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While I am not new to turbo cars (I own one - Renault Megane 225) I am pretty new to actually fully understanding how they work.
I have some technical questions about turbo operation & characteristics that I am hoping that you guys will help me with.

Unless stated otherwise, please assume that questions imply all else being equal
S - statement
Q- question.

S: Or all else being equal, a turbo's maximum efficient boost psi is determined by the turbo size. , big turbo's can flow at a higher boost psi than little turbo's.

Q: With reference to turbo charged motors, 'spool up' is the time it takes for a turbo to reach the maximum programed psi. 'spool up' and 'lag' are the same thing.

Q: With relation to a turbo charger, what is flow? Can turbo's of the same size have different flow capacities?

Q: Does increasing boost psi increase or decrease lag?

Q: Can a turbo reach max set boost pressure without the WOT?
Ie, changing into third at 100kph will mean that rev's are high, even though not at WOT. is full boost achieved in this situation.

Q:Once a turbo has reached it's maximum boost psi, there is no lag?

Q: Is maximum boost psi achieved at a certain rpm in any gear?

Im at the cinema now but tonight I should have time to answer.

What are you planning on achieving with your turbo build?

Dann

The boost is regulated by the wastegate. You can run lots of boost on a small turbo but it probably won't make more power because it hits a flow ceiling.

Generally speaking, the smaller the turbo the quicker it will spool (less lag) all else equal.

A car engine is essentially an air pump, so flow usually equates to power.

You can hit the wastegate pressure without going WOT if the engine is under enough load. You can also easily have high revs and be making no boost if you don't have the throttle open much.

Generated boost is dependant on load not RPM.

Sorry I'm pretty tired, I don't know if I am much help?

S: Or all else being equal, a turbo's maximum efficient boost psi is determined by the turbo size. , big turbo's can flow at a higher boost psi than little turbo's.

Well yes, probably an oversimplification but essentially correct.

Q: With reference to turbo charged motors, 'spool up' is the time it takes for a turbo to reach the maximum programed psi. 'spool up' and 'lag' are the same thing.

Sort of. You are right about spool, but lag is the delay in engine response created by slow spool and is characterised by a steep torque curve as the turbo begins to spool at higher RPM. The longer it takes a turbo to spool, the greater the lag. The reality is that all turbos lag, but it is less noticeable (or sometimes not noticeable at all) in smaller, better flowing and/or well-tuned turbo systems, or with bigger engines, as the turbo is spooling effectively at a lower relative engine RPM.

Q: With relation to a turbo charger, what is flow? Can turbo's of the same size have different flow capacities?

"Flow" (as you seem to be referring to it) relates to how much air you can push through the exhaust side of the turbo's turbine, and how efficiently you can push it through. The exhaust side of the turbine drives the intake side of the turbine, which in turn determines how fast a turbo can spool and how efficiently it can compress the intake side of the engine - as such, "flow" is a very important concept and effects spool and lag speed, as well as intake-side flow efficiency.

There is also an intake-side "flow". This simply relates to how much theoretical air a certain turbo can push on the intake side (assuming maximum ideal exhaust flow and spool conditions), and the theoretical horsepower that an ideal engine could produce having regard to this amount of air flow on the intake side. It isn't a very helpful measure in my view.

Q: Does increasing boost psi increase or decrease lag?

Obviously, any turbo will achieve a steady state at 4psi much sooner than it will achieve a steady state of 20psi.

The lag effect, being the delayed torque response resulting from the rise in boost, will be magnified by the increase in boost. But this doesn't necessarily mean more "lag" as you call it. The nature and intensity of this effect will depend on your turbo and tune, and your boost level. All things being equal, the engine will produce the same power up to 4psi, but from there the 20psi setup will just keep on giving right to 20psi from which point the torque curve will rise at a more uniform rate.

Q: Can a turbo reach max set boost pressure without the WOT?
Ie, changing into third at 100kph will mean that rev's are high, even though not at WOT. is full boost achieved in this situation.

