Is my engine low on vacuum????????????? Help?????

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duster360

duster360

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Engine specs first.
340 ci
10.2:1 compression
Edelbrock Rpm heads ( box stock no porting)
Edelbrock Air Gap intake
Holley 650 DP
20-227-4 - Xtreme Hi-lift Hydraulic Flat Tappet Camshafts

What kind of vacuum should I expect with this cam at idle?

My vacuum gauge is showing 7 in. of vacuum at 800 RPM with 14* initial timing. What are your thoughts?
 
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I'm no expert but try a little more initial and see how much it changes. Seems a little low to me.
But the cam is meant to operate 2000 to 6000. Cam degreed in per specs?
 
add at least 6 more degrees initial and see where it's at. idle speed should increase. reset idle keep adding initial until the vacuum no longer increases, then back off a degree or two.
 
DesertRat said:
I'm no expert but try a little more initial and see how much it changes. Seems a little low to me.
But the cam is meant to operate 2000 to 6000. Cam degreed in per specs?
Click to expand...

Supposed to be degreed, I had a professional shop build and dyno the engine.
 
duster360 said:
Supposed to be degreed, I had a professional shop build and dyno the engine.
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I hate to tell you but the sad fact is there are some shops who don't actually know how to degree a cam.

What do the dyno sheets say? And did they give you a build sheet which shows where the cam was installed?
 
quit chasing your tail ! get the timing and idle speed up where it wants to be and the vacuum should increase.
It's not the time to question the cam, yet. when there is more in the tune.
 
yellow rose said:
I hate to tell you but the sad fact is there are some shops who don't actually know how to degree a cam.

What do the dyno sheets say? And did they give you a build sheet which shows where the cam was installed?
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No build sheet. Dyno sheets just have HP and TQ numbers.
 
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Just curious how much hp and torque did you make. I had a slightly similar engine done that made 10" or vaccum at 750 rpm and 14 and 2600
 
There could be a few reasons for it. Start with what they've said. Add timing. See what it does.
 
Well, the link to that cam in Summit didn't tell Me any more than You provided, but the math in my head says 61deg overlap and My gut says they want it installed @106deg.
That's not a candidate for a good idle vac @800rpm. As already stated above, getting the initial timing up to best vacuum in tandem with the correct throttle blade opening
will maximize what You've got, but without knowing where the cam is installed................just FYI, I've seen 340's w/the 292/.508, a 6pak, and less than 10:1 idling ~1K &
they pulled 9" there..........................
 
Killer6 said:
Well, the link to that cam in Summit didn't tell Me any more than You provided, but the math in my head says 61deg overlap and My gut says they want it installed @106deg.
That's not a candidate for a good idle vac @800rpm. As already stated above, getting the initial timing up to best vacuum in tandem with the correct throttle blade opening
will maximize what You've got, but without knowing where the cam is installed................just FYI, I've seen 340's w/the 292/.508, a 6pak, and less than 10:1 idling ~1K &
they pulled 9" there..........................
Click to expand...

Here is a link to cam card at comp cams.
20-227-4 - Xtreme Hi-lift Hydraulic Flat Tappet Camshafts
 
OK, the card shows IO@32degBTDC & EC@30degATDC, so 62deg overlap & yes, 106 centerline.....4deg advanced......is that where they installed it? For reference, the
Purple Shaft std.292/.508 cam is [email protected] w/76deg overlap & rec'd installed on 108deg centerline.................You should have more vacuum, and prob. need a bump
in idle speed to 9-950..........................whenever I install a cam, I always "cheat" the centerline advanced a tick of the target, for the simple fact that it is usually done
w/o all the valvesprings loading the chain & only being rotated by hand. During running & as soon as the chain starts to relax, the CL will "come in" to it's target......
 
