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2001 Lotus Exige
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Discussion Starter #1
Hi all,

First post here, looking for some help as to what might be the limiting factor in my engine setup that's causing the torque drop off after 6700rpm.
My setup is as follows, all in the back of a Lotus Exige:

K20A2, Forged rods and pistons, 9.6:1 CR
Fully stock head cams and valve train, PRB intake, 70mm TB
GTX3076r, on 1.06 AR turbine housing, sidewinder manifold, 3" exhaust no cat.
Twin barrel water to air charge coolers
GearX 1-4th, LSD, uprated clutch
45mm waste gate, 4 port boost controller, Kpro4, ID1050x injectors

Running at 22psi, 550bhp/400lbs/ft (crank) on pump fuel, but the torque is just dropping like a stone at the top end. Had a very similar previous dyno with torque drop off running a 0.83 AR, the larger housing gained 20bhp, but didn't fix the shape of the curve. Didn't have any issue when running 440bhp on a GT3071 0.63 at 14psi.

Is this a back pressure/cam/srings issue? Any other thoughts? I just want to get the setup optimised and get it to hold the torque to closer to the redline.

Many thanks

101810
101811
 

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Arouse the DAMPFHAMMER!
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I would need calibration and WOT logged files to see more of the engine. In a first order assumption it seems to me boost can't be hold due to some possible reasons:
  • restriction e.g. by the cooler bottles
  • waste gate closed loop system can't hold MAP constant
  • increased MEP leads to timing retard at the higher end
  • not optimal VTC and ign timing mapping
More and more precise only possible with the above mentioned files.
 

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2001 Lotus Exige
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Discussion Starter #3
Hi, thanks for the reply. Unfortunately I don’t have that data, and the dyno run was quite a while ago. I had a fuel pump failure since so have had to rebuild with new pistons.
Another way to look at it would be; while the engine is out is it best to upgrade the valve train or larger turbo for lower back pressure, or something else, if I can only do one?

When it gets tuned I’ll be sure to ask the questions/get the data you mention to know for sure.
In a perfect world I’d tune it again first to get the data, but it’s a time/money issue.
 

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Arouse the DAMPFHAMMER!
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...have had to rebuild with new pistons.
Were there any indications of a ring failure or piston seizure? When it leans out without safety shut down by ECU then it is very likely the hone structure is affected. Just curious on that if you don't mind to share pictures, I would be glad. Not to forget to do a compression or better leakdown test before and after break in of them.

...Another way to look at it would be; while the engine is out is it best to upgrade the valve train or larger turbo for lower back pressure, or something else, if I can only do one?
I would think first, analyse the situation in 2nd order and than take conclusions out of the recognitions. We need the tune and logg for that. To get those files is duty. If there is no chance, log it and pull the calibration from it. All other stuff is hunting a duck in the fog with a catapult 😉 .

Just an offer, if you get the data, I do the analyse and say you if the calibration caused it or the A/R ratio of the trubine or working line of the engine in the compressor for zero cents!
 

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looking at the compressor map of this TC, it seems you are getting somewhere close to the choke line.

101847


Depending on the air filter used and its pressure drop, you are somewhere near a pressure ratio of 2.8 to 3 at 1.5bar of boost. At 550 HP you are also somewhere around 55 to 60 lbs/min of air flow. if you were severely knock limited or lots of timing pulled due to high IATs, you could have also flown 60 or 65 lbs/min.
From the diagram you can see efficiency and pressure ratio dropping fast as airflow increases.
You turbine sees tip speed exceeding the speed of sound.
In a log you will likely find boost not increasing or the waste gate duty cycle changing as less and less gas has to be bled.
You should also a see massive increase in air exit temperatures. Going from 75% efficiency to 60% does not sound a lot, but it means the heat (energy) pumped into the gas increases by 60%!! And that does come with enormous temperature increases.

IMHO, and I have little clue, I think you need a bigger compressor that can pump more air.
 

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2001 Lotus Exige
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Discussion Starter #6
LotusElise - No piston seizure, it just needed re honing. The garage when ahead and put it all back together before I had a chance to upgrade any of the valve train. It was back on the dyno yesterday, I'm waiting for the graph and the data to come through, but apparently it's holing torque better now. If I'd have known I'd would have been there for the tuning session and made sure to get all the data logs!

