I have a SBC 350. I am putting in an MSD Street Fighter HEI Distro. I already set the vacuum advance stop to B which is 11-14 degrees of vacuum advance.
A = 14-17 degrees
B = 11 - 14
C = 8 - 11
D = 5 - 8
This thing has 22 degrees of mechanical advance built in out of the box. It comes full on @ 4K RPM. The stock springs are way too stiff.
I am getting the MSD 8428 advance tuning kit which comes with three color springs: Copper, Chrome, Steel (Matte). Light/Medium/Heavy.
If I read this right, I rip the weights and springs out of the stock unit, put in the weights and bushings from 8428 kit and use the LIGHT spring from the 8428 kit so I am all in mechanical advance at 24 degrees @ 3000.
Is that what everyone has done? It seems like the out of the box setup is not optimal for an SBC 350 at all.
It is odd to think at idle I will be on mechanical advance already. I idle at 900 in N and ~600 in D. TH350.
DO I have to replace the weights? Looks like the factory setup uses medium springs according to this chart b/c they say it comes w/ 22 degrees all in. The stock weights are chrome colors while the replacements are black oxide.
Usually a stock GM distributor comes with the weights and springs so that the mechanical advance will be all in by 4000. It is better to use the stock weights with just a spring change. If you have an aftermarket camshaft you will need to find where you want to set your initial timing. As a starting point use one heavy and one medium spring or two medium springs. Use these springs to give you an advance curve that starts at about 800 RPM and ends at 2800-3200 RPM. Once you have the new springs installed check the advance curve with a dial-back timing light. Swap springs until you get it close to the above specs. As the HEI distributor ages the centrifugal advance weights wear their pivot holes into an "oval" or eat a groove into their pivot pins. So check this before starting and fix it or get a new distributor. Look at "Hot rodding the HEI distributor" in the Wiki articles in the top Header. This articles gives you important info about the HEI distributor. I have a 496 big block Chevy with a MSD Pro Billet distributor. These are the instructions that came with my distributor. https://static.summitracing.com/global/images/instructions/MSD-8360_frm30246.pdf Reading these instruction might help you better understand choosing an Advance Curve. The graphs should help you choose an advance and which springs to use. In the Wiki articles it states the advance curve of the factory GM distributor starts around 800 rpm. Usually the idle is set at 750 rpm. The graphs show that the MSD distributor's advance starts 1200-1300 rpm. My idle is set at 900 rpm and the advance curve starts around 1200 rpm. This helps with big camshafts. The graph I chose to work with was C using a black Advance stop bushing and 1 silver heavy spring and one light silver spring. This adds 18* mechanical advance with all in around 2900-3000 rpm. I have 38* total timing. These advance stop bushings make MSD distributors so easy to tune except the Street Fire. It's like a Chevy HEI.
Usually a stock GM distributor comes with the weights and springs so that the mechanical advance will be all in by 4000. It is better to use the stock weights with just a spring change. If you have an aftermarket camshaft you will need to find where you want to set your initial timing. As a starting point use one heavy and one medium spring or two medium springs. Use these springs to give you an advance curve that starts at about 800 RPM and ends at 2800-3200 RPM. Once you have the new springs installed check the advance curve with a dial-back timing light. Swap springs until you get it close to the above specs. As the HEI distributor ages the centrifugal advance weights wear their pivot holes into an "oval" or eat a groove into their pivot pins. So check this before starting and fix it or get a new distributor. Look at "Hot rodding the HEI distributor" in the Wiki articles in the top Header. This articles gives you important info about the HEI distributor. I have a 496 big block Chevy with a MSD Pro Billet distributor. These are the instructions that came with my distributor. https://static.summitracing.com/global/images/instructions/MSD-8360_frm30246.pdf Reading these instruction might help you better understand choosing an Advance Curve. The graphs should help you choose an advance and which springs to use. In the Wiki articles it states the advance curve of the factory GM distributor starts around 800 rpm. Usually the idle is set at 750 rpm. The graphs show that the MSD distributor's advance starts 1200-1300 rpm. My idle is set at 900 rpm and the advance curve starts around 1200 rpm. This helps with big camshafts. The graph I chose to work with was C using a black Advance stop bushing and 1 silver heavy spring and one light silver spring. This adds 18* mechanical advance with all in around 2900-3000 rpm. I have 38* total timing. These advance stop bushings make MSD distributors so easy to tune except the Street Fire. It's like a Chevy HEI.
