4500rpm in 5th is similar speed as 6500rpm in 4th
4500rpm in 4th is similar speed to 6200rpm in 3rd
4500rpm in 3rd is similar speed to 6800rpm in 2nd
4500rpm in 2nd is similar speed to 7500rpm in 1st

The only time you should be below 4500rpm is for the first second or so after the start, if you find yourself under 4500rpm at any other time, it is time to hook a lower gear - after all thats what gearboxes are for is it not???????????

 

I'm suprised Ted hasn't brought up his famous "if i raced him with a 4000rpm limit i'd beat him easily" speech

As Peter has said, if you aren't using 8000rpm you aren't getting the true potential outta your motor, if you have a restrictive turbo, you aren't getting the full potential outta your motor.

 

Agreed, HWO.

B

 

There's also tire management and chassis dynamics at different rpm levels. I think it's pretty easy to "say" something like that, but the reality on the track is often very different depending on what course you're running. You could abuse the hell out of your car by doing so, but I'd rather not replace engine/gearbox every 20K miles. Sometimes being at max torque/hp coming out of a turn is not the quickest way out if traction is a problem. The best example of this is the dominance of V Twin bikes over inline fours in WSB. Different sort of racing, but it's a dramatic point none the less. Your point is taken however. As stated earlier, I also will be runniing the car on the street and would prefer not to get blown away by a Civic Si waiting for the damn turbo to spool at 4500rpm. :-)
Michel

 

There has been much wisdom spoken here. A bigger exhaust port WILL allow more power. It WILL lose low end torque and have worse emissions. If you don't have to pass emissions testing then go for it. Just remember that for every 1 psi of exhaust pressure you have, it takes 2 psi of boost pressure to make up the difference. This is why a free flowing exhaust on a turbo is so much faster. It also explains the power differences between different a/r housings on turbos. A larger exhaust port has less back pressure and therefore is more efficient. But a larger port isn't always the answer. More flow is the goal not a larger port. Look at the stock exhaust sleeves and how fast they expand. This is terrible for airflow. I have stock sized exhaust ports on my engine but have custom machined exhaust sleeves installed that have the exact same size and shape as the exhaust ports in the housing itself. The result is 30% more airflow through the same size hole! Paul Yaw told me how to do this and it works! I didn't increase the size since I need to pass emissions and want some good low end. If I were building a mega horsepower monster I would make them as large as possible. BDC knows how these work since we have had many hours of discussion on the subject. I'm not even going to get into my rationale for intake ports but we'll just say mine are as large as I could get them in streetport fashion.

Fred

 

I much like the large exhaust ports also. I do leeve the sleeve in but my port is as big as the casting will allow. the intake is less important on a turbo motor because the the air coming in is compressed the air going out is expanding and there for needs a larger area to expand and excape. Just like on cams for piston engines that are turbo they always go after the larger exhaust valve and lobes, althow they do keep a small overlap but for use that is not an option" you can't have yor cake and eat it to"