View unanswered posts | View active topics It is currently Tue Aug 22, 2017 10:21 pm



Reply to topic  [ 43 posts ]  Go to page 1, 2, 3, 4, 5  Next
 Vac advance rates 
Author Message

Joined: Tue Aug 23, 2016 10:38 am
Posts: 84
So, in a continuous search for performance and mpg I have been messing with my stock 74 350s timing. The A/F when it was a little hotter temperatures was in the 18-19 range at steady state cruise at 65 mph (2241rpm) at 16-17in/hg. This is just a bit richer than the point of surging. As we get lower in the vac it has a a/f closer to 15-16 and as it drops below 10 it goes to 14 and is 12.5-12 once below 6 in/hg. So the A/F, though a bit rich should be fairly close.

The timing is currently 17 initial with 24 deg of mechanical advance in at 2400 rpm. The vac can has 15 deg of advance for a total right around 56 degs. At this point there is a slight bit of rattle on one hill as I go to work. Previously it was set at 16 initial and the pinging was not audible and at 18 it is much worse. The thing is 1 deg has yielded a 1/2 a mpg proven over 3 tanks. The one tank I ran the 18 initial or approximately 58 degrees total, showed a gain of 3/4 mpg with no back up to prove it but required me driving very light footed (that may also account for the gain).

The point of ping is usually a slight grade with enough throttle input to lower the vac to 8 in/hg. At this point the vac advance is still pulling in about 10 deg of additional advance. I was hoping to find a can that would drop off severely under 10 in/hg and advance the 15+ degrees between 10-15 in/hg. Looking at the curve listed for the few advance cans it appears most are topped out by 10in/hg and start at 5 in/hg or so.

I have a limiter cam from crane(not installed) but was hoping maybe some of you would be able to suggest or have specs for a vac advance that would be close to what I need.

Otherwise if I am out to lunch on what I am doing please tell me.

It is still a points dist but I was planning to buy a HEI can so I can use it when changing to a HEI this winter.


Fri Oct 07, 2016 12:13 pm
Profile
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
I think your problem is the A/F getting richer as the load increases (vacuum drops). It can actually, in fact should, progress leaner as intake vacuum drops as load is increased. The most detonation prone A/F is near stoichiometric because that is where the combustion temperature is highest. Leaner than stoich. combustion temp drops faster than it does on the rich side. You should be able to configure your carb to get leaner as you add load, or at least not get richer as it does now, up to the point of high load where you want a maximum power A/F. Probably needs a smaller MJ or a larger metering rod. You should notice better performance as you lean it in the moderate load range when crowding the throttle in the 8" to 10" vacuum area. The reason for this is hot Oxygen improves combustion, and the leaner it is, the more hot Oxygen there is in the residual gasses. I think as you lean it you will notice improved part-throttle power with less throttle opening.


Fri Oct 07, 2016 3:55 pm
Profile

Joined: Tue Aug 23, 2016 10:38 am
Posts: 84
It is a 2g. It gets richer as the power valve starts to open. it is a "400" cfm and has the equivalent of a 57 jet right now. I deleted the APT and reduced the power valve channels by 10%.


Fri Oct 07, 2016 4:00 pm
Profile
Online

Joined: Thu May 08, 2014 7:47 pm
Posts: 236
I drive a 75 c10 350 daily, and I have never been able to run the kind of curve and total advance you are at.

They make an adjustable vacuum advance that has an allen screw in the vacuum port.

My Q~jet is way past due for a refresh, as soon as i can make the time for it, i will be going threw the ignition as well.


Fri Oct 07, 2016 4:28 pm
Profile

Joined: Sun Jun 23, 2013 1:41 pm
Posts: 375
Location: Phila. Penn.
80427 wrote:
So, in a continuous search for performance and mpg I have been messing with my stock 74 350s timing. ... So the A/F, though a bit rich should be fairly close.

