Head Hunter (turbo build)

The problem with the larger turbo is, if you get into the throttle (Off Road) at a very low rpm and the turbo tries to spool, it will go into surge and over-rev.
I have been there when our twin turbos were driven into surge. Not Good!

If you cam the engine for low rpm torque and only do minor port work on the head, the engines volumetric efficiency will drop off enough at higher rpm to prevent a smaller turbo from overworking at higher rpms.
 
The idea of undersizing turbos is a Japanse and US Holley 4bbl carb style of idea, and IMHO not what you should want to do. Like carbs and cams, there are actually no wrong turbo's, but wrong engine combinations for turbsos, wrong engine combos for carbs, and wrong engine combos for cams.
I favour always adjusting the combination to suit the parts if you can. That forces you to actually understand what the heck you are trying to achieve, ie, defining your purpose. But don't ignore the real adivice from pmuller9. Its based on reaching into what your reall goals should be to sucessfully nail the engine build.

Small cams, carbs, and turbo's are conservative, and normally give the guy what he actually needs. Its very easy to over size the cam or turbo for the engine combination, and you must fully Consider doing what he says, and if you do that, you'll get a good or better result. Period.


I would add that the late Todd Wilkes was an ealy exponent in the twin turbo V8 powerhouse cars, and that throttle body failure under boost is a major issue, and turbos are a whole new regime of learning still. You can count on Three Fingered Joes right hand how many sucessfull carb turbo in line engines there are.

Probably Three.

1. Maybee the foreign JDM Misubishi Mirage, Cordia and Tredia 2-bbl carb Turbo's (one engine in 1419 and 1594 cc Hemi in line OHC 4 form) and


2. the Turbo Solex 1-bbl Carb Renault R5, R18, Fuego and the non EFi R9 and R11 turbos in 1397 or 1655 cc forms with basically the same Hemi in line 4 engine were okay, but nothing else comes to mind.



The vast aray of distant Thirds are:-

JetFire F85, Corvair Turbo, 79-81 Mustang Carb Turbo 2.3 and Buicks Regal 231 and the Firebird 301 Turbo...all service nightmares due to the complication of cold running warm up issues, vaccum links, auxillary enginering solutions not well understood, carbon build up and a labyrinth of other production engineering challenges.

pmuller9's advice is prudent since blow through turbos are potentially dangerous, and the whole set up is more likely to be unsatisfactory, not just because of turbo sizing, but because the application of a carb turbo is always a problem. Fuel injection or full feedback fuel control is required to ensure the engine responds to charge density.


I would ask you to consider another way in addition to pmuller9's advice on keeping the Volumetric efficency down, with the turbo he suggests. ....and that is to look at the whole application with respect to what the engine would be like Before Turbo.

A 4-bbl 300 with that intake is already prone to flooding, with poor secondary circuit chracteristics, and poor cold running because all the changes are ruining low speed off idle progression.
 
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