408 ignition curver advice

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mbaird said:
I don’t consider 246/250 to be a long duration for a 4” crank but maybe you’re right
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partsmonsta said:
You're over thinking this.
Your problem is your Carb.
Despite being a 408, an 850 is too much Carb.
And your sluggishness is typical of too much carb.
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You would be outright amazed at the pickup in your car once you switch to a healthy 750 cfm.
You may even pick up on the top end.
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If your street driving a lot, a spread bore may be the way to go. Compression seems to have an effect on port velocity.
 
partsmonsta said:
You're over thinking this.
Your problem is your Carb.
Despite being a 408, an 850 is too much Carb.
And your sluggishness is typical of too much carb.
-------------------------------------
You would be outright amazed at the pickup in your car once you switch to a healthy 750 cfm.
You may even pick up on the top end.
Click to expand...
This 408 is running ported W2s flowing 297 @ .650 and 1 7/8 TTi headers . I have run my 800 AVS on it and found it had a bit more low end but worse everywhere else .
I could set up a 650 DP for running around town and use the 850 if and when I go to the track I guess .
 
Oh yeah … I dropped a set of 3.91s in it and that helped low end . Lol
 
mbaird said:
This 408 is running ported W2s flowing 297 @ .650 and 1 7/8 TTi headers . I have run my 800 AVS on it and found it had a bit more low end but worse everywhere else .
I could set up a 650 DP for running around town and use the 850 if and when I go to the track I guess .
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Two 850’s aren’t too big for a 408.

I’m still surprised that people still believe the decades wrong carb sizing nonsense.
 
RustyRatRod said:
Here you bees.
www.forabodiesonly.com

Manifold Vacuum Experiment

I've been noticing a slight stumble off idle and reduced fuel economy on my 75 F250. 400 engine, T18 4 speed transmission and Dana 60 with 3.73 gears. So I started checking it out and quickly found a bad vacuum advance canister. Got a new one and got it installed this morning. I decided to try...
www.forabodiesonly.com www.forabodiesonly.com
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I read through your thread a few days ago (great info!), as I'm gettin' back into my ignition and timing curve stuff again. I'm sure you remember chatting with me about it a couple years ago, which I greatly appreciated.
Man, did life ever get busy and crazy and challenging for a good long while. I barely had any time to even drive my car, much less work on it. I caught a break this Spring to get back on some stuff with my car. I should be receiving a curve plate and springs kit from FBO Ignition tomorrow so once I get my initial and total dialed in, I'll be trying to add in some vacuum advance on top of that. I added a vacuum advance limiter screw in the side of the dizzy a couple of days ago also but it's still plugged off for right now.
 
MobileCustoms said:
I read through your thread a few days ago (great info!), as I'm gettin' back into my ignition and timing curve stuff again. I'm sure you remember chatting with me about it a couple years ago, which I greatly appreciated.
Man, did life ever get busy and crazy and challenging for a good long while. I barely had any time to even drive my car, much less work on it. I caught a break this Spring to get back on some stuff with my car. I should be receiving a curve plate and springs kit from FBO Ignition tomorrow so once I get my initial and total dialed in, I'll be trying to add in some vacuum advance on top of that. I added a vacuum advance limiter screw in the side of the dizzy a couple of days ago also but it's still plugged off for right now.
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I decided to cheap out with my current distributor and welded the governor slots in the distributor instead. lol I filed them to achieve only 5 at the distributor which is 10 at the crank and it worked out really good. I said cheaped out, but the FBO plate doesn't go down that far, so I really had no choice. Slant sixes don't like much more than 30-32 total and mine likes about 20-22 initial without the vacuum can attached, so I had to weld the slots. It really surprised me that it still ran fine with the vacuum can on manifold vacuum at about 40 initial. In fact, it cleaned up the idle a little more than it was. Of course, all that extra drops out on acceleration.
 
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RustyRatRod said:
I decided to cheap out with my current distributor and welded the governor slots in the distributor instead. lol I filed them to achieve only 5 at the distributor which is 10 at the crank and it worked out really good. I said cheaped out, but the FBO plate doesn't go down that far, so I really had no choice. Slant sixes don't like much more than 30-32 total and mine likes about 20-22 initial without the vacuum can attached, so I had to weld the slots. It really surprised me that it still ran fine with the vacuum can on manifold vacuum at about 40 initial. In fact, ir cleaned up the idle a little more than it was. Of course, all that extra drops out on acceleration.
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I have the original distributor that came with my car, which I had welded up the slots to limit the advance before the engine rebuild to a stroker, though I don't remember what that number was. I may end up revisiting that setup before all is said and done. I believe that one has a vacuum can, if memory serves me.
Learning lots as I go!
 