Lotus - I agree exactly with you thinking, and to make it worse it's a gen 1 not gen 2, so I think it's much more likely to be close to the choke line. I was thinking a G30-700 would be a better match for it, but I'm not sure they've releases the 1.01 ar hotisde housing yet, I wouldn't want to go back to a 0.83 or up to a 1.21.
However I was told by a couple of tuners that the curve doesn't look flow limited (as then power would flat line and the torque would drop, here both drop), and to look to the valve train for issues.

Anyway, should get the car back tomorrow and boost by gear has been setup which should help with the drivability, as before I could only put power down in 4th despite it being mid engine RWD.
 

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2001 Lotus Exige
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Discussion Starter #9
The car is back and driving fine, BBG is setup in the lower gears. Annoyingly the garage lost the dyno from the tuner, and can’t get a copy. Also was told it had a Kpro, but turns out to be a K100, so I can’t get any data or view the tune. So no idea what boost it’s running in what gears, or what the torque curve looks like.
from limited driving it doesn’t spin the tyres as much in 1-3rd, also the Rev limiter seems to have been lowered from 8600 to 8000-8100, difficult to tell while keeping the thing on the road.
The torque drop off doesn’t feel as bad on my butt dyno, but I would expect that in the lower gears if the boost has been limited. Also the lower limiter would disguise it in higher gears.
I’ve got a VBOX, so will get some actual acceleration data through the gears at some point and I’ll be able to work the torque curve from that fairly accurately as I know all the gearing and drag maths.

It’s just good to have the car back on the road, and it’s driving well (or as well as something with 600bhp/ton+ can).
When funds allow in future I’ll either get all the head work done, or get wheel speed sensors and traction control set up. To be honest it doesn’t need more power on the road, I just want to get it reliable and drivable now
 

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Arouse the DAMPFHAMMER!
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Also was told it had a Kpro, but turns out to be a K100, so I can’t get any data or view the tune.
Sorry to read that. That's a huge difference in invest and adaptability in future. Stepping up to something where you can log and revise the calibration would definitely help to check the status quo and to improve reliability if necessary.

...I’ve got a VBOX, so will get some actual acceleration data through the gears at some point and I’ll be able to work the torque curve from that fairly accurately as I know all the gearing and drag maths.
That's a good idea 🆙

When funds allow in future I’ll either get all the head work done, or get wheel speed sensors and traction control set up. To be honest it doesn’t need more power on the road, I just want to get it reliable and drivable now
Reliability and Drivability are properties of the calibration tune. Access to that would give you or a different Tuner or anyone else with the specific knowhow about this properties and what it need. My recommendation would be to invest exactly there to achieve your goals, it would also a door opener for traction control (wheel speed sensors, harness are still necessary if you want a closed loop control of it, otherwise gear wise limited boost can achieve a sort of first step into that direction).

BTW, if you upgrade to KPro, it would be simple to copy the calibration from the K100 to the KPro, would save you the money for the necessary tune of it.
 

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yeah, you have to send the ECU in anyhow to have a KPro board fitted. Outfits like CPL with access to the K100 can copy the map off that for you while upgrading. The Kpro v4 seems like a decent kit.
 

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Discussion Starter #12
So if I was thinking about ECU and traction control keeping with Hondata, I’d need a Kpro4 and the separate traction control unit, which is going to total around £1400. For that amount would I be better off looking at a Haltech 1500 or Link ECU or similar?
Syvecs looks nice bit is a big step up again in price.
I have got some VBOX data, looks like it’s just running wastegate (1 bar) for the first 4 gears then up to 1.5bar for 5 & 6, so not the most sophisticated calibration.
 

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Arouse the DAMPFHAMMER!
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So if I was thinking about ECU and traction control keeping with Hondata, I’d need a Kpro4 and the separate traction control unit, which is going to total around £1400. For that amount would I be better off looking at a Haltech 1500 or Link ECU or similar?
Syvecs looks nice bit is a big step up again in price.
AEM Infinitiy from 5 on, Haltech Elite from 1000 on, Link ..., MaxxECU from Mini on, and so on.... Definitely a better step than keeping the Honda original ECU, as it wasn't designed for race or modified stuff. A lot of hold backs like knock control, fuel control are unbolted when switching to a standalone. A lot of more, but also the need to get it tuned again.

I have got some VBOX data, looks like it’s just running wastegate (1 bar) for the first 4 gears then up to 1.5bar for 5 & 6, so not the most sophisticated calibration.
Indeed highly sophisticated 😂. First gear with 1 bar of boost in that light car makes really sense...even my NA'd stock K20 engine in my '02 Elise was able to spool the wheels (Semis) hard in first gear.
 
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