how many of you guys have put a gauge on the up pipe and metered your back pressure?

im a little less than 2:1 ratio, @15psi boost i have about 28psi in the exhaust.....i think that may be a little higher than perfect but still not too bad.

 

What is the ratio that one would shoot for? and what are the advantages of knowing the backpressure...Im guessing that if it has alot of lag then the backpressure isn't enough.

 

I say under 2:1 is perfectly fine. Get to 2.5-3:1 and you need to look at some fixes. I did post in your other thread wondering if anybody has ever measured factory turbo cars to see what they are. Would be interested to know if factory cars are like 3:1 just to keep lag to a very minimum.

 

I'm at 2.2:1 with a little pair of .82 T-3 turbines at 15 psi on 462 ci.
Probably a little high but comes with near instant boost response.

 

I monitor the back pressure all the time.
22.5 # back pressure makes 27 # of boost.

A long downpipe will increase the back pressure significantly.
On the boat the down pipe is only about 16"

 

Monte, Am I reading/understanding you wrong or did you mis type? Wouldn't the lower number help limit lag? Please help a new guy understand, Lorne

 

he is talking PRE turbine pressure not post turbine in the downpipe

 

Ezacry.

 

Any post turbine pressure will increase turbine inlet pressure as well.
I think Mark55 is stating turbine pressure and is running under 1:1
I doubt he has 22.5 lbs in the downpipe

 

i just noticed he mentioned boat so yea doubt he has that much bp in the dp but you never know with some of these setups

 

Boats that run the exhaust underwater have to have a pretty good amount of back pressure I would imagine. I don't think his is like that.

 

I understand the subject.
The pressure gauge is connected to the header about 2" before the turbo.
In the boat the pipes out of the turbos are only about 16" long and there is little if any restriction there.
That made the 22# backpressure to 27# boost ratio.
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When we had it on the dyno and had 8' sections of 5" flex pipe on the exhaust to get it out through the wall the backpressure IN the header went up significantly, to 40# backpressure @ 29# boost.

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I learned something about exhaust pipe sizing there.

 

I would prefer to stay under 1.5:1. I don't like it when the turbine pressure starts getting close to 1.75:1.

 

Ok, Thanks Guys! I have to admit...it took a bit of thinking & re-reading for it to "pop" out & understand what was going on.
I really appreiciate the interest most of the guys on here have with trying to help the new guys learn.
Safe to assume as the exhaust (pre-turbo) backpressure goes up for an extended timeframe (endurance racing, boats, ect.) overheating headers, exhaust valves, ect. could become a negative side effect?
Without building quite a few of a similar combo (with controlled changes) is it rather hard to know in advance where this ratio will play out? Beyond...small pipes & a.r will yield a higher number & big pipes, low compression & larger a.r would typically produce down around 1:1? Again thanks to all in helping me learn, Lorne

 

Am I understanding this incorrectly? If you're running more pressure in the intake vs. exhaust you'd be in the surge area of the turbo.

How does 22.5psi overtake 27psi? Shouldn't the exhaust pressure always be higher then the intake pressure?

 

No, not necessarily. Typically, such an occurance requires big turbo(s) in relation to the engine, so it's often the case that you'd be on the left side of the map, but you don't have to be so far left as to induce surge.

I just tried to answer this but realized there was a distinct possibility I could be talking out of my ass.

Although I think I understand the concepts at play here, I'm not confident in my ability to explain them. But the short answer is no, exhaust pressure doesn't need to be higher than boost. I'm going to go do some searching now.

 

no, the exhaust wheel is smaller than the intake side,so it makes more cold side pressure with less hot side pressure.

 

If the turbos are 1.32 S475's, the turbine wheel is bigger than the compressors.

 

I guess I just don't understand. How does the lower pressure (preturbine) overcome the higher pressure in the manifold? That doesn't make any sense to me.

 

It's possible. The lowest I've seen is .84-1. Spool is baaaad though lol