I recently changed out the cam in my car for a milder one to get better freeway manners in 6th gear. This cam has 10 degrees less intake duration and a wider lobe separation angle of 114 degrees which definitely improved the performance at low rpm (1400-1700) eliminating the need to down shift when traffic slowed. At the same time I put a larger pulley on the blower to reduce the boost and changed out the Flowmaster muffler for a Dynomax. The old cam made massive torque (543 ft lbs at the rear wheels) and the horsepower was 450 but at only 4800 and then it dropped off rapidly. I suspected the exhaust was restrictive and then I saw on another forum the attached chart comparing the flow of various mufflers to that of an open pipe. I had a 50 series Flowmaster on it and switched it out for the Dynomax Ultra Flow Welded. The Flowmaster flowed only 53% of an open pipe whereas the Dynomax was 101%. With the smaller cam you would expect the horsepower be lower and to peak at a lower rpm but it actually raised the peak almost 1000 rpm peaking at 466 rear wheel hp at 5750 rpm (that's 536 flywheel hp). What a difference a muffler makes! After this post the name "Flowmaster" will be forever stricken from my vocabulary.
MufflerComparisionChart.jpg
Before the horsepower and torque peaks the new curves were flatter and lower as I expected but it still makes over 300 ft lbs at 2500 rpm and 425 ft lbs at 3000 rpm and it stays over 400 ft lbs past 6000 rpm (peak is 479 ft lbs at 4300 rpm). All the new numbers were with the water/methanol injection turned off and the old ones were with it turned on so they will probably be closer with the system on. So am I disappointed with the lower performance now? It still has plenty of power to scare me and I still need to be sure to have it pointed straight ahead when the boost comes on. What is nicer is the normal driving and I think it will be much easier to control in autocrosses, I just need to control the acceleration rate by shifting early.
I am going to keep the old cam because if I build another engine with forged internals ( I have a stock shortblock with cast pistons and 11:1 compression now) I would love to see what it would do with the less restrictive exhaust. My guess is well over 500 rwhp and close to 600 rwtq and I could regain some of the low end performance with a set of Rhoads lifters http://www.rhoadslifters.com/Pages/Articles.html The engine just won't take that kind of hp now.
I second the statement on Flowcrapster! Nice information Jim. I need to ask......Indy this year?
Never say never but I have my doubts. That is quite a drive in an open car.
Yes it is. When I made that drive I had a Parish Plastics top on the car.
Jim
Hah, like that is going to fit his car! ;)
Yeah, guess not.
Jim
Possible to trailer it? We can meet up on the way, and caravan out. We can't get any better than now on fuel prices? I know....even myself want to see your ride in the flesh. It would be way cool for sure.