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BongoBennie 12-02-2019 01:19 AM

Looking for expert turbocharger advice for Honda K20
I am putting together an eco minded K20 for commuting. Engine is a hybrid K20...... K20a2 block, K20a3 head, K24a4 Intake manifold. CR sits at 11.7:1. While the redline of the engine will be capped at 7500 im not too worried about max performance at high RPMS, what I DO want is a torquey K20 with low RPM snap and as little turbo lag as possible. The original plan was to use the Garrett GT2554R which would be good for 220WHP but I'm thinking I can get more power with the same spool with a slightly larger turbo. I've tried to ask this same question in the large K series turbo friendly forums but since I'm not talking about 500+hp 9800 rpms it's hard to get someone who knows to chime in.

Here is a link to the build thread

Thanks for any input.

oil pan 4 12-02-2019 02:27 AM

Whats your horse power target?
Compression like 11:1 really limits boost.
If you want low end torque that compression ratio and no turbo may be the way to go.

BongoBennie 12-02-2019 03:15 AM

A stock K20a2 at 11:1 is known to handle over 18psi and put out 500HP on pump gas, I would be happy hitting 300 at the crank but max HP numbers are not goals but rather just hoping to get what I can for this type of setup. No matter what the turbo I will only run low boost while daily driving but it a bigger turbo will spool just as fast as a smaller I will step up accordingly. In other words if a 2860 wil spool just as fast as a 2554 Ill go bigger even if the max hp is tuned the same.

oil pan 4 12-02-2019 11:35 PM

Well there's a lot that goes into spool times.
Compressor size, trim A/R, turbine size, trim and housing type and A/R, oil flow to the turbo, bearing type volume of piping and intercooler.

If you only intend to run single digit psi then compile a list of smaller turbos that have a high trim number on the compressor. Those will be more efficient at lower psi and will move more air faster at lower engine loads.

Ecky 12-03-2019 08:43 AM

What kind of fuel do you intend to use? Those 11:1 K20A2's are using alcohol. On a stock engine, you have to pull a lot of timing already at WOT with 87 octane which hurts both efficiency and torque.

Just as an example, on 87 octane the 9.7:1 Accord motor actually produces more torque and power (at high load) than the 10.5:1 TSX up to the point it runs out of air (somewhere around 6000rpm) because the TSX needs to run 3-5 degrees less timing with that fuel. Unless you're intending to run alcohol in your DD, I would stick with a lower compression ratio or just have boost not come on when using a low octane fuel. Higher CR helps with efficiency at part throttle but if cylinder pressures are too high (high load + high CR + boost) you have to retard timing such that you're sending a lot of unused energy out the exhaust.

BongoBennie 12-03-2019 10:51 AM

In the first stage it will be just 91 octane and 5 psi/low boost but on a typical drive to work I can see making it there without actually going into boost (90%freeway), the idea is the efficiency 'switch' is the gas pedal itself, soon after the car is running I will start adding in E85 and playing with it. I also have plans to play with the Hondata injector driver and a second fuel system and/or CNG still too soon to tell but the first step is to get the car running on 91. Keep in mind I'm under no illusions that making huge power with a small turbo and high compression is possible or ideal.

FWIW the original idea I had with my drinking buddies was an ICE engine that will outlast gasoline but still run on the stuff while its prevalent. Also it it IS a fail it's easy to change up/disassemble/sell/etc.

Ecky 12-03-2019 10:56 AM

A more nuanced explanation I found:


Turbos have an effective range where they move the most amount of air with the smallest heat generation. The translation to how many PSI that is depends on the size of the engine.

Now, how does this help to make power? It's a combination of putting more oxygen in the cylinder, keeping the intake charge cool [so that you can go back to the first item of having more oxygen in the cylinder] , and most importantly, raising the peak cylinder pressures during compression.

So far, I haven't mentioned compression ratio at all [current case exlcuded], and that's because for the most part, the info so far on this thread is accurate. So why, then, do most people view a lower compression engine better for boost?

