From Faq
Forced Induction Basics
By Tom Doherty
I thought I’d post this article in the Tech/FAQ section due to the general info on Forced Induction. This originally was posted in:
TTORA Forum > Tech > T-100 & Tundra Tech
http://www.tacomaterritory.com/forum/showthread.php?p=189189#post189189
tundra S/C or Turbo??
Trying to decide what would be better. Any opinions?
http://www.ststurbo.com/tundra_4_7l_dyno
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- Be wary of data selected to make a product LOOK good...
I didn’t like they way they presented the dyno charts... Check out the RPM at the bottom of the chart... They START at 4,000 RPM! Missing is the power curves for 99you’re your driving needs... Power starts at 4,250 rpm, peaks at 4,500 rpm... Almost looks like a MX bike... How much time are you planning to run at 4k+? Is this where you drive? Is this where you want your power to be made?
I especially didn’t like the Tacoma dyno charts... http://www.ststurbo.com/tacoma_dyno
In my opinion, this is Outright fraud...
The Supercharger is flat across the chart. Interestingly, they only show the Stock and Turbo from 3,500 up... I suspect that even the Stock makes more torque than Turbo at the lower operation rpm range... and that’s the reason I said fraud... Why didn’t they dyno the Turbo and Stock for the range of the supercharger? Obviously they did have the data and the results did NOT support the turbo, EVEN WHEN COMPARED TO STOCK!
Frankly, I wouldn’t trust a company that doesn’t publish the full dyno results... At all!
- Basic difference between the Supercharger and Turbo
So which one, Supercharger or Turbo? Well, that depends on where you want the power. If you want it at 3,500 and up, then the turbo is the way to go... If you want it where you spend most of your driving time then the supercharger. The supercharger will reduce the need to shift dramatically! The Turbo will require downshifting at least two gears to get the power...Heck, if I wanted to shift a lot, I wouldn’t have gotten the blower...
There’s no mention about the disparity in pricing in the above website! If you spent the same money on a supercharger and tuning, I think you would see much closer power levels... $4,000 for the turbo? I think I’d look for supercharger... You will get more time on boost in your day-to-day driving and a heck of a lot more power down where you live...
- What about a supercharger’s parasitic drag? What does it do to fuel costs?
As far as fuel, modern Eaton type superchargers use a bypass valve. This greatly reduces the parasitic drag of a supercharger not on boost; the only power consumed is in the rotational losses. Without the bypass valve, the blower would be constantly compressing the air INTERNALLY and this WOULD waste engine power... With the bypass, the airflow under an intake manifold vacuum takes the easier path leaving the blower compressing a partial vacuum, which takes very little power... However, the basic efficiency of the Eaton supercharger IS LOWER than the Turbos AT HIGH BOOST LEVELS...
- Doesn’t a Turbo use wasted exhaust power so it’s more efficient?
A turbo actually introduces backpressure, think about it, it’s and obstruction in the exhaust plumbing! There’s NO FREE LUNCH... How much? Well consider this... The term TURBO MUFFLER originally meant Forced Induction by Turbocharger. It came from the fact that the TURBO IS ALSO THE MUFFLER! In other words, when you put the Turbo on, you take off the muffler... The backpressure created by the Turbo acted as a muffler... Marketers took this highly specialized buzzword, and the respect it carried, and applied it to their muffler for sales appeal... The term originally meant force induction by Turbo...
- Don’t they do the same thing, force air into the engine?
Actually calling a belt driven supercharger a Supercharger and an exhaust driven supercharger (turbo supercharger) is a mater of convenience. They are both superchargers... Belt driven centrifugal Superchargers have the similar characteristics as the turbo supercharger; I.E. they produce boost at higher RPM and they are more efficient doing so, but they both do poorly at low rpm and display lag...
- So, what are the differences in use?
For normal on or off road driving, the belt driven Eaton blower is far better, for drag style driving the turbo shines... If your thing is stoplight to stoplight, get the turbo... For seamless power from idle up, get the Eaton style supercharger.
- I’ve heard that Turbochargers can work at low RPM also like the (whatever) for example.
Yes, a Turbo system CAN be designed for low-end power but since there’s no free lunch, the top end suffers. I like the Eaton style because it closely matches the engine requirements. It provides boost right off idle and the faster the engine spins, the faster the supercharger spins proportionally MAINTAINING the level of boost.
The turbo on the other hand obstructs the exhaust at low rpm, produces no boost and actually decreases power down near idle. As the engine rpm increase, the turbo gradually starts producing boost and finally reaches the desired boost. It comes on the pipe so to speak. As the engine rpm increases, the boost increases until it reaches the max-predetermined level that the blow off valve opens and vents the excess pressure (waste gate) to atmosphere. The waste gate is appropriately named because it also wastes the power used to over pressurize the system. This is accounted for by increasing the standard boost to a level that makes up for the wasted power, but the tradeoff is higher gas consumption/heat.
The innate problems with Turbos can be seen by the fact of the hybrids that have become popular. Twin Turbos allow the use of two small Turbos with less lag yet still produce the needed air volume required for hi rpm... Less lag being the operation word... Also, there are twin Turbos that have one large Turbo for top end and a small Turbo for low and midrange power... These are good, but read TWIN as expensive and/or complex.
