Performance 101
03-06-2010, 01:26 PM
#1
Burning Brakes
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Performance 101
For about a year I have posted a few performance questions and read hundreds more, though I would like to ask a top down question.
My question should apply for many owners and is simply this: To maximize performance, what is the low hanging fruit in terms of cost/labor?
I am not in a geography where I can access 928 experts and I am not a mechanic, thus I must rely on my own skill and that of my mechanic. Please hold the criticisms…
The 928 is an 84 USA L-Jet AT with low mileage and only a cat bypass mod, otherwise OEM across the board. I have changed the plugs, rotor, fluids, filters and tweaked the timing, however she is very slow on acceleration and I know there is more under the hood that I am not getting as I am being hammered by school busses. She starts easy and runs steady, again just slow. That said, I do know that the AT needs adjustment and/or vacuum issues repaired as it shifts to low in the RPM rev.
With all this in mind, I am not looking to turn this into a race car or spend tens of thousands of dollars. What are the next logical steps to take to unlock performance? Injector cleaning? Fuel filters? Compression testing? TC rebuild? Vacuum Tests?
Where would you start?
My question should apply for many owners and is simply this: To maximize performance, what is the low hanging fruit in terms of cost/labor?
I am not in a geography where I can access 928 experts and I am not a mechanic, thus I must rely on my own skill and that of my mechanic. Please hold the criticisms…
The 928 is an 84 USA L-Jet AT with low mileage and only a cat bypass mod, otherwise OEM across the board. I have changed the plugs, rotor, fluids, filters and tweaked the timing, however she is very slow on acceleration and I know there is more under the hood that I am not getting as I am being hammered by school busses. She starts easy and runs steady, again just slow. That said, I do know that the AT needs adjustment and/or vacuum issues repaired as it shifts to low in the RPM rev.
With all this in mind, I am not looking to turn this into a race car or spend tens of thousands of dollars. What are the next logical steps to take to unlock performance? Injector cleaning? Fuel filters? Compression testing? TC rebuild? Vacuum Tests?
Where would you start?
03-06-2010, 01:43 PM
#3
Burning Brakes
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I bought the car a year ago and have done the following with modest gains:
1. Oil Change & filter
2. AT Fluid Change & filter
3. Air Filter
4. Cat bypass
5. Timing Advance (gained some performance)
6. New Plugs
7. New Rotor
8. New Steering Cup & Fluid
9. Rad Fluid Change
10. New Alternator
11. New Battery ground strap
12. Hatch alarm bypass
None of these projects reduced performance... she is slightly better than I when I bought her.
1. Oil Change & filter
2. AT Fluid Change & filter
3. Air Filter
4. Cat bypass
5. Timing Advance (gained some performance)
6. New Plugs
7. New Rotor
8. New Steering Cup & Fluid
9. Rad Fluid Change
10. New Alternator
11. New Battery ground strap
12. Hatch alarm bypass
None of these projects reduced performance... she is slightly better than I when I bought her.
03-06-2010, 01:56 PM
#4
Administrator - "Tyson"
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The lowest hanging fruit is making sure everything is working 100% as the factory intended.
1. Is the WOT switch working (very common and will rob HP - I have a dyno somewhere...)
2. Vacuum - any slight leak will kill your vacuum advance. My friends 84 had this problem. He replaced everything....never checked the vacuum lines. 5 minutes with my mighty-vac and some new lines he was good-to-go
3. Is the throttle opening 100%? Sounds odd but another common issue.
What plugs did you install?
How good are the plug wires?
What condition is the green ignition wire in? (Runs from the distributor to the passenger side of the engine compartment. Usually the outright fail, but sometimes they can short out causing an ignition miss).
Are the fuel pressure regulators working? There is a test procedure for this, I'll have to find my WSM to refresh my memory.
Is the rest of the exhaust in good shape? I've seen more than a few sluggish cars that ended up having a plugged exhaust or a collapsed inner wall that was blocking output.
Best bang for the buck is swapping out the exhaust manifolds for SS units from an 85/86 car. Your CAT Bypass is a step in the right direction since that is #2 after the manifolds
Next step - supercharge it
1. Is the WOT switch working (very common and will rob HP - I have a dyno somewhere...)
2. Vacuum - any slight leak will kill your vacuum advance. My friends 84 had this problem. He replaced everything....never checked the vacuum lines. 5 minutes with my mighty-vac and some new lines he was good-to-go
3. Is the throttle opening 100%? Sounds odd but another common issue.
What plugs did you install?
