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Old 11-12-2003, 10:00 PM
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Sb Chevy 400 Uneven Feel 2800-3000 Rpm

I have a 77 chv 3/4 ton 4x4 with 35s, 410s, th 350 with stock conv, and a sb 400 with 9.8 to 1 hyperutes, 993 heads with 194 ints, performer cam and manifold, stock exh manifolds going to 2 1/2 " pipes and glass packs, stock hei, and 74 pontiac 455 qjet. I set up the carb and dist to Cliff Ruggles recomendations (ABSOLUTE GENIUS!!). I pull 19" of vacuum at idle. This setup really runs. I have never had a sb powered rig pull throughout the rpm range like this one does, My problem is that I get an uneven feel in the engine at about 2800 rpm that goes away a little after 3000. It doesnt lose any power, but it drives me freakin crazy. It gets worse with reg gas, and even worse when I tow. The dist has 12 deg initial, 20 mech all in by 2700 rpm. Crane adj vac adv is set at 12 deg, in by 10" hooked to manifold source. I had to use both the heavy crane springs to get The curve to 2700. It was at 2500 with med and heavy springs. Do I need a slower curve with a rig this heavy? Is the uneven feel detonation? Also, would headers be beneficial to this setup? Im a first timer, so your input would be appreciated.

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Old 11-14-2003, 07:29 AM
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Now that is a thorough explanation of his problem, somebody help this guy!!!


Todd


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Old 11-16-2003, 11:35 AM
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Hey Sly,

Sounds like you have done some homework on your motor, the combination you have put together isn't half bad. You only have a couple of small problems to address to get rid of your "dead spot." The heads you chose give great low end and off idle response, only problem is that the same air ramps in the intake runners that give you this also cause an uneven fuel distribution problem in about the rpm range you complained about. What happens is that the a/f mix hits these ramps (which were by the way put there to break up the mix coming from a fuel injected setup, solving this delima in that format) and it actually falls out of suspension, first making a lean condition then a rich one when the fuel acummulates enough to pass the ramps and is all picked up by the air. This is probably the cause of it "running worse" on regular gas, lower octanes are a lot more suseptable to lean conditions. The best way to solve this is to sacrifice a little bit of your off idle performance by opening the bleed screws a wee bit (17 inches of vacuum works on my blazer with the same setup) and softening up the timing curve some, try the medium springs and let it come in a little sooner, then adjust the vacuum advance to back it up around 13 inches. Its either this or go to a head that's a little more user friendly to carbeurators. You can also remove the heads and do some grinding on the intake ports, just enough to straighten them out a bit, and if you do this don't forget to shine up them exhaust runners too (don't be afraid of doing this, we aren't talking about an all out racing engine where CNC machining is required, when you get in there you can see what you need to grind to make it work better). I'd also recommend a good set of 1 1/4 tube headers with long colllectors.
Hope this helps, it did for me.......
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Old 11-17-2003, 09:41 AM
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Thank you very much for the extremely intrelligent response. I have a set of 441 cast heads I was planning on trying to pocket port. Will give them a try soon. How does Changing the idle mixture help acceleration?
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Old 11-18-2003, 08:16 AM
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The stock HEI wont cut the grade.
The full timing should come in total 36*@ 3000 rpm on a 400.
rainmaker hit just about everything.
Site for porting.
http://www.sa-motorsports.com/diyport.htm
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Old 11-18-2003, 09:56 PM
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Hey guys, I fixed it! I put some heavier springs on to make the curve max at 300o instead of 2700, and the uneven spot went away!!! Is there any weight combo that makes 24 deg mech adv in an hei? I am running 13 deg initial now, and that is the most the starter can handle. This would allow me to get 36 deg total instead of 33. I am using 41 weights now. You guys have been great. This forum is really cool.
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