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Holley 390 CFM Info


EZDUZIT

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Hey guys, My brother needs some help that I can't provide. Does anyone here have any experience with the Holly 4barrel set up? We have got everything hooked up and have the car running. The problem is adjusting, I called Holley with our problem and they told me it sounds like the powervalve is too big. He said it comes with an 8.5 which is probally too big for a 2.8 liter. He said we need to find a flow meter, find out what idle cfm is then divide that number by two and thats what size powervalve to run. We can't find anyone witha flow meter! Do any of you guys know how many CFM's a L28 pulls idling? Thanks in advanced, Josh

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I have never heard of that but then again you learn something new everyday. You could calculate the cfm very easily based on rpm and displacement. You need a vacuum gauge hooked to manifold vacuum and check it at idle and part throttle. The powervalve that you have will open at 8.5 inches of vacuum and add fuel. if it has a stumble and you get low vacuum at part throttle then perhaps a higher number power valve like a 9.5 or 10 will help. I belive that they make them all the way to 11 or 11.5

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If Holley said the valve was too big, I assume that means the number is too big. I believe this means that the valve stays closed at higher than 8.5" of vacuum, ant that as the vacuum drops below 8.5 inches, the valve will open (At WOT for example) allowing a richer mixture. This then tells me that if Holley thought it was too big, then a smaller one (say a 6.5 for example) would keep the mixture a little leaner by not allowing extra gas until vacuum dropped even further.

 

So is your problem being excessively rich mixture. In this case, perhaps vacuum is not strong enough to keep the valve closed (at the times it should be closed), so a smaller number should be used to correct this? We need more information on what you base your comment, "The problem is adjusting".

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If it excessively rich, the go with blue oval, if it's lean then go the other way. In any event, you are going to have to buy a power valve or two. Find out excactly what power valve is in it, then put the vacuum gauge on and put it by the windshield somewhere where you can see it while driving. Notice the vacuum reading when you encounter stumble or hesitation, both may seem about the same if you arent' used to diagnosing this type of problem. This'll tell you what you need to know whether it is opening too soon or not soon enough. Try changing power valves one way or the other and repeat.

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Thanks for the input guys, I have played with it alittle. The problem is I have to lean it out to idle and richin it up to run higher rpms. If I leave the carb rich enough to run say 2-3k rpm and let it idle it will pour black smoke out. Then I turn the fuel screws in a 1/4 turn and it quits smoking as bad, but if you try to rev it up it hesitates (starves for fuel). This is what I told the Holley rep and then he gave me his answer. Now I know that the L28 will usually pull about -20psi at idle. But I don't know how much flow it pulls at idle. So I think the powervalve is the best starting place, then move on to tuning and timing! I have got to find somone with a flowmeter.

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You should be able to line this out with a simple vacuum gauge. If at idle you're at or below 8.5 inches of vacuum, then the power valve needs to be reduced to a number below this so that the valve stays closed until you open the throttle up under load.

 

But I wonder if the power valve is only part of the problem. Assuming you still have the OEM cam in place, and timed correctly, you should have a vacuum of well above 8.5" at idle, so this tells me you may have a bad power valve (ruptured diagram or one not sealing closed) or idle screws are not adjusted correctly. Hesitation when you open it up can also be associated with an accelerator pump/squirter that is too small, plus you may have too small a main jet sizing as well. Timing could contribute to this as well. I'd work on all this before looking for the flow meter. The flowmeter will help in getting it really fine tuned, but you need to get in the ballpark first.

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