Tuning Help for a Freshly Rebuilt 350 - Pontiac GTO Forum
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post #1 of 3 (permalink) Old 04-20-2017, 11:59 AM Thread Starter
 
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Tuning Help for a Freshly Rebuilt 350

I need advice on tuning a freshly rebuilt 350 from my 1970 LeMans.

With the vacuum advance disconnected:
- At 6 DBTDC (17" vacuum) has poor performance and little to no ping
- At required 9 DBTDC (17.5" Vacuum) has mediocre acceleration with a slight ping and still runs at near 200 degrees temp.
- At 10.5 DBTDC (18" vacuum), runs hotter (215 Temp) with more ping
- At 12 DBTDC (18.5" vacuum), runs even hotter with even more ping.

When the Vacuum advance is reconnected, the performance drops and the pinging is much greater. I have resorted to running the LeMans without the Vacuum advance connected for now.

I have checked for vacuum leaks and found none. The Quadrajet was freshly rebuilt but originally tuned for a 400.

1969 XS Block 350 with the std 9779067 grind cam. Rebuilt Quadrajet from a Firebird 400 and an HEI ignition.

I have struggled with this for weeks and am at the end of my rope.

Any advice would be greatly appreciated.

Please help.

George
Arlington VA
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post #2 of 3 (permalink) Old 04-20-2017, 06:04 PM
 
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As always, this is my opinion and hopefully others will chime in with theirs so you can resolve the issues with your car.

You mentioned a Torker intake on your engine in your other post. Is it one of the early "original" single plane intake or the Torker II?

In either case - BAD CHOICE. 1.) - A single plane intake will have no bottom end and will be sluggish UNTIL you start getting up over 2,500 RPM's and have an engine able to pull 6,500 - 7,000 RPM's to take advantage of this manifold. 2.) This intake is too big to be used on a 350CI unless you have a high revving engine, 2,500 RPM or better stall converter and 3.73 rear end gear or better. 3.) This intake needs a way bigger cam. The "067" grind is a good cam for a street engine, but way too small to take advantage of the Torker.

So, your first problem is your intake selection. Until you lose this, your engine will be very sluggish on the bottom end. Find a factory Q-jet cast iron dual-plane intake. A dual plane intake is what you want for a street engine. This change alone will wake up your engine. You can go other dual plane intakes in aluminum/aftermarket, but you would be throwing your money away as the factory intake is one of the best performers.

Next, you did not say what transmission or gears? The "067" cam may actually be too much for a 350 having a stock converter trans and 2.93 - 3.08 gearing. It may be soggy on the bottom end because the cam does not develop enough velocity within the intake runners of the intake/heads. Stock heads? Bigger valves?

Your factory compression is listed as 9.2, which is probably too low to take full advantage of the "067" cam. It has a 113.5 lobe separation angle (LSA) will will bleed off some of the compression in the lower RPM range - again making the engine sluggish. The "066" would have been a better pick with a 111 (LSA). The Comp Cams XE256 (with matched valve springs) might have also been a better choice with a 110 LSA and a little more lift than stock. I would think you won't have any problems with valves hitting the top of the pistons, but this is where you contact Comp Cams to confirm.

Hate to say it, but not so good a choice with the single plane Torker or the wide LSA "067" cam - both of which will kill your bottom end and make the car very sluggish on the bottom end which is where your cruising RPM's are.

On the distributor advance curve, as you retard your ignition, it will make the engine run hotter. You want to advance the distributor as much as you can without the engine "pinging" - which can destroy an engine if you don't back out of it to get the pinging to stop. Pinging means the distributor is too far advanced for the gas octane/load you are putting on the car.

Setting up the distributor advance curve and adjusting the springs/vacuum advance has been very well covered in several posts. A search of the forums should pull this up. The vaccum advance when connected will do as it is called, advance your timing using engine vacuum - as you learned. At this point, I would leave it disconnected and time the engine where it does not ping under load - it may be 6,7,8 degrees as you found out.

