Shape of Acceleration Curve
 

Howdy fellas. Now that I have my MT installed, I've noticed something that kind of disappoints me. As much as I want to have nothing but good things to say about the car, I gotta bring this up ;)

How does everybody's acceleration feel above the power peak @5800 RPM? What I mean is, does your engine keep pulling strong to redline, or does it feel like it is struggling up over 6k RPM?

I've always noticed some struggling at high RPM, but with the 5MT it seems more pronounced. I get a nice strong accel up to about 6kRPM, then it seems to level off. This all makes sense, of course, looking at the SVX TQ/powerband. But, on your SVXen, is it very noticeable?

Can this be overcome with the MAF mods (either MAF bypass or Stage 1), or can it only be addressed via cams? Ideally I'd like to be able to spin up to Stage 1's 7400RPM redline without feeling like the engine is straining so much.

This is more of a discussion thread, I hope, rather than a "fix my problem" thread. So, let the information flow! :)

Mat

Get LAN's stage II chip and z32 MAF....your engine is having problems breathing.

Mat, I have noticed the same thing that you have noticed. I hardly ever (maybe a total of 4 or 5 times) take the engine above 5,500rpms, but I have noticed that the few times I have, the power just drops off into nothing... It pulls very strong below 5,500rpms though... Perfectly fine for me... Except in AutoXing when I might want alittle more speed before I shift... :rolleyes:... Guess I'll have to think about the STi Ra Gearset if I want to do that...

I am curious about the LAN Stage 1... interested in exactly what that could do for me on the AutoX course... Unfortunantly I am broke as a bad joke... So there will be no buying of new things for my SVX anytime soon :(:(

Mine seems to accelerate linearly up to about 6500 after which anywhere from 6750 to 7200 I believe I'm maxing out the MAF with Stage I only and it feels like I'm hitting the rev limiter.

I have the STi non RA gearset in my MT and the rev range is good for around town driving. Especially good is the 5-3 downshift for passing at highway speeds. I'm not sure if 3rd is different from regular WRX trans or not...I think it is.

I'm planning to do the MAF bypass mod to see if that helps out at all. I'll let you know my results. I was also perusing through the thread about using dual runner intake with a single MAF and some sort of signal multiplier coming off the MAF. Sounds interesting. I'm curious as to how the ECU would handle voltages higher than the max MAF voltage. Can it handle that at all without Stage 2 programming?

I'd like to buy stage 2, but it's just so expensive. We'll see. I can get a Z32 MAF for a good price, so I may have to do it.

By the way, Earthworm, I'm jealous that yours pulls strong to 6500:mad: :p One day...yes...one day mine will also pull strong to redline.

Anybody else...how's the 6000 RPM + range feel?

Just thought of something else...

In my sound clip I just made, you can hear where the power starts to drop off. From launch to ~6k RPM, there is a strong sound to the engine/exhaust, but above that it actually becomes almost silent. Not like the sound goes away, but instead of a rasp or growl like what is heard through the rest of the clip, it's just like a loud white noise. Not sure if thats indicitive of anything. Give it a listen if you need clarification. Is that a symptom of some sort of breathing difficulty or timing "issue"?

http://www.subaru-svx.net/photos/user.php?SVX10

It's the file with the long, descriptive name :)

Where did the top end power go?
 

Mat I think you have answered your own question, that you said here:

While I had my car apart for the 5MT install, I fiddled with the exhaust a little bit. In some of the exhaust related discussions we've had here, there was some discussion as to whether or not there would be any benifit to gutting the pre-cats and using them as an expansion chamber. I never saw any truly conclusive results from those forums, so I did a little experiment

I think the expansion chambers that you replaced the front cats with, have killed the sonic action of the header pipes. The wave is being terminated at these chambers, which have shortened the effective tuned length of the header pipes. This would raise the resonate peak up out of your rev range, so the torque that it provides would disappear.

Harvey.;)

Mine pulls strongly up to 7000 rpm. Have even hit the rev limiter at 7200 a couple fo times on the track.

Thanks Harvey. I had considered that. In actuality, I've been without the cats for a while. I had them cut out about 3 years ago, but the guy who did it did a horrible job putting the pipe back in. All kinds of pipe overlap at the joints, so there were some pretty serious restrictions there. Thus, I figured I might as well give it a try :)

I can't say whether or not I noticed a difference from when I had the cats to when I had them removed.

So, in reality, I guess I've kind of done the opposite of what has been said to be effective for the SVX (moving the cat/y-pipe/1st expansion chamber toward the rear of the car).

Would that also explain the exhaust sound at high RPM?

