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Discussion Starter · #1 ·
Looking at upgrading the turbo on my (mostly) stock GXP.

As some folk have been returning to stock and selling their cars, I was wondering if anyone is looking to sell their aftermarket turbo?

Just throwing it out there :)
 

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Werks EFR 6758 kit
 

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Discussion Starter · #4 ·
I know there are new ones KG.
Just hoping I can catch someone “returning to stock” or with an extra. Seems to happen with some minor frequency with this car.
If ya don’t ask :)
 

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You may find that a used turbo is false economy. Depending on how much use it has had, the bearings may be on their last legs. A nice step up is a stock K04 turbo with a larger wheel installed - they either use new units or completely rebuild a used unit (bearings seals etc.). Usually $1000-1200 new.

If you do buy a used unit, do yourself a favour and take it in to a local turbo shop (there are all sorts that rebuild commercial and truck units) for assessment. A layman may not recognize trouble before it happens.
 

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Discussion Starter · #6 ·
I have read your threads on your “big wheel” KO4 (I think?) in the past. Would love your feedback on the below…

I have searched all over and read a multitude of opinions on this but here we go (opens can of worms):
What turbo upgrade causes the least amount of low end torque loss?
I understand the big wheel allows the KO4 to drop off at a higher RPM than stock, which is great.
But I do wish there was a way to increase horsepower for the Sol that didn’t cost (or possibly even improved) low end torque..
 

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Bottom end torque can be improved by upgrading the cams to get the valves to open further. The problem is the ECM is going to limit the torque output at low speeds/off the line. It does this to keep from damaging the drive train. Low RPM torque is really married to engine displacement, and I am sad to say that the LNF doesn't have a whole lot of that. You could go dual boosted and add a small super charger, but that really would be a waste of money unless you are running a monster sized turbo where the lag is HUGE.


You can get a rough estimate of what the torque for a given RPM needing to know only a small number of things, the engine RPM, the intake air temperature and the absolute manifold air pressure.
In order to calculate the torque we need to know the horse power and in order to get the horse power we need to know how much air is moving into the engine.

calculation to get the amount of air flowing into the engine you would use the calculation below
MAF = ((Displacement * RPM / 2) / (12 ^ 3)) * 2.7 * (P / (T + 459.67))

where
Displacement is the engine size in cubic inches
RPM is the engine RPM
P is the absolute MAP reading in psi
T is the IAT (Intake Air Temp) in Fahrenheit

I have a log form my vehicle and using the RPM, absolute MAP and IAT this is what the calculation looks like. This is with the throttle plate at just under 50% and the turbo is not boosting at all.

Displacement = 122
RPM = 1711
P = 10.9
T = 83

MAF = ((Displacement * RPM / 2) / (12 ^ 3)) * 2.7 * (P / (T + 459.67))

MAF = ((122 * 1711 / 2) / (12 ^ 3)) * 2.7 * (10.9 / (83 + 459.67))

(122 * 1711 / 2) = 104371
(12 ^ 3) = 1728
(83 + 459.67) = 542.67

MAF = (104371 / 1728) * 2.7 * (10.9 / 542.67)

(104371 / 1728) = 60.399884259259259259259259259259
(10.9 / 542.67) = 0.02008587170840473952862697403579

MAF = 60.399884259259259259259259259259 * 2.7 * 0.02008587170840473952862697403579

MAF = 3.2755976813717360458473842298272

My log reports 3.01 lb/min which is pretty darn close.