How good are the plug wires?
What condition is the green ignition wire in? (Runs from the distributor to the passenger side of the engine compartment. Usually the outright fail, but sometimes they can short out causing an ignition miss).
Are the fuel pressure regulators working? There is a test procedure for this, I'll have to find my WSM to refresh my memory.
Is the rest of the exhaust in good shape? I've seen more than a few sluggish cars that ended up having a plugged exhaust or a collapsed inner wall that was blocking output.
Best bang for the buck is swapping out the exhaust manifolds for SS units from an 85/86 car. Your CAT Bypass is a step in the right direction since that is #2 after the manifolds
Next step - supercharge it
03-06-2010, 02:11 PM
#5
Nordschleife Master
When these busses are passing you are you holding the pedal all the way down and the RPMs are going to near 6K?
My suggestion is to take it to a chassis dyno and get the numbers.
Cheapest/easiest performance mod is to make sure the throttle is opening all the way. Have someone hold the pedal down and see if the throttle plate will still turn.
My suggestion is to take it to a chassis dyno and get the numbers.
Cheapest/easiest performance mod is to make sure the throttle is opening all the way. Have someone hold the pedal down and see if the throttle plate will still turn.
03-06-2010, 02:12 PM
#6
Burning Brakes
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Bosch Platinum
Seem ok, no fireworks in the dark
Ok, interesting, though I need to find out what is broken first, thanks!
03-06-2010, 02:17 PM
#7
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Wide Open Throttle
There are two switches on the drivers side of the throttle body. One is the idle switch, the other WOT.
Reach down there (car off) and see if you can hear a "click" at some point when opening the throttle. Or try and just close the switch with your finger (sometimes it's hard to reach).
Even if it does click, might still be malfunctioning. Test with a milti-meter.
This shouldn't hurt anything, but I would go to Bosch Copper.
There is zero performance to be gained by using anything other than Copper plugs. My friends 84 developed an idle miss using iridium plugs.
Silver, Platinum, Milti-tipped etc.. plugs extend the life of the plug, nothing more. Copper offers the best performance, downside is they do not last very long.
Gap is important too, .028-.032
Since you are in SoCal, using a plug one range colder than stock should be considered. I'm using 2-3 ranges colder than stock in Wisconsin, havn't had any issues but I do park the car in winter.
Stock is WR7DC (7 is the heat range, C is for copper). If I were you, drop is a set of Bosch WR6DC plugs.
I spritz some water all over the engine when testing this. Unless there is a metal source near the break in the wire, might not see the sparks.
Isn't there something with the automatic transmission, some vacuum line and the kick-down? I'm not all that familiar with diagnosing "roger boxes".....
There are two switches on the drivers side of the throttle body. One is the idle switch, the other WOT.
Reach down there (car off) and see if you can hear a "click" at some point when opening the throttle. Or try and just close the switch with your finger (sometimes it's hard to reach).
Even if it does click, might still be malfunctioning. Test with a milti-meter.
This shouldn't hurt anything, but I would go to Bosch Copper.
There is zero performance to be gained by using anything other than Copper plugs. My friends 84 developed an idle miss using iridium plugs.
Silver, Platinum, Milti-tipped etc.. plugs extend the life of the plug, nothing more. Copper offers the best performance, downside is they do not last very long.
Gap is important too, .028-.032
Since you are in SoCal, using a plug one range colder than stock should be considered. I'm using 2-3 ranges colder than stock in Wisconsin, havn't had any issues but I do park the car in winter.
Stock is WR7DC (7 is the heat range, C is for copper). If I were you, drop is a set of Bosch WR6DC plugs.
I spritz some water all over the engine when testing this. Unless there is a metal source near the break in the wire, might not see the sparks.
Isn't there something with the automatic transmission, some vacuum line and the kick-down? I'm not all that familiar with diagnosing "roger boxes".....
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03-06-2010, 02:25 PM
#8
Burning Brakes
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Thanks, Hacker-Pschorr, this is a good starting place...
I would add that I am not getting any kind of hesitation or lull during the acceleration climb, simply slow all the way up…
I would add that I am not getting any kind of hesitation or lull during the acceleration climb, simply slow all the way up…
03-06-2010, 02:34 PM
#9
Burning Brakes
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When these busses are passing you are you holding the pedal all the way down and the RPMs are going to near 6K?
My suggestion is to take it to a chassis dyno and get the numbers.
