72 Chevy 350

overbore,10.5-1

Cast iron or aluminum heads?

Doc

Sorry I should have mentionedthat. They are iron w/ new valves and seats installed for unleaded gas.

Charles

I'd say 10.5 on iron is a bit much. also the added temps for this time of the year don't help. Probably will have to run premium with octane booster and retard the timing if there aren't any vacuum leaks or egr issues. hth

>

My thoughts exactly. I'm suprised it hasn't pinged from day one running that compression with iron heads. 9.5:1 is pretty much the highest you can go on unleaded gas with carb and vacuum advance on a cast iron block and heads. You could squeeze more out of an EFI/electronic ignition controlled vehicle but that's just cause the ECM would be retarding the timing.

Anyhow, back to the question, you could drop the timing down to TDC and try to richen the mixture just a fuzz via carb adjustments. Also, if you're not already, a 160* t-stat might get the core temp down enough to stop the pinging without sacrificing performance by retarding the timing.

Doc

Thanks for all the info, we will try backing off the timing some more . We've been tossing around the idea of changing out the pistons back down to

9 or 9.5 as a last resort. He also picked up a complete factory TPI setup that we plan on installing in the next 6 months.

Charles

. . ===== =====

72 Chevy 350 Group: alt.trucks.chevy Date: Fri, Jul 2, 2004, 6:32pm From: snipped-for-privacy@pld.com (Charles) My son rebuilt the engine in his truck 2 years ago w/ a .030 overbore,10.5-1 pistons and all the assorted goodies including a performance cam(specs not available at the moment). It has a 700r4 trans hooked up correctly and working great.

Lately it has started pinging........

on any hill

or light acceleration in 3rd, OD, w-w/o lockup.

We dropped back the timing to about 4 degrees (vacuum disconnected) but it hasn't seemed to help. He used to run the middle grade fuel until this started but he runs premium fuel all the time any more. Trying to figure out the next step. Already used gas from several sources with no help. Charles ========== ==========

Want a couple uneducated guess's????

plugs were changed...and they're not in the right heat range.

or....

sucking air through the intake gaskets that were changed during the valve job.

or...

the Vac Advance is malfunctioning

MarshMonster always remember.......and never forget.....

~if it were'nt broke before you fixed it..... you broke it......... fixing it~

======= =======

Get a decent set of aluminum heads and leave the bottom end alone! That mo-fo would be nasty running 10.5:1, with aluminum heads and base timing set at 6-8* BTDC.

Doc

are the same also. It has about 10K on it. Also running a stock HEI. Vacuum advance has been checked and is working correctly. Mech advance is also working right.

Charles

There's a missing piece of information here. We don't know the heads combustion chamber size. To make that kind of compression they need to be

64cc heads. in 1972 the most common size would have been 76cc which equate to about 9-1/4 to 1 compression with 10-1/2 to 1 pistons. I doubt the problem is the iron heads or the compression unless they are the closed chambers heads.

Brian

4 degrees (vacuum disconnected) but it hasn't seemed to help. He used to run the middle grade fuel until this started but he runs premium fuel all the time any more. Trying to figure out the next step. Already used gas from several sources with no help. Charles Cast iron or aluminum heads? Doc Sorry I should have mentionedthat. They are iron w/ new valves and seats installed for unleaded gas. Charles I'd say 10.5 on iron is a bit much. also the added temps for this time of the year don't help. Probably will have to run premium with octane booster and retard the timing if there aren't any vacuum leaks or egr issues. hth -------------------------------------------------------- There's a missing piece of information here. We don't know the heads combustion chamber size. To make that kind of compression they need to be 64cc heads. in 1972 the most common size would have been 76cc which equate to about 9-1/4 to 1 compression with 10-1/2 to 1 pistons. I doubt the problem is the iron heads or the compression unless they are the closed chambers heads. Brian "

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D I agree with Brian above.My last 350 of that vintage was 9-1 comp. and ran on mid- grade gas. You need to check for carbon in the combustion chamber causing an artificial rise in compression.If you run an EGR check that as a malfunction will cause knock.If it used to run OK at a certain timing advance you shouldn't have to change that radically.Finally ,if the above doesn't work then get an adjustable vac-advance (about $20.) because knock during light acceleration as you described is Vac adv,adjust out the knock and hopefully you will still have full throttle power. Brian O

. . no daddy, i don't know how the front end got tore off on yer 72 t-bird.

