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BlueBull
03-22-2009, 03:57 AM
I recently fired up my new engine combo, a 406 CSB with Brodix IK200 heads (64cc and 200cc int.runners). I have flat pistons with 12cc dish and a hyd roller cam from comp (XR270HR) with following specs:
duration @ .050 lift: Int/218 Exh/224
Lift: Int./0.495 Exh/0.502

The selection of the cam was based on the moderate compression and the use of the car (daily driver). The distributor is an MSD streetfire (HEI) with adjustable vaccum. My redline is 5500 rpm.

Now I need advice how to setup the ignition timing. The present timing is 10 degrees on idle (1000 rpm) and a total (without vacuum) of 36 degrees. How shall I setup the adj.vaccum depending on the rest of the configuration. Shall I use ported vacuum of not?

Thanks

MonzaRacer
04-27-2009, 09:06 PM
On street you should use it for mileage/driveability. That setup should be able to handle more initial, try little more like 16-18, light springs on centrifical advance and see if it rattles, if it does not rattle and likes it , then hook up vacuum advance to vacuum, manifold vacuum. This way it will loose the advance timing at full throttle.
If it pings under load try swapping one of the advance springs thats stiffer, one at a time to try to slow the advance in the centrifical, than if it still pings drop back 2 degrees with light springs. Most Chevys will take anywhere from 34 to 42 n ot including the vac advance, just listen for problems and if you see black smoke and no power its probably too much. As for vac advance I try to let it run as far as engine will allow with no rattle at cruise or tip in.
IF you here it just split second your close and try tyo tune with 87 then up to 89 octane, if it likes the octane and price is no object tune for 89 then whe nracing jump to 91,93,94 depending on area your from.
Also your going to want to spin just little more rpm with that cam, say 6000.
Email me if you have anny issues or questions at [email protected]



Lee Abel
AFTERMARKET PERFORMANCE

L n L
04-30-2009, 06:45 AM
I personally set my cars up to use manifold vacuum.

This allows me to back way off on the idle adjustment on the carb, which also increases the manifold vacuum. At the track, this set-up has also given me my best 60' times.

With your heads, I would suspect that you don't need as much total timing as you have.

I would use either (preferably) a dragstrip or a chassis dyno to figure out what the best total timing would be. For initial timing, I would try different settings to see which gave me the best throttle response and MPG's. Then, I would adjust the curve to "marry" those two settings.

Lee Atkinson

MonzaRacer
05-31-2009, 09:51 PM
Same as I said, add timing and listen to what its doing, add timing to base till it quits getting any better, I always start with light springs then slowly add in stiffer ones if I have ping, the vacuum advance I keep loose and keep it on manifold vacuum and use it to smooth out wilder cams. Most people wont believe my 402 bbc had 58 to 62 total on 10.5 to 1 on 94 Sunoco.

68Formula
06-01-2009, 03:35 AM
With that size engine and cam, it would probably run good with ported. Manifold will tend to smooth the idle out with big cams. I wouldn't consider yours a big camshaft.

First try about 16 -18° initial with the vacuum advance disconnected. With then engine fully warmed up, keep increasing intial until you hear the starter beginning to "lug." This means the spark is coming in soon enough to fight against the starter. Then back it off 2° from there. This applies whether you're going to run with ported or vacuum. You should find you can decrease your idle speed by a couple 100rpm after doing this.

Total depends on the combustion chamber design. I haven't seen the Brodix, but the newer style combustion chamber designs typically like 30 - 34° total at wide open throttle (without vacuum of course).

Remember if you increase your initial timing and then reduce the corresponding centrifugal so that the total doesn't change, the rpm at which you'll reach total is lower. In other words, more initial, and less centrifugal makes for effectively a faster curve with more at each rpm point. In that regards, I always start with the stiffest mechanical advance springs. Once I set my total and find out what rpm in comes in at, then I try lighter springs carefully listening for detontation and/or loss in mph in the quarter (mechanical total needs to be adjusted for wide open throttle conditions). If damage is going to occur due to over advance it's going to be at WOT, so it's important to start on the conservative side and work your way up.

Couple issues using just too light of a mechicanical spring. One is they can start adding timing way down at idle and there cause more erradic timing. Secondly, they are more likely to fatique, which will cause problems later on. This is another reason why it's better to set your initial with the highest allowable first, and then try lighter springs. Instead of trying to compensate for lack of enough initial by using lighter springs. Start heavy, and then try one heave/one medium, then two mediums, etc.

Finally, once you have your mechanical all set (again, doing all this with vacuum disconnected), then adjust the vacuum advance. You can try both ported and manifold, but switching between the two will require resetting the idle rpm and mixture. Start with ported and run the lightest setting to listen for detonation. Since you'll be testing at part throttle (never full), this should be safe. You want to just get it to the point where the pinging goes away. Try some small hills just to be sure.

If you decide to try manifold vacuum after this, a good rule of thumb is with vacuum advance at idle not to exceed the total timing at WOT. For example, on my car with the camshaft but low static compression ratio, I have initial set to 20° without vacuum with a total of 36° coming in at 2600rpm (using factory mechanical spring rates). Then I have the manifold vacuum setup such that the canister addes another 16° at idle. Then do the drive again and listen for detonation. Again, with vacuum connected, you should be doing low load and low to medium rpm accelerations.

Let us know your final settings.

MonzaRacer
06-06-2009, 02:33 AM
OK ported timing is stupid on performance engine as the normal pressure rise will be getting advance when you want to stay in the power range.
Ported vacuum was a non useful crutch in the garbage emission 70s
Tune for best power production less the manifold vacuum, then add it back for mileage.
Vacuum advance is simply a mileage tool, with out you get great power, but crap mileage.
Most engine baring super small/efficient combustion chambers and super high compression will use the 28-36 combined initial/centrifical timing.
Heck I have my 402 running 58-60 total and in a 4000lb + 70 Monte with 355s ran 14-15 mpg driven hard.
I set my initial in at 16-18, started with heavy weights and light springs,it now has one light and one medium spring to tailor the curve, I enlongated the centrifical curve slot to gain a max of 40 and its all in at or below my 55mph cruise speed, then added in my vacuum advance.
Lee Abel
AFTERMARKET PERFORMANCE