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Customer: Innovative Tuning

Car: 1999 Pontiac Grand Prix GTP 4 Door

Story: Auction’s can be rewarding!

Here is the condition of the car at time of purchase.  It was a little rough around the edges, having some scrapes on her, but a closer look revealed this car to be a diamond in the rough.  All this car needed was some TLC.  So I bought it, brought it home & thats exactly what it got!

The interior was in incredible condition.  Just a quick vacuum/shampoo  job in here.

In less than a years time, the car has changed dramatically. It is no where near the same dull, scraped up, family car it used to be.

Apparently the previous owner had some issues parking the car!

That got fixed up very quickly!

Ditched those old dull, pitted headlights that were full of condensation.

Refinished the calipers and brakes…

Brought her closer to the ground…

2″ drop in the front, 1.75″ in the rear via Super Sport Chassis lowering springs.

Finally got the windows tinted out…..a must on a black car!

Removed the pathetic components GM calls an exhaust…

And replaced them with something… a little more appropriately sized…

XSPower stainless headers with a 3″ downpipe.

Also, she got a bit of a heart transplant. Dropped the OE supercharger pulley for a 3.4″, an Overkill SBC cam, and LS6 valve springs.

A Dynomax Super Turbo cat-back exhaust has now replaced the OE system so you can actually hear that cammed engine’s lumpy idle, & help with flow.  The car has now been re-tuned & is ready to run on the dyno at the end of April.

Dyno results coming soon…..

April 26, 2010 – UPDATE

Well, the GTP has met the dyno today.

Just a note about the chart you are looking at above.  On a car with a waste spark ignition system like the Grand Prix, using an inductive pick up on a plug wire for the rpm signal on the dyno results in the rpm reading double what it actually is & the torque being half of what it actually is.  The data from the dyno pulls was exported to Excel in order to make everything read correctly (rpm & torque), & the corrected Excel graph is what you are viewing, not the Dynojet graph.

Once you take drivetrain losses into account (normally about 20% on these cars), crank numbers would work out to be roughly 345 hp, & 320 ft/lbs of torque.  This represents an improvement of 105 hp & 40 ft/lbs over a stock GTP’s 240 hp & 280 ft/lbs.  Not bad.

This week a new cold air intake has been installed in place of  the factory airbox finally.  A Wizaired cold air intake was the intake of choice.

I’m very impressed with the build quality, & fit & finish of the product.  Datalogging shows this unit flows only about 5% less than a wide open throttle body with no filter at all!

Up next is  upgrading the pulley further.

The 3.2″ pulley has arrived today & should bring boost levels back up to about 10 psi.  Time to go out & retune it & see what she’s got.

Update:  June 28.

Now that the new 3.2″ pulley has been installed & re-tuned, the car pulls hard!  I’ve recorded several instances during datalogging of the car spinning the tires to 80 km/hr +!!!  Additionally, last week the throttle body was removed and treated to a mild port job…..airflow readings appear to have improved approx 3% from porting, so definitely a worthwhile mod!

Virtual Dyno results with the intake, ported TB & 3.2 pulley added (current setup) vs. bone stock with only a 3.4 inch pulley.

Stock vs Current Dyno

Assuming a 20% drivetrain loss through the automatic transmission, 308 whp would translate to  385 hp @ the crank.  Impressive!  Reviewing the datalogs reveals that the car is still only making 9 psi of boost on average though, so we are suspicious that we may be seeing some belt slip on the supercharger now with the smaller pulley….that will get rectified soon enough.  There is probably some additional hp in there once that is fixed.

One other interesting note about the car.  During a recent road trip of approx. 600 km’s, even with power the car has, the car delivered 37 MPG! (or 7.5 L/100 km’s).  Awesome!

Update June 26, 2011.

Well, over this past winter the GTP finally got  some paint.

Update Feb. 6, 2012.

As of when the car was put away for the winter last fall, the upgrades had stopped due to the injectors being maxed out……she was just plain out of fuel!  Not particularly wanting to pop for a full set of new injectors, progress halted.

Well this week the fuel system has been upgraded enough to allow for dropping down another pulley size to a 3.0 inch.  This should bring boost levels up to 12’ish psi.  The fuel system upgrade was achieved by substituting a 4 bar fuel pressure regulator in the factory fuel rail (& it took hours of research to figure out how & where to get one that would work) instead of the OE 3.5 bar regulator.

A quick reflash of the pcm to accommodate for the higher rail pressure has brought the car’s fuel trims back into line & it is now ready to make even more power!  The 3.0 pulley should arrive shortly!