I would not mess with the distributor advance curve until I swapped out the intake first, and then considered a different cam. Do one thing at a time. Try a factory intake and see what changes. If this does not pick up the engine, I might swap out the factory torque converter for a 2500 RPM "tight" stall converter. You may see a slight drop in gas mileage if this is of any concern, but it will pull the engine quicker to a better/higher RPM range where the cam will perform better. If this is not something you want to experiment with, then a cam change may be in your future to increase the bottom end/mid range power band for street use.

You did not say if you had dual exhaust? A Pontiac engine needs to breathe and dual exhaust is a must with low restriction mufflers.

Hope this helps.
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post #3 of 3 (permalink) Old 04-21-2017, 09:59 AM
 
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Originally Posted by pontiacjim View Post
as always, this is my opinion and hopefully others will chime in with theirs so you can resolve the issues with your car.

You mentioned a torker intake on your engine in your other post. Is it one of the early "original" single plane intake or the torker ii?

In either case - bad choice. 1.) - a single plane intake will have no bottom end and will be sluggish until you start getting up over 2,500 rpm's and have an engine able to pull 6,500 - 7,000 rpm's to take advantage of this manifold. 2.) this intake is too big to be used on a 350ci unless you have a high revving engine, 2,500 rpm or better stall converter and 3.73 rear end gear or better. 3.) this intake needs a way bigger cam. The "067" grind is a good cam for a street engine, but way too small to take advantage of the torker.

So, your first problem is your intake selection. Until you lose this, your engine will be very sluggish on the bottom end. Find a factory q-jet cast iron dual-plane intake. A dual plane intake is what you want for a street engine. This change alone will wake up your engine. You can go other dual plane intakes in aluminum/aftermarket, but you would be throwing your money away as the factory intake is one of the best performers.

Next, you did not say what transmission or gears? The "067" cam may actually be too much for a 350 having a stock converter trans and 2.93 - 3.08 gearing. It may be soggy on the bottom end because the cam does not develop enough velocity within the intake runners of the intake/heads. Stock heads? Bigger valves?

Your factory compression is listed as 9.2, which is probably too low to take full advantage of the "067" cam. It has a 113.5 lobe separation angle (lsa) will will bleed off some of the compression in the lower rpm range - again making the engine sluggish. The "066" would have been a better pick with a 111 (lsa). The comp cams xe256 (with matched valve springs) might have also been a better choice with a 110 lsa and a little more lift than stock. I would think you won't have any problems with valves hitting the top of the pistons, but this is where you contact comp cams to confirm.

Hate to say it, but not so good a choice with the single plane torker or the wide lsa "067" cam - both of which will kill your bottom end and make the car very sluggish on the bottom end which is where your cruising rpm's are.

On the distributor advance curve, as you retard your ignition, it will make the engine run hotter. You want to advance the distributor as much as you can without the engine "pinging" - which can destroy an engine if you don't back out of it to get the pinging to stop. Pinging means the distributor is too far advanced for the gas octane/load you are putting on the car.

Setting up the distributor advance curve and adjusting the springs/vacuum advance has been very well covered in several posts. A search of the forums should pull this up. The vaccum advance when connected will do as it is called, advance your timing using engine vacuum - as you learned. At this point, i would leave it disconnected and time the engine where it does not ping under load - it may be 6,7,8 degrees as you found out.

I would not mess with the distributor advance curve until i swapped out the intake first, and then considered a different cam. Do one thing at a time. Try a factory intake and see what changes. If this does not pick up the engine, i might swap out the factory torque converter for a 2500 rpm "tight" stall converter. You may see a slight drop in gas mileage if this is of any concern, but it will pull the engine quicker to a better/higher rpm range where the cam will perform better. If this is not something you want to experiment with, then a cam change may be in your future to increase the bottom end/mid range power band for street use.

You did not say if you had dual exhaust? A pontiac engine needs to breathe and dual exhaust is a must with low restriction mufflers.

Hope this helps.
x2!!
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