Cheapest/easiest performance mod is to make sure the throttle is opening all the way. Have someone hold the pedal down and see if the throttle plate will still turn.
My suggestion is to take it to a chassis dyno and get the numbers.
Cheapest/easiest performance mod is to make sure the throttle is opening all the way. Have someone hold the pedal down and see if the throttle plate will still turn.
03-06-2010, 05:32 PM
#10
Rennlist Member
OK,
Before all the other stuff, have you had the car checked with a Hammer or laptop??
You may have a knock sensor off-line or the Hall Sensor is done. Easily checked.
The retard in the ignition timing will have a huge effect on your performance.
Before all the other stuff, have you had the car checked with a Hammer or laptop??
You may have a knock sensor off-line or the Hall Sensor is done. Easily checked.
The retard in the ignition timing will have a huge effect on your performance.
03-06-2010, 05:35 PM
#11
Administrator - "Tyson"
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To advance the ignition he just has to turn the distributor, old school.....
03-06-2010, 05:36 PM
#12
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Once that's checked (and confirmed good) then the next step is your vacuum checks.
I hope you've got the Moorehouse collection, or at least a good manual for your model year.
I hope you've got the Moorehouse collection, or at least a good manual for your model year.
03-06-2010, 05:37 PM
#13
+1 Go back to basics and set the car up to original specs, such as throttle settings, timing, check for induction leaks , check fuel system leaks by pressure rentention test, chean and calibrate injectors, check out ignition components, measure the resistance of each component to ensure they meet specks to ensure that you are getting the correct ignition and spark intensity etc., etc.
Check out the auto transmission for shift points etc. carry out torque converter stall test, this will indicate whether the internal drive bands etc., are worn or slipping, check out cable adjustment, vacuum and modulating pressure settings.
A dyno test will confirm whether you are developing HP.
Have you done a compression test and leak down test on engine to confirm that you are getting the correct air induction, compression and if the injectors have been cleaned and calibrated then you should have the correct air fuel ratio to develop the HP as specified.
When these are confirmed, as meeting specks, then you can progress onto the more adventurous items to increase HP mindful that Mr Porsche got good performance from the engine in the first place.
Tails 1990 928S4 Auto
Check out the auto transmission for shift points etc. carry out torque converter stall test, this will indicate whether the internal drive bands etc., are worn or slipping, check out cable adjustment, vacuum and modulating pressure settings.
A dyno test will confirm whether you are developing HP.
Have you done a compression test and leak down test on engine to confirm that you are getting the correct air induction, compression and if the injectors have been cleaned and calibrated then you should have the correct air fuel ratio to develop the HP as specified.
When these are confirmed, as meeting specks, then you can progress onto the more adventurous items to increase HP mindful that Mr Porsche got good performance from the engine in the first place.
Tails 1990 928S4 Auto
03-06-2010, 06:11 PM
#14
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OSP, you missed something. At least from your list.
Bowden cable. I'd bet a 100% sirloin burger lunch that you have Grandma Buick syndrome. Its different than the kickdown switch.
Put two turns on the turnbuckle to shorten the cable. It'll hold its shifts longer. It should have just a bit of tension on it, no slack or very little once its reseated on the ball after adjustment.
(At the throttle body, not the throttle connection, the other turnbuckle thats near the throttle, I think is down lower. You will need to pull the airbox and reach around the back under the spider, and maybe reach in front. A bit contortionist, but doable.) A long thin screwdriver helps pop it off the ball, and can be used to pop it back on. You will need a couple of small open end wrenches, can't remember 6,7,8 mm, to undo the jam nut.
On the timing, set it to specification, don't use abnormal advance.
Bowden cable. I'd bet a 100% sirloin burger lunch that you have Grandma Buick syndrome. Its different than the kickdown switch.
Put two turns on the turnbuckle to shorten the cable. It'll hold its shifts longer. It should have just a bit of tension on it, no slack or very little once its reseated on the ball after adjustment.
(At the throttle body, not the throttle connection, the other turnbuckle thats near the throttle, I think is down lower. You will need to pull the airbox and reach around the back under the spider, and maybe reach in front. A bit contortionist, but doable.) A long thin screwdriver helps pop it off the ball, and can be used to pop it back on. You will need a couple of small open end wrenches, can't remember 6,7,8 mm, to undo the jam nut.
On the timing, set it to specification, don't use abnormal advance.
03-06-2010, 06:24 PM
#15
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