While waiting for the pulley to arrive, the old corroded, generally crappy looking throttle body was treated to a bit of a touch up.

As it came off the car.

After media blasting all the corrosion/grime off of it.

And finally after a paint job & re-assembly.

Feb. 14, 2012.

The new 3.0 inch pulley has arrived!  Hopefully I’ll get a few minutes today to go press it on.

March 16, 2012.

Well, we finally got nice enough weather to get the car out & see what the 3.0 inch pulley had in store for us!  After doing a ccouple of WOT runs to dial in the air/fuel ratios we noted that we got quite a bit more boost than we should have, & had to pull more timing  than we should have had to also……something was clearly wrong!  Investigating the problem revealed that the perforated tubing inside the OE exhaust resonator had split along a seam & the metal curled in creating a massive restriction in the exhaust system.  The opening in the resonator was equivalent to approx. a 1 5/8″ pipe……new parts are on the way.  When I get some time I’ll try to get some pics…..very bizarre.

Finally had a chance to take some pics of the inside of the OE resonator.   Unusual to say the least & definitely not helping flow at all!

Update Apr. 6, 2012.

A new Jones Exhaust glasspack muffler has been installed in place of the original resonator.   Very impressed with the fit of the new unit.  The car once again has its nice low rumble at idle & surprisingly enough, the car seems to be even quieter than with the factory resonator installed… extra bonus!

A few WOT runs with the 3.0 inch pulley installed showed that the car was clearly not happy with that much boost getting rammed down the intake un-intercooled, so the 3.2 inch was put back on.  Datalogs show the car to be quicker with the 3.2 pulley on with a little more timing than the 3.0 was with the amount of timing that needed to be pulled.  The car is definitely liking the exhaust flowing as it should though.

Update July 2, 2013.

I have  replaced the headers that were installed several years ago (an OBX copy of Pacesetter headers).  Right from day 1 they fit poorly.  The front header hit the transmission, the rear header hit the exhaust tunnel. The Y-pipe was an ongoing source of leaks, so I finally got fed up trying to fix the leaks & ordered a set of Speed Daddy headers. I am happy to report that pretty much everything fit & lined up as it should!  The only issues I ran into during installation was the angle of the crossover pipe wasn’t quite right to mate up to the front header easily.  With a little persuasion & a quality band clamp, everything is sealed up first time out, just as they should!  The second issue I ran into is that the upstream O2 sensor will not clear the exhaust tunnel.  I have a fix in mind, but I’m in no particular hurry to get into that.  The reason being that I went out a few days ago & did an open loop MAF tune to get the car running top notch without the front O2 sensor, & the car is now running the best it has ever run as long as I’ve owned it!!  So a big thumbs up to Speed Daddy headers!

Update July 22, 2013

After putting the new Speed Daddy headers on, I elected to press on my 3.0″ pulley again & see if the car likes it any better now.  To my surprise, its taking the extra boost (13+ psi in some cases!) better than I thought it would.  Its taking it well enough that I’ve elected to leave it on.  I’m thinking its time for that Dynomax catback to be replaced with a full 3 inch system.  The 2.25 inch Dynomax is the big bottleneck at this point, so plans are in place to uncork that soon.  Details later when that gets dealt with.

In the mean time, I’ve been attending to little detail stuff.  Spent today replacing all the clips on both front doors to get all the squeaks & rattles out of them & they are now pleasantly quiet once more.

I also did some modification to the WizAired intake that is in the car.  I spend quite a bit more time datalogging this car than your average Joe does, & have noticed that driving around town, our IAT’s slowly creep up & up until they are way up in the 120-130 deg. F range at which point the computer is pulling timing making the car feel lazy.  Furthermore, even once out on the highway, it would take a considerable amount of time for the IAT’s to recover down to close to ambient temperatures.  After studying the design of the intake, I came to the conclusion that location of the IAT sensor seemed to be less than ideal.  Its located in the side of the heat shield, directly beside the upper rad hose.  It would also seem that there is little air circulation in this area of the airbox given the long time it takes the IAT’s to recover.  My solution was to relocate the IAT sensor from that position into the intake air stream……after all, I want to read what the temp of the actual air coming into engine measures, not what the air in the back of the airbox measures. Picture below illustrates my modification.

