I have a 1990 Seville 4.5 engine, and a 1991 Seville 4.9 engine. I have both cars tuned the same...running premium gas with base timing at 14 degrees, TPS base throttle angle set to 2.1 degrees. I have experimented with various TPS angles from -0.5 to 3 degrees, as well as the max extension value of the ISC to find the perfect combination to eliminate the infamous sail-on condition these cars experience. The ideal combination that works with zero sail-on, and cold idle start is 2.1 degrees withe ISC fully retracted and throttle plate resting on base idle screw, and 15.8 degrees when ISC is max extended. I have not altered the base idle screw from the factory setting as suggested in the TSB. If I recall, even with advancing timing 4 degrees from 10 degrees base factory timing, idle speed is 500-550 rpms. TBS calls for 450rpms. Close enough. Shift points, idle, and coast down work fine with my adjustments.

I figured the stroked 4.9 engine would beat my 4.5 engine, but, surprisingly, I have taken 0-60 runs with each...the 4.9 engine I didn't even bother figuring the time. My 4.5 engine, with stale gas, clearly gave me a 6 sec 0-60, possibly 5.7 secs. The only mods I have made to the car were replacing stock injectors with mustang injectors, Denso O2 sensors, K&N air filter, and reducing load on alternator by replacing all incandescent bulbs with LEDS. The 4.5 engine has 160,000 miles and 4.9 engine has 60,000, and I'm can't believe the smaller engine is not even a contender. It will slam the doors off the 4.9. I have taken a Northstar as well, up until 65-70mph, where the N* can breath and takes over. Through diagnostics, both the 4.5 and 4.9 engines are giving me the same values...very surprising the stroked engine isn't the winner. As far as low end torque...either car will light the tires up with ease.

My 1990 is in better condition, surprisingly. I take care of my cars. The 1991 was my grandmas, and the sun did a number on the paint and interior. My car is very clean. I recently replaced the factory wheels with 16" custom wheels with high performance goodyears. Painted calipers...looking sharp. I would like to add the visual effect of having dual exhaust outputs, as well as giving the car some growl. I know the cars can not accommodate true dual exhaust, nor will that, or having dual outputs increase performance, but stock muffler is rusted out and I would like to replace with stainless, giving car some look and sound.

The 1990 STS shows dual exhaust outputs, but, I can not find a picture of how this is set up. The Seville uses a transverse muffler with single output from factory. I would like to install a stainless flowmaster 40 series....single in, dual output. Obviously will need to have a shop bend me some output pipes. I'm thinking having the pipes straight out the back with some chrome tips. Is this possible? What's the best method to achieve a dual output look? With the transverse muffler design, there is limited room to work with. Thanks.