quite predictably, the question asked by the OP has diverted into opinion wars about stuff that happened or didn't happen 50 years ago. Some thread topics can't escape this phenomenon, vacuum advance is definitely one of them
The OPs problem is real and in the present.
Coming back to the original topic, I was as ignorant as most about carb tuning when I had to get my new 350 crate engine tuned 2 years ago. Selected publications became my ultimate and reliable references for distributor/vacuum setting: the ones from JohnZ and Lars Grimsrud, both widely recognised C2 hotshots. They have been referred to many times. To help out the OP , I included them. John's papers are the best I ever read to get to understand in layman's words what's going on in a distributor. Lars's method of tackling the timing setting in a real world is my favourite. If all that worked for me, no reason why they wouldn't work for him. To be clear: I tune for all round smoothness of driving, not racing, and I have done 10.000 miles in the last 18 months. My engine is a plain vanilla 350 crate with a Holley 4160. It idles at 650, no stumbling or hesitations, acceleration as good as it gets with the modest 290HP.
Yes, we do have a cadre of folks stuck in the past - decades in the past.....many own these cars and very seldom drive them.
And, they think they were manna from heaven as built and the GM engineers were all Einsteins and its blasphemy to deviate from the original configuration.
Your distributor isn't installed correctly. Is the engine timed correctly? Vacuum advance aside?
All the bench racing rhetoric aside, I don’t understand this comment?
How is it not installed correctly? It looks fine to me and I don’t see how you can tell from this simple picture. I also have heard a couple of people on other threads say their distributor was installed “off a tooth”, yet they have a carb and distributor controlled ignition. There is no such thing. If you fully meshed the distributor gear to the cam gear and then turned the distributor body to set base timing you are done. Yes, you might want to install it in a way that allows the vacuum canister to clear the carb etc. to give yourself plenty of room for tuning, but there is no such thing s being off a tooth in a non-computer controlled engine.
To the OPs question, engine timing and the advance curve are comprised of three components: base timing, mechanical advance, and vacuum advance. The interaction of these three components at every rpm and engine load determines your engine timing at that point. By the way, the more radical your cam in terms of duration and overlap the more concerned you have to be about these factors. Some people remove vacuum advance thinking a “race” engine doesn’t need it. The reality is that full race engines typically don’t use it simply because it is one more thing to break and often the radical nature of the engine provides little vacuum at idle and wot where the engine spends most of its time. Assuming a cam that is not too radical for the street, your carb can be tuned. While I prefer a Holley, my C1 has two Edelbrocks on a built 350 that perform well. You can tune out that stumble, but you need to ensure that all engine components are working correctly.
I also have heard a couple of people on other threads say their distributor was installed “off a tooth”, yet they have a carb and distributor controlled ignition. There is no such thing
...... but there is no such thing s being off a tooth in a non-computer controlled engine.
All the bench racing rhetoric aside, I don’t understand this comment?
How is it not installed correctly? It looks fine to me and I don’t see how you can tell from this simple picture. I also have heard a couple of people on other threads say their distributor was installed “off a tooth”, yet they have a carb and distributor controlled ignition. There is no such thing. If you fully meshed the distributor gear to the cam gear and then turned the distributor body to set base timing you are done. Yes, you might want to install it in a way that allows the vacuum canister to clear the carb etc. to give yourself plenty of room for tuning, but there is no such thing s being off a tooth in a non-computer controlled engine.
To the OPs question, engine timing and the advance curve are comprised of three components: base timing, mechanical advance, and vacuum advance. The interaction of these three components at every rpm and engine load determines your engine timing at that point. By the way, the more radical your cam in terms of duration and overlap the more concerned you have to be about these factors. Some people remove vacuum advance thinking a “race” engine doesn’t need it. The reality is that full race engines typically don’t use it simply because it is one more thing to break and often the radical nature of the engine provides little vacuum at idle and wot where the engine spends most of its time. Assuming a cam that is not too radical for the street, your carb can be tuned. While I prefer a Holley, my C1 has two Edelbrocks on a built 350 that perform well. You can tune out that stumble, but you need to ensure that all engine components are working correctly.
I prefer the Holley also, they are much easier to tune if you are so inclined.
I just want to speak to your statement about vacuum advance in a race engine. A race engine spends all it's time at WOT, rich A/F mixtures make best power at these conditions, they burn fast and keep the point of maximum combustion at the top of the pistons power stroke so there is no need for spark lead beyond the centrifugal advance in the distributor.
Frank, I am listening to Frampton right now, I hope it's OK and I am not living to far in the past, "Decades" you know...:-)
If the 1966 425hp engine used ported vacuum advance and GM had the manifold vacuum advance right by1964 then what's the reason this engine used the ported vacuum. I suspect the whole thing was a moving target, the full manifold vacuum advance and retarded initial timing did help these high compression engines crank over better when hot.
I understand the idle thing using manifold vacuum but you don't drive the car around at idle rpm's.
I can't say for sure, but here's my theory. L-72 and '67 L-71 were low volume options, and needed to meet CA tailpipe emissions, which required AIR and ported vacuum advance. So as a cost and inventory saving measure they used the same carb with ported vacuum advance on both CA and 49-state models.