Relocated IAT

Datalogging after this modification showed a night and day change!  Over the course of a half hour drive around town, the IAT’s varied maybe 5 deg. from what ambient air temp was, & even after having the temps rise a few degrees after sitting at a light for a few minutes, the IAT’s recovered right back to pretty much ambient within a minute or so of leaving the light.  Definitely a successful modification, & highly recommended to anyone who has one of these intake systems!

Of course while I had the lid of the intake off to do the above modification, I decided to spend half an hour or so sprucing it up a bit too.  The Wizaired intake when new, is delivered in a natural aluminum finish….I thought it would look wicked polished, so that’s exactly what I did!  Came out pretty nice! Also decided to take some rubbing compound with my polisher to the plexi & clear it right back up to as new condition too.  Pretty pleased with the results!

Polished intake

Update July 25, 2013.

As I alluded to above, I feel that the Dynomax catback is in over its head at the power level this car has climbed to.  While fine for a less modified vehicle, experience tells me that the 2.25″ piping is inadequate for this car.  However, being the person I am, I like to have hard data to base my decisions on, so today I installed my backpressure tester into the rear O2 sensor bung & went out to measure how much backpressure the car really had.  During normal driving around town there was 0 psi of backpressure, but as soon as I did a full pull at wide open throttle through 2nd gear, I observed almost 20 psi of backpressure!  Holy crap!  So, with my suspicions confirmed, a full 3 inch system will be going on soon!

After searching around for quite some time, the only data I could find regarding studies on backpressure is the article written by Thrasher.

It would seem that article is also the source of the apparent myth in the Grand Prix community that the OE exhaust system is not a big restriction.  The testing performed in that article was done on a stock, normally aspirated 3800 V6, & in that particular application I have no reason to question their results.

A highly modified supercharged 3800 is a whole different ball game.

In their article they state “It is generally accepted by automotive engineers that for every inch of Hg of backpressure (that’s Mercury – inches of Hg is a unit for measuring pressure) approximately 1-2 HP is lost depending on the displacement and efficiency of the engine, the combustion chamber design, etc. Our sources indicated that in the case of the L67 3800SC, 1HP per inch of Hg is reasonable.”   I have not read (or found) any other data on this matter, so I have to make the assumption that this is correct.  Based on that, my observed 20 psi of backpressure would convert to 40.7 in/hg.  This information would suggest that there is upwards of 40 hp being lost in the exhaust setup currently on the car, & while achieving 0 psi of backpressure is highly unlikely (nor would it be tolerably quiet),  it does suggest that the improvement in power with a better flowing system could be considerable.  Getting back pressure down to 10 psi or less would free up 20+ hp.

I will be doing further testing once the new system is installed to see how valid this data is.

Update Aug. 11, 2013

At the stage the car is at right now, I have been interested to evaluate how much of an impact  the throttle body is having on the car’s performance, so today I set out to find out!  What I wanted to do was get a MAP reading from the car at WOT, but not from the normal location from the intake manifold.  I wanted to get a MAP reading from between the throttle body & the supercharger so that I could compare that reading to barometric pressure.  This would give me an idea of what kind of restriction the throttle body was creating.  So after switching some hoses around to have the MAP sensor sample from where I wanted it to, I went out do some WOT pulls.  Before starting the car I made note of the MAP sensor’s reading…..96 kpa on this particular day.  After doing a few pulls & looking over the data, I noted that the MAP reading fell to 88 kpa at full throttle.  Doing the math shows there is 8 kpa of vacuum between the throttle body & supercharger at full throttle, indicating that the throttle body is in fact a sizable restriction.  Normally I would like to see MAP readings on normally aspirated cars to be be 1 – 2 kpa max. below barometric pressure for best performance, so 8 kpa is something that will have to be dealt with in order to achieve best performance.  I would also expect that eliminating this restriction before the supercharger will likely increase boost slightly by virtue of the supercharger not having to suck air under such vacuum.   Doing a little more math we see that 8 kpa = roughly 8% of 96 kpa, so therefore to minimize the restriction of the throttle body, it would stand to reason that the throttle body area would need to be increased the same 8%.  Even more math tells us that the stock 69 mm throttle body would have an area of about 3739 mm^2.   3739 *1.08 = 4038 mm^2, or the area of a 72 mm throttle body, so we now know the minimum size we will be looking for to solve this issue.

