84-86 owners remove CAT for 10 HP

[Kurt B]

Dunno.
On page 90 of Anderson's Handbook, paragraph 3, he suggests that Porsche got the additional 10 HP out of the 87 model by "wring[ing] a little more power out of the catalytic converter version."

This is where the 217 comes from over the 207 HP for US 84-86 owners.
Anyone care to add to this or confirm or reject it? Unless some kind of perfect back pressue increases HP. It seems to me removal of the cat altogether at least gives the 217 HP, or even mroe.

[tight as a bull's ass]

Well, even if that's the case, you're talking about a HP increase of about 5%...

I doubt you could be able to tell the difference when driving and may not be the worth the trouble to mess with the cat.

[ChrisBennet]

Bruce has also stated that they dynoed about the same (-86 & 87+) and that there wasn't any real difference. The earlier numbers were just conservative.
My own pet theory is that the G50 should have more inertia and thus would show more losses on a rear wheel dyno.
-Chris

Don't forget that the euro cars with 217 hp do have a cat.

I've come upon some German homepages that offer a socalled sports cat for these engines. That should bring hp up around the 231 of the non cat cars. But the sports cat doesn't do it alone. They supply a chip with it also as far as I can remember.

I'm sorry that I can't remember any homepage adresses right now.

[dickster]

if you have a cat. the best thing appears to be replacing it with a euro pre muffler pipe, which is what i plan on doing.

it seems it is more effective than a "straight" pipe because of the back presure issue.

richard

[Thomas Owen]

Interesting topic...What about the combination of no cat and stock muffler vs. no cat and sport muffler? Or, pre-muffler and a sport muffler? I run no cat and a Bursch on my '88 and really wonder what the gain(loss) has been. It 'feels' like it pulls better, but that could just be the sound (that I like)...
Regards,

[Dug89]

I just replaced the cat on my 84 with a straight test pipe. Still have the stock exhaust. All I noticed is a bit more noise and less heat out of the left rear.

Not very quantitative but I figured less heat and more space for valve adjustments were advantagous. Can't say I feel more power.

My 12 year old says it sounds more like a Porsche now.

[Mike the mechanic]

Before my turbo conversion, I had installed a B&B muffler with the cat in place and there was a seat of the pants difference on the top end. After i got used to that, I installed the test pipe and noticed the same power increase, again at the top end. I found myself going to the redline beore shifting, as opposed to before. The difference in power the bypass pipe and B&B made was less than my Racing Dynamics chip gain.

Mike
86 Carrera t

[MRM]

Oops. Double post.

[MRM]

I have read at several explanations.

Paul Frere says in The 911 Story says that the increase comes from more aggressive mapping of the factory chip once Porsche had a couple years' experience with the Motronic system. I've also read Bruce Anderson say that he didn't know what the difference between the ratings were, that he thought they just looked at their dyno a little closer and were less conservative in 1987.

As Kurt said, Bruce Anderson writes at page 90 of his Performance Handbook, "Also for 1987 Porsche was able to wring a little more power out of the catalytic converter version of the engine, increasing it to 217hp." But I think he gives it away right there because he immediately changes the subject without explaining where the increase in power came from. For all other years he explains the engine changes in great detail and identifies where the performance increases came from.

I suspect there is no real explaination here. Maybe after a few years' modification and experience Porsche just remeasured the hp rating and felt comfortable that it was an honest 217.

[ChrisBennet]

Originally posted by Mikkel
Don't forget that the euro cars with 217 hp do have a cat.

I've come upon some German homepages that offer a socalled sports cat for these engines. That should bring hp up around the 231 of the non cat cars. But the sports cat doesn't do it alone. They supply a chip with it also as far as I can remember.

I'm sorry that I can't remember any homepage adresses right now.

The Euro cars ran a higher compression (in addition to a premuffler instead of the cat) which accounts for the higher than US horsepower figure. I think stock Euro was 231HP (DIN). This makes me think that the 217HP Euro cars w/cat probabably didn't have the high compression pistons either. (Just speculation on my part, I didn't even know they had catted/217hp Euros.)
-Chris

[Kurt B]

it is the cat. Read on..there was a 207 HP 87 delivered to australia because of emissions requirements of some type!

ChrisBennet

I think you're right. The euro 217hp cat engines did have a lower compression than the non cat 231hp.

I just checked my Cargraphic catalogue out of curiousity. They offer a metal sports cat which would offer a hp increase of 8 by replacing the stock cat. No chip, no sports airfilter, just the cat.

[masraum]

The difference in the '84-86 US cars and the '87-89 US cars is a slightly different mapping in the DME and also that Porsche just was conservative on the initial hp claims. They did the same thing with the SC that went from 180-188hp with no changes.
Reading BA's book and the section you describe, the book isn't saying that Porsche added a cat and got another 10 hp. In the US they had a cat before and after the difference. "...wring a little more power out of the catalytic converter version of the engine," just means that they were able to take the US version that came with a catalytic converter and more highly tune it to get a few more hp out of the engine, not adding or removing parts. If you read the rest of the paragraph it says that the Australian version detuned the car to be able to use lower octane fuel which is why the car was back down to 207hp.
Basically, Porsche designed the fuel and spark curves in the software for the '84-86 version conservatively because there is no knock sensor and so no way to prevent detonation if they were too aggresive with the igntion curves. When after some testing and realizing that the US has fuel of a high enough octane and quality they realized they could go more aggresive on the fuel and ignition mapping. That's where the extra hp comes from. But when they did that they thought that you couldn't get the same high octane, high quality fuel in Australia so they basically left the fuel and ignition mappings the same so there would be no danger of detonation with the lower quality fuel. All Carrera's in the US from '84-89 had cats. Removing them would probably, at least, change the power curve and probably increase it at some range. There are high flow metal cats that you can get that are better than the stock, but they are pricey.

[MRM]

Originally posted by Kurt B
it is the cat. Read on..there was a 207 HP 87 delivered to australia because of emissions requirements of some type!

The Australian version was "detuned" (Bruce Anderson's word) to take lower octane gas, not for emissions concerns. At the time Australia did not have high enough octane for the Carrera.

[Mike the mechanic]

On the Carrera DME box, there is a little knob that you can supposedly click to alter the fuel and timing maps. I don't know if this is true, I have seen the knob but I haven't messed with it. Each click is supposed to add 3*, remove 3* of timing and add/remove 3-6% of fuel. It is there for the factory to fine tune each car for it's destination. I think there was a post about this a while back. Maybe thats all the difference in 84-86 and 87-89.

Mike
Carrera t

Mike that is interesting. So would it be possible to tinker with it and make the car accept another grade of fuel that way? Mine runs on 98 octane, but that is hard to get so changing it for 95 octane would make things a lot easier. Still I'm worried how much power that would cost?