Anyone want to take a guess at where my initial timing should be set?
I just stroked out my 350 to 383 and am running the comp 270H cam.
Automatic with stock tq converter. Also, any suggestions on distributor
springs and weights (non-HEI).

Trying to dial this motor in.

Blake - 73

 

You've got more cam and cubes than me but I think that if you'll look for about 16 degrees of initial timing you can work from there to get the advance set. On my motor I set the idle down to 600 RPM when setting the initial timing even though I set it at 750 RPM to drive.

 

16 BTDC & all in by 1800-3000 rpm.

 

What CR aer you running? I am at 11:1 and I am getting alot of valeve noise
at WOT on any timing setting over about 6 degrees. Am I having a fuel problem?

Blake :chevy

 

Disconnect your vacuum advance if you have it connected. You may find that your engine doesn't like the vacuum advance. Buy a dist. weight kit, use the med. or heavy springs to allow for full advance not earlier than 2500 RPM. Set your initial timing back to about 12 degrees rather than my earlier recommendation. If you are still having problems recheck your timing marks and valve settings. Do you have fresh premimum fuel in the tank?

 

My cam has a bit more duration than yours, but I also had more than a little frustration with the cars poor off the line performance. I am using the MSD version of the transistor ignition with mechanical tach drive.

I had set the spark curve according to typical settings advancing the spark about 20 degrees from 800 rpm to 3000 rpm. Since the L88 cam doesn't have very much vacuum, I am not using any vacuum advance. The engine was gutless and rough through the lower rpm's and didn't feel good until about 3000rpm. I was using one heavy silver spring and one light silver spring. Changing to two light silver springs brings in all the advance by 2000 rpm. The engine feels much more responsive and smooth below 3000 rpm now. As I have the opportunity to experiment more, I may even try using just one light blue spring allowing all the advance even quicker. The blue springs are a little heavier than the light silver springs, so using only one blue spring should allow quicker advance than two light silver springs. I was tempted to simply lock the advance and set the initial timing at 36 degrees; but, this would make the car even harder to start - especially with no choke! At cranking speeds, there is not enough rotational speed to advance the spark much until it fires, then the advance can come in.

I may change the distributor advance cam button to allow for more than 20 degrees advance which would reduce the timing at startup even more. I will still have the engine's full spark curve come in very quickly. My tuning is a little different than the book suggests. I set the timing to 36 degrees at 2500 rpm and then verify that it is all in by 1800-2000 rpm. I spend very little time below 2500 rpm anyway so pinging or knock at these lower speeds is not an issue.

Chuck

 

I have been having Lars help me with timing issues on my 11.5:1 406. I order to avoid detonation, I had to back off initial to about 8* and put in very heavy advance springs, bringing the total 34* in well above 3500. I still had ping above 3500 rpm.

I ended up changing the heads from 041 to 493, which came on the factory 400 SB's Hope to get it running again some day :smash:

 

For best performance dont worry too much about "initial timing". Total timing is the more important factor. You want to set your total timing to reach 36 degrees BTDC. Do this adjustment with vacuum advance disconnected. You want it to all come in no later than 3000rpm. Adding timing tape to your balancer will make this job easier. Here is an article that may help>>http://www.angelfire.com/on/geebjen/timing.txt

 

thejaf,

If you are using a shorter duration cam with that compression (I have 12:1) you will very likely have more of a detonation problem than me at lower speeds. 100 octane fuel may be your only solution.

Chuck

 

chuck,

I measured the cam when I had the heads off. It's a solid flat tappet dual pattern cam with moderate lift (0.450 with 1.5 rockers average) with 253/272 duration at 0.050" lobe lift.

I had terrible off the line performance when I first bought the car, found out from Lars that my distributor had no advance at all. Someone had wire-tied the weights and set a moderate initial timing while keeping the idle circuit rich with a Holley 650 DP. I my search for gas mileage above 7mpg, I ended up changing to a quicker advance and rebuilt Q-Jet carb. I noticed a huge increase in low end torque after those changes, however I now had lots of detonation.

The rebuilt heads are on and I'm waiting for a replacement roller rocker from Crane (1.5 and 7/16), so I hope to get the car back on the road in a couple weeks.

Jeff

 

Chuck Harmon nails it on the head. A bigger(longer duration) cam will lower the cylinder pressures and make the engine more pump gas friendly. If your cam vs. compression ratio gives you too much cylinder pressure, you can retard the ignition to soften the knocking. But then you get lower performance as a result. Its important to have the cam matched to compression, especially when considering pump gas. Changing the heads is one option. A bigger cam is another. With a bigger cam and auto tranny, a stall convertor might be needed. I hope you can get it squared away without any major changes anyway. As far as octane is concerned, my brother was mixin up some stuff for his car last weekend. It was some type of thinner he got from a paint store for $6 a gallon. Xylene iI think it was. Its the main ingredient in the octane boost you buy at the auto parts store. Buying it in another form such as thinner is way cheaper. Ill see if I can get some more info from him on it. I.E. how much to mix for what volume and octane desired.

