Getting under low jack points

Hey folks,

I wanted to talk exhaust noises.

My 2001 2.7 Boxster I just got came with these after market mufflers:


Although they sound amazing at higher engine speeds, they are pretty loud and "droney" at lower engine speed when under load. Not having the benefit of having tried a stock muffler, I'm not quite certain what to expect, but I think it could be a lot quieter.

Does anyone else have a similar exhaust setup, and how is it compared to alternatives?

In researching muffler design on these cars, it seems the ones reported to have less drone (including the stock muffler) have a crossover element in the muffler that connects the left and right cylinder banks. I happen to have a tig welding setup here, and was thinking about fabricating a crossover pipe that basically would link the two muffler outlets together in perhaps a straight line. The idea is some of the sound pulse energy will reverberate into the opposing cylinder bank's muffler. (The pulses alternate between cylinder banks, so I think this will dampen the low order exhaust harmonic). I could tune it for lots of dampening (big straight crossover pipe) or little bit of dampening (smaller diameter crossover pipe, fitted further back towards the exhaust tips). I'm not sure if you guys can visualize this based on my description, but what do you think?

The other exhaust noise I want to get feedback about fixing is what I believe to be a rattling catalytic converter. It seems to only really rattle when it's hot. Metallic rattling like a loose heat shield (but I checked and tried to tighten those, but the problem persisted). It seems louder on the right/passenger side - especially with passenger window rolled down when driving near an object on that side (like a wall or in a parking garage).. I read about one guy here having noisy catalytic converters which he fixed by replacing them outright, but he had some other problems (like low power I think, and bad emissions results), and his car also had roughly double the miles mine (which is "only"110k miles. ). Has anyone else here had a fixed rattling catalytic converter somehow other than replacing it outright? Anyone know what exactly is making it rattle as it comes up to temperature?

Finally, is it ok to jack where I have pictured below? Someone at a local porsche shop said they do it there all the time, but I got realizing it was supporting the entire weight of the rear half of the car through the two transmission mounts.




Edited 1 time(s). Last edit at 04/25/2015 12:25AM by jakeru.
(view changes)

There are four factory jacking points. Look in (or if you don't have one, buy one) an owner's manual where they are described.

... but make sure you put some soft material (hockey puck or wood).
The weight of the car is not supported by the transmission mounts, because that jacking point doesn't stress the transmission, it has a bracket that bolts right to the body's "frame".
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 314,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

... I just noticed that the jack is NOT in the correct point.
The only safe jacking point in the rear is right under the 2 rearmost bolts on the aluminum skid plate.
This point has a frame support to distribute the weight right onto the body.
The jack should not lift on the transmission or on the aluminum diagonal braces.
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 314,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

Thanks for the clarification on the rear jack point! I'm going to bring this up to my friend at the local Porsche shop. Perhaps I misunderstood his instruction, or he was confused about Boxster's layout versus the 911 style setup. Although he is not an actual tech there, (he's more the service manager/counter guy), as I know he's been racing Porsches for many years (they are mainly a race car prep shop), I am still a bit surprised he would lead me astray.

The place is pretty high-end, mostly focusing on race car prep. They had a McLaren and a Carrera GT in there when I toured their shop area (among many other very nice Porsches, and many racing-prepped Lotus Elise's). So it's hard to understand why they would lead me to jack my car up through its transmission mounts. I'm glad I noticed and thought to ask here.

It's surprising how little the transmission mounts deflected when supporting the rear of the car.

I did get a polyurethane puck (called a boxster jack adapter) that has a locating protrusion, specifically for using the floor jack on any of the four factory jack points.
And I did find this to be helpful for jacking at the factory points using a hydulic floor jack, without marring or otherwise damaging Aerokit1 side skirts my car has.

However, I find additional jacking points besides the four factory ones seem necessary to lift the car using a floor jack and get the weight safely transferred onto jack stands.

I found some other jacking points described in pelican's article "jacking up and lifting the boxster on Jack stands" (link: [www.pelicanparts.com] )

What do you guys think about using this rear jack point from the article?