Those are reasonable tests for cruise and WOT AFR. However, you may find that slightly richer AFR at highway cruise produces more power, which should translate to higher efficiency and vacuum at those speeds and loads. That would then go well with Tuner's point that normally increasing throttle (load) should be cause the AFR to go leaner.
see
Also keep in mind AFR for today's pump fuel is not quite the same as 100% gasoline as it has varying amounts of things like ethynol in it and evan race gas has varying densities and stoich (but at least they are published and consistant).
Graphs and explanation of AFR for various Loads and throttle opening here.

Quote:
The timing is currently 17 initial with 24 deg of mechanical advance in at 2400 rpm. The vac can has 15 deg of advance for a total right around 56 degs. At this point there is a slight bit of rattle on one hill as I go to work. Previously it was set at 16 initial and the pinging was not audible and at 18 it is much worse. ...

I have a limiter cam from crane(not installed) but was hoping maybe some of you would be able to suggest or have specs for a vac advance that would be close to what I need.

The aftermarket Crane vac advance for GM points distributors that I have somewhere was adjustable with an allen key as mentioned. Even though its for GM, it will work the same as the ones I tested and graphed. See the thread on "Too much Vacuum Advance?"
However, if you have not already seen it, there is listing for GM vac pods in this PDF on GM Vacuum Advances by Lars Grimsrud

I agree with your suspicion that the 1/2 mpg came from light throttle more than the couple of degrees in the advance.
The three ways to deal with the part throttle ping will be to dial back on the vacuum pod if its adjustable, reduce the entire curve by 2 degrees by rotating the distributor back to 16 at idle, or slow the mechanical advance curve by changing one of the springs. The last may or may not help the part throttle rattle depending on whether it occurs at only lower rpms or all rpms.


Fri Oct 07, 2016 5:25 pm
Profile WWW
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
80427 wrote:
It is a 2g. It gets richer as the power valve starts to open. it is a "400" cfm and has the equivalent of a 57 jet right now. I deleted the APT and reduced the power valve channels by 10%.

400 CFM does not compute. What are the venturi and throttle bore diameters? What is the carb #?

JETFAST wrote:
I drive a 75 c10 350 daily, and I have never been able to run the kind of curve and total advance you are at.

They make an adjustable vacuum advance that has an allen screw in the vacuum port.

My Q~jet is way past due for a refresh, as soon as i can make the time for it, i will be going threw the ignition as well.

I also think the curve is too quick.
80427 wrote:
The timing is currently 17 initial with 24 deg of mechanical advance in at 2400 rpm. The vac can has 15 deg of advance for a total right around 56 degs.

I have a limiter cam from crane(not installed) but was hoping maybe some of you would be able to suggest or have specs for a vac advance that would be close to what I need.

Otherwise if I am out to lunch on what I am doing please tell me.

It is still a points dist but I was planning to buy a HEI can so I can use it when changing to a HEI this winter.
Depend on the cam and compression, with mild cam, 12 deg. initial, start 1000 slope to 30 deg at about 3000 then 36 or so at 4500-5000, with spunky cam, 18 deg, initial, start 1200 slope to 30 at 28-3000 then 36 or so at 4500-5000. Your pinging problem is because the mechanical curve is too quick. On a drag race engine which is run cold the timing can come in quick like that, but a street engine that runs fully warmed up 99% of the time does not need that much timing to make good torque, it is better left a but shy to avoid knock.

A vacuum can with close to 10" start is a Niehoff DR-356 marked B-1. It is similar to the can used in high compression Chevrolet and others from '57 on up.


Fri Oct 07, 2016 7:55 pm
Profile

Joined: Tue Aug 23, 2016 10:38 am
Posts: 84
tuner wrote:
80427 wrote:
It is a 2g. It gets richer as the power valve starts to open. it is a "400" cfm and has the equivalent of a 57 jet right now. I deleted the APT and reduced the power valve channels by 10%.

400 CFM does not compute. What are the venturi and throttle bore diameters? What is the carb #?


I don't have it with me but it is factory for my l-65 350. It has the larger throttle plates but is not the largest venturi shown in the Roe book or what the circle track guys call a "500".
See Attached Photo

I was wrong the jets are now 55s I had to drill because I was out of larger jets and knew it was going to be replaced this fall.

tuner wrote:
[
JETFAST wrote:
I drive a 75 c10 350 daily, and I have never been able to run the kind of curve and total advance you are at.

They make an adjustable vacuum advance that has an allen screw in the vacuum port.

My Q~jet is way past due for a refresh, as soon as i can make the time for it, i will be going threw the ignition as well.

I also think the curve is too quick.


I have tried it at a slower rate and the low-end power was noticeably reduced every time. It always felt soggy before and is sharper even after driving the 45 miles to work.

tuner wrote:
80427 wrote:
The timing is currently 17 initial with 24 deg of mechanical advance in at 2400 rpm. The vac can has 15 deg of advance for a total right around 56 degs.

I have a limiter cam from crane(not installed) but was hoping maybe some of you would be able to suggest or have specs for a vac advance that would be close to what I need.

Otherwise if I am out to lunch on what I am doing please tell me.

It is still a points dist but I was planning to buy a HEI can so I can use it when changing to a HEI this winter.
Depend on the cam and compression, with mild cam, 12 deg. initial, start 1000 slope to 30 deg at about 3000 then 36 or so at 4500-5000, with spunky cam, 18 deg, initial, start 1200 slope to 30 at 28-3000 then 36 or so at 4500-5000. Your pinging problem is because the mechanical curve is too quick. On a drag race engine which is run cold the timing can come in quick like that, but a street engine that runs fully warmed up 99% of the time does not need that much timing to make good torque, it is better left a but shy to avoid knock.

A vacuum can with close to 10" start is a Niehoff DR-356 marked B-1. It is similar to the can used in high compression Chevrolet and others from '57 on up.


I tried it with springs that slowed the advance to 24 deg at 3000. This dist only has 24 deg without removing the limiting bushing. I had slowed the curve to avoid detonation when it was running in the 20s after the initial roe modifications suggested in his book. At this point is had a very noticeable lean surge that I finally fixed by going from the factory 49 jet to 50s then to the 52s from a spare carb and finally to drilling the 49 to a 55 finally eliminated it with possibly a very slight lean surge at steady state cruising on flat ground with minimal throttle on a 50 deg morning.

Once I got the jetting correct the mpg was only marginally better at 14.0 vs 13.5 it achieved with the stock carb. I was reviewing my notes I make at each fill up on modifications I have done and noted I had slowed the curve. Returning the curve to max mechanical advance at 24 deg at 2500 was instantly noticeable. The mileage jumped into the 15s and the drivability was greatly increased.

Still unhappy with the mpg I bumped it from 16 initial to 18 initial and the mpg on that tank went to 16mpg but the detonation returned.

Audible detonation occurs at a specific place on my trip. It is where I make a corner off of one highway onto another where there is a grade and am also attempting to accelerate up to highway speeds. This occurs about 1/2 throttle where the vac advance is still adding advance but the engine is under greater load. If I open the throttle more to the point the vac is reduced below 5in/hg the pinging goes away.

I have the initial at 17 now with 24 degrees of mechanical in at 2400 (approx. 20 at 60 mph and 22 at 65 mph) with 15 deg of vac advance. The mileage has increased with only a single degree added and the vac on the single stretch of flat road has improved from 15-16 in/hg to 17-18in/hg. It pinging with this setting IF I am aggressive accelerating on this hill but is easily controlled.

The increase in vac while cruising and increase in mpg indicate to me in wants more advance at steady state but the pinging while accelerating on the grade indicates to me the vac advance does not drop off quick enough under load.

