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Travis Followell
11-29-2005, 05:54 PM
What is Turbo Lag?

lawnmaniac883
11-29-2005, 06:01 PM
The time it takes for the turbo to spool and begin to provide boost pressure to the engines intake manifold. Intercooled engines tend to have more turbo lag that non intercooled because you are trying to pressurize more air...the intercooler holds more air.

Lawn Masters
11-29-2005, 06:01 PM
My understanding, is that its the time between when the motor accelerates, and the turbo spools up to provide the appropriate boost to the motor.

MJM landscaping
11-29-2005, 08:06 PM
Well the turbo runs off spent exhaust gases so the engine has to be revving up to spin the turbo faster to make more power causing a little pause between the turbo to spin and getting it back into the intake manifold.

StealthDT
11-30-2005, 02:09 AM
I always thought it was the observed difference in time between when you put the hammer down, and your engine responded. Used to be very noticable in early turbo systems, especially in passing or crossroads driving situations. This is why my Maserati has two small turbos, one for each bank of cylinders. The smaller the mass of the turbine, the less hangtime at redlights.

lawnmaniac883
11-30-2005, 05:58 PM
StealthDumpTrucks, usually, instead of using two small turbos, you make a compound set or twins. This consists of one large charge on the bottom and one small on the top. The small will spool quickly while feeding the large, then the large comes in for your real boost.

StealthDT
12-01-2005, 11:50 PM
Really? I've never seen a setup like that, got a link or pics? If they are in series, I guess they would need an intercooler rather than the Maserati system dumping into a pressurized box.

Dirty Water
12-01-2005, 11:58 PM
Stealth, There are two ways to set up a twin turbocharged system.

The way your maserati is set up is called Parallel turbo's. It works by having a turbocharger off each engine bank, while this method does not reduce turbo lag, or overall boost generated, it is still able to pump more air in at the same psi (More CFM, same PSI) resulting in more power.

A sequential turbocharger setup is what LawnManiac is describing. This is usually ran off both banks and consists of a small turbo and a larger turbo.

The small turbo spools almost instantly, but goes out of its efficient range quickly, by the time you it does the large turbo is spooled.

The result is a almost the low lag of a small turbo with the top end power of a large one.

The Mazda RX-7 and Toyota Supra both used this setup.

The main problem with it is the incredibly complex vacuum lines/waste gate/plumbing setup for them, they can be picky and hard to get to work right.

StealthDT
12-02-2005, 12:12 AM
I think i understand how it works, but I'd like to see how they handle an obvious pressure/vacuum differential on both sides of the spools. So they don't use the same plenum or waste gate? Ducting must be a nightmare.

Scag48
12-02-2005, 03:14 AM
Usually on turbo'd engines a blow off valve will release the extra boost into the atmosphere so as not to cause harm to the engine by overboosting, which can send rods through the side of the block and all sorts of other nasty stuff. In a sequential setup the turbo's use the same wastegate/dump valve/blowoff valve to release extra boost. The best idea to minimalize turbo lag aside from a sequential setup is Banks splined turbo system. It's very difficult to explain, but basically what happens is that the vanes of the turbo open and close, allowing more or less exhaust in and out automatically. It's a centrifugal system in which the faster the turbo spools, the wider the vanes open and when the turbo is spooling slighty, it lets in enough exhaust to boost quickly without lag. I'm not sure if they've released this product yet, saw it on Trucks or something on TV.

On turbo'd Audi's, a blow off valve cannot be used as the engine needs a constant supply of exhaust to run the engine correctly, so a diverter valve in used. Instead of blowing off the excess boost into the atmosphere, it recirculates it through the intake to maintain a minimum amount of boost. If there's problems with overboosting, there's problems with underboosting and that's what the diverter valve basically eliminates. When an upshift is made the engine negatively boosts, blowing all of the exhaust out through the wastegate and technically sucking some air back in through the turbine. With a constant positive boost pressure the turbo is always spooling in the right direction. Haha. For some reason on Audi's, internally, they need that consistent boost to keep the engine running through an upshift.

Anyone know of any other engines that use a diverter valve instead of a blow off/wastegate? Audi and maybe a couple other car MFG's are the only ones I know of.

