Is a Bigger turbo Better?...
07-17-2016, (Subject: Is a Bigger turbo Better?... ) 
Post: #1
Is a Bigger turbo Better?...
Here comes back the old argument that keeps circulating on this forum ...


- We have tried the bigger holset on the CM871's that had the smaller Holset. With the larger turbo, the fuel mileage and efficiency actually dropped, but the power band it was capable of went up. Smaller turbo's ARE simply more efficient in fuel mileage than bigger turbo's when the average torque demand is not very high. What no one seems to be looking at here is the fact that most highway trucks in the states only average 24 - 28% torque demand long term going down the road. This is assuming they are loaded fairly heavily (close to 80,000 lbs), and many trucks use even less torque that haul lighter freight. I see these numbers all the time when looking at the recorded history of their ECM's,.. these are actual numbers!... This is well within the tolerance of the smaller turbo, and therefore makes them more fuel efficient overall when a smaller, more responsive turbo is used on them.. It comes down to a matter of the operational/average torque needs of the truck. Sure, everyone wants more power so they can stroke their ego, and their brains are always trying to justify it,.. and your brain is always telling you that bigger is better,.. but in most cases,... when you hook a laptop up to a truck, and look at its average load, and you look at the operation of the truck,.. IE. dryvan/reefer or flatbed, and mostly highway miles,... you find that 80% or more of the torque requirements over say, a year,... does NOT justify the use of a bigger turbo,.. and it will simply be a bad investment that does not yield any positive gains compared to the costs of retrofitting one on.

- To someone who does not know much about turbocharger technology,... It would ALWAYS SEEM that a bigger turbo would always be more efficient because of its bigger housing, and they always think "More FLow - Less resistance" but THIS IS NOT TRUE!. More Flow area = More exhaust flow AND Pressure to spool it up -- IE. More POWER from the engine required!,... Not less.

There are reasons WHY there are different size turbo's --- There is a LOT of science behind this,.. and it is REAL!,... If larger turbo's were the answer to efficiency problems, or were simply just as efficient, ALL trucks would have the same size turbo on them! -- AND there would be NO NEED for manufacturers to make different size turbo's!

It is NOT simply a matter of "Better Flow",.... it is a Matter of Efficiency based on BOTH FLOW AND PRESSURE AT DIFFERENT POWER REQUIREMENTS!. Smaller turbo's spool up quicker,.... A smaller turbo takes MUCH LESS exhaust flow and pressure to spool it up,.... AND THEREFORE REQUIRES LESS POWER FROM THE ENGINE to achieve it!. Only when you start demanding more flow than it can reasonably provide, like 75% flow capacity and UP, does it start to loose efficiency. The result is higher EGT's due to flow restriction. This ONLY HAPPENS when you are nearing the upper portion in its band of operation. Think of it like pouring water ino a tube. 25% water poured into the pipe free-flowing downward, and 75% water free-flowing has about the same resistance in the pipe, but when you start trying to pour 80% or 90-95%,.. or perhaps 120% water into the pipe,.. it simply starts overflowing because of the resistance. Until you get to about 70 - 80% of a smaller turbo's charge flow rating,... it is simply more efficient!.

Australian road train trucks have clearly pointed this fact out as well,... Those types of operation demand a lot of constant high torque. Because of this, a turbo with better charge flow IS IN FACT GOING TO HAVE THE ADVANTAGE on trucks that see upwards of 48%+ constant torque demand. The trucks over there also run at 1650+ RPM all day at those high torque demands unlike the trucks here in the states. A HIGH-FLOW TURBO IS IN FACT BETTER WHEN YOU CONSIDER ALL OF THAT, but as well, the vehicle has already been set with proper gearing, etc. to go along with such high demands.

On the other hand,... If you do this to a truck that averages 25 - 28% torque load going down the highway all day, and does not turn 1600+ RPM all the time (Most American trucks), .... GUESS WHAT!!!,... A very large/high flow turbo is not going to have the correct "CHARGE PRESSURE/FLOW" rating for the smaller more constant torque demands and therefore will in fact be LESS efficient overall on that type of operation. As well, a LOT of trucks SIMPLY ARE NOT GEARED CORRECTLY for the task the driver is demanding. Because of improper gearing, Many drivers first thought is to raise power/torque in the engine instead of actually looking at re-gearing the truck to be more suited for a slightly higher RPM and HP instead. Last I checked, it was WAY CHEAPER to put the engine under less long-term strain and re-gear the truck for a higher RPM range than is was to WEAR OUT THE ENGINE PREMATURELY because someone simply wants more power from it to make up the difference.

- It is a balancing act plain and simple!. If your a driver that cares about FUEL COSTS the most,.. and you go down the road all day keeping boost low,... A SMALLER turbo will out-shine a bigger one all day long in fuel mileage!,.... and If your using your truck to pull 63 TONS around all day, and you need lots of torque all day, the truck is actually geared for it (engine running in a higher RPM/HP range)... YOU WILL SEE IT IN YOUR MANIFOLD TEMPS,... They will be high all the time for what your trying to do,... and guess what!!!,... A Bigger Turbo WILL IN FACT BE BETTER!,... SO....



