waterplay

Finally decided to see if I could hook up my vacuum advance. I originally had my initial timing set to 20 with a total of 36 and would idle around 850rpm. Car seemed to run good with that setting. My biggest issue was that I could only pull about 6 in vacuum at idle.

Eng facts..
383cid 10.77 compression, open plenum manifold, hooker side pipes, 4spd, 850 Holley double pumper, Comp Cams HR294, aluminum heads, HEI distributor, Crane adjustable vacuum advance can.

My observations…
The carb is way to large and even with the vacuum can at its lightest setting it starts to pull advance @ 9in.

What I have done…
I replaced the 850 Holley with a 750 Quick Fuel annular discharge carb. May I say absolutely incredible carb. Changing the carb and only quickly adjusting it got my vacuum reading up to 8in from 6in. I also adjusted the vacuum advance limit to about 9 degrees.

What I have now…
With out vacuum advance hooked up I now have 18 degrees at 660rpm and a total of 34 degrees around 2800rpm
With the advance hooked up I now have 27 degrees at 860rpm and a total of 49 degrees around 2800rpm.

This is more initial advance than I have ever run but the more I gave it the better the car seemed to idle and run. It also seems to be running cooler. When I put the timing to what I think is a normal setting (12 degrees or less) the headers start to glow a lot and the idle quality diminishes.

What do all you timing gurus think about this setup?

Sorry for the long post I just wanted to give as much info as possible.

David Ey

I'm no expert, but those results sound about right to me. As long as you don't have detonation that is not too much advance. I assume those numbers without vac. connected the hose is plugged.

waterplay

Thanks for the input.
Yes the hose was plugged. I have not driven the car yet as it was later yesterday when I finished and then I had the wife’s honey do list. That is one reason why I went with 49 degrees instead of 52 as recommended. I want to go somewhat conservative and work up if needed. I don’t want any knock.

427Hotrod

I have my 540 idling at 29* with no vaccum advance...39 total..no vacuum. MSD dist.

The previous dist was a GM one I had modified. It idled at 31* with the same 39* total (mechanical). I had vacuum advance on it and kept it around 6* additional. The motor loved it.

I just haven't messed with tuning the MSD vacuum can yet, so I just took it off. It has too much advance in it.

You're on the right track..put in as much initial as you can..and limit the total to the correct amount. Your throttle repsonse and drivability will go up drastically, it will run cooler etc. as you've noticed. Then throw in whatever vaccum it can stand under light throttle cruising.

Big cams, big intakes and carbs like lots of initial.



JIM

waterplay

Good to know that I am not way off in my thoughts. I am going to keep tweaking it to get the best performance and drivability. My next purchase will be an innovate LM1 with AUX box so I can get both the timing and the air fuel set correctly. That new carb has adjustable settings for everything.

BarryK

Waterplay

sorry, I was gone most of the weekend.
First of all, I'm no expert by any means and grouping me into that classification with someone like Lars is insulting to Lars. He is an expert......... i'm still learning this stuff the best I can manage.

That 850 carb seems like it would be way too much for the motor which seems to make sense since you say you say an immediate difference once you changed it out to a 750.

your vacuum can is still not correct though besed on your numbers. With only 8" vacuum (which is very low!) having your can set at 9" you are not configured properly and not getting full use of the vacuum advance. Your own numbers you listed bearsthis out.
you said you have it set right now at 18º and 34º before vac adv and than it reads 27º and 49º after hooking up the vacuum adv.
That means at idle you are only getting 9º vac adv but the can is giving you 15º advance at total. As you can see from this at edle you are not getting full benefit of the vac can at idle.

If your motor is only giving you 8" of vacuum at idle you need a can that gives you full advance 2" below what your manifold vacuum level is so you need one that starts at around 6", not at the 9" you say yours is currently set at.

The first thing i'd do is make sure your carb is set up correctly and is giving you the bst vacuum you can get.
Use a vacuum gauge, hooked up to a full manifold vacuum port, and adjust your fuel mixture until you get the highest steady vacuum reading you can.

Once you do that use Lars paper on my website to select a correct vacuum can appropiate for that vacuum reading your motor is giving you.
If your vacuum readings do not improve after resetting the carb this way and it stays at a low vacuum reading around 8" you will need to go with a more "radical" model vac can that allows full vac adv to come in early enough. The VC1862 will work for this. If your vacuum readings come up somewhat when you retune the carb than you may be able to use a less radical vac can such as the VC1843.