thanks for the response. everything i have read (on here and other sites) says the sbc should have all in mechanical anywhere from 2400-3000 but no later than 3000 for best performance. so that is what i was going for. if you look at the pdf w/ the advance timing kit I post above it shows a combination of light springs and a timing curve. actually, here is the photo:
the kit doesnt make mention of mixing springs. not sure that is possible on the street fire. i am just afraid the light spring will clearly have me advancing timing at idle. not sure that is ok. the medium spring will be just like the stock configuration.
i am honestly thinking of using one light and one medium spring, but that is unknown territory. wont be able to tell if i am getting mechanical advance at idle.
You can mix different spring tensions (medium on one weight, light on the other, exc.), done all the time, it's how you dial in the timing curve.
Be sure you are below 800 rpm idle speed while setting timing and adjusting curve. If you have set your idle higher to compensate for your transmission's in gear idle speed drop, simply lower your neutral idle speed while doing this, then re-set idle speed when done. Mechanical advance works on engine rpm, so once the transmission pulls the idle speed back down in gear it will be as you want it.
Further, I recommend you use an adjustable timing light or timing tape to accurately check and adjust your timing curve. And experiment with different curves to find which works best for your motor, some like all in at 2000 rpm, some like it later. Always listen for detonation when you get on it after each adjustment and back the curve off if any knock (detonation).
Finally, always check total timing after each adjustment at above 4000 rpm to ensure total timing is not too much (no detonation). On a SBC total timing should not be more than 36° (sometimes less), with vacuum advance plugged off.
One more note, on unmodified distributors with no total advance adjustments (bushings, exc.), timing is set by TOTAL timing (above 4000 rpm) and initial is whatever comes out of it (usually between 10-14). Total timing is the most critical, too much total timing will destroy a motor..
Have fun and discover improved performance.
Another poster (383paul) was asking the the same thing about which springs to use with a Streetfire distributor as it is similar to a GM HEI. I told him to look at the top header and hover over Wiki and then click on engine. Click ignition and then look under H and click on Hot rodding a HEI distributor. Scroll down to 15.4.1, Limiting or locking the mechanical advance mechanism. Read this section carefully as it tells you how to limit the mechanical advance in a HEI distributor. It shows where to put the limiting screw in the first picture. It also tells how to achieve the correct amount of advance in the first picture. I also told you in an earlier post that you would have to use different combinations of springs. I gave you an example of what I used in my MSD distributor. You will have to take a total timing reading after every spring change.
for anyone who finds this thread on a stock sbc 350 i purchased the MSD Advance timing kit and used two medium springs in it (silver) with the stock weights on the street fire. So far so good.
17 degree initial
positioned stop plate on B @ 11 degrees
36 degrees @ 3000rpm
i have about 50 with vac advanced plugged in
it adds about 2 degrees more up till 3500 where it stops which I am not going to drive myself crazy with.
here is what my curve looked like.
RPM - Degrees --- NO VACUUM ADVANCE PLUGGED IN!
950 (idle) - 17
I should have gotten a 1400 sample to ensure there wasnt a massive jump. i am guessing it would be about 25 here.
1800 - 31
2130 - 32
2200 - 33
2750 - 34
3000 - 36
3200 - 38
we stopped there b/c timing stopped getting added. which means adding 11 degrees of timing would put me in about 49 cruising. i could have sworn i saw 52 @ 4000 rpm but could have been mistaken.
this was the old turd. weights would not return to center. missing a spring. no C clip on top of the weight. i am guessing the spring fell off at some point vs the previous owner removing it to adjust the curve.
