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400 W-72 Timing

  
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400 W-72 Timing

 
78TransAm400 78TransAm400
New User | Posts: 26 | Joined: 12/13
Posted: 05/04/14
03:45 PM

Hey Everyone,

I know it's been awhile since I've visited the forums, but my company has me moving to North Carolina from Florida so it's been busy. I've also been away from my 78' Trans Am for about 3 weeks and I miss it more than the girl friend!

But while I have been away from it, I've been thinking about it's timing. The factory recommends it be set 18 degrees before TDC, but as we all know they where de-tuned 400 blocks due to the recent emission laws. Since I'm not running a cat anymore and the exhaust flows freely, do you think I could advance the timing up to maybe 14 degrees or even sooner to gain some extra ponies of out that 400?

Also, below is some Pontiac eye candy, my car was used in a professional photo shoot in Tampa about 2 months ago. Something we all can enjoy! Grin

10155240 10203933981336634 9050367570987133668 N

1622719 10203933984176705 6501188952238120651 N

1797481 10203933981096628 8568566515257939451 N  

wayne712222 wayne712222
Moderator | Posts: 235 | Joined: 10/13
Posted: 05/04/14
04:50 PM

you should probably invest in a detonation sensor and one of the microdisplays that can be stashed where you can observe them while under power if you are going to play with the timing..

you might also since you removed the cat.. invest in an air fuel ratio display.. have an oxygen sensor bung installed in the downpipe..  so you can run an oxygen sensor..

that car probably has an air injection system.. you will want to install an vacuum gauge in the same gauge bracket as the air fuel ratio display.. and you can mount them upside down in a gauge bracket and install a carpet scrap so you can set it on your dashboard.. without scratching the dash. this makes it a plug into the lighter socket.. plug into a quick connector for the few wire and a vacuum hose for the vacuum gauge...

why the vacuum gauge . you will want to tee it into the diverter valve vacuum circuit to see when or if it is dumping air into the exhaust manifolds which will fool with your air fuel ratio readings.. hint.. this is a trouble point in feedback and fuel injected cars also.. but it is only supposed to happen during open loop... but it can make your air fuel ratio readings go crazy and confuse you..

the 78 carb may have a APT adjustment..  that can allow you to find tune the off idle and low cruise Rpm air fuel ratio..  apt adjustment access depends on which 400 you have.. the 400 pontiac carbs are harder to adjust the APT on..
why am i typing all this.. because you have removed the cat..  changed the back pressure and scavenging of the cylinders.  will you need to go a little richer or a little leaner..  that NOBODY knows without monitoring your exhaust flow while the car is rolling along under power..  a video camera that can see the air fuel ratio display.. the vacuum gauge that's now hooked up to the intake manifold.. and the tachometer.

MSD used to have a knob that could be used to adjust the base timing. this would give you a quick turn to change the timing ever so slightly.. to know where its going to ping at..when using the detonation sensor..  there are a bunch of aftermarket distributors out there.. some of them have electronic advance curves built into the module..

pontiac enginnering spent a LOT of weeks of dyno work. cross country fully instrumented road trips to verify emission levels and fuel and spark needs.. its really complicated.. so take it one step at a time..

this is just my personal opinion..    does not mean i know what i am talking about.


just a hint..

http://www.summitracing.com/search/department/gauges-accessories?SortBy=BestKeywordMatch&SortOrder=Ascending&keyword=knock

end edit  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 496 | Joined: 11/13
Posted: 05/05/14
01:17 AM

hi;

so which one is your girlfriend?

your stock timing was likely not 18, it was probably less.

no you will not get more hp out of changing it.

if you want quicker acceleration then change the rear gear ratio.  

Brazil Brazil
Addict | Posts: 3963 | Joined: 06/09
Posted: 05/05/14
06:26 AM

Going from 18 degrees to 14 would actually be retarding the timing. A good starting point would be between 14 and 16 with 92/93 octane fuel. If you hear any ping (detonation) back it down a bit because it will damage your engine. A 3:73 gear ratio will give quicker acceleration, but fine tuning your carb and getting your timing to match will show an improvement.

Is it a 4 speed? I can't see past the chick's legs..  

78TransAm400 78TransAm400
New User | Posts: 26 | Joined: 12/13
Posted: 05/05/14
08:55 PM

Yes its a factory 4speed car, and advancing my timing to maybe 14 degrees while maybe playing with the mixture screws on the carb might give me a slight boost?  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 496 | Joined: 11/13
Posted: 05/05/14
09:12 PM

No...  

Brazil Brazil
Addict | Posts: 3963 | Joined: 06/09
Posted: 05/06/14
07:44 AM

No
Again, the timing is advanced when you go numerically upward. Playing with the mixture screw is just that - screwing it up.

A good starting point would be to not touch those adjustments. Typically they should be approximately two and a half turns counterclockwise from being seated. Set your timing at about 14 degrees and "advance" toward 16 until you hear detonation, then "retard" it a little until it stops pinging. Once you have the timing close, drive it to see if there is any bog down upon acceleration.

It's essentually a delicate balancing between the carb mixture and timing.

