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Going by PSI vs HP for engine limits? I don't think PSI matters much?

Going by PSI vs HP for engine limits? I don't think PSI matters much?

 
Old 01-06-2019, 03:28 PM
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Default Going by PSI vs HP for engine limits? I don't think PSI matters much?

I'm having a hard time understanding how high psi can cause damage. Like running a stock bottom end B series, small turbo at 30 psi to make 300 whp vs a larger turbo at 15 psi to make 600 whp. I know generally, the higher psi you run after a point, efficiency drops, turbo has higher wear and IATs rise. But there's tons of turbos now that can hit 70%+ efficiency around 40 psi etc.

To my understanding, PSI doesn't matter, HP does. PSI tells us how hard the turbo is working and how far we can push it based on its compressor map. I'd like to go with the smallest turbo I can find with a nice compressor map for higher PSI but I wanted to run this by the folks here and get your understanding on it first, thanks.

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Old 01-06-2019, 04:04 PM
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Default Re: Going by PSI vs HP for engine limits? I don't think PSI matters much?

Allot of it depends on the engine your putting the turbo setup on. Lower flowing motors seem to do better with a small turbo running higher boost, example ls motors do great with crappy cast iron manifolds on small turbo's running high boost, a b16 on the other hand likes higher flowing parts. The higher the boost the more back pressure your exhaust will see causing heat build up and forcing hot exhaust gases to remain in the cylinder, this kills power and increases chances of detonation as well as generally you have to run less ignition timing to make the engine run safely.


Over the years when I first got into hondas everyone would beat their meat about a massive turbo making moderate horsepower numbers on low boost. Like a 6262 on a b16 on 8lbs making 370 or something along those lines. This sort of engineering leads to a terrible powerband.

You have to find the right size to optimize your engines powerband and complement it in my opinion. I'm sure the shodan can give a much more thorough explanation with science and **** to back it up.
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Old 01-06-2019, 07:22 PM
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Default Re: Going by PSI vs HP for engine limits? I don't think PSI matters much?

This new CA exhaust law that just passed is insane. Everyone needs to sign the petition only 80k out of 150k signed. Instant $1,000 ticket, mandatory court and can only be corrected by a state ref. Also, they expect you to not drive the vehicle until it's fixed, they can keep giving you another ticket every time. Makes me sick, not putting anymore time or money into the CRX until this **** is figured out, pretty bummed. Cousins friend just got pulled over and cited in a 997 turbo in Sacramento.

I'm reading more and more articles of folks running higher pressures. Like 40+psi. Mostly racing stuff but why wouldn't these guys just run a giant turbo and anti lag? Interesting stuff.
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Old 01-06-2019, 09:03 PM
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Default Re: Going by PSI vs HP for engine limits? I don't think PSI matters much?

The higher the boost the harder it is on the valve train, ring lands, and rod bearings. If high IATís, valve float, and detonation arenít your thing you better build the engine to handle what your throwing at it.
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Old 01-07-2019, 06:07 PM
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Default Re: Going by PSI vs HP for engine limits? I don't think PSI matters much?

If the engine doesn't like the high boost pressures, wouldn't there be signs like not making anymore power on a dyno? From what I understand, detonation comes from high IAT's usually from running past a turbos efficiency range. Not saying you're wrong, I just don't understand how 40psi and 300whp could cause more stress on valvetrain, ringlands and rod bearings vs 15psi and 600whp. (I do understand why a smaller turbo could cause more stress than a bigger one, huge low rpm torque for the main part vs dividing the load with rpm.)

Cams/LSA could be adjusted to aid in scavenging of the high exhaust pressures, only way I'm seeing high psi causing a problem is when it becomes a restriction. Generally 15psi makes more power than 10psi on your average turbo, if the compressor map says it's still efficient at a pressure ratio of say 3.5 with a goal of 400lb/min of air I don't get why it's harmful to do so. The turbo is happy, IAT's are reasonable, shaft speeds in check, etc.

I can't remember off the top of my head but I know lately there's more and more turbos that are happy at higher pressure ratios, which in my mind is great, less lag and more power. But all I've ever heard is high psi is no bueno, just never understood what makes 25psi okay and not 35psi.
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Old 01-07-2019, 06:29 PM
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Default Re: Going by PSI vs HP for engine limits? I don't think PSI matters much?