If you were using a timing light with built in advance you could start at idle and increase it 100 rpm at a time. I would check it at 800-1000 to see when the advance starts and then at 3500 or where ever the advance curve stops being added. What type heads are you using iron vortec or old Gen 1 heads? I would set the total at 32*-34* with Vortecs and 36* iron Gen 1 heads.
You can look under the valve covers, they have casting numbers in the rocker arm valley. Then look up the casting numbers on Mortec.com. I would want to know that I don't have a set of smog heads that do not flow that good. BTW that is a nice timing light. Have you changed to a bigger camshaft in that engine? Just wondering why you are using 17* initial advance. If it has the factory camshaft in it I would lower the initial to 14*-15* so that will drop the total back to 36*. Once you plug the vacuum back to the distributor it might have some detonation under load at 38*
2X
950rpm will already have some mechanic advance. 38° total is too much an may lead to detonation.
Check TOTAL timing at 4000-4500rpm, this will be above any mechanical advance curve and more accurate.
TOTAL timing is critical...
just got back from mechanic to verify my timing. took timing light. went through curve. I am actually closer to 18 initial.
she literally won't idle at anything less that 17
he says it's cammed. we know nothing about the origins of this car. it has a drive shaft cage and we suspect it was dragged so the cam makes sense.
anything above 3000 I am at 38 and stay 38
2750 ish I am 36 and from what I hear that's perfect
I can't limit mechanical advance with this dizzy
with vacuum advance I am 54 @ 4500-5000+ . I get ZERO ping. when I left off the gas I hear a little popping but it's no cats, hooker headers, 3" open exhaust. hearing that should be normal.
this is on 87 marine gas too. no ethanol. if I am not pinging on 87 I should be fine at 54 all in.
Thought I'd share this. My dad and I just got done building a 383, 11.2.1 drag only motor. Right now we ourselves are tinkering/tuning with timing. This is what jumped out at me on your post (and keep in mind what i typed above):
"She literally won't idle at anything less than 17." Right now, mine idles very fine at about 11 degrees, despite the fact i'm probably going to bump it another two degrees.
"He says its "cammed." So is mine. Are you above 645 lift, 257 duration, etc?
Your initial seems high to me. let's say even if you think your initial is PURE initial and you might even have a few deg of mechanical in on 'that initial' that you're not aware of, it still seems too high.
I'm just trying to throw some things out there as i'm no timing guru but have slowly learned about it. If you literally cannot idle below 17 degrees, something seems off bud. Will also add, your spring combos just dictate how fast you reach that all in setting and the advance curve.
I have read that article no less than ten times. the guy is in favor of greater initial timing and lower mechanical advance. facts are this engine won't idle below 17 degrees. it gets supper rough and the vacuum drops and is sporadic.
i am going to change my vacuum advance stop plate to position B which limits me to 8 degrees. that should put me at 49/51 all in.
If the StreetFire is just an HEI copy as far as the mechanical mechanism is concerned, the limit screw idea contained in the WiKi entry "Hotrodding the HEI Distributor" , section 15.4.1 should give you an easy way to limit the mechanical advance travel, using the head of the screw as a travel stop. Diameter of screw head determines total travel, head can be filed smaller on the correct side to change the limit amount.
people are wigging out about 38 degrees at highway speed. what does vacuum advance do? I would be running above 36 with vacuum advance. I pulled the cap and rotor and switched my vacuum advance stop to D which is 5-8. bc she needs so much initial and is all in at 38... another 8 on top of that should bring me to 46 cruising with vacuum advance. just hoping that won't cost me power up top. I will have to floor it and see if she moves.
on the plus side... touching the throttle she wants to leave hard. she starts right up too. idles good. has good vacuum. we will see what happens this weekend when I pull out the timing light.
I could drop initial and up idle by screwing in my idle screw. what does that do? opens the throttle plate so I will likely start pulling vacuum.