Update Aug 12, 2013

Spent a little more time investigating the throttle body on the  car today.  The throttle body was treated to some further smoothing & blending today as well as narrowing the throttle body shaft somewhat  to try to improve the performance of the stock TB.  Testing afterwards showed a small improvement.  The vacuum behind the TB has now dropped to 7 kpa from 8 kpa, so it was at least a step in the right direction.  In order to further evaluate the throttle body, I decided to ensure that it wasn’t actually the air filter/intake that was causing the issue, so I removed the intake for 1 quick WOT pull.  The data showed that only 1 kpa was being caused by the intake/filter, meaning that the remaining 6 kpa can be attributed to the throttle body.  In the grand scheme of things, 6 kpa converts to 1.77 in/hg….so not horrible.  Room for improvement for sure, but at this point there are bigger fish to fry on this car right now that are certain to provide bigger gains than swapping throttle bodies will (namely the cat back exhaust with its 20 psi of backpressure!).

Update Sept 16, 2013

The new catback is finally on!  The system I chose was the Pacesetter Monza system.  3″ mandrel bent all the way back into 2.5″ dual mufflers (if you want to call them that, LOL!)  The system is definitely louder than the old Dynomax setup, & a little too loud for my taste, but I’ll deal with that later as the system unquestionably provided the performance gain I was looking for.

Hopefully a large resonator will tone things down enough to satisfy me with the sound of the car again.

Update Sept. 22, 2013

Today I dove into solving my issue of the upstream O2 sensor not clearing the firewall.   Rather than bashing in the firewall as I’ve read many people with these headers have done, I chose to tackle the problem in a little more elegant fashion.  My solution was to utilize one of AEM’s no weld O2 sensor band clamps.  Excellent product.  Was a snap to install, lots of clearance now for the o2 sensor, & my firewall isn’t mangled!


Today wasn’t a total win though…..discovered a bit of an issue with the flex section of the Speed Daddy headers.  Not decided on how I’m going to rectify this problem!

Speed Daddy Flex

That flex definitely isn’t supposed to be imploded like that!

Update Sept 27, 2013

Today I tackled the issue with the flex in the Speed Daddy headers.  Inserting my pipe expander into it allowed me to work the flex back out to full diameter, so the issue seems to be resolved for now.

While under the car fixing the flex, I decided to stick my backpressure gauge into the rear o2 sensor bung to see what sort of difference the Pacesetter catback made.  A few pulls showed the car now has 0 psi of backpressure!  Unfortunately it sounds like it too.  Luckily with 0 backpressure, even if the way I plan on quieting it down adds a couple of psi, I’ll still be WAY ahead of where I started.  More details on that once install & testing commence hopefully early next week.

Update Oct. 1, 2013

Spent time today attempting to deal with the excessive noise issue of the Pacesetter catback today.  The way I chose to try to tackle the problem was with a Dynatech auger insert.

Dyantech Auger

After installing the insert into the mid-pipe, the noise level dropped to a much more pleasant level, so their claims of reducing noise levels by 6-7 db appear to be valid.  All the drone & raspiness was gone!  Unfortunately my satisfaction with the modification only lasted until I went out & did some WOT pulls with the car with my backpressure gauge installed.  Backpressure readings were right back to the 20+psi I was seeing before installing the Pacesetter catback……so out it came.   So with this modification proven a fail, I’ve ordered a 3″ Jones Glasspack muffler to insert as a resonator.  I’ve chosen this specific model as it is the same model I used with the previous catback, just in a 3″ inch version rather than the 2.25 inch version I used before.  The noise reduction with the previous system was exceptional, so I’m hoping to get the same results with the new unit.  More testing will follow once parts arrive & installation happens.

Update October 25, 2013

Finally got some time to install the Jones Glasspack Muffler on the car!  Much to my relief the decibel level has been reduced during normal driving & idle.  The irritating rasp it had at certain rpms is gone & I can once again leave my subdivision without waking up the neighbors!  At WOT the car is still a little loud for my liking & a little raspy, but I can live with that as I can choose where & when I go WOT.  Although time didn’t allow for backpressure testing yet, the butt dyno & noise levels at full throttle suggest backpressure isn’t an issue.

Update October 29, 203

Had some nice weather today so I decided to get out one last time with the laptop & Virtual Dyno to see if the new resonator was hampering performance at all.

The result appears to be a definite NO!

Nov 2 2013 Dyno


Update Feb. 10, 2014

At the end of last fall after installing the 3″ Pacesetter catback, the car was taking the 3.0″ pulley with out much drama, so today I went out & decided to step it up a notch further!  I’ve installed a 2.9″ pulley!


Its a pretty snug fit!


Now I just have to wait for at least another month to see how the car likes it!

