[rocketman]

Originally Posted by J2SpeedandCustom

Ok so let's take that 500+ CID motor and the 1000hp goal.
Can I use twin 67mm's? I've heard they can make 1000hp?"

Nope! There's nothing wrong with twin 67mm's they will definitely make 900hp, just not in this scenario. Here's why: 900hp represents a fairly constant amount of air/fuel mixture, regardless of whether it's being made by a small motor at high boost (eg. 380ci at 20psi) or a large motor at low boost (eg. 500ci at 10psi).

The first problem is that most turbo compressors are only able to reach their maximum airflow when they are running at high boost levels. For example, a GT42 is able to flow about 94lbs/min of air at 32psi of boost, but it can only flow around 64lbs/min of air at 10psi. Often people are quick to assume that high boost means high heat and therefore decreased efficiency, but in reality, it takes higher boost levels to put most turbos into their "sweet spot". In this particular example, the turbo is capable of almost 50% more HP at high boost levels than it is at low boost levels.

The other problem is related to backpressure. If the exhaust system (headers, turbine, downpipe, etc.) is the same between both motors, the backpressure will be roughly the same. Let's say the backpressure measures at 48psi between the motor and turbo's exhaust turbine. The big motor will run into a bottleneck because there is 48psi in the exhaust and only 10psi in the intake (a 4.8:1 ratio). This keeps the cylinder from scavenging/filling fully and therefore limits power. The small motor, on the other hand, has 20psi of boost (only a 2.4:1 ratio) to push against the backpressure. Therefore it is able to be much more efficient under these conditions.

The bottom line is, as your motor size increases, your boost level will go down (in order to achieve the same power level). In such a case you will need to maximize the flow potential of your compressor and minimize the restriction of your exhaust system (including the turbine) in order to reach your power goals.

So in summary you need to have a turbo compressor that is efficent at a lower boost like 10psi. In a perfect world on a 500cid a big single like a 101mm would be ideal.

Thanks,

So in a nutshell if you are going to use big cube's might as well make big hp,rather than choke it down.i have been playing with some #'s and a single big turbo on a large cube motor could easily make 2k hp.