Detonation can come from a number of things. Not just high IATs, though they can contribute. Overly advanced ignition timing, poor fuel, excessive oil in the combustion chamber, among other things can also lead to detonation.

More boost will cause more stress on the valve springs. 20psi is 20psi, regardless of turbo size and airflow at a given pressure. If you're running 20psi of boost you have 20psi of pressure pushing down on the back of the intake valves, effectively reducing your spring rate.

The bottom end doesn't care about PSI in the intake tract, assuming the engine is tuned correctly and the setup operating within efficiency limits. The extra air coming in just allows more fuel to be burnt, thus making more power. Generally speaking, 300hp is 300hp to the bottom end whether it's coming from a 6262 or disco potato. Total cylinder pressure(NOT boost pressure) required to make 300hp is not going to change. Torque delivery is a different story. The harder and faster boost comes on(like with a small turbo), the harder the rods are shock loaded because of increased torque output at low rpm. Not really an issue with a built bottom end that has forged rods but a stock set of Honda rods won't like making a ton of down low grunt. Ultimately the limiting factor is how much power/torque the engine can handle, not the amount of PSI it takes to get there.
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Old 01-08-2019, 07:22 PM
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Default Re: Going by PSI vs HP for engine limits? I don't think PSI matters much?

Interesting thank you, starting to understand. Didn't know psi in charge pipes was same as wastegate, need to look into that. Thought everyone goes by wastegate pressure when talking about psi. Was going to go with forged internals, new bearings, air/oil separator and stiffer springs. Maybe some cheap delta regrinds. Want to avoid sleeving the block, proper machining adds a ton to sleeve cost.
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Old 01-08-2019, 09:19 PM
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Default Re: Going by PSI vs HP for engine limits? I don't think PSI matters much?

The wastegate is what regulates boost pressure by basically being a metered exhaust leak so they are directly related. If you are running a 9psi wastegate spring then you should have ~9psi operating pressure(depending on a number of variables) from the turbo. You can change springs, use boost controllers, etc to manipulate the wastegate which will speed up/slow down the turbine. This will effect the speed of the compressor(which is what produces the airflow/boost) as they are connected directly via a shaft.
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Old 01-09-2019, 12:25 AM
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Default Re: Going by PSI vs HP for engine limits? I don't think PSI matters much?

Psi is a combination of the amount of restriction the head creates for the asked air volume and the size of the turbo.
Like bigger superchargers bigger turbos can move more air without even creating boost. Every rotation moves more air.
And every turbo rotation spends energy to create the compression.

Super simplified:

You have a 16 liter balloon.You want to fill at 4 seconds.
Every person can inhale nature the size of his lungs. A small person will have like 4 liters capacity. An athlete will have 8 liters capacity
The athlete will have to take 2 breaths each 2 seconds long to fill the ballon.(Less turbine shaft rpm)
The smaller person will need more effort as he has to breathe 4 times in 4 seconds.(more turbine shaft rpm)
The "boost" has to do with the size of the balloon opening. Smaller opening higher restriction created and the stronger (again more effort) you ll have to push
for the air to go past it to fill the balloon. The bigger the opening the lest restriction and the less force required for you to push since the opening is larger.

Ill tell you this VERY few people actually know why bigger turbo makes more power...

Its also a matter of efficiency. Since the power is made from the turbine and the bigger turbine wheel creates bigger lever
the less the restriction it creates on the exhaust gases gimping the engines ability to "exhale" to empty cylinders from exhaust gases to make room for
clear air fuel mixture.
~~~
Imagine 2 pipes coming out of the ground/concrete. The on is 5 feet long the the other is 2 feet long. The same person will need much less effort
to bend the longer pipe due to bigger lever.
~~~
Also the story goes on and on because of the less RPM need on the turbine shaft on the big turbo. Bigger % efficiency for each rotation thats also a compression etc etc.

Last edited by Balor_Gr; 01-09-2019 at 02:11 AM.
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Old 01-09-2019, 09:24 PM
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Default Re: Going by PSI vs HP for engine limits? I don't think PSI matters much?

Thank you for the explanations, appreciate it.
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