Okay, let me attempt to word this intelligently. Over the years, it has always been my understanding that the power valve should be rated at 2-inches less than what the engine pulls at idle (or in Drive, if it's an auto trans). My old engine always seemed to like that, but on Holley's website, they say to use a power valve rated at half the number that the engine pulls. So what do you guys think (what is your experience?). My old engine pulled 10.5 inches in Drive (remember, I am at 6000 feet, so it's gonna pull less up here), so I always ran a 8.5 power valve. My Street Avenger came with an 8.5, and ran great with the old motor. Well, this new motor, with the Comp Cam, pulls 7-inches in Drive, so I assume that the power valve is always open at idle, which can't be a good thing. So what should I do? Install a 4.5, which is 2-1/2 numbers below idle vacuum, or a 3.5, which is half of 7?... :confused:
I do the two below idleing in gear method. Also like to drive around with the vac gauge on and watch were the motor tends to drop to when stopping at lights or idleing along in a parking lot letting it just pull the car like in traffic. Just my way theres probally better but that works for me
I wish I knew more to help...I am running a Holley but i'm not at the tuning stage yet . It does intrest me to read about issues though. Hope you get an answer and/or get it fixed.
I would try the 4.5 for starters and see how it reacts.I have always used the 2 below method and it worked fine for me.But to contradict what i just said i have also ran a 6.5 pv in an engine with 12 inches of vacume and it still runs fine today.
I have about 9-10lbs vacuum and have used a 4.5 for years with no problems at all. My carb is an AED modified 750 Holley DP though.
For the most part I prefer the Edelbrock on a street car. Not trying to start a flame war. I have a Holley on my drag car. But,... The reason you run a PV with a lower setting is so it will open when the vacuum drops, i.e. when you open the butterflies. The reason for that is so you don't get a lean stumble. So, if it's not stumbling on initial acceleration then why do you care what the number is on it? Let the car tell you what it needs. What Holley is telling you is a rule of thumb, a good place to start. Generally the carbs are calibrated around a 350 Chevy at sea level. A 302 at a higher altitude will require less/more power valve, less/more accelerator pump shot, smaller/bigger jets, etc. Again, let the car tell you what it needs. Install one or the other, 3.5 or 4.5 and see how it performs. Then go from there.
Use the 3.5 power valve. You aren't going to notice any difference in performance and the extra inch is worth a tiny bit in fuel economy. The real reason is that at altitude you will lose vacuum faster than if you were at sea-level and you will constantly be running in the valve at times when you don't need it. (as my memory is as old as I am - I have been known to be wrong) I like the idea of following the recommendations of the manufacturer - especially Holley - nobody has more experience with their stuff than they do.
power valve You stated the issue very well. answer is easy - get both... start with the 4.5 and see what happens. How do you adjust the carb air bleeds? with a vacumn guage? Are the plugs fouling or does the exhaust show a rich condition? Is it loading up at idle or low speeds? I would think the 4.5 would do the trick if the carb is otherwise properly adjusted.
The plugs look they way they should, and the engine idles fine ('sounds great too). It does seem a bit sluggish off-the-line compared to the old original worn-out engine. 'Not what I expected at all. I am running 12-13 degrees of initial timing (just like I always did on the old engine. This engine has slightly over 9.5:1 compression (compared to 8:1) and a Comp Cam 268H (compared to the stock cam). It's also bored 040 over, has roller-tip rockers and a windage tray. I expected more spunk out of it. It has just slightly over 100 miles on it now. All I know is that the power valve is not supposed to be open at idle, but probably is, since it's pulling 7-inches with an 8.5 PV...
The accelerator squirters and pump cam have alot to do with the sluggish feeling. I have a black on Tammy's car witch correted that feeling.
I agree...I played with my pump cam and it corrected my slight stumble feeling. (about 11 years ago )
I have never run less than a 4.5, bit I have never lived at altitude as well. Requirements are leaner up there. While "half" is Holleys recomendation, I would start at the 4.5. I am surprised your carb has an 8.5. In my experience most Holleys seem to run around a 6.5 from the factory. My 390 has an 8.5, but that is an economy carb. Most 4160 600s I have run have come with 6.5 power valves. Putting a Comp 268h cam in moves your power up the rpm band, which is why they often recomend more aggressive gears and a looser converter. Running that cam with the same gears and converter to the previous set-up will seem softer off idle.
The sluggish acceleration could be due to a nice tight motor.... That worn out engine didn't have to fight any friction to get going. The cam you have in the new engine may make less torque off idle than the one in the old motor - that can make a world of difference. Does it seem to come on with a burst of speedaround 1500? If so you may just have a cam that wants to be higher in the rpm before it turns on the power. You could fix that by advancing the cam timing 2 to 4 degrees. That will shift the power band lower and cut off 600 to 700 rpm from the top too. I lost 1000 rpm and 10 hp when I advanced my cam 6 degrees. I gained a decent idle speed from 1200 down to 800rpm. It would have been a great drag race cam at zero center but not for road race and hill climb where you need that lower end.