Having gone thru that........ that's take a look at your timing numbers.
I feel that an initial timing of 20º is excessive. If Lars jumps in and disagrees than I'll stand down from that, but in my opinion it is excessive. If initial timing is set too high you can have issues with starter kick-back because of combustion pressures before the the piston reaches TDC plus risks of detonation.
yes, a higher initial timing can help throttle response, but you need to always make a compromise when setting up timing on a motor. This compromise is somewhere between "ultimate performance" and "safety" on the motor. The safety aspect regarding to having the motor survive and things like detonation means the motor will not survive for very long.......
I like as much performance out of my motors as much as the next guy, but at the same time i'm willing to sacrifice a little of it to make sure the motor is reliable and doesn't destroy itself because it's on the ragged, extreme edges of performance.
remember, we are talking a STREET motor here, not a racing motor where it only needs to survive one or two passes down the track before it's probably going to get ripped apart and rebuilt anyway.

My recommendation based on what you have siad so far is to first set your fuel mixture with the vacuum gauge as mentioned above and than select a correct vacuum can based on that.
Than set timing for an initial timing around 12º - 14º and a total timing of 36º (both before vac adv). On the total timing of 36º test drive the car before reconnecting the vac adv can and make sure you are not getting any detonation. If you do, back it down 2º at a time until it stops!
Than reconnect the vac can which is going to add another 15º - 16º of advance depending on the can. Than take it for another drive and check again for detonation and especially for any "surging", "jerking" or "chugging". If you get that than the vacuum adv is throwing in too much advance but usually with the other numbers set correctly this shouldn't be an issue.

bobs77vet

scrap the adjustable can and go with a fixed can to match the vacuum readings you have....i scrapped mine and went to a fixed unit

Gordonm

I am running an 825 Mighty Demon on mine and am having the same issues. I don't agree on the carb being to big but I do agree on lots of initial timing. I have had mine at about 24 degrees and it likes it and runs great with this setup. Total is 36 and I am still playing with the vacuum advance. Give it as much initial as it needs. The newer head designs seem to like a lot of initial timing. Mine will not run below 16 degrees initial. Motor specs are in my signature. If you plan on keepingthe rpms below 6000 rpm the 750 might be OK but if you are above this the 750 will run out of air.

CheezMoe

I thinks I may have to agree on two points...1.The 850 it seems is too big a carb for many apps. Look at other recent posts where an 850 was pulled from a 400hp motor and replaced with a 750 and everything got better. There have been at least two in the past week.
2. Based on all tech info available and my limited experience, inital timing at ~18-20 should be fine with the final advance numbers you show. Smooth idle, Low temps, lot's of throttle response, no detonation, thats what you want! Sounds to me like your pretty close.

Gordonm

With his setup if he is not pulling more than 400HP something is way off. He should be 470+ HP with that setup.

Fevre

Did you adj your int timing back deg for deg to match your limiter? In case you did not what the limiter does is increase your int timing by moving the vac adv forward by the number of degrees on the limiter tab thus allowing the vac adv can less travel; this gives you more initial timing so you are now adding your base timing + mech + limiter induced timing (sounds like you are at the 6 limiter mark on the tab). If you did not adj your int timing back 6 deg you will have a total timing of 40 deg without vac adv. From your post this is about where you are: 18 int + 16 mech + 6 timing limiter induced = 40 total + 9 vac = 49.

Sounds to me you need to back off your int timing by at least 4 degrees if not 6.

BarryK

sorry guys, I'm not going to try to pretend that i'm going to convience you of this, but i just can not agree with initial timing setting up in the 20º or higher range.

starter drag, especially in hotter weather can become an issue, and more importantly, the risks of detonation are too great.
By running higher initial timing settings you also need to shorten up the centrigugal advance dramatically which effects how well the motor runs and it's "streetability".
Granted, higher initial timing will give you more "seat-of-the-pant" performance, but at the serious risk of the motor.