.... start by lifting from one of the rear jack points.
Lift until you can place a floor stand under the front jacking point now up in the air.
Lower the jack slowly.
Place jack in rear jacking point on the other side.
Lift until you can place a floor stand under the front jacking point now up in the air.
Lower jack slowly.
Lift from rear jacking point (described in another post on this thread)
Place two floor stands under the two rear jacking points now up in the air.
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 314,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

You have to look at where the suspension and sub-frames bolt to the car's frame rails. These points are heavy, reinforced metal and in fact take the weight of the car as it hits potholes, corners hard, etc. They are very, very strong. That said i always use a wood or rubber cushion. On is quite obvious where the diagonal braces int eh rear bolt to the subframe, which carries the load up to the frame. Another is where the control arms and sway bars bolt to the front frame rails, in front of and inside the front wheels. Very hard to get a jack there since its so low. There are three large headed bolts to help identify the spot. I use this most of the time, but need to use a 2nd jack to get the car up a few inches (i place the other jack at the factory point and remove it after i can slip the main jack under the nose).

I always place the jack stands or my preferred blocks of 6 x 6 x 18 under the factory points, in my case with the wood at 45 degree diagonals for stability.

rear lifting point

On blocks

As Marc warned, be very, very careful of where you lift from. I use these because they *must* carry many, many times the car's weight under impact.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

I often drive up onto a small 2x4 or 4x4 before jacking because it makes the low jack points much more accessible.

The vast majority of dealer and shop lifts wont go under the lowered track cars. Most shops simply put a floor jack under the rear sub-frame, but we greatly prefer your method. I just dont keep all that much stuff laying around in my already crowded garage, but sometime i think i may add a couple of them.

G

Grant

gee-lenahan-at-gee-mail-dot-com

pads or lifting the car by the tires/wheels.

Lifting the car using any other lift point is risking bending suspension/chassis members or worse.

Hi
On my 981 I have a Fabspeed Maxflo exhaust. It has a crossover designed into it, I noticed that the 986 model has the crossover as well. My car does have a slight drone in the 2000-2400 range when under load, usually in top gear on the highway. It seems to be a common issue which doesn't bother me but I can see how it could. I have found the simplest solution is to shift to a lower gear. Usually shifting down one gear solves the problem and normally its only for a short time till the load comes off the engine.
Paul

Thanks, interesting stuff.

I'll bet our 986 and 981s aren't entirely different, given the rough same overall vehicle dimensions, engine location and configuration (mid-engined boxer 6) and exhaust system termination point (at rear center).

So I went ahead and did some calculations using a tuned organ pipe harmonic equation, where I think the drone would be most affected by the presence (or lack) of a crossover that couples the two separate exhaust systems of the 3 cylinder banks. I measured (by eyeballing with measuring tape, and guesstimation) that there is in the ballpark of an 8 foot long overall exhaust path length, from the exhaust valve through the cylinder head ports, headers, and rest of exhaust system out to tailpipe. (That's 2.44 meters.) An assumption made is for the speed of could of the exhaust gasses being 500 meters per second (based on an estimate that the average exhaust gas temperature would be about 500 F throughout the length of the system).

And guess what frequency came up at being the 1st harmonic (the loudest and lowest)? 51hz at the exhaust valves per bank, which equates to 2,040 crank rpm. I was surprised how close this was to where you mentioned you heard the drone at!

So given the data matches my model, I will go ahead and conjecture/hypothesize from this that if you were to close off or remove the crossover (e.g., by welding the crossover tube shut), it would make your 2000-2200 drone significantly more pronounced. (I have a feeling you won't be rushing into trying this, but it does make me want to further consider adding the crossover to my system, which currently lacks any crossover.)

I paid more attention to what RPMs the droning was happening on my 986. 2200 rpm seems like the peak that happens when cruising (which can sound "droney" ) , and there is another sound peak around 1100 rpm (which I would say can sound "boomy" ). Although the sound pressure seems most pronounced at those two points, it's fairly loud anywhere close to, or in between those points. Above 2200 or below 1100, the sound gets quieter. I'm seriously thinking about adding a crossover.



Edited 1 time(s). Last edit at 04/28/2015 12:33PM by jakeru. (view changes)