Boost availability is entirely dependent on LOAD level, not throttle position (though generally throttle position has a major effect on load).

My setup is running 10psi at present. I see 10psi at anything about 50-60% throttle levels in any gear and at any engine speed above 3000rpm (as low as 2000rpm at WOT). If I want, I can run my engine at redline at low throttle levels and see zero boost or vacuum readings. I don't do this often (because it is retarded) but it can be done.

Q:Once a turbo has reached it's maximum boost psi, there is no lag?

Again, I think you mean lag as in the lag in torque resulting from spool, and of course if this is the case then yes, once max boost is reached lag is GOOOONE (and so are you, in a hurry).

Q: Is maximum boost psi achieved at a certain rpm in any gear?

Again, boost has nothing to do with RPM and everything to do with load. Throttle position is the most critical element here, not RPM. Gear has nothin to do with it, except as far as it affects load on the engine.

NitroDann wrote:Im at the cinema now but tonight I should have time to answer.

What are you planning on achieving with your turbo build?

Dann

In a nutshell, make it feel like an N/A, so eliminate lag.
Certainly not chasing huge hp numbers.
Ideally I would like a set up that was on full boost by 3k and didn't die off until 7k (or higher if possible).
My megane dies off at about 6kish... Id want higher rpm power for the mx5.

Other thing that I don't like about my Megane is the throttle response (eg heel toe) is very very dull. You do get used to it (just give throttle a big shove) but I love how i only have to breath on the mx5 to get the revs zinging up. This could be due to it having a dual mass flywheel, or no physical throttle cable - Or is this just typical of a turbo set up?

I think it would be a combination of all 3 factors.

My mate had trouble with his sports bike when we were out riding. In the rain, so Ill answer tomorrow.

However zossy's answers are great.
Ill be elaborating, specifically to deal with your perception of lag, and Ill give definitions of lag and boost threshold and explain the differences. Common misunderstanding, or at least its common for people to misuse the terminology and confuse a conversation like this.

Dann

cooper1896 wrote:While I am not new to turbo cars (I own one - Renault Megane 225) I am pretty new to actually fully understanding how they work.
I have some technical questions about turbo operation & characteristics that I am hoping that you guys will help me with.

OK.

cooper1896 wrote:Unless stated otherwise, please assume that questions imply all else being equal
S - statement
Q- question.

OK.

cooper1896 wrote:S: Or all else being equal, a turbo's maximum efficient boost psi is determined by the turbo size. , big turbo's can flow at a higher boost psi than little turbo's.

Yes, a bigger turbo will usually be able to flow efficiently at higher levels of boost (in psi) than a smaller turbo, however PSI of boost is irrelevent, it isnt important how much boost you are running, whats important how much airflow you make, airflow is power.

cooper1896 wrote:Q: With reference to turbo charged motors, 'spool up' is the time it takes for a turbo to reach the maximum programed psi. 'spool up' and 'lag' are the same thing.

Lets get this straight-
Boost threshold is the lowest RPM that a setup starts making boost. Just 1psi counts.
Lag is the amount of time the car takes when above the boost threshold to achieve enough boost to make good torque.
If a cars boost threshold is 3000rpm, and by 4000rpm its at full boost (say, 12psi) it takes 1000rpm to hit full boost. However at 5000rpm it may take only 400rpm to hit full boost, so lag isnt quite as clear cut as many make it out to be.
People also commonly refer to boost threshold as lag.
Also, if you are running a smaller turbo at a very high boost level, say a Nissan S13 T25 at 20psi then at 20psi its FAR out of efficiency range. You may do this to get the last 10 hp out of the system over running 16psi.
In a setup like this its not important to achieve full boost quickly, as half of full boost will net you 80% of the torque.
SO... the LAG between 0psi and 20psi, above the BOOST THRESHOLD isnt important, the LAG between say, 0 and 10psi is whats important.

cooper1896 wrote:Q: With relation to a turbo charger, what is flow? Can turbo's of the same size have different flow capacities?

