On motors that aren't build up, IE extreme valve jobs and head porting, gasket matching, etc the effect of a full exhaust system creates what is called a scavenging effect. What this means can be though of in Newton's laws, an object (exhaust gases) in motion tend to stay in motion. Your gases flowing through the pipes create a slight vacuum behind them as the rush out and hence helps to pull the exhast gases out of the next cylinder to dump them. When you go with a completely open system or a system with shorty headers and dumping right behind the door or something you lose that scavenging effect. Now for race motors that you see with straight pipes dumping right out the side on individual pipes and such, they are build so that the gases are forced out before any scavenging can begin. The cam profiles are setup so that the intake and exhaust valves are open for a short period at the same time. I hope that answers some of your question.

 

scavanging doesn't help pull exhaust gases of the next cylinder, scavanging causes the incoming air to be forced in more, that's why the intake and exhaust are open at the same time. the exhaust leaving causes a vacuum causing the incoming air to be forced in more. another reason why race cars run open headers or very short pipes is becuase they are running very high compression. A blower or a turbo raises your copression ratio. Nascars are running around17 to 1 compression. our stock motors i believe are around 8.9 to 1....pretty big difference uh? also scavanging helps out more in lower rpms then in higher rpms. so if your running in the rpm range of 4000 to 5000 rpms all day long, then no backpressure might be good. guess how much rpms nascars are at??? close to 15000 rpms all day long

 

The two go hand in hand, if the cylinder hasn't exited the maximum amount of exhaust gases then the maximum amount of intake mixture cannot be pulled into the cylinder. The vacuum of the flowing exhaust gases does one which in turn causes the other. Good call on the compression ratios, I should have mentioned that and I didn't.

 

nascar doesnt come close to 15000

they dwell in the vecinity of 6500-just shy of 10 most of the racing occurs (dependant upon track) at 8-9500

now indy cars, thats another issue all together, they run up about that high(15000), with comp ratios up to 23:1

 

no they run around 13000 to 15000rpms during races.. we had a couple of nascars come to school while i was in the chassis and high performance motor class at wyotech. we didn't get to hear it run at 1500 but he reved it to 8000 and it sounded pretty damn nice and got there pretty quick.

 

acutually i hate to tell ya but IndyRamMan is right...i did a quick google and found this about nascars

"...To coax their pushrod designs to rev higher without running out of breath, engineers have designed lighter, stiffer, lower-friction valvetrains. Pushrod V-8 engines racing in Nascar routinely rev to 9,000 rpm. Thanks to natural trickle-down, the lightweight valves, low-friction lifters and high-tension valve springs made of exotic steels have filtered into production pushrod engines, giving them the speed and stamina to compete with Ferrari’s V-8. While the F-430’s screaming 8,500 r.p.m. maximum speed is still out of reach, the 7,000-r.p.m. redline of the Corvette outdoes the 6,600-r.p.m. maximum of the Porsche 911 Turbo. The new Viper engine revs to 6,200 r.p.m., 200 more than before...."

from http://www.nytimes.com/2007/01/14/au...yt&emc=rss
(just over half way down the page easest to find by searching "nascar" on the page)

 

hum...maybe he said they dyno the cars at 13000 and race the cars at 10000....it's been a couple about a year ago...my bad though....i just know they rev hella high for a pushrod v8