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TDI Power Enhancements Discussions about increasing the power of your TDI engine. i.e. chips, injectors, powerboxes, clutches, etc. Handling, suspensions, wheels, type discussion should be put into the "Upgrades (non TDI Engine related)" forum. Non TDI vehicle related postings will be moved or removed. Please note the Performance Disclaimer.

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Old March 1st, 2018, 09:07   #1
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Join Date: Apr 2014
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Default Question about Nozzles, HP, and RPM

So I have a question about how increasing the rpm while having the same size nozzles effects the hp. I understand that the nozzle size is the limiting factor on how much fuel in injected each cycle but how much does the rpm factor in?

Say that someone has a car with stock nozzles. The best I've heard from stock nozzles is just under 130hp at the crank. But this is never past 4500rpm it seems. I'm guessing the hot side on the stock turbo is already starting to choke the engine even at that low of power. So if the stock turbo was replaced with something that allowed for unrestricted flow to 6000 rpm and stock nozzles run with full fueling and a lift pump would I see a hp increase up to 6k or would it drop off much earlier without any increase in power? Does the pump start to inject less per stoke at higher rpms due to cavitation ect? Assuming sufficient air what else limits fueling besides the nozzles?
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Old March 1st, 2018, 12:32   #2
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Trying to force more fuel through a small nozzle increases the length of the injection window (time) and there is a point of diminishing returns where more fuel is just making smoke and heat but no more power.

Other than that, the rpm capability and plunger diameter would be limiting factors.

Iím sure DBW will be around eventually to elaborate, he certainly knows more about it than me. Lol.
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Old March 1st, 2018, 13:42   #3
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With stock nozzles and a big turbo you would notice a drop off. Even with the timing advanced as much as you can there would not be enough time to stuff that much fuel out of the stock nozzle at high rpm. The stock nozzles are very small to decrease nox and other emissions even though slightly larger ones are a little more efficient. If you are trying to decide between turbo or injector nozzles pick the nozzles. I am not an expert though so keep that in mind.
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Old March 1st, 2018, 15:12   #4
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Originally Posted by Exenos View Post
So I have a question about how increasing the rpm while having the same size nozzles effects the hp.
Your first step is to understand how and what HP is, seen as your question is lacking the knowledge of the relationship of HP and Torque have with each other over RPM, you need to understand this first.
Here is a great video for you to watch.

Now that you understand, your question is not about RPM, its about the power curve relationship with the function of torque, as HP is calculated from torque and RPM.

in a nut shell, you want more power, make the fuel you have more efficient, or add more fuel and make it burn most of it vs rolling coal.

you want higher revs, do all you can for head work, port and polish ect, Frank0 is the place to go for head work and a jazzy port, put a monster cam in it, and up your fuel and get a new tune. otherwise higher RMP is pointless and less efficient and lower power output because torque drops off, also diesels dont like RPM so much, im reving up to 5500 or higher and your just trying to get more out of the gears you have.

TDLR, dont, you want more power, plain and simple, nozzles are #1 and a tune will go miles on a budget.
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Old March 1st, 2018, 18:05   #5
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Originally Posted by Mongler98 View Post

in a nut shell, you want more power, make the fuel you have more efficient, or add more fuel and make it burn most of it vs rolling coal.
And that is the point of asking my question. If I can add an extra 1000 injection events per minute by running it at 5.5k vs 4.5k then in my mind that does qualify as extra fuel. Really what I was meaning is does the fueling from the pump itself fall off at higher rpms from cavitation ect even when fed by a lift pump? Even with the shorter injection window?
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Old March 1st, 2018, 18:11   #6
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Here's an example of a TDI dyno with a high rev tune (RC3 for 6000 RPM) This is in my '99.5 Golf, ALH engine. Dyno operator got the wrong model year.

You can see where the HP and torque numbers cross at 5250 RPM.

This car had stock internals, PD150 breathing mods (airbox, snorkel, airbox to intake, MAF, Intake manifold), PP520 nozzles, and a lift pump. Stock turbo. This is a conservative dyno.

The benefit of the higher RPM tune is that you can hold gears longer. This was a track day car and it was very helpful for me to be able to run to 80 in 3rd and 115 in 4th. But the engine isn't in its most beneficial RPM range above, say, 5000. It also makes a lot of heat at those revs. I've seen coolant temps in excess of 240.
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Old March 1st, 2018, 18:44   #7
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The available SOI timing is your limiting factor for RPM. For an ALH (degrees camshaft) you need to advance timing 3 degrees per 1000 RPM in order to keep the same start of combustion. More or less of course. There are other factors at play that determine the exact ignition delay, but that's a decent thumbrule to consider as a starting point for analysis.

then you are limited by the heat of compression being hot enough to actually ignite the fuel. At STP and 19.5:1 compression using the crank-slider geometry of the ALH, that works out to *about* 17.5 degrees BTDC (camshaft) or 35 degrees crank or thereabouts. (which also happens to be the degrees before and after TDC where the combustion chamber volume is double what it is at TDC, assuming a median head gasket thickenss and blah blah blah.) So it's no mistake that max timing limiter on a stock ALH is about 15 degrees (cam) and the RPM limiter is about 5000 rpm. Strange how it all works out like that, eh. Boost adds pressure above STP so more heat of compression so a bit more to play with (potentially)
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Last edited by KERMA; March 1st, 2018 at 18:48.
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