John

When I increased my exhaust to straight through I lost power....initially.
I had to faf about with fueling and boost pressures. Eventually I gained power, so I'm with you on the reducing backpressure.

I guess there are exceptions to every rule.

I dug out the correction values that the ECUs use if the boost is over the target value so you can see what I mean about running excessive peaks on the PPP.

Compensation duty cycles which are subtracted if the boost value (as seen by the MAP sensor) is over target (15.7 PSI in midrange for PPP, 13.7 PSI for AE800-802):

3.0 PSI 10%-12%
1.7 PSI 4%
0.8 PSI 1-2%
0.3 PSI up to 1%

The result is that if the boost goes to 1.7 PSI over target (ie 17.4 PSI peak) then there is a rather heavy handed 4% drop in duty cycle (this is quite noticeable) to sort it out and it drives horribly. The PPP, standard and Ecutek ECUs all use these values (as in the 0.3, 0.8, 1.7 and 3 PSI offsets from their held boost targets), although they can be altered. It is nice to have a car not spiking appreciably above target at all in the summer so that in the winter (or if you go and drive it really hard) if it spikes by 1 PSI it is all smoothy controlled. The further you get away from target the more heavy handed the correction is so that these cars on factory boost control nearly always manage to avoid full blown fuel cut in the event of overboost. The 3 PSI error usually results in swings of boost from about 10-18 PSI about twice a second as the response is so heavy handed it leads to instability and oscillation.

The closed loop control is also the reason when you setup a fuel cut lifter using the standard boost control that you let it see the boost target it has - so if you want to run 18 PSI you make it see 13.7 PSI on the standard ECU when it really runs 18 PSI, and then it controls it nicely around 18 PSI for you.

[Edited by john banks - 6/28/2002 6:41:33 PM]

John
the biggest change you made to your exhaust was the Magnex downpipe. This removed a restriction that we all know about. After this restriction is removed, then the rest of the exhaust is by far the restrictive part and is best replaced as soon as possible with a straight through design to allow better scavenging in the cylinder and therefore more potential in the cylinder for increased peak pressure and better combustion.

I think Andy's comments are more appropriate to what I have found than any other. i.e. with the SS BB on my car peak and held boost decreased, probably due to better boost control. If this boost had then been readjusted to what I am getting currently then obviously I would generate more power.

I would say the PPP back box is doing what it is designed to,by giving increased low down torque and high end BHP. The whoosh that comes from it suggests that some kind of scavenging is taking place at this point and the increased boost pressure may substantiate that. After all if my MAP sensor was reading incorrectly then you would think that levels of boost would be the same no matter what exhaust I had on.

I am currently a bit restricted by my warranty, and I know if I put a SS downpipe on I will be on stony ground. However, that is definitely my next step, and once this is done I would run the full SS system and drop the PPP BB.

Beastie :D

SCO
you ask about the Cone effect. I really do not know what this is, (I was hoping someone on here might know). However, when I asked the dealer fitting the PPP to put my SS BB on instead of the PPP one at time of fitting. He would not do it, and said I would lose the cone effect. After having the PPP backbox on and trying it but finding it a bit quiet, I put my SS on. It was then that I noticed something had radically changed.

I can only think that the cone effect refers to some change in the gas flow caused by the PPP backbox.

Beastie :D

PS I see you are going for the SS system. Good on ya!!

http://dyno.scoobynet.co.uk/uk/sunil_patel3.gif

Dotted Line to red line - swapped PPP centre and backbox for magnex and SS. The other line is a Link remap.

So it looks like the "cone effect" loses you torque across the range - however, this car had a decat DP which could make a difference.

http://dyno.scoobynet.co.uk/uk/jonathan_rigby1.gif

This from switching from the PPP setup to a full exhaust.

Sounds like you made a good decision SCOSaltire. Beastie when does your warranty run out? Looks like a full exhaust makes about as much difference again as the PPP in the first place? Just a downpipe on mine added considerably.

However, are you saying that if the original downpipe is still in place then changing the PPP backbox to a SS makes things worse from your impressions?

[Edited by john banks - 6/29/2002 2:42:26 PM]

John
that pretty much sums it up.
My warranty expires May 2004 (5 years)
I dont really think there is much more mileage in this subject, it just seems to be an interesting anomally.
Thanks for your input.
Beastie