Eclipse
12-02-2005, 09:36 AM
After reading this thread I think some are confusing a blow off valve with a wastegate. These are two different devices used generally for different reasons.


The best idea to minimalize turbo lag aside from a sequential setup is Banks splined turbo system. It's very difficult to explain, but basically what happens is that the vanes of the turbo open and close, allowing more or less exhaust in and out automatically. It's a centrifugal system in which the faster the turbo spools, the wider the vanes open and when the turbo is spooling slighty, it lets in enough exhaust to boost quickly without lag.


I think you are trying to describe a variable geometry turbo. This idea is already in production on certain vehicles.


When an upshift is made the engine negatively boosts, blowing all of the exhaust out through the wastegate and technically sucking some air back in through the turbine. With a constant positive boost pressure the turbo is always spooling in the right direction.

Negatively boost? Sucking some air back through the turbo? The turbo only spins in one direction? I think you are referring to the variables that can cause a turbo to "bark"?

Dirty Water
12-02-2005, 11:10 AM
Scag, You are confusing a waste gate (A mechanical valve that opens at a set boost pressure and redirects the exhaust flow around the turbine), with a blow off valve.

A blow off valve is a valve that opens when the engine throttle butterfly closes. It has nothing to do with the exhaust flow at all. You also will never see it on a diesel (No throttle valves :) )

The concept behind it is that when the engine throttle valve closes (for a shift, or deceleration) the boost pressure will slam into it, and quickly travel back down the line and put reverse pressure on the turbine. This will cause the turbine nut to back off, or the blade to shatter.

A Blow off valve is placed in the line between the turbo and the throttle body (or if inter-cooled, between the inter-cooler and the throttle body). When the throttle closes, the engine goes from boost to negative pressure (Vacuum) this vacuum opens the blow off valve, and the excess boost pressure is vented out to the atmosphere (Or recirculated in before the turbo in most stock setups).

You seem to almost have an idea with the Audi, but your totally wrong. :)

Eclipse
12-02-2005, 12:37 PM
A blow off valve is a valve that opens when the engine throttle butterfly closes. It has nothing to do with the exhaust flow at all. You also will never see it on a diesel (No throttle valves :) )

Actually BD Power does make a BOV for a diesel application.

Also some use a BOV (of a different nature but still referred to as a BOV) to limit maximum boost. They will place it inline between the turbo and the intake and use it to bleed off excess boost pressure before it reaches the intake. This is the "poor man's" way of limiting boost.

Dirty Water
12-02-2005, 12:41 PM
Actually BD Power does make a BOV for a diesel application.

Got a link? A Bov won't work without vacuum.


Also some use a BOV (of a different nature but still referred to as a BOV) to limit maximum boost. They will place it inline between the turbo and the intake and use it to bleed off excess boost pressure before it reaches the intake. This is the "poor man's" way of limiting boost.

I'd call that a bleeder valve, and I've never heard of doing it that way...Again, got a link?

I have heard of people putting a bleeder on the wastegate vacuum line, so that it holds the wastegate shut longer, resulting in more or less boost.

Eclipse
12-02-2005, 03:56 PM
Got a link? A Bov won't work without vacuum.

That was the hurdle to overcome when BD designed this unit.

http://www.bd-power.com/ram/product.php?pn=BD%20TurboGuard&tt=ram


I'd call that a bleeder valve, and I've never heard of doing it that way...Again, got a link?

I agree a bleeder valve would be more accuate terminology but the valve being purchased is referred to as a BOV also, but not a BOV in the same sense that is typically referred to when discussing turbocharged motors. I agree this is misleading terminilogy but I was just adding it because it is being done.

No link for this one as it is not a "commercial" product/application but it is in use. As I said it is a "poorman's" solution.

Dirty Water
12-02-2005, 04:03 PM
That was the hurdle to overcome when BD designed this unit.

http://www.bd-power.com/ram/product.php?pn=BD%20TurboGuard&tt=ram


Wow, thats a total marketing BS product there. With no throttle valve, theres no turbo backsurge. Thats why it has a microprocessor controller to make it blow off.

Basicly, its a expensive toy ($660, a comparable vaccum based BOV is around $200) to make the woosh sound. When you decelerate or shift the engines drops in RPM and the reduced load slows the turbine down naturally....you won't have any backsurge on a diesel.