Killer6 said:
OK, the card shows IO@32degBTDC & EC@30degATDC, so 62deg overlap & yes, 106 centerline.....4deg advanced......is that where they installed it? For reference, the
Purple Shaft std.292/.508 cam is [email protected] w/76deg overlap & rec'd installed on 108deg centerline.................You should have more vacuum, and prob. need a bump
in idle speed to 9-950..........................whenever I install a cam, I always "cheat" the centerline advanced a tick of the target, for the simple fact that it is usually done
w/o all the valvesprings loading the chain & only being rotated by hand. During running & as soon as the chain starts to relax, the CL will "come in" to it's target......
Click to expand...
I hope so. They had the cam card at the time.
 
I did pull the initial up to 16 and the vacuum came to 10" to 11". It wants more initial but I already put the the 18* mechanical advance in the msd distributor. Which is the lowest msd makes. But I do have 10* and 14* bushings on the way from FBO ignition. Then I will tune some more.
 
^^^ this. I have 19 initial in a bone stock 85 318 ex for cam advanced 4 crank deg with a timing chain replacement. Now I would degree it.
 
My 340 combo is close to yours except I run the XS274S solid version and J heads. Cam installed at 105 ICL. I run 30*/35*/48* ported vac. Idle in nuetral 1000 rpm 12", in gear 850 9", tight converter. Lots of folks will say that it's to much timing but I tested enough and that's what the motor wants. Get a magnifying glass and read plugs as you put timing in it.
 
duster360 said:
I did pull the initial up to 16 and the vacuum came to 10" to 11". It wants more initial but I already put the the 18* mechanical advance in the msd distributor. Which is the lowest msd makes. But I do have 10* and 14* bushings on the way from FBO ignition. Then I will tune some more.
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Too bad, I have them for a lot less than FBO charges. CNC machined as well...
 
I may call and ask just to be sure. They handed me the cam card back when I picked up the engine. I am assuming they did because they build some pretty fast race engines and have for 50 years.
 
I would have expected 11-12 in of vacuum without advancing the ignition timing. Where did your idle RPM's end up with the change in timing? If at 850-900 now, then the vacuum increase is just due to that IMHO. If still at 800, then good.

For reference: My son's 340 is almost identical, with the same heads and intake and 10.0 SCR: 268 Crane cam and 112 LSA and slower ramps so the overlap is less, which helps the vacuum, so it is a bit different there. It is at 14" at 800 RPM with 15 initial. Cam timing is at 107 ICL. We had a lower vacuum initially (11"), but fixed the PCV and also fixed the secondary stop screw setting and that brought the idle vacuum up to the 14" range.

What PCV do you have? Do you have headers?

Also, are you racing this on the strip or just cruising? I ask because with your SCR and the cam and other parts, you're gonna have plenty of low and mid RPM torque, and I'd not be pushing the initial very hard with that SCR, unless I was trying to wring out just a bit more time on the strip. But, I suspect with your engine, getting the tires to hook up with all the torque that you already should have (and that the dyno torque numbers says you have) is a much bigger deal, and putting earlier timing into it just makes that harder to manage.

So understanding what you are wanting to achieve here is important.
 
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nm9stheham said:
I would have expected 11-12 in of vacuum without advancing the ignition timing. Where did your idle RPM's end up with the change in timing? If at 850-900 now, then the vacuum increase is just due to that IMHO. If still at 800, then good.

For reference: My son's 340 is almost identical, with the same heads and intake and 10.0 SCR: 268 Crane cam and 112 LSA and slower ramps so the overlap is less, which helps the vacuum, so it is a bit different there. It is at 14" at 800 RPM with 15 initial. Cam timing is at 107 ICL. We had a lower vacuum initially (11"), but fixed the PCV and also fixed the secondary stop screw setting and that brought the idle vacuum up to the 14" range.

What PCV do you have? Do you have headers?

Also, are you racing this on the strip or just cruising? I ask because with your SCR and the cam and other parts, you're gonna have plenty of low and mid RPM torque, and I'd not be pushing the initial very hard with that SCR, unless I was trying to wring out just a bit more time on the strip. But, I suspect with your engine, getting the tires to hook up with all the torque that you already should have (and that the dyno torque numbers says you have) is a much bigger deal, and putting earlier timing into it just makes that harder to manage.

So understanding what you are wanting to achieve here is important.
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1. Idle is 900 now. The vacuum bounces around between 10'' to 11''.
2. Should I expose the Transfer slots in the secondaries. I know when I square them in the front venturies, I lose that because I have to turn the idle screw and expose them even more to get the right idle speed.
3. PCV is just whatever the parts store handed me for a 340. I am now starting to realize that doesn't always work in the fine tuning world, which is what I am trying to learn now. I grew up in a world where you just set the timing by ear adjust the idle speed and let it go. If the gas fumes burn your eyes, oh well. Lol. But those were also mostly stock engines.
4. I have hooker competition headers.
5. Just a street cruiser.
6. As for torque, I have not had this engine over 4000rpm yet but I did get it rolling in first gear ( manual trans ) up around 15 mph and nailed it, started to boil the tires but I let out of it before the rpm's jumped up. Gonna be a fun toy when I learn more about tuning.
 
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Hee-hee.....#6 is to be expected from your combination LOL. Good deal on that part.

I suspect that the increased vacuum is just from the increased RPM's with the added timing. If you put the timing back and turned the idle screw up, then I bet you would have the same vacuum.

Yep, probably time to do a very different PCV. Do you have the PN that was given to you? The stockish Mopar PCV's are wide open to the higher cruise flow opening at your level of idle vacuum and typically putting too much air into the intake. It makes the vacuum low and messes up adjustments.

Try a Mopar Performance PCV valve # 4343581. Also the Fram FV191 works for your size cam but has a large base. The objective is to get the right PCV that will close down to the low idle air flow level when you have idle vacuum levels.

Do that first and see where your vacuum is. It is important to get the PCV working right before other tuning adjustments.

As for the secondary transition slots, you may have a to close the secondaries a bit to start; that is what we had to do with both a 675 and a 600 cfm Holley on our 340 engine. The point is to get the secondary transition slots completely out of the picture, so that they are not adding any fuel to the intake at idle. Then the primary idle and mixture screws will have complete command over over idle operation, and the primary transition slots will control the part throttle, off-idle response, as they should.

And as for your use, I personally would not go too aggressive on the ignition timing. The higher compression ratio that you have makes up for a lot of what strongly advance timing does for a lower compression engine, and you are not after every fraction of a second on the track.

The cam timing just has to be assumed unless you want to go back into the front of the engine. It can't be far off, if at all, with the TQ and HP numbers that you list.
 
I looked at the PCV and the part number is 2103. Rear venturies are completely closed. I have to expose a lot of the front transfer slots to get the idle up. At the moment I have 16 initial 34 total timing
 
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I took a long shot and googled "2103 PCV" and came up with this cross-reference listing:
http://www.teamlucor.com/stores/generaldocuments/Novo 2006 Catalog.pdf

So it crosses to a Mopar 3671076, which looks like a plain-jane Mopar PCV, and I suspect it is wide open at idle in your engine, and not at the lower idle flow level. BTW, if it is open to the wide open cruise flow at idle, you can pull the pcv from the valve cover at idle, still connected to the hose to the carb, and if it has a loud 'whooooooosh' all the time, then it is wide open. In that case, put your finger over the open end and the vacuum should jump up a couple of inches.

The 4343581 crosses to a 2193 in the same catalog, and that shows it is for some mid 80's cars & trucks. Odd....Here is a thread that discusses the FV191 and 4343581; just filter through the nonsense LOL:
Good quality PCV valve

OK on the secondaries being all closed; I was just making a guess based on what we found. If excess PCV flow is closed down, then I bet you can/should open that secondary up. You'll know you are doing good if the idle mixture screws are more effective. It may take a few tries to get the secondary transition smoothed out.
 
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