Softened the curve a bit from 1500-2500 . All in by 3k @ 3200.
Played with the VA as well . My idle vacuum is 9” and light cruise 15” . Higher speed max it out around 18” . I have an 8.5 VA can and set it to come in around 10”.
Next up is more carb tuning . Idles in low 13s afr. Cruises in the low 14s but rises to 15-16s if I creep into the throttle. Iam thinking this may be the PV . I will go up 1 to a 9.5 and see what happens .
 
mbaird said:
Softened the curve a bit from 1500-2500 . All in by 3k @ 3200.
Played with the VA as well . My idle vacuum is 9” and light cruise 15” . Higher speed max it out around 18” . I have an 8.5 VA can and set it to come in around 10”.
Next up is more carb tuning . Idles in low 13s afr. Cruises in the low 14s but rises to 15-16s if I creep into the throttle. Iam thinking this may be the PV . I will go up 1 to a 9.5 and see what happens .
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How did it feel driving with the curve softened?
 
mbaird said:
Softened the curve a bit from 1500-2500 . All in by 3k @ 3200.
Played with the VA as well . My idle vacuum is 9” and light cruise 15” . Higher speed max it out around 18” . I have an 8.5 VA can and set it to come in around 10”.
Next up is more carb tuning . Idles in low 13s afr. Cruises in the low 14s but rises to 15-16s if I creep into the throttle. Iam thinking this may be the PV . I will go up 1 to a 9.5 and see what happens .
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Didn’t your engine go across a dyno?? If so, where did peak torque happen?
 
Also changed my pinion angle today .
Pinion was up 10* and trany was up 2* ..wtf ?????? Pretty sure beer was involved in that . Lol
I got the pinion to 2* down and installed an adjustable snubber .
We will see how she drives now . Might have to tweek the angles a bit more .
 
mbaird said:
Softened the curve a bit from 1500-2500 . All in by 3k @ 3200.
Played with the VA as well . My idle vacuum is 9” and light cruise 15” . Higher speed max it out around 18” . I have an 8.5 VA can and set it to come in around 10”.
Next up is more carb tuning . Idles in low 13s afr. Cruises in the low 14s but rises to 15-16s if I creep into the throttle. Iam thinking this may be the PV . I will go up 1 to a 9.5 and see what happens .
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Make sure your transfer slots are not over exposed. Cause if they are that will cause rich idle and lean as throttle is cracked open.
 
340sFastback said:
Make sure your transfer slots are not over exposed. Cause if they are that will cause rich idle and lean as throttle is cracked open.
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Idle bypass air is a key element in tuning idle without transfer slot exposure. An easy way to check you bypass air is to pull a vacuum accessory like power brake port and cover it with tape. Poke a 1/16 hole in the tape and close the throttle blades a bit. AFR cleans up. repeat with 3/32 and increase with 1/8 all the while dropping throttle blade angle until you achieve best afr and idle. Check your corrected vacuum and install correct power valve. One you have the diameter of idle bypass figured out you'll know how big of an orifice must be drilled into the throttle blades, or you may open the secondary blades slightly with the set screw in the base plat mounting flange.
 
Illahe said:
Idle bypass air is a key element in tuning idle without transfer slot exposure. An easy way to check you bypass air is to pull a vacuum accessory like power brake port and cover it with tape. Poke a 1/16 hole in the tape and close the throttle blades a bit. AFR cleans up. repeat with 3/32 and increase with 1/8 all the while dropping throttle blade angle until you achieve best afr and idle. Check your corrected vacuum and install correct power valve. One you have the diameter of idle bypass figured out you'll know how big of an orifice must be drilled into the throttle blades, or you may open the secondary blades slightly with the set screw in the base plat mounting flange.
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I opened the secondaries on mine to correct transfer slot alignment. Now I can breath when I stand behind the car whens its idling.
 
340sFastback said:
Make sure your transfer slots are not over exposed. Cause if they are that will cause rich idle and lean as throttle is cracked open.
Click to expand...

Illahe said:
Idle bypass air is a key element in tuning idle without transfer slot exposure. An easy way to check you bypass air is to pull a vacuum accessory like power brake port and cover it with tape. Poke a 1/16 hole in the tape and close the throttle blades a bit. AFR cleans up. repeat with 3/32 and increase with 1/8 all the while dropping throttle blade angle until you achieve best afr and idle. Check your corrected vacuum and install correct power valve. One you have the diameter of idle bypass figured out you'll know how big of an orifice must be drilled into the throttle blades, or you may open the secondary blades slightly with the set screw in the base plat mounting flange.
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Great tip !
My carb is a custom built Pro Systems 850 so it already has bypass holes drilled into the throttle blades.
 
I skimmed through these posts and saw the timing and carb suggestions which are all good, but have you told us what torque converter you're running? A proper converter matched to your engine will make a huge difference in that bottom end 'feel'. Get that stroker up in the rpm range where it's happy.
 
CopperheadDart said:
I skimmed through these posts and saw the timing and carb suggestions which are all good, but have you told us what torque converter you're running? A proper converter matched to your engine will make a huge difference in that bottom end 'feel'. Get that stroker up in the rpm range where it's happy.
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His car is a 4 speed.
 
j par said:
It's the cam...
My last cam had a long duration low ls and wasn't worth crap under three grand but after three grand hold on for dear life...
Had the cam reground and a different animal...
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As the cam duration goes up you need to crank up the compression or you loose cranking and dynamic compression which is what keeps the lower RPM torque numbers up. With that cam I’m guessing without doing the math it needs another point of compression to really keep any low RPM torque. Cam selection, head flow and compression need to be matched to have an engine that works great across the band.
 
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