I agree a bleeder valve would be more accuate terminology but the valve being purchased is referred to as a BOV also, but not a BOV in the same sense that is typically referred to when discussing turbocharged motors. I agree this is misleading terminilogy but I was just adding it because it is being done.

No link for this one as it is not a "commercial" product/application but it is in use. As I said it is a "poorman's" solution.

I just don't see the reasioning behind it, you already have a wastegate, thats very easy to control, why not just use the boost/vaccum (well, boost only on a diesel) line to it with a bleeder to control boost instead of creating lots of extra lag by blowing out of the intake ducting?

Eclipse
12-02-2005, 06:53 PM
Wow, thats a total marketing BS product there. With no throttle valve, theres no turbo backsurge.

When you decelerate or shift the engines drops in RPM and the reduced load slows the turbine down naturally....you won't have any backsurge on a diesel

By backsurge you are referring what is commonly called "barking" the turbo. When you let off the thottle abruptly and the turbo makes a "woof" sound from the air traveling back at the compressor. I just want to make sure that we are referring to the same phenomena with different terms.

If in fact that is what we are referring to it is a reality on a diesel engine. It can and does happen. It is not all that uncommon either. If you lived closer I could take you for a ride and you could see what I am talking about :)


I just don't see the reasoning behind it, you already have a wastegate, thats very easy to control, why not just use the boost/vaccum (well, boost only on a diesel) line to it with a bleeder to control boost instead of creating lots of extra lag by blowing out of the intake ducting?


One reason is not all aftermarket turbos have wastegates. Also, often times with heavily upgraded fueling the wastegate on the turbo cannot bypass enough air to keep the boost in check. Perhaps the most common place I see this, at least on a Cummins, is on a compound twin turbo setup because the secondary turbo is often times not wastegated and total boost control cannot be achived by the wastegate on the primary.

No additional lag is present. The turbo(s) are already spooled and you are at desired peak boost when the "bleeder valve" pops off.

Scag48
12-02-2005, 07:52 PM
Yes, a variable geometry turbo is what I was talking about. I get confused with BOV's, I forget that they are controlled by the throttle body whereas a diverter valve is not.

lawnmaniac883
12-02-2005, 09:56 PM
[QUOTE=JonHolland]Wow, thats a total marketing BS product there. With no throttle valve, theres no turbo backsurge. Thats why it has a microprocessor controller to make it blow off.

Like the guy before me said, there is backsurge when fueling is instantly reduced to the engine, the excess boost will actually reverse the rotation of the turbochargers compressor wheel making a "bark" like sound.

Eclipse
12-03-2005, 12:29 AM
the excess boost will actually reverse the rotation of the turbochargers compressor wheel making a "bark" like sound.

It is a common misconception that the wheel stops or reverses but this is not true.

Here is a excellent description of surge and an explination as to why it is a reality on a diesel engine.

"The actual engineering term is not "bark", but compressor surge or stall. The noise is caused by airflow reversal through the compressor. Since the diesel engine, unlike a gasser, does not have a throttle valve in the intake system, backing off the accelerator pedal does not immediately cause a high head/low flow condition that is the classic cause of compressor surge. Rather, chopping the fuel causes the turbocharger to lose drive energy on the turbine side, so the compressor impeller begins to slow down. With the impeller decelerating, since it is a dynamic rather than a positive displacement air compressor, it cannot maintain the head (high boost pressure) seen at the compressor outlet, and the airflow actually reverses through the compressor section of the turbocharger (Note: the rotation of the turbocharger rotor itself does not reverse - there's too much rotational inertia in the rotor assembly for that to happen!) This reversal of airflow unloads the impeller, shaft and turbine of the turbocharger. Once the excess head has been dumped, the turbocharger compressor will pressurize the intake system once again, but if the turbocharger is still decelerating, its speed will fall to a point where it once again cannot maintain the head it has created, and flow reversal will occur again. This process can continue to repeat itself as the turbo continues to decelerate - each time it does, the "whoomp" is audible. The repeated loading and unloading of the compressor impeller, shaft and turbine can cause cyclic fatigue failures."