Hello. I have been on here only a short time but noticed there are quite a few knowledgable advisors and help on here. Ive been a 3rd gen owner since '95. The car is now just a weekend fun ride and toy. I love it!! I had a question about engine vaccum and how much is typical? What determines the overall amount of vaccum on a given motor? General combination and condition I assume. I want to know what I should expect from my steup or a similar setup. Also how does the vaccum react to changes in timing and/or carb tuning? How doea the vaccum look when accelerating and at WOT? I appreciate the input! My setup is a 355ci. , Edel PerfRPM cam : .488/.510 and 234/244, Add for 1.6 roller rockers, SportsmanII heads cast and unported, Edel PerfRPM #7101 intake, BG Demon 750 mech sec carb, MSD billet dist/6AL/Blaster2/8.5s , 10.2:1 CR, 700R4 w/3200 stall and can get you anything else you need to know from gaskets to bolts. THX

 

Hey! That's a good question! I mean, you've given a lot of info to let us help ya, and it's a thorough intelligent question! Very refreshing

Many things can effect the amount of vacuum your engine creates, camshaft duration (advertised) is the largest factor.

A big cam, with other intake restrictions (small head intake ports, small intake, small valves, small carb etc) can increase the vacuum, basically by having a restriction. The higher CR you have (10:1) helps a bit as well.

More timing will give you more vacuum. Higher RPM, with restriction, will give you more vacuum. So a 1200RPM idle with 24* timing should give TONS of vacuum vs a 750RPM with 4* timing. Oh, and you normally adjust your carb's idle mixture for best vacuum. That basically tells you when you're at the "peak" power. Highest vacuum means the engine is burning the fuel most efficiently and making enough power to try to raise the RPM, and create more desire for more fuel (more vacuum).

You've got a big cam, which is also an old inefficient design. It's no wonder you're asking this question, it's a common malady of that EXACT cam (in all it's many 'skins'). It won't give you much vacuum, i'm not sure how big your sportsmanII heads are, (200cc?), so I wouldn't expect you to be able to get over 10" idling in park. Less in gear eh?

At WOT you *should* have 0" vacuum. The only thing that might make you have more, means you've either got too small of a carb (probably not your case) or some other restriction in the intake. