Custom sidepipes - Phase I

[scottj]

Dean,

Here is alot more than you asked for but, to simply answer that I didn't make jetting changes wouldn't make sense.

Keep in mind that my whole combination is designed for open exhaust.

I did't make any jet changes and here is the reason why: with the 3" perf core or open pipes the fuel curve is perfect and throttle response is great. When I want the car quiet I put the Lobaks on. With the Lobaks and the increased back pressure the correct fuel and air ratio is still entering the motor but, a couple of bad things happen. Because of the combination of cam overlap and the back pressure the intake charge is now diluted by exhaust. The intake charge dilution by the exhaust slows the burn, making the throttle response very lazy and because the exhaust isn't helping to pull fresh charge in during the cam overlap the carb now feels too big, again killing throttle response. That is not something I can correct with jetting, it's just one of the evils of excessive back pressure and a big cam.

There are ways around it. Here is something I do that works: with the Lobaks I'll advance the timing 6 degrees (I run the distributor lock out). This gets back most of the throttle response that the Lobaks lose. For easy driving (which is the only time I would use the Lobaks) this works very well but, with the timing that advanced I'll now hurt the motor if I start doggin it hard. So, I run a retard box on a toggle switch that knocks the timing back to where it belongs and now I can get on it hard. This is the same principle as vacume advance only with the driver control and reliability of the electronic age.

Sorry to ramble
Scott