- What about reliability of the two different systems.
The Eaton style blowers have been stone reliable, usually lasting about as long as the engine they straddle. Because of the heat involved, the Turbos haven’t faired as well.
Actually the reliability problem with a Turbo comes from improper operation. If they are NOT cooled properly on shutdown the oil in the Turbo cooks.
The fix, ALWAYS allow the engine to idle till the Turbo cools. Nobody does this all the time. The answer, if you go with a Turbo, is to get a quality Turbo Timer... The Turbo Timer will automatically keep the engine running for the cool down period after you have left the car and eliminate this particular problem and greatly extend the life of the Turbo.
- Can I have it all, the low end of the supercharger and the high range of the turbo?
THE ONLY WAY I KNOW OF to overcome the problems associated with each system is to run a Hybrid system that uses an Eaton style blower for low RPM and twin Turbos for mid and high end power... IF YOU HAVE TO ASK HOW EXPENSIVE IS THIS, YOU CAN’T AFFORD IT! http://forums.vwvortex.com/zerothread?id=2180560
- What other factors can influence my decision?
Another point to consider is the design of the various kits. A poor designed/tuned supercharger will suffer compared to a good twin turbo kit that is highly tuned. When talking about the efficiency of any supercharger, your dealing with mathematical formula using observed data to compute the PHYSICAL characteristic of that particular supercharger. There’s no question that at higher boost levels Turbos ARE more efficient. However, there’s a point that the ENTIRE driveline needs to be strengthened to be reliable at these boost levels. When your talking about boost levels suitable for stock components and reliability, the differences are fairly close. Consider that most automotive companies have gone with the Eaton blower; they have good reasons like cost, reliability and operating characteristics. When you see this reverse and the auto giants switch to turbochargers, then you know the technology has arrived...
In a normally aspirated engine; vacuum is created by the pistons and regulated by the throttle. (A valve that regulates the flow of a fluid, such as the valve in an internal-combustion engine that controls the amount of vaporized fuel entering the cylinders.)
When the throttle is closed, airflow is blocked and the engine produces a high vacuum. When the throttle is then opened wide, the airflow is at it’s maximum and vacuum is greatly reduced. In a properly designed, high performance engine the vacuum should approach ‘0’. In short, the engine is getting as much air as it can. (There is a lot more involved than this...)
With forced induction and an Eaton style blower sucking through the throttle body.
The throttle still controls the airflow, naturally. When not under boost, the system is in the normally aspirated mode. When the throttle is closed, the intake manifold again goes to high vacuum, the blower isn’t getting any air to compress, the bypass valve opens and the small amount of idle air available routs through the bypass. Since little air is available to the blower, there is no internal compression by the blower and the resulting parasitic drag is eliminated. The vacuum extends from the piston right up to the throttle plate.
When the throttle is opened wide, the air now rushes to the bypass valve and the supercharger; and fills the intake manifold which goes to atmospheric pressure... almost. Without a vacuum, the bypass valve closes and all air goes to the blower. This happens FAST! The supercharger is already spinning at the correct speed and creates boost instantaneously... No downshifting, just instant boost... no lag.
What happens if the throttle is partially opened...
Everything is controlled by the manifold vacuum, which is in turn controlled by the amount of throttle opening... The engine will accelerate normally until the throttle is opened enough to trip the bypass valve... then the supercharger goes right to boost at THAT throttle level... More throttle, more boost. This is seamless, you wont feel the transition if it happens gradually. The important thing to consider here is there are NO obstructions in the Intake or Exhaust systems like with a turbo!!
An example is cruising at highway speeds. You hit a grade and press the throttle harder you get a proportional increase in power...The harder you press, the more power until your WFO... Remember the supercharger can only produce the amount of boost (up to max) that the throttle allows it airflow.
- Well, what are the drawbacks of the Eaton type supercharger?
Eaton style superchargers are NOT as efficient at high boost rates, but sufficient for stock engines, transmissions and other stock driveline components when kitted properly.
Probably the biggest design flaw I see in ALL the supercharger kits is the lack of provision for an intercooler and insufficient tuning... If someone came out with an intercooler supercharger kit I’d probably jump on it... Intercooling would seriously up the ante for the Eaton blowers... You would have a kit that could take you right up to the edge of engine destruction if you wished.
As far as rock crawling or any other application, it all boils down to torque... You either have enough or you don’t. If you don’t then you can increase it by gearing down or increase the engine’s output... There are tradeoffs to both methods. Gearing down is great, but you loose top end, which doesn’t matter if you have a dedicated rock crawler...
I once saw a buggy with a Brigs & Stratton engine and two automotive transmissions mated back to back... The available Torque was astounding. Figuring how to select the two gear selectors perplexing and when both transmissions were in reverse the top end was measured in feet per hour... All Torque, no speed...
For a dual-purpose truck this may be a problem and a crawl box might be the way to go. If you have the bucks, you can add the S/C and have it both ways... There is always trade off’s... I have found that for a dual-purpose truck the S/C 2.7L even on 33’s has met my needs... I’d go with lockers and suspension before I would feel the need to regear... and even then, I’d probably want the crawl box.
This entire article can be boiled down to; test drive each style installation you are thinking about and pick the flavor you like best... May take time and effort, but you will be happy with the results...