One more question for you, Harvey. In other exhaust threads I read on this forum, you and Mychailo sort of concluded that the 3rd cat and resonator were both in locations that would provide torque peaks based upon sonic tuning/pressure tuning. Is that based upon the assumption that the cat is expansion for the sonic waves while the resonator is expansion for the actual pulse of air? In doing my expansion chamber project I thought I would be creating a new torque peak for high RPM, but it sounds like I just replaced the OE sweet spot into a range where I can no longer access it.

Depending on the distance to the chambers, what about 22" or so? the resonate peak would be around 14000rpm, a bit high.:)

Yes I believe that the front end of the 3rd cat is the volume that reflects the sonic wave back to the cylinder, and the resonator is the point where the high pressure gas plug is expanded.

The two different effects help at two different parts of the cycle. The sonic resonate effect, produces the low pressure in the cylinder at the end of the exhaust stroke towards TDC, and the high pressure gas plug expanding at the resonator, allows the inertia of the flow to drop the pressure in the cylinder just after BDC.
With the expansion chambers up close to the engine, both effects are terminated there. So the resonate peak is too high, and the Inertia run is too short.

The intensity of the sound, is a measure of the gas pressure that is being released from the cylinder, listening to the sound is always a good indication of VE.

Harvey.;)

high intensity = high volumetric efficiency? Thus quietness at high RPM = low VE?

Yep thats the way I see it.

Harvey.;)

My car was the same as yours, and I was also a bit disappointed. However, the specs on the SVX say that peak power is at 5500 rpm, so perhaps it shouldn't be so surprising that power drops off after 5500 rpm.

I've tried a bunch of different exhaust configurations, and none of them ever made a significant change in the position of the HP peak.

Assuming that you want the reflected exhaust pulse to come back to the exhaust valve at about 180-200 deg after the exhaust valve opens, then for sonic wave scavenging at 5500 rpm, a reflector is needed at about 54-60" from the exhaust valve (if the speed of sound is about 1500 fps). This is the location of the factory 3rd cat on a US SVX. If you look at my last exhaust system before I sold my SVX (see "custom exhaust 2004" in my locker), I ran single 1.75" ID pipes to that location and then put two 2.5" ID tube mufflers at that location. If anything was going to improve scavenging at 5500-6000 rpm, that would have done it, but you can see that the HP/TQ really isn't much different than any of my other 5MT HP/TQ curves.

IMHO, the 5500 rpm peak power with a stock motor is controlled more by the cams and not by the MAF maxing out. If you look at a few of my LM-1 data logs in my locker (see tuning data) where I measured the MAF voltage with the MAF bypass installed, first you'll see that the MAF voltage never reaches the 4.8 V limit that the MAF can produce, so the MAF is not maxing out. Second, you'll see that the voltage reachs its peak at about 5500 rpm which is right where peak power is made.

I think that if you want more high rpm power, cams are needed. However, the MAF is not far from maxing out, so either a MAF bypass or Stage II MAF is needed to realize the effect of the cams. With that said, I don't have an explanation for why other people's cars pull strong after 5500 rpm. I'd like to see dyno curves, MAF signal voltages, and AFR values for those cars. It would be useful in making sense of everything.

Yea, that's good info. I've been thinking alot about cams and going through alot of what Tom has been sorting through in his thread. I'm trying to figure out the combination of parts (cams, valve springs, exhaust, etc.) that will allow for better high RPM performance without destroying low end. As part of my job I get to drive alot of the BMW 330ci alot, and occasionally the E46 M3. Both of those pull so smoothly to redline...it's addicting. Sure, they have MUCH more advanced cam/valve technology, but the E36 M3 didn't and it was a sweet motor also. The H6 is sweet as well, but I want to improve it :D :D

With that in mind, I had contemplated a single or dual butterfly valve system that would allow for 2 different flow paths. I was thinking the valve could be activated when the IRIS system was activated. In theory that would allow (with cams) for low end comparable to stock, but much improved high end. Cost isn't a big factor, but I want to have some high probability of satisfactory return on investment. Already found butterfly valves that will work. Now I just gotta pin down the cam specs, whether or not I'd need valve springs, and how to layout the 2 flow paths in the exhaust.

Mat

My first mods were the Ecutune Stage 1 chip and the exhaust system. I remember that the chip made an enormous difference. When I took it to the track, and put it in 2, it stays in 2 until you hit 6400 rpm, no matter the position of the throttle pedal, it also seemed very willing to pull all the way to tranny redline. Felt no inclination to try to shift up before it did so automatically. I would also pull out onto Route 1 in Princeton, and floor it, it would go up through first, second, and then hit 3rd, about 80+ mph before the next stop light, shifting at about 6400 rpm. Seemed to me, it pulled all the way. Now with the cams, the Stage 2 chip and MAF, and the 5 speeed, it pulls solidly all the way up to 7000.