Ok, here is an interesting theory i came up with the other night.

The stock turbos have a boost pattern of 10-8-10. So the first turbo is perfectly capable of making 10lbs of boost from about 2800rpm on. The second turbo in stock form makes no more than 10lbs or boost. Why then, did Mazda decide to put twin sequentials on the car. Wouldn't it seem logical to put one turbo on the car, that boosts to 10psi and spools at 2800 rpm, i.e. the first turbo?? It seems like a waste of time, energy, space, heat, and everything else under the sun to use a second turbo when it does no more than the first turbo.

Is this the logic behind going single?? I though it was to get a huge turbo that makes like 18-20psi, something that the stock twins are not (easily) capable of. But after considering that, maybe that is the logic. It just seems stupid a waste of time and power going through a boost drop on the transition only to acheive the same boost after. The boost could be 10. Instead of a drop, couldn't it?

 

you are on one turbo down low, then on both turbos up high. It doesn't just switch over to the second turbo alone.

The drop is because at some point you have to give up some energy to get the second one spinning.

If you go with one big turbo - you loose the kick down low. Some like the tradeoff - a lot. Some less so.

 

Indeed.

I believe the 10psi you are feeling after 4500 is the combined boost from both turbos.

I dont know what would happen if you only ran the primary turbo.
Would boost just begin to fall off?
Or would boost hold at 10psi and the turbo burn out?

 

I understand that both turbos produce boost when the second one comes online. The drop is experienced because the computer starts routing some of the exhaust gas to the second turbo, leaving the first turbo partly deprived, and the second turbo then makes up for the missing 2psi when it comes online (probably the second turbo does more work, and only a small amount of the exhaust gas is routed to the primary turbo.)

But you missed the point of my question. The first turbo is clearly capable of making 10 psi. why not use it the entire way through the rev range, and eliminate the need for a second turbo altogether. Ech, i wasn't saying go to one big turbo. I was saying go to one small turbo (like the stock primary) becuase it is capable of acheiving the maximum boost that is made in stock form.

Ok, analogy attempt here. Imagine having two identical light bulbs in the center of a room. Someone walks into the room, and flips a switch for the first light bulb. It makes x amount of light. A second person walks in, turns on the second light bulb as well. the room only has enough power to power both bulbs at half power, so even with them both on, it makes x amount of light, the same as with one on. So why have 2?

 

You are only accounting for boost but ignoring flow. The primary turbo is already running out of steam on highly modded cars by the 4500 RPM switch point, it would be way beyond its range to provide 10 psi at 8000 RPM. The car would choke on the restriction and the intake air would be super hot. The turbo probably wouldn't last long that way, either. Notice the big jump in power after the switch, too -- bringing the second turbo online increases the efficiency of the system, so you get a bunch more power even at the same boost level.

The car has two turbos from the factory so they could do the sequential setup. That gives good boost response down low (minimal lag) while still having enough flow to boost the car as you approach redline. If you don't want sequentials, a larger single turbo is the logical choice.

-Max

 

Ahh, an answer i was looking for in a roundabout way. Sorry, excuse my ignorance. I was unaware turbos had a "window" of rpm that they functioned effectively in. I thought once it starts spinning, it's good at any (turbo) rpm. But after a certain rpm (again, turbo rpm) point it can no longer make appropriate boost? That would make sense, thanks for the help.

 

Hmm ... good deal!

An answer to a question that has always been in the back of my mind.

Thanks for asking it 911GT2

 

I believe that even though it is the same amount of boost there is now more volume because of the second turbo. Kinda of how a bicycle tire can have 100 psi in a small area and a car tire has 45 psi in a large area

 

you are on the right track, but i will attempt to add some clarification...or confusion.

an engine running at 4000 rpm is only flowing approximately half of the air that it would flow at 8000rpm. to maintain 10psi at the higher engine rpm is going to take twice as much air. there is a point where the turbo will be spinning so fast that it is creating an extreme amount of turbulence, and actually flowing less or the same volume as it would at a lower turbo rpm.

so, it's not only related to the efficiency range of the turbos, but there is a maximum amount of volume that a turbo can flow, regardless of how many hundreds of thousands of rpm it is turning.

 

Think of it this way... if you only had the primary turbo, you'd be lucky to get 200 hp out of an FD.

Wade

 

I also hadn't thought of that. More air going into an out of the engine = a larger volume of total air. More volume = less pressure. Yeah, i get it now.

 

Sounds like everyone has it worked out, but just in case, here is another analogy:

Imagine two water hoses - one with twice the diameter as the other. Both can have water pumped through them at 100 psi, but the larger hose will allow more water to flow. In the end, it's all about how much air the engine gets, not necessarily the pressure that gets it to the chamber.

I guess it's also kinda like bandwidth on a computer.

There must be some other good analogies too. Anyone?