- The Answer to all this speculation is this.... WHAT IS YOUR MANIFOLD TEMPS ALL DAY?,... And I am NOT referring to when you climb that mountain or big hill,... but the AVERAGE!?,... This will tell you if a bigger turbo might actually help instead of hurt you. Assuming that your truck does not have boost issues, etc. and everything is dialed in correctly, If you lug around so much weight that your manifold temps are upwards of 700+ degrees all day and it is difficult to keep them down because your torque demand is high all the time, A bigger turbo will likely bring up fuel mileage/efficiency slightly. But if your running an average of 450 - 600 and rarely get to 800+, only on that occasional big hill or mountian,... A bigger turbo is NOT going to make any difference, and will likely make fuel mileage fall off instead.

- Another way to check is with Insite Software. It will let you know what your demands are long term. All someone has to do is look at their "Duty Cycle Monitor" and see where most of the torque/rpm demand is for the truck. If your torque demand is below 30% most of the time, and a lot of your driving shows 1500 RPM and below, then a very large turbo is NOT going to benefit the truck or its operation in any positive way that will increase its profitability, nonetheless pay for the retrofit costs. -- Last I checked I was out here to make a living, and NOT STROKE MY EGO!. Turning more profit and keeping the money in my pocket is what strokes my ego,.. NOT how fast I can sling around 80,000 lbs+, straining my truck in ways that only leads to increasing all my other maintenance costs too.

My truck, its operation, and how much demand I can MINIMIZE to get the job done is first and foremost. Next to that, A good hard look at the REAR END RATIO AND ACTUAL SPECS OF THE TRUCK to match these demands, and how it can be adjusted for best operation would be next. --- ONLY AFTER I know I have done everything to the truck as a whole to lower demand and increase efficiency would i start considering the engine, and perhaps the turbocharger, etc. -- AFTER I had already done the truck.

-- Too many guys take their truck and instead of trying to squeeze more money out of them,.. they try to squeeze more power and torque and bigger badder ego-stroking CRAP! -- Only to trade long term income away so that they can keep up with the neighbors!.

( Not directed towards anyone in particular...)
The VERY MOST DIFFICULT THING to adjust towards making money that you can KEEP --- IS IN FACT -- ADJUSTING THAT NUT BEHIND THE STEERING WHEEL! -- I.E. -- YOU!.


* First take a hard look at what you really do with your truck to make your living and what can be done to minimize your costs WITHOUT any modifications. Things like better trip planning so that you can operate it at lower speeds, consuming less fuel and torque.

* If high torque demand cannot be avoided through better planning and better driving habits, and you are doing those things already, the next thing is to ensure the truck is geared for what you are doing. A lower gear ratio and higher engine RPM will help the most towards moving heavy loads down the road. Many road train operation companies have already figured this out and spec the truck to run in the 1650 - 1750 RPM range for a reason. It provides MORE OVERALL HP to keep things moving and reduces strain in the engine when torque demand is always high.

* Monitoring exhaust manifold temps and boost levels via a pyro and boost gauge set up the way that I describe in my youtube video(s) ( ) and determining if in fact a bigger turbocharger would be beneficial to you.

* Checking your ECM data through Insite or some other software to see what the long term/average demand on the engine is AFTER you have minimized it through the methods mentioned above. With this information, you can see what your torque demand really is and if a larger turbocharger will in fact benefit you.

* Weighing the costs of "how much it might help you" against the cost of having it done.

* Collecting some solid data both before and after to see what the resulting effect in fact was.

- The problem I see with truck owners getting caught into the rut of "More Power" and/or " Bigger Turbo" and all those things they try to justify, is that most of the time the truck simply is not geared or set up properly towards meeting their needs. Instead of actually correcting this, being it is the most effective solution, they focus solely on the engine to make up for the differences and this is wrong. Everything you do from how you drive, the specifications of the vehicle, to the higher costs of operation due to increased load stress needs to be considered long before your ego. Unfortunately though for most, the ego is what gets in their way because they have been brainwashed onto thinking that bigger, faster, and first somehow seems to be more important than how much money they take home and keep.

Don't get caught up in that cycle. Make wise, informed decisions that are based on fact and relevance to what your demands really are towards making money with your truck. Anything else is just a waste of effort and costs you in the end.

These are my own thought on it -- Rawze

User's Signature: ->: What I post is just my own thoughts and Opinions! --- I AM Full Of S__T!.
 Thanks given by: trucklogger2 , PuroCumminsPower , LargeCar , Toolguy
07-17-2016, (Subject: Is a Bigger turbo Better?... ) 
Post: #2
RE: Is a Bigger turbo Better?...
Rawze , thank you very much for this nice thread !
I wish more people are thinking like you . But to think like that , or at least in the right direction, they have to be educated sufficiently to make right decision(s) .
And I think this is where all the problems come from, and arise further!
Now days been an O/O isn't simple task. It involves pretty much all your skills , and demand even more . A simple wrong decision , made because of lack off knowledge , can lead to very difficult and costly situation!

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