The two most important aspects on timing to consider are the total timing and the centrifugal advance. You really want both of these set properly for you motor to run it's best.
We all seem to agree (for the most part) that 36º total timing is optimum so lets leave that as a given. that now means that if you push your initial up too high you are going to do one of two things: either have to reduce the centrifugal advance which is not an optimum choice or to exceed that total timing of 36º which is also not a good choice.
I still feel that an initial timing in the range of 10º-14º is your best choice - this gives you the ability to stay at 36º total and a proper centrifigal advance. With the total timing and centrifugal advance set to proper levels the initial will automatically fall into a safe range - typically in the range that I suggest anyway.
Depending on the motor you may lose a small degree of ultimate performance, but at least it's safe.
To *me* a motor that runs and keeps running no matter hard hard you decide to push it while driving it is always a better performing motor that one that has a bit more performance dialed into it but than destroys itself and fails. That motor that you dialed in too much timing to make it go faster just burnt up a piston or two and isn't running very good anymore is it?

just my opinion and like I said, for those of you that have high initial timing setting and are enjoying the added performance I don't expect to convience you to change, but just be aware of the added risks you are putting on your expensive motor.

Gordonm

OK Barry, Why is it so bad to run a lot of initial timing on a motor? Yes you want to keep the total at 36 or slightly less depending on cylinder head design. The initial timing might be needed to be more due to high overlap cams intake design or carb selection. If the motor can handle the high initial timing without detonation and runs very smoothly from idle to the upper rpms than what is wrong with it. We are talking motors here tha tare far from stock also. The stock motors with mild cams will allow you to run very little advance.
Starting it might be a small problem but you can always put on a retard box for starting. Not the best option but an option none the less.

waterplay

BARRYK
First thanks for all the info. Sorry to insult LARs but you are getting close if not already there to guru.
WOW lots of info! I keep going back and forth to make sure I answer all your questions. I did set the carb both the 850 and the 750 with the vacuum gauge and that is all the vacuum she has. It is a relatively decent size cam. The adjustable vacuum can that I have is set as low as it goes. Does the VC1862 can work with the HEI or is that a point style can? I have the same issue as Gordonm where the car just won’t idle worth a lick if I set the timing back much further that where it is. Now if the Vacuum is not hooked up it idles poorly. I don’t think I can bring the timing back more. I have also checked for vacuum leaks and there are none. I think I answered all the questions?

GORDONM
I make max power at 6500rpm so I should have enough carb. I think I have this correct? All advertised cfms are not the same. My 750 Quick Fuel flows more than a 750 Holley and the Holley flows more than a 750 B.G.. I may have the B.G. and the Holley backwards? If I am wrong I am sure that someone will chime in. So your 830 and my 750 are going to flow around the same in real world.

FEVRE
It took me a little while to figure out what the limiter was doing. (Advancing the initial to limit the total) But that is a good catch as I did not mention that in my initial post. Yes I did place the limiter on its last setting and I did re-check and reset the initial timing.


To All
Thanks for all the great advice please keep it coming as I am still not there yet.
Waterplay

olescarb

the following link is to the barry grant web site it shows the suggested initial timing by cam duration at 0.050 valve lift
http://barrygrant.com/demon/default.aspx?page=5

I have found this a very good stating point for initial timing. a cam with between 240 and 260 duration at 0.050 valve lift would use a initial timing of 18 to 20 degrees btdc
the vac advance i use most for a hei is a d1370a from ac/delco it gives 10 degrees of advance

henry @ oles carb

BarryK

Initial timing is generally determined by what ever is left after the mechanical advance gives you the timing you want at WOT. this is the old "set total and let initial fall where it may" idea.

For example. if you have a distributor curve set to give 24 degrees total mechanical advance at a given rpm, then you set the timing to give 36 degrees with all the mechanical in and you get 12 initial. If you change the mechanical advance curve to give a different mechanical total then the initial will change accordingly.

The timing must change as engine conditions change. By using load (vacuum advance) and engine speed (mechanical advance) schemes, the amount of timing the engine requires to maintain peak combustion pressures at the proper time can be controlled.
If you put in more initial timing than is needed, you essentially have no control over this as engine conditions change. What is there is there and you limit your variability via vacuum and mechanical inputs to the upper limit of timing to prevent detonation.

basically, with initial timing set excessively high, you may feel an increase in performance but as stated previously you add a lot of additional risks of detonation.
You also limit the ability of the distributor to make the changes to the timing as required (hence my prior statement that it makes the motor less "streetable").

