1984 Turbo. Go bigger.


Active member
Mar 24, 2008
I bought my Corvette in april 4th 1998, 10 years ago, my first american car. I did not know much about Corvettes then, but I liked the C4 look. It was all stock exept for a ADS Superchip, a rear wing and a black paintjob.

I started with some minor modifications.

I removed the AIR-pump and made a bracket for the pulley.
More pictures: http://www.joby.se/corvette/mods/2000_2_alternator/

Later I also started thinking about a supercharger. I decided to move the alternator to make room for a supercharger on the drivers side. With the AIR-pump gone that looked like a good location.

More pictures: http://www.joby.se/corvette/mods/2000_2_alternator/

In october 2000 I was in the US and I bought a Vortech supercharger at a swap-meet. That winter I made supercharger brackets and made an air-box to replace the air filter box. I also replaced the fuel pump and installed a progressive fuel pressure regulator to get more fuel during boost.

The first day with boost I lost 2nd gear in the th700. Within a month I broke the rear end.

In may 2001 I was finished with my Corvette modifications. At least I thought so at the time.
More pictures of the 'finished' Corvette: http://www.joby.se/corvette/2001-05-23_lawn/index.htm

I measured 65% hp increase over the stock engine.
Blue is hp/tq with supercharger. Yellow is without supercharger.
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Early 2002 I went to a friend i Houston and I brought a lot of parts home.

AFR 190 aluminum heads, a new Cross-Ram intake manifold, Hooker super comp headers, new cam, lifters, roller tip rockers, new rings, bearings and gaskets.

I installed the new heads and cam, but I used the stock Cross-Fire intake.
I also removed the A/C pump and moved the alternator to the passenger side.

More pictures: http://jobyteknik.homeip.net/corvette/2002-05-12_ready_for_start/2002-05-12_ready_for_start.htm

Later that summer I installed the Offenhauser intage that I bought. I made a custom lid and mounted one TBI backwards to center it on the intake and still fit the aircleaner for a stock look. I replaced the windshied wiper motor with a smaller modified Blazer wiper motor.




More pictures: http://jobyteknik.homeip.net/corvette/2002-11_offy_installation/

The spring 2003 I modified the fan housing when I removed the A/C evaporator.


More pictures: http://jobyteknik.homeip.net/corvette/2003-04-13_fan_housing/
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With the new intake the air-box was a bit higher and more to the rear. I had to modify the air-box to clear the distributor and hood.

Pictures: http://jobyteknik.homeip.net/corvette/2005-11-17_airbox/

Later in the summer 2003 when I had sorted out some problems and finally installed the supercharger again the tranny broke again within a few days.

Instead of throwing more money into the th700 I decided to get a 4L80E instead.

I spent the summer 2004 building a garage while the Corvette waited for a better tranny.



I installed the 4L80E early 2005.

Old and new:

Read all about it: http://www.joby.se/corvette/mods/2004-12-26_4l80/
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I had some problems the summer 2005. The cam lost a lobe and a seal in the supercharger were bad so it would have to be rebuilt again.

I decided that it was time to try something new when I had to replace the cam anyway.

I sold the supercharger and Offy intake.

I bought a Holley Stealth Ram and a turbo from a Volvo semi truck. I also bought a big front mount intercooler.

I had to modify the HSR to clear the hood:






Building turbo Headers:





I made a surge tank and mounted two BOCSH fuel pumps and filters where the spare tire used to be.








I was more or less ready to install the turbo during the 2006 seson, but with the new cam and intake I wanted to sort everything out fist without boost.

I had one problem after another. I had some fuel problems and rewired to semi-sequencial injection. (The injectors fires i pairs) I lost spark and installed a MSD 6AL and converted to crank trigger. When I finally was ready to install the turbo the tranny started acting up with black oil.

I went to the dyno anyway to get a pre turbo run and that killed the tranny for good. I only managed to get one good run on the dyno with 230 rwhp.


After that I parked the Corvette for the winter.

It was the converter that had a major failure. The oil cooler and transmission was full of aluminum shavings.

I bought a new 9.5" Vigilante converter with a 3-disc lock-up.

I finished the turbo installation early in the summer 2007.




At the time I ran 10 psi boost.
On the street I had to go faster than 60 mph to be able to step on it without traction problems. My tires might not be the best but they are not that bad.

I went to a local racetrack on the 11th.
This is a picture from the 'pacecar' lap before we could have some full speed runs around the track.