The car is a 1974 Malibu with 36k miles (29k when I purchased it). It has a stock L-65 350 with factory specs of 8.5 to one compression, 2g 2bbl Rochester, stock single exhaust with 2.25" into 2" tail pipe , stock A.I.R., stock stall speed, stock turbo hydromantic 350 transmission, stock 929 camshafts, 2.73s gears and 16.6" tall tires. I am sure the compression is actually closer to 8.0 with stack tolerencing and just for the fun of knowing the heads are 993s (much better than the 882s the 75s received). It does have the egr disabled only because with I purchased the car it was disabled because it had a broken thermostatic valve and I have been unable to find one in my stash and it has been discontinued. The intake I am changing to does not have egr provisions, so it will no longer be used.

I will probably just purchase an adjustable canister.

Also I will be dynoing my car to get a baseline before the winter modifications I have planned so I should be able to do a steady state pull and adjust timing to see what it actually desired at a 65 mph for timing.

p.s. the car was factory ordered without a radio so I have a chance to listen to detonation the entire trip and it isn't just something I hear over other noises, of course knowing that detonation occurs before it is audible.

Thanks for taking the time to answer my question.


Attachments:
2G.jpg
2G.jpg [ 1.12 MiB | Viewed 628 times ]
Mon Oct 10, 2016 10:28 am
Profile
Online

Joined: Thu May 08, 2014 7:47 pm
Posts: 236
if you are dead set on running as much lead as possible, you need to reduce the heat range of your spark plugs, so you can retard the plug, and pull it out of the flame, and/or increase the knock rating of your fuel with higher octane. This will give you some room for what you are trying to do. other then that, you have to reduce timing or compression. Gearing, load, intake air temp, altitude also effect timing requirements, just cause its pulling a lot of vacuum doesn't necessarily mean it needs or will actually take more timing. AIR fuel ratio also has a huge affect on timing. A slow rich burn needs timing to accelerate, a lean fast burn will uncover an over timed/low octane situation, and cause it to rattle too. You need to look at the whole picture. not just your timing window. the most my fuel injection tunes have is 45* I have never been able to run anything 55* plus. A cast iron headed harley is the only thing i have ever had that kind of timing in, and that is one of the most inefficient cylinder heads i have ever worked with.


Tue Oct 11, 2016 12:16 pm
Profile

Joined: Tue Aug 23, 2016 10:38 am
Posts: 84
I guess I don't insist on anything. I felt a little beat on but the reality is you are all suggesting something is a miss with my set up. I don't feel it is but I am willing to try it your guys way. I will attempt to change the advance curve to the style suggested. The only way I can jet up further is to drill the 50s I have.

I guess I feel my fuel curve is ok but I can try and map it to see if there is any disagreement. For the most part it is only in the 18-19s at very little throttle input crusing and is in the 16-17 range for the majority of the non power driving range only richening once it gets to approximately 7/16-1/2 throttle.


Tue Oct 11, 2016 5:45 pm
Profile
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
I realize you have made the effort to A-B-A-B to determine the advance that is working best for your situation. I gave you suggestions (did you see the vacuum can part #?) but you haven't responded to some of the requests I made. What is the carb number and what is the venturi diameter? It looks like a 1-3/16" x 1-11/16" but the picture doesn't tell the story.

I suggest you read my post above again to see that I think the part-throttle knock is because it is too rich at the load point it occurs. Nearly all engines can run leaner as load is increased. You describe the A/F as progressing richer as load is increased. Most engines like it to get progressively leaner as load is applied up to the point, perhaps near 75-80% load, where you want the full power A/F and open the PV.

The original tuneup in your carb had a power piston spring that opened the PV at about 10" (as I recall, it has been a long time since I worked on an original car, but when they were new I tuned a lot of them).

Does that carb have the high speed pull-over, a brass tube in the lid next to the power piston that connects the float bowl to a pair of discharge orifices in the choke horn? If it does, plug it. Next, can you tell when the PV opens by comparing the A/F gauge to the vacuum gauge? What vacuum level does the mixture definitely switch to a rich A/F? You may need to "adjust" the spring on the power piston.


Tue Oct 11, 2016 6:49 pm
Profile
Display posts from previous:  Sort by  
Reply to topic   [ 43 posts ]  Go to page 1, 2, 3, 4, 5  Next

Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group
Designed by ST Software for PTF.