You can mess with the timing map and improve performance but at the cost of possible damage to the engine.
As I've said many times on this issue, yes, you can many times notice an increase in performance by setting the initial higher, but at the same time you are greatly increasing the risk to the motor also.
That's the reason I don't like very high initial timing settings.

Again, I'm NO expert, but this is the way I learned and understand the different aspects of timing issues.
If someone like Lars comes on anf tells me i'm full of sh*t than I'm open to listening to reasons why i'm wrong but so far Lars and other experts that I respect haven't done that. In fact, Lars sent me an email after I posted my timing thread that's a sticky above and told me that my technical info was correct and it was a good post. He has seen me post my views on initial timing settings i'm sure as i've mentioned this plenty before this thread and he has yet to tell me that i'm wrong. Other people that opinions I greatly respect also confirm this same information.

As I mentioned before, you noticed a performance improvement with your initial timing set higher so I don't expect to convience you that i'm completely right and you are completely wrong. My point is that if you have your initial timing set very high like that there ARE disadvantages to it, the most important being greater risk of detonation so you should be aware of this and careful.

BarryK

again, I'm NOT AT ALL an expert so it's best to not group me in with people like Lars........

yes, the VC1862 is an HEI vac can.

it's typical to have a poorer idle with the vacuum line disconnected because you are removing about 15º-16º of timing!
On my '65 motor when I pull the vacuum hose off the can I need to plug is very quickly or the car stalls and than still have to adjust the idle screw to get it to stay running while I set my timing than bring the idle level back down again after I reconnect the hose back up to the vac can.

If the biggest issue you are saying is that you can't bring the timing down because it won't idle well, did you try bringing the timing down to 12º-14º and resetting the idle adjustment?

you are running off full manifold vacuum rather than ported vacuum correct??

waterplay

Yes I am running off manifold vacuum. My new carb has no ported capabilities. I also never tried it hooked up there with the old 850 which did have that capability. I also set the RPMs back to 850ish before setting timing and after. I understand what you are saying about excessive initial timing and have followed that for years without a hiccup. (8-12 degrees) But this is my most radical motor, a small block making over 550hp at 6500rpm and it has a hard time with what you and I call normal initial timing. That is why I put out this post to see if I was the only one who can’t get there car to idle with “normal” timing. I was partially hoping that I set something else wrong and that was why I now need more than normal timing to keep it running. It sounds like this my not be the case though. I am at a loss!
If I stop at NAPA on the way home today if I give them the CAN # that you listed will they be able to help me or do I need another #?
Sorry for the slow responses I am in and out of meetings.
Thanks,
waterplay

68coupe

I usually pop my head in on these timing posts.

I think there have been some really great recommendations made here. (I have met Lars and I have also met Barry and I worked with Gordon on his other post.) Here are my thoughts:

1.) Initial timing - I think that the initial timing CAN NOT be set in the 10 to 14 degree range. It will most likely cause the engine to run very hot and from a performance perspective, it will be lousy. Do not try to set your timing by initial timing setting. The biggest worries with initial are detonation and/or excessive starter load. Most likely, if those are not happening (based on your post I do not see any issues with either), then you are in good shape.

I would recommend this timing setup:

Total without vacuum advance= 36 degrees
Amount of Centrifugal = 18 ( I think that is what you have with your HEI based on your post.)
Makes you initial around 18
Amount of vacuum advance = 16
Makes total (not under load) with vacuum advance = 52
Makes your initial with vacuum advance around 34
Have it fully advanced by 2500 RPM
Plug the vacuum advance into manifold vacuum

MAKE SURE YOU HAVE NO DETONATION OR EXCESSIVE STARTER LOAD…..Barry is correct, the best day to check is on a hot day.

2.) Vacuum – what is the duration of that cam @.50? I think that 9 sounds really low. I would try to keep the idle under 900 and then try to tune the air fuel mixture screws of that carb. Is it running rich/lean at idle? What type of carb? (Manufacturer/model)

We can go from there…….

Joe

waterplay

Joe,
I need to head out again!!! They are killing me! I will get that info out to you in about an hour and a half. hope thats ok.
Thanks,
Dave