Converting to full time vacuum reduces EGT at idle and low speed driving, which reduces heat rejection to the cooling system for lower operating temperatures in around town driving. It's okay to use the OE 360 12 VAC on L-72, but the OE 201 15 VAC on L-71 must be replaced with a B26 to meet the Two-Inch Rule.
Everyone who has made the conversion reports results as I indicated above with NO exceptions.
Well, maybe, just maybe, their design criteria wasn't the same as yours?
It is very easy to criticize design engineers releases when you establish targets other than what the original guy had, many of them set by Product Planning. And you should know that as well as anyone here.
Quote:
Originally Posted by SWCDuke
I can't say for sure, but here's my theory. L-72 and '67 L-71 were low volume options, and needed to meet CA tailpipe emissions, which required AIR and ported vacuum advance. So as a cost and inventory saving measure they used the same carb with ported vacuum advance on both CA and 49-state models.
Duke
I thought that is what I said in post #34?
Engineers got it wrong. Bean counters got it right?
I prefer the Holley also, they are much easier to tune if you are so inclined.
I just want to speak to your statement about vacuum advance in a race engine. A race engine spends all it's time at WOT, rich A/F mixtures make best power at these conditions, they burn fast and keep the point of maximum combustion at the top of the pistons power stroke so there is no need for spark lead beyond the centrifugal advance in the distributor.
Frank, I am listening to Frampton right now, I hope it's OK and I am not living to far in the past, "Decades" you know...:-)
Frampton rocks.
My 61 came factory with NO vacuum advance on the dual point distributor. The solid lifter Duntov motor was expected to spend most of its life accelerating or at WOT.
I knew a lot of guys back in the day that ran that engine in pass cars and Corvettes but I didn't know a single one that ran their car as you described. Just occasionally.
I think the reason might have been more likely that (to my knowledge) Delco didn't build a dual point vacuum advance distributor in that era. But, in the interest of longer coil dwell time, Chevy/Pontiac wanted dual points that gave a hotter, more reliable spark. When it might be needed and when needed, the vacuum advance wasn't important.. Besides, it was commonly thought that the moving breaker plate casused inaccurate dwell and timing. Many used to weld the breaker plate solid for this reason.
I'm getting some slight hesitation off the line and some popping thru the exhaust. I've been told by 2 different mechanics that my '64 327-300, manual trans should be receiving vacuum from the manifold, not the carb (as I have it). Is this true? If so, is this where the vacuum connects? (see pic below). Thanks for the input and have a safe and happy 4th.
I have a hole like this also...does that go through to the manifold as a hole, or is it a mounting post? Because I do not have a plug in it...but never thought it would lead all the way through.
I have a hole like this also...does that go through to the manifold as a hole, or is it a mounting post? Because I do not have a plug in it...but never thought it would lead all the way through.
Yes, that hole for the vacuum fitting goes right into the runner in the intake manifold; if it isn't plugged or the fitting in it isn't plugged or capped, you'll have a MASSIVE vacuum leak - I'd be surprised if it ran at all.
quite predictably, the question asked by the OP has diverted into opinion wars about stuff that happened or didn't happen 50 years ago. Some thread topics can't escape this phenomenon, vacuum advance is definitely one of them
The OPs problem is real and in the present.
Coming back to the original topic, I was as ignorant as most about carb tuning when I had to get my new 350 crate engine tuned 2 years ago. Selected publications became my ultimate and reliable references for distributor/vacuum setting: the ones from JohnZ and Lars Grimsrud, both widely recognised C2 hotshots. They have been referred to many times. To help out the OP , I included them. John's papers are the best I ever read to get to understand in layman's words what's going on in a distributor. Lars's method of tackling the timing setting in a real world is my favourite. If all that worked for me, no reason why they wouldn't work for him. To be clear: I tune for all round smoothness of driving, not racing, and I have done 10.000 miles in the last 18 months. My engine is a plain vanilla 350 crate with a Holley 4160. It idles at 650, no stumbling or hesitations, acceleration as good as it gets with the modest 290HP.
We put a crate 350/290 in a c3 a few years back. Tried a few fixes but couldnt achieve greater than 15-16 full manifold vacuum and because of it (or so we thought) the idle really suffered when set at less than 850-900 rpm. Off idle the engine was terrific and performed really well. Contacted GM for info and were told the lower vac numbers and suboptimal idle was not something that needed to be fixed and was due to cam / valve overlap and that was that. I'd be curious to know if your vacuum numbers were the same.
I have a hole like this also...does that go through to the manifold as a hole, or is it a mounting post? Because I do not have a plug in it...but never thought it would lead all the way through.
Is this a picture of your engine you posted in another thread?
We put a crate 350/290 in a c3 a few years back. Tried a few fixes but couldnt achieve greater than 15-16 full manifold vacuum and because of it (or so we thought) the idle really suffered when set at less than 850-900 rpm. Off idle the engine was terrific and performed really well. Contacted GM for info and were told the lower vac numbers and suboptimal idle was not something that needed to be fixed and was due to cam / valve overlap and that was that. I'd be curious to know if your vacuum numbers were the same.
slightly off topic, but cause you asked: I get 16-17 vacuum at idle. Talking Idle, the 650 is when the AC compressor is on. Compressor off it runs at 750. I could have it even lower than the 650, but then it may quit when loading the engine with the Power steering.
