I need help deciding on a turbo....

[WizardBlack]

Wow lots of misinformation on this one.

 

Lag - Time to build boost when it is floored. Has nothing to do with how low in the rpms it will build boost. You can rev up to 5000 rpms and then floor it and measure lag.

 

Boost Threshold - Get out on the road at 1000 rpms in top gear and floor it. When it builds "x" boost, that rpm number is the boost threshold for that boost level. It's the minimum rpms needed for the turbo to build that boost. A lot of people like to quote boost threshold for 1 bar of boost in 4th gear, etc.

 

Compressor Surge - The engine can feed the thermodynamic energy to the turbo to make "X" amount of boost, but the turbo itself doesn't like to make boost at that low of shaft speed (for the turbo).

 

Backspin - Lack of a pressure relief device (BOV, BPV, etc.) or a poorly configured one wherein when the car has been boosting and the throttle is shut, the pressurized air hits the throttle plate and flows in reverse back to the compressor where it physically stops the spinning of the turbo and forces it to spin backwards as the pressurized air tries to escape. This doesn't always get so bad as to spin the turbo backwards (it can just slow it down) but any way you look at it, it's very hard on the turbo and increases respool time (lag). A lot of kids think this sound is cool and incorrectly tighten their aftermarket BOV too tight until it isn't opening and backspin results.

[WizardBlack]

Anyone else care to comment on this?

 

I have a stock engine/turbo with the same late 280ZX 5 speed with 3.545 rear gears, and I've gone well over 80 without getting near boost.

 

Does the RPM make that huge of a difference? Because the load/RPM ratio is different, and the amount of power required to propell the car at 80mph doesn't change no matter what RPM you're at.

 

So it comes down to volumetric efficiency. And to be at 0hg at 80mph I think you must be at a much lower VE than I've ever reached on the freeway...

 

I might have some of this backwards, I just found it odd stumbling upon a post like this, as I've only ever hit boost when putting the engine under load.

 

...maybe my wastegate is cracked open?

General rule of thumb is: larger the turbo, the more vacuum you will have while cruising under load. The turbo isn't working as hard to feed the ~100 hp it takes to keep your car at 80 mph, etc.

[Gollum]

Well I've got a stock T3 and I can get to well over 80mph and still be in vacuum, hence why I thought the post was wierd.

[Guest mafiaz]

ya the other poster also has different gearing which makes a big difference ya 80 mp but what gear and what rpm

 

this post has gone into an all out debate

[Pyro]

ya the other poster also has different gearing which makes a big difference ya 80 mp but what gear and what rpm

 

this post has gone into an all out debate

 

a 3.90 gear with a late model 5 speed (more od) is about the same overall ratio as a 3.54 with a early model 5 spd, or a stock turbo manual setup (3.54 with T5). You need to consider the trans gear in the overal gear equation (not just the rear diff). 5th gear x diff gear= overall gear ratio

 

I think it is weird that you guys don't have the same thing going on with the stock turbo. My friend also had near 0 inhg manifold vacuum at 80mph with his turbo L28. 80 mph is around 3,000 rpm and that rpm the stock turbo can easily make full boost under load so almost boosting at that point during a light load doesn't seem weird to me.

[Gollum]

it's not a matter of when can it MAKE full boost, it's about how much air the engine needs to maintain a speed. I can make 7psi by 2000 rpm going uphill in 5th, but that doesn't mean driving at 2000psi I'm anywhere near 0hg/psi. Manifold vacuum is directly related to how much air is going into the engine. If you're at 0psi just cruising then you've got some serious load going on.

 

Remember, turbos are a dynamic power adder. They're in no way connected to the engine, and driven by gas/thermodynamics. If the engine isn't moving air the turbo doesn't spin and create PSI.

[Pyro]

well, 80 mph isn't exactly a no load situation. I have seen the same situation (near 0"hg at 80 mph) in a number of L28 turbo's. If you aren't close to boosting at 80 mph with a stock turbine then I would check for a leaky wastegate are maybe retarded ignition timing or a rich fuel mixture.