I am ported right now. so I have no vacuum at idle. if someone said they are 12 at idle non mechanical non vacuum then swear by manifold vacuum for smoother idling... they will be at 18-20 at idle bc of vacuum advance plugged into manifold vacuum will they not?
I just happen to be there without vacuum advance. I may have some mechanical in there. I don't think I do knowing my dizzy puts out 22-24 of mechanical. so 22+17=39 which is what I see (38ish) maxing out on. then vacuum plugged in adds that 11-14 per the stop plate chart.
i mean honestly I was thinking of backing down to 12 and seeing if I could compensate with throttle... but when I did that last time even with a 1050rpm the car wanted to die when shifted into drive. dropped almost 600rpm.
with my setup now of 17 degrees and 950 idle when I go into drive I am at about 650. exactly where I should be.
the car also has amazing initial throttle response. I have to check the total timing again with vacuum advance now that I changed the stop plate.
and I do remember I got timing down to 12 initial and she would idle, but even with idle at 1050rpm when going into drive she would start to idle rough and want to die and drop to 600 rpm or lower.
That is because backing down the initial timing then opening up the throttle to compensate exposes too much of the transfer slot in the throttle baseplate.
You may find it runs even better with the initial up around 22-24°....but you WILL then have to find a way to limit the mechanism so it doesn't add too much mechanical advance.
By the way, 38-39° total mechanical is reaching into a risky area for pump gas in a SBC
17° initial doesn’t bother me, but raising the throttle speed is not the answer, it will only take you off the carb’s idle circuit and screw the tuning further.
38° total is what bothers me, because it’s not at cruise as you say it is with vacuum (cruise can take higher timing), but rather it is at 38° under load which can lead to detonation and destroy the pistons. So while cruise can take higher timing because their is no or very little load on the motor, a motor under load can only take so much, and usually a SBC likes no more than 36° on the street.
Now, to the real problem your having, the idle speed dropping when you drop it in gear from 950rpm to 600rpm, read this; Talking TORQUE - Street Rod Life
If a new torque convertor is out of the question, believe it or not, an inexpensive vacuum solenoid can be used with some creative wiring to manipulate your vacuum advance to raise and lower your idle speed as you shift in and out of gear automatically. And, this trick would allow you to drop your total timing and let the motor idle. With the added benefit of allowing you to adjust your carb better, and maintain your COOL lower idle speed cam lope even in neutral. Really not trying to be a smart ***, be glad to show you how if you want.
i shouldn't have bought this car. i just dont know enough and now hate it and wish i never bought it.
we were off a tooth on the distro so i went to TDC. lifted up the distro. rotated it by one tooth back. re-set timing. NOW THE CAR SOUNDS LIKE CRAP!
i was at 17 degrees initial before the rotation and it was running smooth as a kitten at 950rpm. now at 17 degrees the idle is like 650 and it wants to die. i upped the idle screw a tad, but that is not a solution. if i rotate to some 23 degrees it seems to like that, but that is way too much. vacuum is at about 15 inches of mercury which says "hey ur timing is too advanced" but it likes more timing.
i pulled the rotor. brand new distro. springs look ok. weights sling in and out. vac advance i locked out at 5-8 degrees.
vaccum advance is unplugged. when i plug it in to ported, i get no advance which is good b/c it means i don't have the throttle plates open causing a vacuum when i added more rpms.
i am honestly 100% at a loss here. 17 degrees before I went back a tooth was 100% fine. now, it isnt. what the hell?
obviously when i rotated the distro i had to move the crank so it would rotate the oil pump tooth so the distro would drop. i am 100% sure the distro is all the way seated.
I checked the plug order 500 times. It is right.
All the vacuum ports are plugged so no vac leaks.
I am truly at a loss how the change of one tooth could screw this turd, but not surprised.
Could I have damaged #1 plug wire when I tightened down the air cleaner? It left a mark in the boot.
Before and after photos of where the rotor was and currently is located.
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