Engine Vacuum and Tune Questions

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Engine Vacuum and Tune Questions

Post  SLord82 on September 28th 2017, 4:21 pm

I've been trying to tune up my recent cheap build and I've been encountering some weird behavior.

Build Info;
Used factory 1996 shortblock with unknown miles. Bores looked clean at the time of assembly
Ported Big valve DOVE heads
Lunati 31604 cam 275/285, Lift .550/.560 110LSA
Comp Magnum 1.7 rockers
Stealth intake
Demon 850 carb
Truck Manifolds
Duraspark recurved by The Mad Porter hooked to a Crane HI-6 box and coil

Its a pretty basic missmash of old/used parts but it was cheap and fun to slap together.

First of all, I can only get about 12" of vacuum from my build. I'm not sure if this about as good as I should expect from this combination @ about 1000 RPM idle speed, WITHOUT the vacuum advance hooked up. The vacuum gauge does fluctuate around 10-12inches though. Should I expect more total vacuum with this setup and/or a steadier needle reading?

Secondly, when I hook up the vacuum advance it goes from about 10* initial to about 26* and the engine goes from running a little rough to running pretty smooth. How much vacuum advance should I be looking for at idle? Is 26* about right or should I be looking for more? I haven't tested the total timing yet, I think the distributor was locked out at 20* of mechanical advance by Scott, but its been years since I bought this from him so I could be wrong.

Third, when i try to use a tee fitting to hook up both the vacuum gauge and the vacuum advance I get no reading and no vacuum advance. After playing around I found the tee I have and the extra vacuum hoses I bought at NAPA are too restrictive. Is there better or specific vacuum hose that I should have asked for? The short length of hose that came with my Harbor Freight vacuum gauge works fine.

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Re: Engine Vacuum and Tune Questions

Post  supervel45 on September 29th 2017, 11:16 am

SLord82 wrote:I've been trying to tune up my recent cheap build and I've been encountering some weird behavior.

Build Info;
Used factory 1996 shortblock with unknown miles. Bores looked clean at the time of assembly
Ported Big valve DOVE heads
Lunati 31604 cam 275/285, Lift .550/.560 110LSA
Comp Magnum 1.7 rockers
Stealth intake
Demon 850 carb
Truck Manifolds
Duraspark recurved by The Mad Porter hooked to a Crane HI-6 box and coil

Its a pretty basic missmash of old/used parts but it was cheap and fun to slap together.

First of all, I can only get about 12" of vacuum from my build. I'm not sure if this about as good as I should expect from this combination @ about 1000 RPM idle speed, WITHOUT the vacuum advance hooked up. The vacuum gauge does fluctuate around 10-12inches though. Should I expect more total vacuum with this setup and/or a steadier needle reading?

Secondly, when I hook up the vacuum advance it goes from about 10* initial to about 26* and the engine goes from running a little rough to running pretty smooth. How much vacuum advance should I be looking for at idle? Is 26* about right or should I be looking for more? I haven't tested the total timing yet, I think the distributor was locked out at 20* of mechanical advance by Scott, but its been years since I bought this from him so I could be wrong.

Third, when i try to use a tee fitting to hook up both the vacuum gauge and the vacuum advance I get no reading and no vacuum advance. After playing around I found the tee I have and the extra vacuum hoses I bought at NAPA are too restrictive. Is there better or specific vacuum hose that I should have asked for? The short length of hose that came with my Harbor Freight vacuum gauge works fine.

1.I would bump the initial timing up to 14-16 degrees. This should smooth out the idle some and help vacuum a little.

2.You are likely uncovering the vacuum port in the base plate that feeds the ported vacuum port on the side of the metering block. You want this covered by the throttle blade at idle.

3.Close the primary throttle blades enough to cover this port and open the secondary blades some to maintain your desired idle, and see how it does. About .020"-.040" is a good place to be on your primary transfer slot exposure.

4. Reset your idle screws for highest vacuum in gear.

5.Make sure your total timing is not too high, so as not to get into pre- ignition detonation problems.

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Re: Engine Vacuum and Tune Questions

Post  SLord82 on September 29th 2017, 3:21 pm

1. I'll give it a try.

2. I don't think the throttle blades are that far open. I had to open them up to get it to idle when cold without vacuum advance. But after leaning out the idle mix screws and the engine warming up I was able to close them back down a bit.

3. I might try opening the secondaries a bit if needed. Guess that depends on what it wants after bumping up the advance.

4. This is a manual trans car. I've been trying to max the vacuum at idle in neutral but the most I can get WITHOUT the vacuum advance hookup is 12", and that number fluctuates between 10 and 12. Maybe I can find a better tee fitting and hoses to so I can read the vacuum with the advance hooked up.

5. ok

I feel like a lot of these issues will be alleviated once I can read the vacuum with the advance hooked up. Thanks for the suggestions.

Does anyone else know if 12" is about the most I should expect from this cam/setup or am I just bad at tuning or maybe something is setup wrong or other?

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Re: Engine Vacuum and Tune Questions

Post  supervel45 on September 29th 2017, 5:02 pm

SLord82 wrote:1. I'll give it a try.

2. I don't think the throttle blades are that far open. I had to open them up to get it to idle when cold without vacuum advance. But after leaning out the idle mix screws and the engine warming up I was able to close them back down a bit.

3. I might try opening the secondaries a bit if needed. Guess that depends on what it wants after bumping up the advance.

4. This is a manual trans car. I've been trying to max the vacuum at idle in neutral but the most I can get WITHOUT the vacuum advance hookup is 12", and that number fluctuates between 10 and 12. Maybe I can find a better tee fitting and hoses to so I can read the vacuum with the advance hooked up.

5. ok

I feel like a lot of these issues will be alleviated once I can read the vacuum with the advance hooked up. Thanks for the suggestions.

Does anyone else know if 12" is about the most I should expect from this cam/setup or am I just bad at tuning or maybe something is setup wrong or other?

1. On a Holley baseplate you can see the ported vacuum hole on the passenger primary ventury in the front center of the throttle blade, the other hole lower down is the idle feed hole. You will have a lot of the transfer slot exposed if it is uncovered on a Holley. Barry Grant I have no experience with but, if it is drilled right I would think it is the same.

2. Always set the idle screws with an engine at normal operating temperature. Never cold or very hot.

3. Good idea. I would also try to avoid exposing the secondary transfer slots if it can be avoided.

4. Lumpy cams tend to be less stable then stockers and timing and fuel also play a role. Not many good ways on a manual car to load the engine by yourself either.

5. I would not expect a lot of vacuum with that cam, if you get 12-14" I would be happy. I also believe you may need to take some mechanical advance out of your distributor with the compression you likely have (9-10/1 ?)

6. Some people  like to run manifold vacuum for a vacuum advance source. I have never done it but, it may have a few advantages you may want to consider.

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Re: Engine Vacuum and Tune Questions

Post  supervel45 on September 29th 2017, 5:16 pm

PS: Vacuum gauges are good and all but, don't forget to use you ears and noise when fine tuning idle screws. Highest rpm at idle as well as smoothest idle, is also a decent rule of thumb sometimes.

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Re: Engine Vacuum and Tune Questions

Post  SLord82 on September 29th 2017, 6:01 pm

Sorry I wasn't clear in my first post, this is all manifold vacuum, not ported. I'm reading manifold vacuum from the carb and then hooking the vacuum advance to the manifold vacuum.

I want to find that happy spot in the tune where it runs smooth and crisp. But I'm a little new to tuning a carb so I'm not sure if I should expect more vacuum with this cam. I know several people have run this cam in the past and I was just hoping for feedback from them about their experience. It doesnt seem like a big cam so I was expecting more like 16" of vacuum.

I had a build a few years ago using these same heads, intake and carb but with a 'mild' solid roller that made 15 or 16 inches. However I didnt tune it, a friend tuned it for me.

Should I expect similar results from a used short block and heads and a smaller cam? I dont know


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Re: Engine Vacuum and Tune Questions

Post  colorado ford man on October 10th 2017, 9:36 pm

What is your elevation? that plays a role in vacuum, as well as how is your valve train geometry/pushrod length? Finally, is your carb/intake all sealed right? and finally check the air bleeds in that carb. There are probably changeable ones that could effect this as well as one being restricted.

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