This is a short movie. I am having some traction problems coming out onto the straight.
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I raised boost to about 15 psi and got even more traction problems.

I bought a pair of Hoosier Drag Radials (dot) and what a differance !!

New best 60" = 1.72 sec.
New best ET 11.14 sec with a trap speed of 203.7 km/h ( 126.6 mph ).
New best trap speed of 210.9 km/h ( 131 mph ).

This was on plain tarmac, and some wheelspin, so with improved traction a high 10 is possible.

I found some pictures from the event.



A movie with all my runs. ( 70 mb )

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After that event I went to the dyno again and it was a big surprice.

627 rwhp at 4200 rpm.
1086 Nm ( 801 lbs-ft ) at 4000 rpm.


For some reson it did not hold boost at higher RPM. Peak boost is 1.3 bar (19psi) at 3900 rpm and drops to 0.9 bar (13psi) at 5000 rpm


Engine specification when I did the dyno run.

Stock engine block.
Stock crank.
Stock rods.
Stock pistons.
ARP bolts.
CompCams 12-404-04 cam (LSA 114, duration 222/225 @0.050, lift 0.464/0.464
CompCams 858-16 Pro magnum Hydraulic lifters
CompCams 1412-16 magnum roller rocker arms
AFR 190 heads with 74cc chamber size.
Holley Stealth Ram that I modified to clear the stock hood.
Haltech E6K
120 lbs injectors.
SX fuel pressure regulator.
Twin BOSCH fuel pumps and a surge tank in the rear right fender.
4L80E controlled by a TCI T-Com
Vigilante converter
Front mount XS Power intercooler with 24"x12"x4" core.
HKS 'style' 50mm wastegate
Garrett GT4288N turbo from a Volvo semi truck
E85 ethanol fuel.

It is not a stock engine, but the short block is stock except for ARP bolts.

The 1984 Corvette has the CrossFire Engine.
The stock engine was rated 205 hp @ 4200 RPM and 290 lbs-ft @ 2800 RPM.

After the dyno I did some measuring of the exhaust pressure and at 5000 rpm, where boost was down to 0.9 bar, exhaust pressure was over 3 bar.

The 2007 seson ended with the conclution that the big semi truck turbo was too small.

The search for a replacement began.

I bought a bigger turbo.

A comparison between the Garrett GT4288N that I used last summer and my new Garrett TBP7501.




I thought that it was tight before but I was wrong ... With the new turbo it's tight but the hood still closes.




I made a fixture for the new flange when locating the turbo.




This stuff is the bomb :) Awesome Jonas. Gotta love that famous Swedish ingenuity :1st:
That is some awesome engineering on a CFI motor. The video was awesome as well. You gotta love a CF that runs low 11s... LOL It looks like it comes out of the bottom like a scalded dog. Thanks for the great pics JoBy.
Wow, that's fantastic.

What made you go with the truck turbo?


It thought that it was a good sized turbo and the price was right.

Usually a big diesel turbo has a larger turbine housing than what would normally be used on a gasoline engine. It will spool at a higher RPM but once you get boost it is more efficient because you get less exhaust pressure at the same boost.

I maxed out the GT4288N at 4000 RPM, both compressor and turbine side. That it the reson for the larger turbo. The new turbo is probably good for 1200 hp but I an not aiming that high. The new turbo is almost brand new ant the price was rught on that one too.

Data from the dyno:

(Rear wheel)
1087 Nm ( 800 lbs.ft.) @ 4000 RPM (this is rear wheel converted to crank scale.)
627 hp @ 4200 RPM

I have a 4L80E transmission so transmission losses might be higher than with the smaller th700. Also I had to make the dyno run in overdrive because I maxed out the dyno in third gear.

If transmission losses are 15% then the crank numbers are:
1280 Nm ( 942 lbs.ft.) @ 4000 RPM
738 hp @ 4200 RPM

If transmission losses are 18% then the crank numbers are:
1325 Nm ( 975 lbs.ft.) @ 4000 RPM
764 hp @ 4200 RPM

Anyway, Rear wheel nubers are what I know and what counts.

I want to increase the power range to higher RPM with the new turbo.
I also think that the tourqe peak of 1087 Nm is a bit high to be safe.

If I lower max boost to about 1.1 bar ( 16 psi ) then rwtq will probably go down to 900 Nm ( 660 lbs.ft). If I maintain that tourqe to 5250 RPM then that is 660 rwhp will less strain on the engine. With a flatter tourqe curve it will be easier to maintain traction too.