 

Jeff,

You have two issues with your timing. 1) how fast the timing advances, and 2) How much timing advances. If your car now runs much better down low, but detonates above a certain rpm, you need to limit the advance at the point of detonation if this is happening at about 3000 rpm. On my MSD distributor, there are different bushings that can be used to limit the total amount of the advance. i.e. if your distributor is now advancing 22 degrees, try limiting it to 18 degrees. In this example, the distributor would advance 18 degrees just as quickly as before, but it would stop there. Changing the springs determines how fast it will advance. Heavy springs advance more slowly than when using light springs.

If you can't lower the amount of distributor advance enough, you may need to go to lighter springs and retard the timing by a few degrees. Because the distributor will now be advancing more quickly, you should have the same ideal timing at the lower rpm's even though your total timing is now reduced to avoid detonation above 3000 rpm.

Determine when you want all your advance to be in and make sure your distributor is in fact fully advanced at that rpm . If you have a timing light see at what rpm all advancing stops. It really shouldn't be much over 3000 rpm.

Perhaps (not necessarily recommended) your settings are now 16 degrees initial timing, 18 degrees advance with the distributor for a total of 34 degrees at 3000 rpm. Leave the vacuum advance disconnected. Try flooring the car at 500 rpm increments between 1000 - 3000 rpm to listen for pinging. If no pinging replace with lighter distributor springs. If for instance it pings at 1500 but not at 2500, you need to retard the initial timing, but allow it to advance perhaps a little quicker and allow the distributor to advance a little more. In this case if you would set the initial timing to perhaps 12, change to a lighter allowing advance more quickly, and change advance limit on distributor to 22 degrees. The all in timing is still good at our 34 degree example, initial timing is retarded to avoid ping at 1500, and advance is greater at 2500 to allow for better power there. Life is now very, very good.

If you hook up vacuum advance and get pinging, you need to limit its advance or consider as I did, not using it. Since I don't floor the car below 2500 rpm anyway, I use even more advance below 3000 rpm with no problems.

Chuck

 

Jeff,

My advice (worth at least twice the price! :smash: ) to get your spark advance curve just below ping levels will still probably require you to get much better gas. Do all you can to reduce octane requirements like lowering intake air temp, very good cooling system that keeps engine at 180 degrees, etc. While I'm getting mine sorted out I am adding about a quarter of a tank of 100 octane gas to the 91 premium offered here in California. Eventually I plan to go to a water (alcohol mix) injection system http://www.aquamist.co.uk/ . This is a much more precise and expensive system than the wiper washer pump systems of the past. The system is map able to vary flow rates at different rpms and engine demand levels. Cost is somewhere around $800-900. Cheaper than rebuilding (emasculating) my engine to crummy gas specs and more desirable if it really allows me to use the 12:1 on the street for those short full throttle emotionally satisfying bursts of power. I will still fill up with race gas when I play at Laguna Seca or Sears Point raceways.

Chuck

 

Chuck,

Thanks for the info! I've been working with Lars on this, and I've already changed the heads to lower my comp ratio to about 9.7:1 This should allow me to run the 36* quick curve points distributor he set up for me on pump gas. I'm just waiting on a replacement roller rocker from Crane, should be here this week, so I'll get my not so qualified hands :smash: :smash: :yesnod: dirty and see how she fires up! :cheers:

 

Sounds like my choices are basically this:

Mix race gas with pump gas or use octane booster
Change heads to 68CC to bring down CR
Live with really low timing curve and lower performance

I think I need to buy some heads.....

 

Do I hear a vote for thicker head gaskets? If you have a .020 gasket now, a .041 will drop the c.r. almost a 1/4 of a point. Changing the cam timing will have an effect on the c.r. also. Someone else will have to answer which direction is correct for cam timing. Check closely for valve clearance before running the engine.

 

Blake,

From what I've learned (I had the same heads as you) was that those heads are great for a 302/327. Lars set me up with a pair of 493 heads from a 400 block, which reduced my CR to 9.7:1, which is still on the "upper end" for iron heads but should work great.

In the long run, the most painless thing to do (relatively speaking :crazy: ) will be to change the heads. I am/was fortunate enough to have a Cam2 pump about 3 miles from my house. Yes, adding a decent amount (5 gallons per tank) did help detonation, but that means you are limited in you driving distance, or you can keep a 5 gallon fuel container in you storage compartment. I wasn't too keen on that idea :nono: