Increasing Performance the Cheaper Way


Supporting vendor
Going real techie here, so tell the wife to shut up, drink your tea and stay awake ! :) :)

Ever since GM began using controllers to control like your engines, the time they start designing the next Gen Corvette to when it hits the streets takes years
That means whatever the controller will be used is already obsolete as to its design
In the case of the controllers they have a coprocessor CPU (computer) that is too slow to process in real-time and that can be an issue when the engine is at
high RPMs and loads.

This leads up to the fuel injection system, where the engine has 8 fuel injectors.
GM chooses sizes that due to the controller being slow to smaller injector flow sizes as the controller (PCM) can command the injectors ON longer so that it can go do other things as CPU speed allows
There is some madness to this, so hang on :)

I will use CCCUK member Mark's 1999 Corvette
Let's add another part, injector design has a limit rule as they do not want the PCM to command a fuel injector beyond 80% duty cycle of its ability to spray a correct fuel mist pattern
Commanding longer and the spray into cylinders is not a very good burn charge

So let's assume Mark wants to haul ass in his C5 on a hot, humid day at sea level ( where more fuel is needed than at higher elevations)
and engine starts having knock/ping

IF Mark was using a OBD-II scanner that was recording and then later replayed and analyzed, he would find the fuel injector duty cycle was like in the 90s
and that is because the injectors GM selected are not large enough for higher engine loads ( some is GM knows many Corvette owners are waxers and never
go fast :)
As an example, I show here some recorded OBD data analyzed :
For Marks drive with stock fuel injectors which are rated as 26 lbs/hr IF holding to the 80% duty cycle rule, those injectors can only support 330 flywheel HP
which is too small for even the stock LS1 engine
So in Mark's case it is not about gaining performance but instead of losing it due to engine knock produced due to his injectors going to 100% DC, which causes the PCM to yank timing out, that reduces torque, which in turn kills off HP :(


So Mark could do is replace the fuel injectors from the 26 lbs/hr to lets say 36 lb/hr
With those installed and leaving the stock GM tune in the PCM now see how much the engine can produce :

Now look at what those fuel injectors could allow the engine to produce and be below the 80% DC - 461 FWHP !


It would take only about 1 hour for Mark change injectors but now for the kicker

IF buying 36 lb/hr injectors for a set of 8 would be somewhere around $500 plus as example
Say picking 40 lb/hr from one vendor, $520


NOW do it my way

There are business that you send them your stock injectors,
1 Cleans them
2. Change the screen filter
3 Put new O-rings on
4. You tell them you want them reflowed from the stock 26 lb/hr to 36 lb/hr ( at 4 bar MAP at 58 PSI fuel rail pressure, what Mark's engine does) and
5. Flow match them so all 8 flow the exact same flow in cc's
6. Tests them when done to what flow they did and the results will look like this set I sent vendor and told them to reflow them to 55 lb/hr (570 cc)


Your way cost $520 plus

My way : Sending them your injectors to do above :

Cost $25 each = $200

Nice saving and there would be techie reason why also to use yours,
but I assume most of you are sound asleep by now to read more :)

Let's Go a Racing, Boys ! :=)


I'm well awake and up to speed with you Team-ZR1.........problem is that my engine is old school and carb'd. Did have plans at one time to go with Enderle 8 stack with electronic injection (Kinsler being far too expensive) but time and lack of funds took control......


Supporting vendor
I'm well awake and up to speed with you Team-ZR1.........problem is that my engine is old school and carb'd. Did have plans at one time to go with Enderle 8 stack with electronic injection (Kinsler being far too expensive) but time and lack of funds took control......

Good Roscobbc
I'll go a bit more techie than

Each maker of fuel injectors has their own design specs and could be each flow size of injector is also different specs

In this case, the question most people do not know what to ask a vendor is for the injectors they are buying from them is
what all the spec values are
Injectors inside have a coil, the coils will have their own reaction time and latency
This is called "injector offset Vs battery voltage vs MAP" ( inverse of engine vacuum)

Depending on what the voltage level is at the injector and the value of MAP the PCM uses the below table ( is same as Mark's C5)
and all those values are telling the PCM as when it commands the injector pulse width ON time to add that offset voltage delay to how long it commands the injector ON

Now if my case in last post if sending the stock injectors to a vendor to reflow (make then flow higher rate) then also that means being the same
injector ALL those values in that table are still correct

This means in design, the correct values assures that the PCM commands exactly at the same time a sparkplug is fired off which makes a good burn charge

IF instead buying other injectors then you would NOT know the specs for the coil in that injector and thus the offset values in the tune are now off and now when injector pulse width is commanded ON, it could be too early or too late, crap burn charge
Also, as you see on that table to the left is the values are different depending on what the voltage is at the injector
so if no maintaining the vehicle's battery and thus voltage is lower than the PCM in using that table uses the offset times at that voltage level

Another reason for vehicles with controllers to assure proper voltage levels

So I suggest doing my way, but if having to buy 3rd party injectors, do NOT buy them IF the seller will not supply all the values for offset and
then as seen on the right is the PCM commands injectors as to their flow rate, thus if you're looking for in Mark's example about and wants to change from 26 lb/hr that is the rate injector is for the C5 fuel rail pressure at 58 PSI and 4 bar map (104 KPA)
Now if Mark wants to change to 36 lb/hr

If not knowing to ask the questions, you really could be selecting an injector designed as 36 lb/hr BUT that was at 43 PSI (as old C4s were)
and thus that injector used in Mark's C5 is really, (let me compute)

So you see selecting the wrong injector because of the designed at 43 PSI that 36 lb/hr in Mark's C5 would be flowing
at 41 lb/hr and engine's AFR would be way rich, esp if having a tuner change the flow rate in the table that was on the right side as
shown above


So my job in doing custom tunes is much more complex than people may think and to do a good quality tune I have to make sure if someone wants a tune from me, they must answer in part what the specs are for injectors they buy

Most selling fuel injectors have zero clue of any injectors they sell

Vendor I suggest to my customers is the same as where injectors are cleaned and higher flow rates because
if buying injectors from them included in the shipping is a memory USB dongle and on it has the specs for the injectors that were bought


CCCUK Member
I'm old-fashioned. To my mind a bit too much fuel doesn't matter much, so long as you're not chasing ultimate economy and emissions, but a bit too little fuel matters a lot. Hence I think it's normal for the margin of error to be towards the richer side in order to keep the engine internals ..... inside the engine.

Back in my days in the industry (shortly after dinosaurs roamed the earth) people were still trying to work out combustion chamber shape, squish and so forth. Mind you, I've no doubt there are still a good many Corvettes on the road from that era! The joys of a nice fat accelerator pump in the carburettor. Very different world from plugging in a laptop and tweaking a tune.


An 'old school' big carb doesn't always result in excessive fuel consumption (if jetted and set up correctly). Mine uses a 950 cfm race spec double pumper...... in hindsight a 750 cfm would really have been more than adequate.......yet even with more 1000 cc's and 180 hp more than factory 427 cu in figure it has fractionally better mpg figures.


Team ZR-1 - say (hypothetically) I sourced a LS3 Vette, manual preferrably and using your suggestions re. replacement injectors and perhaps one of your custom tunes to the ECU etc, and looking to retain or improve driveability (and perhaps even economy) but still retaining OEM cats, exhausts etc what would you reasonably expect the HP gains to be over stock?


Supporting vendor
Final piece of the puzzle when choosing fuel injectors

Pre C5s GM used the known standard of fuel rail pressure of 43.5 PSI
Other nameplates also used that standard, so there were tons of fuel injectors made, but you have to keep in mind they were designed at that PSI

So now going back to Mark's 1999 C5, say he needs to go a bit bigger and decides on using the injectors used on the SVO Fords rated at 32 lb/hr
But since the C5s GM upped the fuel rail pressure to 58 PSI so those SVO injectors were rated at 43 PSI, now makes them like 37 lb/hr due to the higher rail pressure.

This can be done because of another standard for injectors as their designed size
So even though the SVO were for Fords, but they also were a EV3 style
That spec EV3 then defines the shape, height and even the shape of the wiring connector

So this part is when selecting an injector, make sure the one you select is the same design style and was their flow rated at 43 or 58 PSI
to know, you're selecting the right style and flow rate you want.
Also, the higher rate of flow you need the more costly the injectors cost

Again another reason if possible is do it my way,

as this way you know the injector style is right and that they were rated for in Mark's case fuel rail pressure of 58 PSI
And let's say you want 44 lb/hr but all you can find are 50 lb/hr that might be too large so by reflowing yours you can tell
the vendor to reflow them to the 44 lb/hr and are only paying about $25-30 to get yours reflowed to rate you need


Supporting vendor
Team ZR-1 - say (hypothetically) I sourced a LS3 Vette, manual preferrably and using your suggestions re. replacement injectors and perhaps one of your custom tunes to the ECU etc, and looking to retain or improve driveability (and perhaps even economy) but still retaining OEM cats, exhausts etc what would you reasonably expect the HP gains to be over stock?

Hi Roscobbc,

Let's look at a 2013 C6 that has LS3 I tuned back in 2015

GM specs : Note the flywheel HP and TQ


Keep in mind the final results was the C6 was at 1,400 feet elevation, so performance numbers would be less

In this case seen often, the stock fuel injectors were a bit too small and would exceed the 80% duty cycle rule
I had the C6 owner have his injectors reflowed from 36 to 42 lb/hr
This also meant having access to more fuel flow (richer) would reduce chance of engine knock and PCM yanking power killing timing out
I also then could even command more timing than what values GM used, so with a stock C6 with just larger fuel injectors

GM specs again stock at flywheel : 430/425 HP/Tq
With my tune completed: ( all results captured with a OBD-II scanner under a testrun, and then I analyzed all that data)
All values can be seen in my results' sheet below

