn't seem all that great. sure it will run and drive, but a tune will make it tons better.

Elistan

The profile of the red trace is 100% normal for an S2000, albeit slightly lower on the peak end. My initial thought is that the odd shape of the blue trace is an artifact of the dyno itself, not something going on with your engine.

Your max torque, 130 to 133, it's absolutely normal for a 200 rwhp F20C engine. You're just loosing some on the top end for that "peak" reading. I wouldn't be surprised if one of the I/H/E setup is all that's involved there.

And you won't be able to get the VTEC transition down to 4000 rpm with the VAFC. The VAFC does fuel trim only, and that's not enough to support a VTEC transition below about 5500 or 5600.

hondaphreak

the vafc doesnt mess with the fuel at all. it only tricks the ecu into sending more or less are to lean it out or make it richer. depending on what the tuner sets it at. plus vafc can allow you to set vtec as low as 4000 rpms in increments of 100 rpms.