To get an estimated horse power we multiple the MAF by 10

HP = 3.2755976813717360458473842298272 * 10
HP = 32.755976813717360458473842298272

To calculate the engine torque we use the equation below
Torque = HP * 5252 / RPM

Torque = 32.755976813717360458473842298272 * 5252 / 1711
Torque = 100.54610767132880019164501446553 lbft


so at 50% throttle and the engine at 1711 RPMs and an intake air temp of 84 °F the engine has a calculated horse power output of 33 (rounded) and 101lbft (rounded) of torque


Here is a script that you can run in Windows PowerShell, just replace the values of the 3 top variables with given parameters.
Code:
$RPM = 1711
$IAT = 84
$MAP = 10.9


$MAF = ((122 * $RPM / 2) / 1728) * 2.7 * ($MAP / ($IAT + 459.67))

$HP = $MAF * 10

$Torque = $HP *  5252 / $RPM


"Horse Power: $HP"
"Torque: $Torque"

Just remember that when the turbo is not spooling and you are pressing down on the throttle the MAP pressure reading needs to be below barometric pressure because the engine is sucking air in so there would be a vacuum (pressure less then barometric)
 

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If I use the above calculation and only change the engine from the LNF to the LS3 this is what you get as an output

Horse Power: 100.767158846359
Torque: 309.309829492155
 

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You can test how close the calculations are since we know that the GXP produces 260 HP and 260ftlb torque @ 5200 RPM. We also know that the car has a boost pressure of about 14PSI at sea level and the absolute air pressure at sea level is 14.7.

So if we use an RPM of 2500 and a MAP of 28.7 (14 + 14.7) we get

Horse Power: 261.637344344915
Torque: 264.253717788364

I would say that's pretty damned close.
 

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I have read your threads on your “big wheel” KO4 (I think?) in the past. Would love your feedback on the below…

I have searched all over and read a multitude of opinions on this but here we go (opens can of worms):
What turbo upgrade causes the least amount of low end torque loss?
I understand the big wheel allows the KO4 to drop off at a higher RPM than stock, which is great.
But I do wish there was a way to increase horsepower for the Sol that didn’t cost (or possibly even improved) low end torque..
The stock tune drastically limits low down torque - a good thing as it lessens the chance of causing driveline issues from drag racing starts. Once you retune to take that impediment out of the mix, there is no shortage of torque. Assuming that you don't want to play around with cams, you can do what I did - look at the alternatives for modified or replaced turbos and choose the one that you think will work best for you. For me, after exchanging correspondence with several helpful members that had done various turbo mods, I opted for the fairly modest big wheel K04 which I am quite happy with.

If you are concerned with moving your power band up too much, that might be a good stopping point for you as well. The guys that turned to larger turbos liked them and usually said that they were down below the torque onset so briefly that it didn't bother them, and that the trade off was worth it. I guess it depends on how you intend to drive the car.
 

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I have an EFR 6758 and maybe for the first 30-40 feet or so the car is lacking. Once that turbo starts to spool it's game on. and once the engine hits the 4,000 RPM range ya batter hold on. It reacts much like the car does bone stock except you get a HUGE HP boost when that turbo spools up.
 
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While I understand the thrill of the power coming one, I have always preferred less lag as a fair trade for less power up top. I think it is just a matter of personal preference. I sized the turbo on the Fiero V6 that I built to perform right from lower rpm as well, even though giving up some top end power, on the rationale that I wasn't into drag racing, but on real world street driving and keeping an engine up in the best rpm band all the time was tedious. In fact the resulting limitation of the continuing production of increasing power higher up in the range never bothered me.

But bear in mind that on the Solstice I was shooting for a nice balance based on a stock bottom end - I didn't shoot for 450+ bhp, I 'settled' for in the 375 neighborhood and stock internals. I had about 300 bhp on the Fiero, which was a nice bump over the stock 140, and was also much harder to do as the heads on those engines were pretty bad an needed a fair bit of reworking.
 

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Discussion Starter · #13 ·
While I understand the thrill of the power coming one, I have always preferred less lag as a fair trade for less power up top. I think it is just a matter of personal preference. I sized the turbo on the Fiero V6 that I built to perform right from lower rpm as well, even though giving up some top end power, on the rationale that I wasn't into drag racing, but on real world street driving and keeping an engine up in the best rpm band all the time was tedious. In fact the resulting limitation of the continuing production of increasing power higher up in the range never bothered me.

