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3.4L swap in a '97... Issues... Please Help

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I picked up my '97 Boxster from a local shop today after they performed a 3.4L swap and I have a few issues.

First off, my redline is 6600 RPM(i thought the 3.4L was supposed to be 7100 rpm redline?). The dyno sheet they gave me (on a dynopack) shows 250hp to the wheels (right at the 6600 rpm redline!). The dyno sheet also looks to me like the 3.4L hasn't even hit its peak HP yet. I was told by the shop owner (after the swap was already complete! btw), that because of my older ECU (part # ends in .01) that they couldn't reflash my ecu. He said they were able to reprogram it with the stock 996 3.4L program, but were unable to overcome the 6600 rpm redline. Can that be true? If he couldn't overcome the 6600 rpm redline, then how did he get the stock 996 3.4L software in there? Is this car possibly running on the stock 2.5L program?

Can anyone confirm this?

What are my options?

Can I get a GIAC performance program for the 3.4 motor on my stock ecu or do i need to get another ecu (and if so, what ecu do i need to look for?)?

Also, the motor idles a little rougher than the 2.5L motor did. sometimes shaky idle, but not enough to move the tach needle around.

the Torque curve in the dyno graph is extremely erratic (see dyno printout below). The excuse that was told to me by the Owner of the shop for this was that it was normal for all stock 996 manifolds to have torque curves like this. This doesn't make sense to me. Can anyone confirm/deny this?

Some Backround on the car:

It's got Fabspeed headers (the ones without the cats in them) and a custom L pipe without cats. It also has that Stebro extremely ltw race muffler.

The intake is stock.

It now has a 996 sport clutch and a ltw flywheel.

It's a stripped out caged up race car that I'm building for NASA's GTS3.

Any help would be greatly appreciated!


Dyno Pack printout below:


Edited by Stee
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Sounds like you are being scammed. If they are using the factory PST or PIWIS to reprogram the DME, either it is reprogrammed or not, you can't put the 996 maps in without changing the other parameters. Easiest way to check is to open the engine compartment and rev the engine up. You should see the vacuum actuator on the second plenum cycle. If it doesn't you have a 986 program in the DME. I've heard rumours that the early DME can't be reprogrammed but I have never personally tried to do it. I have several later 5.2.2 DME's that if you provide me the programming codes I could program to match your car for a reasonable cost. Based upon that dyno, I am willing to bet you are running a 986 program in the DME.

I doubt your car has a 996 sport clutch as well since the 996 clutch and PP won't work with the 986 transmission.


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Thank you for your help Todd.

I spoke to the shop owner this morning and he did the right thing. He asked for the car back and said he would figure out what it takes to get it right.

I will keep you posted.

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  • 3 weeks later...
Thank you for your help Todd.

I spoke to the shop owner this morning and he did the right thing. He asked for the car back and said he would figure out what it takes to get it right.

I will keep you posted.

Hi and congrats on the 3.4 conversion.

I also have a 98 boxster 3.4. I also noticed a little rougher idle between the 2.5 and 3.4 motor, but it is nothing to worry about.

I have driven a few 3.4's and the stumble in idle seems common or normal. Just as long as the rpm's stay around

800-900 during idle.

I also created a small spacer at the very end of the throttle cable where it attaches to the throttle body

in order to take up some of the "slack" in the cable. It made a nice little difference in throttle response.

As far as ECU... When I purchased my 996 3.4 engine (used, 18,000 miles on engine) we also got the 996 computer

that came from the same car with that motor. Then added the "powerchip group" ecu upgrade.

Now that I think about it, we had to also use a new key "computer chip" inside the key that the 996 ecu would recognize.

same key blade, just different chip in head of key. I have had no problems or "check engine" lights since the conversion

and 10,000 miles.

3.4 loves to rev up to 7200 rpm's all day long. Obviously our boxster rpm gauge shows redline at 6600, but the motor should

pull even harder all the way to 7200 rpm.

I agree with an earlier response as well... while the car is idling

look inside the motor on the top at the intake manifold. There should be 2 "resonance" pipes connecting across

the left and right intake manifolds. (2.5 liter motor only has 1)

1 resonance pipe is part of throttle body plenum, the 2nd one behind it has a little valve actuator arm that you

should be able to see swing open when the rpms rev to over 3000 rpm. Rev the car slowly by twisting the

throttle body where the cable attaches and see if the valve actuates.

hi flow intake is a big plus. Especially when reving to 7200 rpm. 2.5 liter airbox is too small for those higer rpms.

I saw a 10-15 hp increase between 5500-7000 rpm just from adding a cone high flow intake and modified intake pipe.

I also modified the air pipe that goes to the throttle body. The stock 2.5 mid section has a lot of "crimps and restrictions" in it.

I basically got my hands on a extra pipe, cut off the ends and used the ends to connect to the throttle body and "MAF" sensor housing,

but cut out the middle and replaced it with a nice, smooth 3 inch diameter by 10 inch length section of pipe.

I then used rubber couplers and hose clamps to connect the new sections together. It worked out great. similar to this design.. http://www.rahul.net/dennisp/boxster/boxst...ds-intake.shtml

Dyno's... need to be taken with a grain of salt. I have seen 10-15 hp difference between 2 different dynojet locations

even though temp and humidity were similar.. go figure. I would not be surprised if you are seeing 250-275

hp readings depending on dyno's. I purchased a "Gtech Pro RR" that is an in car "accelerometer" and measures

g-force in order to give you 1/4 mile times and 0-60 mph times etc. Because that's what it comes down to, acceleration.

Currently my 3.4 liter boxster measured these times on the Gtech Pro... with a good launch ofcourse...

0-60 ft 1.9 sec

0-60 mph 4.4 sec

0-100 mph 10.6 sec

0-130 mph 17.3 sec

quater mile 12.8@110 mph.

based on a online dyno calculator:


I entered these values for my car

3000 lbs (2700 + driver+fluids) guess-tamate

trap speed 110 mph

elapsed time 12.8 seconds

=318 hp crank and 280 hp at wheels... cool... I can live with that.

This is what will drive you batty on dyno's...I have to admit I never really saw

280 hp at the wheels on any dynojet or dyno machine. Yet my acceleration times are

still pretty fast.

I really hope you get all the bugs ironed out and can really enjoy the ride.

I just drove through the mountains this weekend and I love my 3.4 boxster.

After my 3.4 conversion I made myself crazy trying to find 10-15 hp more because

of my dyno readings being lower than expected... After you get all the bugs out

you will get use to the additional power of the 3.4 all over again and then wish you could

somehow afford a 3.6 liter gt3 engine in a boxster... The need for speed never ends ;^)

best of luck.


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  • 2 weeks later...

Very interesting as when I was doing some testing on our 3.4 swap I noticed that the vacuum actuator was not moving when I reved up the throttle. We have a flat spot occuring where it is not pulling hard until you get past 4.5K rpms. Then it takes off like crazy. I thought it was a vacuum leak at the intake couplers though does not seem to be the case now as I have that adjusted out where a clamp was falling off. Anyone know why the actuator would not be moving? I verified the wiring is correct, is there a vacuum line routing somewhere to check out?


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  • 2 years later...

One reason the resonance flap isn't isn't moving could be that it's not hooked up. The original Boxster harness doesn't come wired for it, so you have to add your own harness wire (from the DME control unit to a new vacuum switch under the intake plenum.) A vacuum "T" to the vacuum switch is also required to get the system up and running, so it could very well be that the installer simply neglected to add the extra parts required to take advantage of the resonance flap on the 3.4 liter intake.

Edited by res1afbe
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