It was brought up on another forum about duals vs single, without knowing much about turbos I'm wondering how the "hit" or boost is handled. GH was running twins on his Stude, Dave B runs a single on his...Is it a matter of setting up the wastegate and than gearing around it?
There are lots of factors involved in how the turbo comes on boost. The most important probably is the size of the turbo (and its trim) and the displacment of the engine it is feeding. On factory turbo cars they typically run a turbo that is a bit small for the engine so it comes on boost quickly (single turbo) and it runs out of breath on the top end which naturally tapers off the boost a bit at high rpm. Most of the time a larger turbo will come on boost slower because it has more inertia to spin up, but if sized properly it will have ample capacity to feed the engine and it can run full peak boost (or very nearly so) all the way to red line.
Other factors are how the boost control is set up. You can run just a simple waste gate spring, a basic boost controller which senses the manifold pressure and throttles the boost signal to the waste gate, or a fancy electronic boost controller where you have multiple things you can control including on some of them you can set independent boost limits in each gear. That is probably a good idea at Bonneville, limiting boost in low and second gear to very moderate levels to keep from blowing the tires off, higher boost in 3rd gear and only allow max boost in 4th and 5th if you have that many gears.
The physical plumbing of the boost control circuit can also affect the way the boost comes on. Very short large diameter tubing tends to hit harder than a longer run of smaller diameter tubing (just like a NOS system). It also depends on where you take your boost signal, some have the boost signal tube just down stream from the turbo outlet, others have it plumbed so the boost signal comes from just down stream of the intercooler or in the intake manifold. The later can cause problems if you get too close to the intake valves and the boost controller starts to see pressure pulses from the opening and closing of the intake valves.
Last the diameter and length of the intake tract from the turbocharger to the intake manifold can soften the hit if it is large enough as it takes time to fill and fully pressurize a large volume.
Bottom line, you need to talk to someone with lots of experience with different setups and tell them exactly what you are trying to accomplish.
A turbocharger needs to flow a certain amount of airflow volume at any given boost pressure to stay out of surge, so the cubic inch displacement and rpm range of the engine, plus the needed final boost at max power, are important parameters for the proper sizing of the turbo. There are literally dozens of combinations that will work on a given engine but all those variables will impact how the turbo is sized, and what trim it needs to give you the boost profile (and thermal efficiency you need).
Larry
