<VV> Turbos / OverHeating

djtcz at comcast.net djtcz at comcast.net
Tue Oct 1 18:58:03 EDT 2013

For those curious or skeptical about some manufacturers today reserving "full" turbo boost for short term (much less than a minute) special events, here is a 2012 mainstream report, allegedly with input from several auto manufacturers, and even Honeywell, the turbo maker. 
(FWIW The rep from Honeywell of unknown technical qualifications and background, said " If the extra turbo boost were left on longer, an engine could overheat.") 


Mini forums talk about ~ 15 seconds of "overboost" (and about 15 lb-ft of torque) being part of the program since at least 2007, and at least as recently as 2012. 

GM is using 10 seconds of overboost on the 2014 Cruze turbo diesel 

In 2011 BMW allegedly told Fox news the 335 could muster about 3 psi extra boost (and ~ 30 lb-ft of torque) but only for 7 seconds. 

According to a few Internet sOuRceS (maybe the same guys that say Corvair axles or steering breaks, or Corvairs rollover), Lotus Esprit back in the 90s allowed full "over" boost all the way to peak HP of 284, but limited its use to 30 seconds or so. 

Searches for Subaru overboost pretty much invariably get hits for overboost as a problem triggering CEL/OBD codes, and frequently brought on by modifying the stock exhaust system. So at least in 2012 Subaru was comfortable hitting WRXs with 14-15 psi of boost continuously. 

Here is a link to an opposed engine automaker who felt in 2007 they could get away with a little more boost, but for less time. 
First, remember that the latest Turbo has an overboost mode—it ups boost on the 3.6-liter flat-six by 2.9 psi, increasing torque from 460 pound-feet to 505 for 10 seconds when the dash-mounted "Sport" button is ticked and the throttle is pinned 
This Canadian dealer says overboost (with no mention of time) is still on at least one 2014 model, along with a 26 gallon fuel tank. 

Here is a 2010 BMW paper about the turbocharged 3 liter inline 6, N55 engine. On page 91 it describes when in non-economy mode they rev up the electric water pump or something to pull the coolant temp in the cylinder head down almost 29 degrees C, because " This temperature reduction promotes improved volumetric efficiency, thus resulting in an increased engine torque." 

For my money it is relatively easy to just crank up the boost to a level that improves volumetric efficiency to the point I "run over my own crankshaft" (as the late Great Smokey Yunick wrote a few times) so if we take the marketing spin out, I think the description would reverse to include more technical stuff about reliability when running that much boost, or ability to use pump gas or something. Earlier in that section they talk about when in economy mode raising the temperature for reduced internal friction. Sounds like at least PART of the reason may be the inscrutably thorough Germans believe oil viscosity is thin, perhaps too thin for serious work at the higher temp. 

If Chevy had access to similar computer controls in 1962-1966 I believe they could have, and would have done things a bit differently. 

Dan T 

----- Original Message -----

From: djtcz at comcast.net 
Subject: Re: Turbos / OverHeating 

Corvair sensitivity/vulnerability to boost versus time is not unique. Engine management-wise Operationally things are so different nowadays, in large part because so many decisions are made by a Hal 9000. Lots of functions can be easily be optimized and run retty close to the ragged edge. Several modern turbo cars allow several extra psi of boost for 10 or 20 seconds of fun, then cut it back to "normal." The first one I can recall was a buddy's mid 80s Chrysler Lebaron Turbo. 


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