If I remember correctly, it was SWCDuke who told me these 350/290 crates are not the best idlers in the world.
slightly off topic, but cause you asked: I get 16-17 vacuum at idle. Talking Idle, the 650 is when the AC compressor is on. Compressor off it runs at 750. I could have it even lower than the 650, but then it may quit when loading the engine with the Power steering.
If I remember correctly, it was SWCDuke who told me these 350/290 crates are not the best idlers in the world.
Thanks.
So maybe GM wasnt blowin smoke my way when they said it was the cam profile. Overlap = intake and exh valves briefly open at same time = lowered manifold pressure = wonky idle.
Part that bothered us is yes it is a very nice engine but certainly no beast so why not put a smoother cam in it.
slightly off topic, but cause you asked: I get 16-17 vacuum at idle. Talking Idle, the 650 is when the AC compressor is on. Compressor off it runs at 750. I could have it even lower than the 650, but then it may quit when loading the engine with the Power steering.
If I remember correctly, it was SWCDuke who told me these 350/290 crates are not the best idlers in the world.
FROM GMPP SPEC SHEET: 350/290 Crate Engine:
Camshaft:
350/290 HP engine uses an aggressive flat tappet camshaft of 222 degrees duration both intake and exhaust @ 0.050" lift. Intake lift is .450 and exhaust is .460. Lobe centerline is 114 degrees. Normal manifold vacuum for the 350/290 HP engine is 10-12 in-hg at 650-750 RPM.
The 350/290 engine is a case of mismatch of parts and poor design. The above excerpt indicates that your engine uses a 3896962 (L82 350/350) cam, and the reason that this particular engine is such a dog, is that this camshaft was designed to be run with 10.5 - 11.0:1 SCR, not the anemic 8.0 SCR used with the 350/290.
Your best bet would be to swap in a 327/300 (929) cam which was originally intended to be run in an engine of 9.5 - 10.0 SCR. You will notice a much smoother idle, quite a bit more torque with a minimal loss of power above 5000 RPM.
Camshaft:
350/290 HP engine uses an aggressive flat tappet camshaft of 222 degrees duration both intake and exhaust @ 0.050" lift. Intake lift is .450 and exhaust is .460. Lobe centerline is 114 degrees. Normal manifold vacuum for the 350/290 HP engine is 10-12 in-hg at 650-750 RPM.
The 350/290 engine is a case of mismatch of parts and poor design. The above excerpt indicates that your engine uses a 3896962 (L82 350/350) cam, and the reason that this particular engine is such a dog, is that this camshaft was designed to be run with 10.5 - 11.0:1 SCR, not the anemic 8.0 SCR used with the 350/290.
Your best bet would be to swap in a 327/300 (929) cam which was originally intended to be run in an engine of 9.5 - 10.0 SCR. You will notice a much smoother idle, quite a bit more torque with a minimal loss of power above 5000 RPM.
That sounds like a good winter project, thanks. I only very rarely go above 5000rpm, And although my idle is stable and low, silky smooth it isn't
Is this a picture of your engine you posted in another thread?
Yup, that is me. Let me take a look at that hole again. The car runs, just not 100%. I would be shocked in the distributor was in wrong, but I am a little knew to this.
Yup, that is me. Let me take a look at that hole again. The car runs, just not 100%. I would be shocked in the distributor was in wrong, but I am a little new
to this.
I just picked up on this. That was the old picture. Indeed the distributor was in wrong. Here is the updated picture with the distributor fixed.
Camshaft:
350/290 HP engine uses an aggressive flat tappet camshaft of 222 degrees duration both intake and exhaust @ 0.050" lift. Intake lift is .450 and exhaust is .460. Lobe centerline is 114 degrees. Normal manifold vacuum for the 350/290 HP engine is 10-12 in-hg at 650-750 RPM.
The 350/290 engine is a case of mismatch of parts and poor design. The above excerpt indicates that your engine uses a 3896962 (L82 350/350) cam, and the reason that this particular engine is such a dog, is that this camshaft was designed to be run with 10.5 - 11.0:1 SCR, not the anemic 8.0 SCR used with the 350/290.
Your best bet would be to swap in a 327/300 (929) cam which was originally intended to be run in an engine of 9.5 - 10.0 SCR. You will notice a much smoother idle, quite a bit more torque with a minimal loss of power above 5000 RPM.
To the honour of Joe, who unfortunately passed away only days after his post, I did what he recommended. I replaced the cam of my standard Goodwrench 350/290 with the cam he recommended ('929') from compcams. All he predicted is true: way smoother idle, and oozes more torque there where I drive most of the time (below 4000rpm). My 64 is going stronger than ever. May end up with better MPG too. Vacuum at 750rpm increased to 22" (from 17" previously) on 4160 Holley carb. The carb required re-jetting (from 66 to 67). Thanks Joe