Zossy's explanation is pretty good. Of course a bigger intake wheel or a larger a/r will flow more but it weighs more and in the case of a bigger a/r it takes longer to pressurise the larger housing. Many people dont understand why big turbos make more power at the same PSI, stating that they flow more air. They dont usually, they usually flow more exhaust. Its the small exhaust housing that restricts power most.

cooper1896 wrote:Q: Does increasing boost psi increase or decrease lag?

In the lag vs threshold response I gave I showed a car making 1psi at 3000rpm and 12psi at 4000rpm, of course its making 6psi at 3500rpm, so in this case if it were instead limited to 6psi max, it would take only half the time to hit full boost, but as zossy says, thats not important as torque output will remain the same up to 3500rpm.

cooper1896 wrote:Q: Can a turbo reach max set boost pressure without the WOT?
Ie, changing into third at 100kph will mean that rev's are high, even though not at WOT. is full boost achieved in this situation.

If a car running 20psi max can make 10psi at half throttle, of course the exact same setup limited to only 10psi will make full boost (10psi) at half throttle.

cooper1896 wrote:Q:Once a turbo has reached it's maximum boost psi, there is no lag?

Usually when a car has made the first couple of psi, and is above the boost threshold by a few hundred rpm, it will make boost as quick as your ankle moves.

cooper1896 wrote:Q: Is maximum boost psi achieved at a certain rpm in any gear?

Short answer yes.
Long answer.... sort of, the turbo spins up to speed over a period of time, if a car makes full boost at 3000rpm in 4th on a flat it may make it at 2500rpm going up a steep hill as it has more time to spin up to speed due to the hill slowing you down. The hill adds more load.
So a car may only have time to spool to 5psi in 1st gear at 7000rpm but in the longer gears may hit full boost by 3000rpm.

Hope this helps,
Dann

Also, the reason that a T25 may make 230rwhp at 16psi but only 240rwhp at 22psi is that 230rwhp worth of exhaust flow is already enough to jam pack the exhaust housing to choking point, which means that the extra 6psi in the manifold you are reading off your boost gauge isnt going anywhere, as the engine is having a hard time expelling the exhaust to let more boost air in.

Dann

cooper1896 wrote:
Other thing that I don't like about my Megane is the throttle response (eg heel toe) is very very dull. You do get used to it (just give throttle a big shove) but I love how i only have to breath on the mx5 to get the revs zinging up. This could be due to it having a dual mass flywheel, or no physical throttle cable - Or is this just typical of a turbo set up?

Having a turbo will not make much difference to that but the electronic throttle and DMF will kill it off.

Plenty of modern cars dont like you pressing the brakes and throttle at once due to the emissions and safety controls in it. Really gives me the shits when a car aimed at a sporting driver doesnt let you heel n toe effectively.

A factory turbo car often has lowered compression ratio to safeguard against poor fuel, boosting an n/a engine requires 98 octane ALL THE TIME.
Because you wont change compression ratio the car will be identical in response as it is now except faster when it boosts.

Dann

Would changing hydraulic lifters to solid lifters be a prerequisite or recommendation for turboing a 94 BP?
Im thinking if your not changing the camshafts then the answer is no, but thought id check.

cooper1896 wrote:Would changing hydraulic lifters to solid lifters be a prerequisite or recommendation for turboing a 94 BP?
Im thinking if your not changing the camshafts then the answer is no, but thought id check.

There are lots of things you MIGHT do to an engine if you are going turbo. Solid lifters are a good thing, but hardly a prerequisite.

I am running an internally-stock 94 BP @120,000kms with a Garrett 2560R and it goes great.

You are correct.

Dann

Andrew you should check out Chris's clubman. It's a little rocket, 10psi probably 150 rwkw!