I have been a lurker for a long time now and have always been impressed with the knowledge of everyone. Now I need some of that knowledge! I have installed a year one crate engine (400+ hp) in a 1974 corvette and and changed to an aftermarket HEI tach drive distributor. I have the initial timing at 12 degrees with the centrifugal giving 20 degrees and coming in at 3200 rpm. I had an adjustable VAC on it and was getting spark knock no matter where I set it. It only knocks from 2800 to 3500 rpm. If I disconnect the VAC the knock goes away. I thought the VAC was bad so I bought one of the VAC cans that Lars recommended (VC1853) which gives me 15 degrees of advance and still I get the knock. That is only 47 degrees of total timing. I am using 93 octane gas also. What am I missing?

 

You'll probably have to accept a slower advance curve and limit the vac can. With some stiffer springs you can delay the mechanical advance so it doesn't come all in until after 3500 rpm. You'll probably also have to limit the vac can to 10 dgr. You can limit it by shortening the slot (a spot weld will do it).

BTW, the adjustable vac can only lets you adjust how much vacuum is needed for full advance, it does not adjust the amount of advance it adds. Your issue is that it adds too much.

Once you have it limited you should try and get a max of 36 dgr (without vacuum) at high rpm

 

welcome to the world of chit gas and HP motors.
47deg? WOW

2 routes to go
fatten the carb fuel ratio to air
Find a dist guy that can curve your dist to a friendly one.

 

set your timing to 36-38 total and plug the vac, can....just run mechanical...

 

Is your vac source manifold or port? If manifold, suggest switching it to port and giving it a go, as tho many a hot-rodder swears by manifold vacuum, it doesn't always work out. Vac advance can be your friend on most street builds, so I would try to solve it if possible.

Here's a link to Barry K's site, where you'll find some informative tech articles on ignition systems, timing & vacuum advance, and more...

http://www.lbfun.com/Corvette/Tech/vettetech.html


TSW

 

The Crane adjustable advance kits (for points) allow for amount of vacuum advance. I don't know about the HEI kits. Limit the vacuum advance to 10*. This is common with today's gas. I can only get crappy 91 octane in CA. You should get a timing tape and plot your curve.

 

The reason I thought I had a problem with the distributor was, I had this distributor in an identical engine with 16 degrees initial and 18 degrees centrifugal with the same VAC and had no issues at all. Was I just lucky?

 

I appreciate all of the input already since I am a noob posting on this site. You guys are great and I have read all the articles on Barry K's site among others until my head spins!!!! On the other engine I was running 52 total advance on the same gas. If i plug the VAC and run 36 total advance I can start hearing the knock again. Why at 2800 to 3500? Is there anything in the distributor that could have worked loose and moved? I am using a dial back timing light and have mapped my curve and it is very stable.

 

The higher the rpm, the more timing it can take. It makes sense, really. At high rpm the piston moves faster, so you can give the spark earlier as seen on the crankshaft (degrees), and you are still the same time (as in milliseconds) from the piston reaching the top. So at 2800 rpm it is still not high enough rpm for your motor to take full advance.

 

The new motor -I guess- is higher perf than your old one.

Different compression, cam timing, ect.

One size does not fit all, too expect your timing needs and
curve to be the same is wrong.

You will need to research your specs and curve the dist to the motor.

 

As for the knock showing up between 2800 - 3500 and not above, what it sound like is happening is the early/high advance curve is out pacing knock tolerance until you get past 3500 for your build, octane & operating conditions. The easiest adjustment to try would be to simply reduce the initial and thus total from 2* to 4*, but in order to keep your initial up you'll have to re-curve for less mechanical advance. 32* is plenty total mech advance for many a modern high-perf build.

As mentioned, there's always the possibility the timing marks on the balancer are out of index, which can throw off readings, so you can't always go by them.

 

I just finished working on it again and I changed the centrifigal advance springs to the next stiffest set. This give me 12 initial, 18.5 mechanical and I put the adjustable VAC back on and adjusted it to give 9 degrees of advance. This give me 39.5 total. I took it for a quick drive before the storms hit and didn't hear any knock. Next I will try advancing base timing a few degrees until I hear spark knock and then back it up 2 degrees. I think what was happening was that max mechanical advance and max vacuum advance was overlapping in the 2800 to 3500 range giving too much total advance at one time. Is this a reasonable assumption? Thanks for all the help and observations, you all are a big help!

 

Hows it pull?

You are on your way to being "the guy"

Sounds like you could tackle some jetting and power valve tweaks.

