Question about how turbos work

[5amp5on]

During a couple of launch control WOT runs yesterday, I noticed that the highest boost I got was 18-19lbs.

The thing I also noticed is that the boost would drop from 18-19 down to 15 lbs of boost after 5,500 RPM up to redline of 7,000. This happens in all gears.

Why does the boost drop like that when the RPMs go over that level? I'm not saying it's a bad thing I'm just wondering why?

Here are few pics to illustrate:

 

[Modfather]

During a couple of launch control WOT runs yesterday, I noticed that the highest boost I got was 18-19lbs.

The thing I also noticed is that the boost would drop from 18-19 down to 15 lbs of boost after 5,500 RPM up to redline of 7,000. This happens in all gears.

Why does the boost drop like that when the RPMs go over that level? I'm not saying it's a bad thing I'm just wondering why?

Here are few pics to illustrate:

Turbos have a certain efficiency range that they work within, on a stock car they are typically sized so that boost comes in quickly so you don't get a lag when you put your foot down, the knock on effect of that is they run out of puff at the top of the rev range. when you go to bigger turbos they might take a bit longer to get to full boost but will pull all the way to the rev limiter

 

[5amp5on]

@Modfather thank you for that concise clear explanation. Much appreciated and good to know this is normal.

 

[Will-Stillen]

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[RobotAZ]

Turbos have a certain efficiency range that they work within, on a stock car they are typically sized so that boost comes in quickly so you don't get a lag when you put your foot down, the knock on effect of that is they run out of puff at the top of the rev range. when you go to bigger turbos they might take a bit longer to get to full boost but will pull all the way to the rev limiter

Do our turbos actually get in the way at highest revs?

 

[Modfather]

Do our turbos actually get in the way at highest revs?

No idea, my car has only got 300 miles on it so I have not reved the motor out yet

 

[FSUZ33]

Do our turbos actually get in the way at highest revs?

Not at all. They’re still moving air and making boost, they just lose efficiency at high RPM. It’s difficult to setup a turbocharged engine with quick/strong initial power and also see power continuing to increase at very high RPM. Nissan’s chosen for this turbocharged, daily driven-capable sports car what pretty much everyone else does, focus the turbo sweet spot on low to mid RPM spirited driving.

 

[RobotAZ]

Not at all. They’re still moving air and making boost, they just lose efficiency at high RPM. It’s difficult to setup a turbocharged engine with quick/strong initial power and also see power continuing to increase at very high RPM. Nissan’s chosen for this turbocharged, daily driven-capable sports car what pretty much everyone else does, focus the turbo sweet spot on low to mid RPM spirited driving.

I realize it’s common sense that turbos still running at redline wouldn’t be tuned in our cars to be dead down low and screaming at the top.

Seems like a supercharged car or two that I’ve owned the boost was all low and then the drag of the useless SC actually got in the way at the top. Maybe that’s what I’m thinking of.

I’d love to hear from people who know the Z stock and aftermarket turbos. I’d rather have a little more lag if it’ll get me more smooth mid to top end boost.

 

[FSUZ33]

I turbo'd an NA car in 1999, a '97 Civic EX with a D16Y8 (1.6L SOHC VTEC). 247/220 at the wheels @ 8 PSI. The oil return line kinked and clobbered up the turbo. Tuner suggested replacing it with a smaller one rather than sending it out for rebuild. When the new turbo came in it was laughable how small it was compared to the original. At 8 PSI it was making the same HP. Torque increased to the 230's. BUT both were hitting at lower RPM.

The first turbo was amazing, but I had no clue how much lag it had until I replaced it. I didn't have any reference. It pulled like a clydesdale, and kept pulling harder until redline (6,500?), but didnt really start to pull until 4200. Power topped out around 5K with the new turbo, but it started to pull hard around 2750.

It was my only car, and a daily driver. I didn't track or autoX, and I wasn't a street racer. Friends messed with me because of all I had done to it, a massive Blitz intercooler crammed into the front clip, and I was still driving around on the stock 15" steelies with OEM plastic hubcaps.

So for that engine, the sweet spot of the larger turbo was, in my opinion, too high. It would be great if you're constantly driving around in the 4,500-6,500 range. The smaller turbo moved the sweet spot down to an RPM range you'll typically be in daily driving, and the lag was like 1 sec compared to the 3 secs of the larger one.

Although the turbo change didn't have a noticable affect on power/torque, the lower sweet spot made it a COMPLETELY different car to drive, and I think that's more beneficial/fun for a daily driver.