But bear in mind that on the Solstice I was shooting for a nice balance based on a stock bottom end - I didn't shoot for 450+ bhp, I 'settled' for in the 375 neighborhood and stock internals. I had about 300 bhp on the Fiero, which was a nice bump over the stock 140, and was also much harder to do as the heads on those engines were pretty bad an needed a fair bit of reworking.
Well that is exactly what has me in the fence with the turbo.
On the one hand K04 turbo with a big wheel is still relatively conservative on a stock block. And at least, from what I have been able to read on this forum will have the least effect on the low end torque.
On the other hand I have read in various threads that the above statement is not necessarily true and that some of the bigger Turbos spin up just as fast or impercetably less so at least…

I’d like to do it once and not go through the work only to wish I had gone bigger later.

tough call as it seems the opinions here vary pretty wildly…
 

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I can tell you that the EFR 6758 spins up pretty damned fast. I am looking at one of my log files right now. From a rolling start in first gear.. 5MPH @ 1694 RPM. Here are my log timestamps, RPM's and the absolute MAP readings.

This log file was early in the tuning process. I was also being gentle on the accelerator, I didn't mash down on it right away because I was driving on a dirt road. I let the car gain a little speed before flooring it. I noted where the accelerator was pushed all the way down.

4.42.357 : 1694 RPM : MAP 11.3 PSI : BOOST 0.0 PSI : SPEED 5 MPH
4.44.249 : 2375 RPM : MAP 12.2 PSI : BOOST 1.1 PSI : SPEED 12 MPH : TP 55% : ACC 93.7%
4.46.443 : 5072 RPM : MAP 27.5 PSI : BOOST 16.2 PSI : SPEED 26 MPH
4.47.349 : 6764 RPM : MAP 42.6 PSI : BOOST 31.3 PSI : SPEED 37 MPH : TP 80%

I do also want to mention that there is an enormous lag in the throttle response. When the accelerator petal showed being pushed all of the way down the throttle plate never managed to open up all the way before I had to shift the car. It only got to 80% open. There is more then 3 seconds of lag in the throttle response. The throttle plate was 55% open when the accelerator was registering as being pushed all the way down. so 3 seconds to open the throttle plate 25% That's how bad the lag is.


This is from a 30mph rolling start in 3rd gear. This is from early in the tuning process as well. about 350bhp

03.08.298 : 2948 RPM : MAP 3.5 PSI : BOOST 0.0 PSI : SPEED 30 MPH
03.09.125 : 2358 RPM : MAP 13.3 PSI : BOOST 2.0 PSI : SPEED 32 MPH
03.13.849 : 3758 RPM : MAP 27.7 PSI : BOOST 16.4 PSI : SPEED 52 MPH
03.19.282 : 6599 RPM : MAP 40.8 PSI : BOOST 29.5 PSI : SPEED 91 MPH
 

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The big wheel turbo is going to react pretty damned close to the same as a stock turbo does. The only difference is you don't get a falloff at high RPM due to the turbo not being able to move a larger volume of air. As engine RPM's increase the volume of air going into the engine must increase. In order for the turbo to be beneficial at high RPM's it must be able to move a larger volume of air. This is the reason why you see the boost pressure spike and then come down 2-3 psi or so. that spike is when you just hit the limit of the volume of air the turbo is able to move. The boost pressure starts to lower because the high engine RPM is using up more volume then what the turbo is able to produce and this lowers the pressure in the pipe. When the boost pressure settles that is the equalization between the volume of air the turbo can produce and the volume of air the engine is using and the turbo is able to maintain that settled pressure for the volume of air that is being moved.

Someone that has a big wheel turbo might be able to comment on if the big wheel turbo's boost pressure peaks and then drops a couple of PSI. I am going to say that it peaks closer to red line and if the boost pressure drops it probably isn't much more then 1 PSI.
 

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the EPA
 
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It could be any number of things. the ECM isn't fast enough. The number of samples to use to get an average was to large. Ultimatly it all points to a single thing the EPA. The auto manufacturers went to drive by wire not because it is cheaper then a throttle cable. It was done to improve the miles per gallon the engine could get. by removing small fluctuations in the accelerator they were able to increase the MPG by a couple of tenths of percent.
 
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