 

It seemed to pull much stronger than before, but then again I only got about 1/4 of a mile before I had to turn around because of storms in the area. Hopefully I will get to play with it some more tonight. Thanks again!

 

Hi There,
Having been through a similar situation recently myself I feel for ya. Its a bloody nightmare when you cant work out why. You can find an easy solution (retard it till it stops) but know that its still not right and you are just hiding the problem rather than fixing it.
I got there in the end with mine, ended up modifying the Vac to do exactly what I wanted it to do.

I am by no means an expert, but certainly have it fresh in my mind at the moment!

Sorry if you have answered these already, I may have missed them. First question is under what conditions are you getting the ping/pre ignition? Is it present with and without VAC connected? If you havent tried it then is it present under a WOT or on a slight throttle?

If its there on WOT it points towards too much Centrifugal.
If its on a slight throttle it points to too much vacuum.

Have you checked at what vacuum pressure (inch hg) the vacuum cannister starts and stops moving at? If so how does this compare to the vacuum produced by the engine at idle?

On mine I have ended up running 13 initial 36 all in @ 2,700rpm and 48 total with vac advance connected (vac connected to manifold).

Hope this is of some help, your welcome to PM me if you want some tech papers or anything, I too spent a long long time downloading and reading them all start to finish.

Lester

 

Another thing to check is that "0" on the balancer really is top-dead-center.

 

It was mostly there during part throttle in the range of 2800 to 3500 rpm. The vacuum starts at 5" and stops at about 11". I have 14" at Idle and am running manifold vacuum from the base of the carb. I did notice on the inside of the rotor cap it appeared that the weights had been bouncing off of the wall of the cap, hence the decision to change to stiffer springs. The puzzling thing as I said before, I had it an identical crate engine, (as identical as you can get) from year one, same cam, heads compression ratio and ran 52 degrees total advance per one of Lars tech papers, and had no issues. I have not checked the balancer to be sure 0 degrees is top dead center, but I will try that tonight hopefully. By the way, between church and rain, I didn't get a chance to play with it last night, hopefully the weather will cooperate tonight.

 

Some thoughts:

Agree, check that TDC on the harmonic is really TDC.

Secondly, vacuum leaks and/or a lean A/F mix will aggravate the pre-ignition.

Thirdly, if she's knocking under a load, remember that vacuum advance doesn't come into play. Under a load, vacuum is reduced to almost nothing.

Lastly, hate to say it, but you may have some hot spots in the chamber (heads) that may have to be smoothed out which means removing the heads.

 

Just re read you original post and noticed 'goes away when vac disconnected' so you have run it with vac plugged and it is good and clean through the range? Again that leads to the vac giving too much advance. Although the weights hitting the cap does not sound good! There should be a stop on them so that they can only travel so far. Do you get any scatter (jumping of the timing mark with timing light connected) once at high revs?

At a part throttle the engine will probaby still be producing enough vacuum to produce the 11 that the vac can needs to be at its full travel. I know you said that the vac can is new and rated at 15, have you checked this? It could be degrees at the cam which is 30 at the crank. I also found that the rating on the can was not always accurate. I modified my vac can in the end to do what I wanted it to do, i tested it by plugging the manifold and using a vac guage to operate the vac can to its stop and then seeing how many crank degrees it gave. This also allows you to confirm that the vac can comes in and out progressively. One of the ones I fitted fould the distributor slightly (as it moved the lever arm dipped down slightly and was binding) this led to vac advance being stuck in when it shouldnt. The vac guage will also show any binding by more pressure being required to operate the can once in situe rather than off the car.

Timing sometimes becomes a compromise, the vac advance will not effect straight line WOT performance, so if thats what your after get the timing as good as possible on centrifugal. Then resrict the amount the vac will advance the ignition (say 12, 36, 45?) If it works go with it, it might sacrafic economy and cooling slightly at idle or constant throttle, but no where near as much as just disconnecting the vac all together which alot of people do as a simple and easy fix.

Like I said originally I am by no means an expert, I have good basic mechanical knowledge and up until a couple of months ago I knew next to nothing about the timing specifics on Vette's. I have had to do a hell of alot of research and speak to alot of people to find the fault with mine and decide what the best compromise was for me.

One more note, I take the it Vette hasnt been standing for a while? Stale fuel will have alot lower octane rating and create faults that may not be present with fresh fuel.

Best of luck, and if your problem goes anything like mine your neighbours will be thoroughly pi*sed off with the sound of the vette at 3,000+rpm by the time you get it sorted.


Lester

 

I am sure when I read BarryK's thread a while ago, he said with MSD you should set your initial at 13deg.

G