Since the car is a manual that changes the equation concerning the rear gear. Do these adjustments first, and we'll guide you through it from there.  

idrivejunk idrivejunk
Addict | Posts: 5162 | Joined: 12/09
Posted: 05/06/14
06:13 PM

Guys, my repair manuals indeed show 18 degrees BTDC as the setting on the 78 Firebird high performance 400. The 220 horse one that has more compression. The standard duty was in the 7s on compression and made five pounds more peak torque, at about a grand less RPM. Initial timing setting for that one is 16. Think "premium fuel only" sticker? On the W72.

To the OP- I would not go any farther away from top dead center than the factory recommendation. Most 400s I have known like around 13 degrees or so. Moving from 18 BTDC to 13 means the plug fires later because the B in BTDC means Before. 18 degrees is WAY advanced by most of our time-tested tuning standards. dangerous territory with today's fuels. Detonation is not always audible. Idle mixture has little to do with wide-open performance so theres no power gains to be had by venturing away from the smoothest idle setting, in my opinion.

Throw a gallon or two of race gas in that thing and see how she runs without changing anything else. No octane booster, just race gas or 98+ octane. I bet you like it.  
idrivejunk

78TransAm400 78TransAm400
New User | Posts: 26 | Joined: 12/13
Posted: 05/09/14
11:24 AM

Thanks everyone for your responses. As usual they are much appreciated.

Ill be changing the fuel filter this weekend, the current filter is probably from the early 90s. When I bought the car and started doing basic tune up tasks I found that the air filter was dated 1986. Poor car sat for so long....but now she's seeing the road every weekend and it feels great.

Im moving still in the process of moving, don't worry,  she's being towed on a full flat bed behind my moving truck...no unessesary miles or car dollys.  But when i get settled in, i plan to advance the timing to maybe 15 or 16 BTDC.  i am running the factory vacuum advance on the distributor though, but that plays with the arc curve at higher RPM.

The carb, as far as im aware, has never been rebuilt. It still runs pretty good, so i haven't fooled with it much. I did run into a slight bog here and there, sounds like fuel deprivation, hence the change of the ancient fuel filter.

Thanks again for the help, always ready to learn more about Pontiac!  

shyrgfuh3 shyrgfuh3
Enthusiast | Posts: 496 | Joined: 11/13
Posted: 05/09/14
01:18 PM

hi;

glad you're getting her going

the bog is probably not your fuel filter.

the vacuum advance does not work under acceleration. it works at steady throttle and decelleration positions.

connect it to non ported vac and disconnect it when checking timing.  

78TransAm400 78TransAm400
New User | Posts: 26 | Joined: 12/13
Posted: 05/12/14
08:17 AM

Thanks everyone for your responses. As usual they are much appreciated.

Ill be changing the fuel filter this weekend, the current filter is probably from the early 90s. When I bought the car and started doing basic tune up tasks I found that the air filter was dated 1986. Poor car sat for so long....but now she's seeing the road every weekend and it feels great.

Im moving still in the process of moving, don't worry,  she's being towed on a full flat bed behind my moving truck...no unessesary miles or car dollys.  But when i get settled in, i plan to advance the timing to maybe 15 or 16 BTDC.  i am running the factory vacuum advance on the distributor though, but that plays with the arc curve at higher RPM.

The carb, as far as im aware, has never been rebuilt. It still runs pretty good, so i haven't fooled with it much. I did run into a slight bog here and there, sounds like fuel deprivation, hence the change of the ancient fuel filter.

Thanks again for the help, always ready to learn more about Pontiac!  

Brazil Brazil
Addict | Posts: 3963 | Joined: 06/09
Posted: 05/12/14
09:02 AM

Just a note - try to avoid the glass see through filters, they look good but the metal canisters do the job better. If your running a Q-Jet that has the brass filter in the fuel inlet, be sure it's put in the right direction also.
Cool  

wayne712222 wayne712222
Moderator | Posts: 235 | Joined: 10/13
Posted: 05/12/14
11:11 AM

i forgot about these articles. they don't go deep enough..



http://www.highperformancepontiac.com/tech/0212hpp_spark_plug_advance/



http://www.highperformancepontiac.com/tech/1308_how_to_optimize_the_centrifugal_advance_in_pontiacs/

http://www.highperformancepontiac.com/tech/1403_the_importance_of_ignition_advance_perfect_timing/


http://www.highperformancepontiac.com/tech/hppp_1302_msd_distrubutor_advance_curves/

http://www.highperformancepontiac.com/tech/vacuum_advance_tech/


some ignited reading here..  

wayne712222 wayne712222
Moderator | Posts: 235 | Joined: 10/13
Posted: 05/12/14
11:38 AM

more info..

http://www.vetteweb.com/tech/1305_how_to_disassemble_a_defective_distributor/viewall.html

one thing.. when finished OHMing the pickup coil.. do not disconnect. flip the meter to AC volts and spin the distributor shaft.. you need to see over just slightly over 1.0 volts AC with the shaft spinning by hand.. or it will not trigger the ignition module..

if you don't have more that 1.0 volts.. inspect the second layer of the pickup coil for cracks or broken out pieces..  or on other than big cap HEI distributors.. you will want to replace the reluctor..

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small cap HEIs have this issue..

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HEI Largecapgroundstrap

you can see the ground strap fitting under IDJs old coil and cap..

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what happens when the ground strap.. the hold down screw is missing or left loose..

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