WOT timing was increased from 21 deg timing to 26
Good amount of airflow into intake manifold at 46 pounds
No Misfires and very little knock even though on this testrun shifts made at 6,200 RPMs and max speed of 108 MPH
Weather was 75 F degrees
With higher flowing or reflowed stock injector even with 100 % engine loads the injector duty cycle was not only 63% where when C6 totally stock, duty cycle was in the 90s %
With the modified fuel injectors now could support up to 537 flywheel HP to 80% duty cycle
AFR at WOT was 12.6:1 (good charge to cylinders) and non WOT conditions the fuel trims were only 4% lean
as if you make it too rich then engine will be sluggish and slower response times

And to answer your question
as to what the engine outputted for this test run, the flywheel numbers are now : 474/450 HP/Tq

Nice gains for just modified injectors and my custom tuning



Thanks for that - so a 38 hp gain for not a lot of money. Guessing economy will remain much the same except perhaps if on 'full tilt'. That increase seems to coincide with increase in rpm - is that where the gain is?


Supporting vendor
Custom tuning is a lot more complex
Gains in performance, yet can increase the fuel mileage as the PCM is not calibrated by GM for perfect AFR so with the damn E10 and E15 gas the engine
could be running way too lean and the O2 sensors see that and causes PCM to command even more fuel, which ends up reducing fuel mileage

So if tuned properly, the AFR is now close to perfect in all engine conditions
As to gains, the PCM is an engine dyno and because of that GM can control and limit the amount of torque outputted to drivetrain
as to what torque values they put in the calibration
This is used to limit torque on the tranny and also for traction control and even has an abuse mode so that if it sees the driver causing
high rear wheel spin, the PCM yanking timing out to kill off the torque

Below, we see the maximum requested torque of 350 ft/lbs
and we see in this 2008 C6 has the A6 auto tranny and as example IF at 3,000 RPMs and in 6th gear PCM will only allow 280 ft/lbs
There are other torque tables but if we look at one table you see IF engine RPM is 3,000 and in 6th gear PCM commands
max torque at only 280 ft/lbs

In tuning, I redo all those tables starting at leaving from a dead stop, to allow for more total torque.
Raise the torque values for all tranny gears and limit their abuse mode.
As example, I could change all the torque values you see in that table by any value I want, could be increase all by another 50%
or maybe for certain gears or RPMs but have to make sure I adjust the fuel needs and timing correctly for those gains

So in the end results the gains now are that the torque is allowed to come up much sooner and higher from low to mid-engine loads
so the HP gains also increase at those engine loads

But yes, as to HP the value in those results above would have been at higher RPMs.


Forrest Gump

CCCUK regional rep
Thanks for that - so a 38 hp gain for not a lot of money. Guessing economy will remain much the same except perhaps if on 'full tilt'. That increase seems to coincide with increase in rpm - is that where the gain is?
430hp to 474hp so a 44hp increase. Almost exactly 10%. There is a 25lb/ft increase in torque too if I’m reading all this correctly.


Supporting vendor
Now to put action to the words in real life

Project – Modify Stock LS1 Fuel Injectors for higher fuel flow ability

Objective was the fuel injectors in my C5 Corvette (with highly modified LS1 that was using 36 lb/hr injectors to now 42 lb/hr) were functioning over the rule of not exceeding an 80% Duty Cycle
Meaning they function less than 80% of their design as to flow

In even a 60 F degree weather, the duty cycle was seen in a testrun and a OBD-II scanner recording the engine parameters going to 82% DC and in hotter weather that would be even higher.

Replacing fuel injectors to a higher flow could cost $500 to $700 depending on their rated flow
This project was about taking stock C5 fuel injectors and sent to a business that cleans, increases the fuel flow and flow matches them, so all 8 injectors flowed the same amount of fuel

Cost to do that was $190 which is far less than buying others and the fact by using stock ones you're assured the design of how GM set up the calibration was the same where buying 3rd party injectors adds a problem is they will have different delay/latency of the internal coil and if not knowing the countless values in the PCM tune to change to meet those then the PCM could command injector on too early or late to when the spark plug is commanded on

Before making the injectors change, the C5 was testrun 24 miles in 30 minutes, and the OBD-II scanner recorded many of the engine's parameters.
That data was then analyzed and recorded results.

All 8 injectors then were swapped to install the modified GM injectors but now flowing 42 Lb/Hr, and then I tuned the PCM for that flow
In the same weather temp, distance, elapsed time and road, using the same drive style to cover all 23 fuel trim cell engine conditions, including close to the rev limiter in RPMs around 100 MPH, the testrun was done, again having the OBD-II scanner record all the real-time engine conditions which I then again analyzed and end results

The results of the before and after tests are shown below

Refer to the red lines I inserted between the two, so you can see all the results differences
In short the toque gain was 12 ft/lbs and the horsepower gain of 23 HP

Where the injector maximum duty cycle went from 82% to 72% and the modified fuel injectors were decreased in how long the PCM had to command them ON
Where the injectors that were taken out would handle 451 flywheel HP, the modified injectors now can handle up to 538 flywheel HP.

All for $190
of cost to have stock injectors modified to higher flow and flow matched

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