The answer is actually pretty simple. You can only increase the cylinder pressures during compression so much before gasoline spontaneously ignites [detonation, pinging, whatever you want to call it, it's bad]. What happens, then, is that when you boost a high compression engine, the boost makes a larger effect on the peak cylinder pressures than the same boost level on a similar engine with a lower compression ratio. So why, again, do most people view a lower compression engine better for boost?

With a lower compression ratio to deal with, you can inch yourself closer to the maximum cylinder pressure for the given fuel you're using easier than you can with the relatively constrictive range of boost you can deal with on a high compression engine. The downside, again, is that you need a turbo that will operate efficiently to provide the necessary amount of air according to the three guidelines I covered earlier.

Another thing to keep in mind is that anytime you compress air, it gets heated, and since you make your power from the rapid expansion of air in the cylinder, the lower the beginning temperature theorhetically yeilds the greatest change in temperature in the cylinder, which means the fastest/most forceful expansion, which equals power. I take this to mean that if you can compress more air more efficiently and then compress it in the cylinder with the least amount of extra heat generated, the more power you will make. So far, most turbo engine manufacturers/racers agree with this.

In this forum, pretty much anyone that mentions CFM talking about a turbo is likely to be someone that I've told/taught about boost. CFM [cubic feet per minute] is a measure of how much air the compressor will flow efficiently [ie- low temperature increase], and so that is the context it is mentioned, not as a literal definition of how much air the engine will ingest in a cycle, only how much air it can move effectively enough to be useful.

I'm no expert. Some of what I say is extrapolation.

You can drop peak cylinder pressure by retarding timing, all else being equal. You can thus maintain the same peak with increased compression by retarding timing - start combustion later and rising cylinder pressure will happen later, peak pressure later in the stroke after peak compression so your total peak cylinder pressure stays the same but happens at a different stroke angle. That peak cylinder pressure is your ceiling with a given octane of fuel. Dumping fuel (really rich AFRs like 10:1) can help by cooling the combustion chamber but that's also a loss.

What I'm uncertain about is if you can remain neutral at WOT with extremely precise engine tuning, or always have a net loss with high compression + boost. That is to say, after a certain point you're losing power and efficiency because you have to start combustion so late (to keep peak pressure from getting too high) that more and more of the combustion energy is just being flushed out the exhaust and never pushes on the piston, but is this equal to or greater than the gains from increased compression? This applies even without boost; more compression is not always better, but it may or may not always be worse.

I'm aware that engine tuners often use exhaust gas temperature (per cylinder) to find ideal timing advance. Advancing it to a certain point drops exhaust temperature because more of it is turning into mechanical energy. Retarding it and watching exhaust gas temps can help find the minimum timing needed to make power with high octane fuels like alcohol, which is typically not knock-limited.

Ideally at a given compression ratio you would find where advancing your timing more provides no more power. If you can do this on the fuel you're using without detonation, you have room to add a little boost OR increase compression, with no negative consequences.

Grant-53 12-03-2019 12:35 PM

Consider what the actual operating RPM range will be. Turbine speed is affected by impeller mass and diameter as well as flow and pumping efficiency factors. Plenty of good information in the previous posts.

BongoBennie 12-03-2019 12:39 PM

On fuels, after the car is tuned and running as best as it can on 91, I will run it for a good 6 months, and document the results, then do the same on E85 and as long as the cost does not prohibit for some odd reason I will stay on E85 and start leaning it out. at that point I will have a 20/30 gallon capacity? keep in mind my commute is (one way) 26 miles in 25-35 minutes. 2 miles Las Vegas City traffic, the rest is open freeway. 6 days a week.

I have also done my homework on CNG conversion (all 4 of my Hondas have aftermarket ECU stuff available) There is a huge pump on my direct commute although it's not much cheaper than gas, ***If ANYONE reading this has knowledge on a home pump please let me know*** That is very interesting to me and hoping to go that route with all my cars however with the civic..... subtract all the swap parts and its nothing but a $500 car even if mint so I'm going to cut it into a pick-up truck for (A) the fun of it (B) dont need 4 seats (C) can easily add tanks in the rear including a proper truck sized CNG tank (Utahns know what I mean)...... crazy, yes I know.

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