parrothead Posted October 11, 2005 Report Share Posted October 11, 2005 About 3 weeks ago I added a Volant CAI and a Corsa cat-back exhaust. My goal was to increase flow through the engine as my car has 117,000 miles. I don't have the ability to measure 0-60 times (yet) and I don't know if it really relevent in such a car. The real improvement was in highway gas mileage. I drive a 720 mile highway loop from Cleveland to Chicago. Generally I set the cruise at 68 mph. The average gas mileage before the mod was 23.0 to 23.9 mpg. After the mod mileage increased to an average of 25.9 mpg. I have only made 1.5 round trips but I am amazed at the increase. All of the averages are calculated by the DIC. My question is this, the tachometer is analog but I noticed the engine RPMs used to run just under 2500rpms at 68 mph. Now it seems to run at 2300 rpms or so. It is definately onder 2500 based on the readout. Is this decrease possible and if so is it caused by the modifications done or am I just crazy. ~for every vision there is an equal and opposite revision~ Link to comment Share on other sites More sharing options...
Bruce Nunnally Posted October 11, 2005 Report Share Posted October 11, 2005 Thanks for the report. I am enjoying my Corsa/Volant combo, although I have not been on a road trip yet. The speed limit is 65 mph along this route, so you set the cruise on 68? Without a change in gearing, engine rpm at a particular speed will be constant. Not sure why there would appear to be a difference in RPM. Bruce 2023 Cadillac CT4-V Blackwing Follow me on: Twitter Instagram Youtube Link to comment Share on other sites More sharing options...
JasonA Posted October 11, 2005 Report Share Posted October 11, 2005 The change in RPMs would not have happened due to the mods. You were driving slower. Possibly the cause for the increased mileage, but maybe not. So how does it sound? Jason(2001 STS, White Diamond) "When you turn your car on...does it return the favor?" Link to comment Share on other sites More sharing options...
parrothead Posted October 12, 2005 Author Report Share Posted October 12, 2005 Speed Limit was 65 set cruise at 68. I pretty much set the cruise at 68 or so, I have a 5.5 hour drive and I tend to daydream half the way. I don't want to worry about blowing past a State Trooper and hitting the brakes.. slow and steady. I need those 2 extra mpg s, to account for the citt driving with my new set up !! Yes it sounds wicked, but it is quite quiet at idle and crusing speed. ~for every vision there is an equal and opposite revision~ Link to comment Share on other sites More sharing options...
etcxtc Posted October 13, 2005 Report Share Posted October 13, 2005 The variation in RPMs could be something simple such as a prevailing headwind or tailwind. Wet vs. dry pavement. Just a thought idea... Link to comment Share on other sites More sharing options...
JasonA Posted October 13, 2005 Report Share Posted October 13, 2005 The variation in RPMs could be something simple such as a prevailing headwind or tailwind. Wet vs. dry pavement. Just a thought idea... As long as the torque converter is locked up, and it should be cruising at 65-70 mph, engine speed is directly tied to vehicle speed. Ambient winds or condition of pavement will not affect it. What CAN affect it are things that changes the effective gear ratio between the pavement and the engine. Things like tire diameters, final drive ratios, transmission ratios, etc. Jason(2001 STS, White Diamond) "When you turn your car on...does it return the favor?" Link to comment Share on other sites More sharing options...
parrothead Posted October 13, 2005 Author Report Share Posted October 13, 2005 I do have new tires and alignment in the past 3 weeks, the old ones had 35k on them. That may be it. There is a head wind when traveling east and tail wind when heading east though. ~for every vision there is an equal and opposite revision~ Link to comment Share on other sites More sharing options...
JasonA Posted October 13, 2005 Report Share Posted October 13, 2005 I do have new tires and alignment in the past 3 weeks, the old ones had 35k on them. That may be it. There is a head wind when traveling east and tail wind when heading east though. If there is a considerable increase in tire diameter, then your engine speed will be slower at a constant road speed. If your old tires were small for their size, and really worn, and you replaced them with tires that were big for their size, I'd say there could be enough of a diameter difference to affect that change in RPM. Even for the same nominal size, different brands/models can vary as much as 1/2" or more in overall diameter...which is probably enough to affect the cruising RPM. Jason(2001 STS, White Diamond) "When you turn your car on...does it return the favor?" Link to comment Share on other sites More sharing options...
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