KEEP COMPRESSOR OPERATIONS OUT OF THE "CHOKE ZONE"

By 460-BBF-Turbo-In-CC (adapted from the legendary Car Craft Forum turbo blog)

"Don't choke now!" -- Grady Seasons (Keith McCready), "The Color of Money" (1986)

The following excerpt appeared on the forum at Bangshift.com:

"[The January 2011 issue of Auto Enthusiast magazing at pg. 30]has a story about a class project at WyoTech's Blairsville, PA campus. They built a crate 350 SBC with a single [Holset]HX35. It maxed out at a paltry 575 h.p. on 15 psi manifold pressure (charge cooled). This shows that the engine was obviously running well into the choke zone of the HX35. [Jay K.] Miller's map [Turbo: Real World High-Performance Turbocharger Systems ]shows that the HX35 goes into choke at ~ 50 lbs/min at 2.0:1 pressure ratio."

 

Sadly, Holset is tight-fisted with its compressor maps and there isn't a good on-line map of the HX35 available today.

 

Referring back to the old T66 map, every point to the right of the mapped area is in the choke zone of the compressor. Choke means just as it sounds -- the turbo compressor is choking the engine because it is not large enough to flow air efficiently in the quantities the engine COULD be consuming.

A turbo operating in the choke zone is too small for the engine at high r.p.m. It's that simple.

That doesn't necessarily mean that the turbo is too small at every r.p.m. For example, the WyoTech Blairville SBC reportedly has excellent low r.p.m. response, with a boost threshold (on-set of measureable manifold pressure increase) of ~ 2,000 r.p.m.

In another famous example (reported in the June 2001 issue of Hot Rod Magazine), Chris Stewart used a pair of junkyard 1986 Ford Thunderbird Turbo Coupe Garrett T03E turbos on a 472 CID BBF. The boost threshold was a mere 1,700 r.p.m. and it pounded out 714 lbs./ft of torque at the rear wheels through a power-guzzling non-lockup C6 automatic. And Stewart achieved this on a mere 10 psi of "boost."

The torque peak was a diesel-like 2,200 r.p.m.

However, the smallish turbos greatly restricted power at high r.p.m., limiting the 8.2:1 BBF to a weak 355 RWHP at a tractor-like 4,200 r.p.m.

Even though a turbo will still flow some air in the choke zone, the mass air density will be lowered by excessive charge heating. The turbo could also be at risk of overspeeding. And if the turbine (exhaust side) is fairly well matched with the compressor (as expected with factory turbos but not always true with aftermarket "hybrid" turbos) the turbine is likely also in an inefficent range of operation when the compressor is in the choke zone. Excessive backpressure and exhaust heat from a turbine opertaing in choke can hurt power production and exhaust valve longevity. In short, operating in the choke zone is akin to a dog chasing its own tail.

 

It should be clear by now, a properly fitted turbo or turbos must avoid the extremes of surge and choke. The "sweet spot" for turbo compressor operation is in the middle portions of the map where compressor efficiency is the highest.

What does "choke" mean? And why does it matter?