I did a lot of math before ordering the 2.9″ pulley.   What I am anticipating is about a 3-3.5% improvement in airflow (3.0/2.9 = 1..035) .  There were several key things I looked at before putting this on.  First thing I looked at was injector duty cycle.  In the very cold temperatures last fall I observed low 90’s duty cycles with the 3.0 inch pulley.  In the cooler temperatures in the early spring that are coming, I’ll be keeping a pretty close eye on the wideband to ensure the car is going to be able to keep up with the fueling.  As the temps come up I anticipate it will be less of an issue, but I’ll be keeping a close eye on it.  Second thing I looked at was my MAF.  My peak readings from last fall was 11,100 hz.  The MAF pegs at 11,500 hz.  Based on the anticipated 3.5% increase in airflow, 11,100 hz X 1.035 = 11,488 hz…….the MAF is going to be pretty much pegged by the looks of it.  3rd thing I looked at was my MAP sensor.  Peak readings last fall were 190 kpa.  The car comes with a 2 bar MAP sensor OE so it pegs at 200 kpa.  Doing the same math 190 kpa X 1.035 = 197 kpa…..the MAP sensor should pretty much be pegged too!  If the anticipated airflow increase actual pans out, the cars peak airflow reading from last fall of 48.02 lbs/min should go up to 49.7 lbs/min.  That’s moving a boat load of air!!!

Now its a matter of hurry up & wait for spring to arrive to go out and see how the car reacts!

Update Mar. 10, 2014

New tires have arrived!  The 10 year old hockey pucks that were on the car have been replaced with a set of Hero Atlanta AR01 tires in the factory size.  Surprisingly enough they actually measure a 235 rather than the stated 225, but I’ll take wider any day!  Hopefully the v-rating will perform much better than the old ones…..just being new has to be a massive improvement!

Hero AR01


Time will tell how these work out!

Update Apr. 6, 2014.

So winter has finally taken a hike & allowed for me to get out in the car & do some testing of the latest changes to the car!

First order of business was to see how the new tires hook up.  0-60 times have gone from 6.03 sec down to 5.3 sec, the best time the car has pulled off yet.  Additionally, the tires are extremely quiet, very responsive handling and ride very comfortably.  Two thumbs up to Hero Atlanta AR01 tires!

As for the 2.9 ich pulley, I was a little more apprehensive about how that was going to go.  Turns out my concern was for nothing as the car is taking the extra boost quite nicely…..14.5 psi now!  The car pulls HARD!   Virtual Dyno spit out a result of 341 whp this morning, a result even better than I was anticipating!

So this winter’s modifications appear to have been a great success!

Update Apr. 17, 2014

Turns out most of my calculations were pretty close to what the car is actually running currently.  I expected a MAP reading of 197 kpa & I’ve recorded 195 kpa, so that was pretty close.  I was expecting to see rougly 49.7 lb/min of airflow & have recorded slightly over 50 lb/min, so that was quite close as well.  Injectors……well, they’re done.  They are maintaining my commanded air/fuel ratio, but there is nothing left….they’re maxed (as I expected they would be).  I’m actually amazed the OE injectors have taken the car as far as they have with only a pressure regulator upgrade.

Update July 1, 2014

This car is maxed out!  Without dumping a considerable amount of money at it (injectors, intercooling, throttle body etc), this car is at its full potential.  With that in mind I’m giving some consideration to selling this car & moving on to something else…..gotta have something to tinker with!  I’ve had an absolute fantastic time over the last 6 years with this car & I’m sure it will put a smile on somebody’s face.

If this car is of interest to you, hit the Contact button & get in touch!

Update Aug 12, 2013

Our ’99 GTP has moved on to a new home!  Hopefully the new owner has as much fun with the car as I did……it will be missed!






  1. WOW Shane That is one SWEET lookin ride man. Very nice Web site.You want to tweek my OLDS?



  2. Hey so don’t know if you still check this page but wanted to give u a up day on the Grand Prix. IM STILL RUNING IT HARD:0 . Dayley dove it since I bought it from u except for last winter car ran great no issues love it . I will admit I have neglected for the last year or so think it’s time for a go over and tune . If your interested in looking over it for me and doing some work on it it would be much appreciated. Gotta say this car has surprised me just goes to show the good work and time you put into it. unfortunately I don’t know much when it comes to modified cars and the care that is needed to maintain them . With that being said I plan on putting some well deserved money into this year and would love for you to do the work if you have time . Don’t think I’ll ever sell this one lol let me know if you are interested thanks .

    Richard Reid


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