~~ oo L O

"ok my little marshmonster....i believe you"

:)

=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D Re: 72 Chevy 350

11:53pm (CDT-2) From: snipped-for-privacy@webtv.net (Brian=A0Orion) says.......

=A0My last 350 of that vintage was 9-1 comp. and ran on mid- grade gas.

=A0=A0=A0=A0=A0=A0You need to check for carbon in the combustion chamber causing an artificial rise in compression.If you run an EGR check that as a malfunction will cause knock.If it used to run OK at a certain timing advance you shouldn't have to change that radically.Finally ,if the above doesn't work then get an adjustable vac-advance (about $20.) because knock during light acceleration as you described is Vac adv,adjust out the knock and hopefully you will still have full throttle power. =A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0=A0

=A0Brian O =3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D

Ditto......I second the preliminary diagnosis. What Brian O said.

=3D=3DTO THE OP=3D=3D as stated before.....

Plugs Vac Advance Leak around the intake

The most important info given to diagnose this rascal, was in the oringinal post. THE SYMPTOM!!

I don't care what your compression is..... what your chamber volume is...... or what your deck height is.....

If the dang thing ran good before, then it should be running good now. You should be able to go from running RACING FUEL to running 87 octane....by simply adjusting the timing and the fuel mixture, without having a prob with spark knock, as long as you're dialing in the fuel mixture and timing. (I would'nt suggest doing this alot though)

~~ oo L O

There's always the possibility that you're ignition is breaking down under load, for whatever reason. (there....I've thrown another unnecessary log on the fire) However....if that was the case you "should" be seeing the symptom pop up even when the engine isn't lugging. And the symptoms you gave has led completely away from that. You're getting some good advice. I'd suggest you take the time to double check the areas that are being suggested as the main likely failure points.

Here's what we know...... what you gave us to go by........

Rebuilt Engine (by your son)

2 years on rebuild 10,000 miles on rebuild (given in a later post) Retarding timing DOES NOT affect symptom Fuel grade DOES NOT affect symptom Spark knock on inclines Spark knock on light acceleration

Here's what you've left us to "ass u me".....

The thing ran good for 2 years The thing ran good for 10,000 Your son can build a kick-arse motor Timings not an issue Fuels not an issue NO problem during cruise with no load

Here's my conclusion....... Plugs Vac Advance prob Leak around the Intake

You need to check..... Good Spark Timing is reacting to load demand Proper fuel mixture

keep in mind..... I researve the right to change my GUESS.... if you change your symptoms.

and.....

It's called a vac advance....not a vac retarder. (hmmm....wonder what an easy test would be)

Respectfully,

MarshMonster ~always remember and never forget...... if it were'nt broke before you fixed it...... you broke it fixing it~

ps.......

how olds your son?

he wouldn't do something stupid..... like overheating the car and not tell you about it .....would he?

=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D

~~ oo L O

"ok my little marshmonster....i believe you"

:)

====== ======

Re: 72 Chevy 350

11:53pm (CDT-2) From: snipped-for-privacy@webtv.net (Brian Orion) says.......

My last 350 of that vintage was 9-1 comp. and ran on mid- grade gas.

You need to check for carbon in the combustion chamber causing an artificial rise in compression.If you run an EGR check that as a malfunction will cause knock.If it used to run OK at a certain timing advance you shouldn't have to change that radically.Finally ,if the above doesn't work then get an adjustable vac-advance (about $20.) because knock during light acceleration as you described is Vac adv,adjust out the knock and hopefully you will still have full throttle power.

Brian O ======= =======

Ditto......I second the preliminary diagnosis. What Brian O said.

==TO THE OP== as stated before.....

Plugs Vac Advance Leak around the intake

The most important info given to diagnose this rascal, was in the oringinal post. THE SYMPTOM!!

I don't care what your compression is..... what your chamber volume is...... or what your deck height is.....

If the dang thing ran good before, then it should be running good now. You should be able to go from running RACING FUEL to running 87 octane....by simply adjusting the timing and the fuel mixture, without having a prob with spark knock, as long as you're dialing in the fuel mixture and timing. (I would'nt suggest doing this alot though)

~~ oo L O

There's always the possibility that you're ignition is breaking down under load, for whatever reason. (there....I've thrown another unnecessary log on the fire) However....if that was the case you "should" be seeing the symptom pop up even when the engine isn't lugging. And the symptoms you gave has led completely away from that. You're getting some good advice. I'd suggest you take the time to double check the areas that are being suggested as the main likely failure points.

Here's what we know...... what you gave us to go by........

Rebuilt Engine (by your son)

2 years on rebuild 10,000 miles on rebuild (given in a later post) Retarding timing DOES NOT affect symptom Fuel grade DOES NOT affect symptom Spark knock on inclines Spark knock on light acceleration

Here's what you've left us to "ass u me".....

The thing ran good for 2 years The thing ran good for 10,000 Your son can build a kick-arse motor Timings not an issue Fuels not an issue NO problem during cruise with no load

Here's my conclusion....... Plugs Vac Advance prob Leak around the Intake

You need to check..... Good Spark Timing is reacting to load demand Proper fuel mixture

keep in mind..... I researve the right to change my GUESS.... if you change your symptoms.

and.....

It's called a vac advance....not a vac retarder. (hmmm....wonder what an easy test would be)

Respectfully,

MarshMonster ~always remember and never forget...... if it were'nt broke before you fixed it...... you broke it fixing it~

ps.......

how olds your son?

he wouldn't do something stupid..... like overheating the car and not tell you about it .....would he?

======= ======= ??Good question but I would think not. Seems to be pretty responsible for his age (25). Also you assumed correctly that it ran great for the last 2 years (10K miles)and I might say I was impressed as to how strong it did run. After re-reading my original post it seems like I forgot 1 thing, when you run it full throttle flat or uphill it kicks a$$ just as good as ever with no indication of spark knock at all. We will see about getting a good adjustable vac. advance unit and a new set of plugs over the holidays and give it another try. He also said he ran some of the good GM combustion chamber cleaner about 3 weeks ago without any satisfactory results either.

Charles

. . . On yesterdays episode of ATC we heard snipped-for-privacy@pld.com (Charles) say........

Seems to be pretty responsible for his age (25).

Also you assumed correctly that it ran great for the last 2 years (10K miles)and I might say I was impressed as to how strong it did run.

After re-reading my original post it seems like I forgot 1 thing, when you run it full throttle flat or uphill it kicks a$$ just as good as ever with no indication of spark knock at all.

We will see about getting a good adjustable vac. advance unit and a new set of plugs over the holidays and give it another try.

He also said he ran some of the good GM combustion chamber cleaner about 3 weeks ago without any satisfactory results either.

Charles ============= ============= glad to hear yer kids a good little feller, shows his upbringing.

Before you go buying one......

1) hook the old timing light up 2) make sure the timing mark ain't jump'n around 3) stick a goodly length of hose on the vac advance and make sure it's working like it's suppose to 4) watch yer timing mark again while yer holding vacume on the advance

You've given another diagnostic clue for us. So be expecting some more "guess's".

...... Another question 4-u.

Was A N Y T H I N G done to the truck just before the pinging started showing up?? Anything?

marsh ~always remember and never forget........ most stuff breaks.....cuz you break it~

======== ========

Done

Nice and steady-gradually advances as throttle is opened so mech advance is not stuck

Hooked up a mighty vac to the vac. adv. and it worked but forgot to write down how far it advanced

Changed from the stock radiator to a Griffin aluminum--cooling system is topped off.

. . . =3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D Concerning pinging on light throttle and uphill driving.....

(marsh) asked.....

Was =A0 A N Y T H I N G =A0 done to the truck just before the pinging started showing up?? Anything?

=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D (Charles) replied.....

Changed from the stock radiator to a Griffin aluminum--cooling system is topped off.

=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D

Charles, why was the radiator changed? Was the truck overheating? and...if so how hot and how many times?

DID YOU CHECK ....for air leaks around the intake?

marsh ~continues to dig for clues~ =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D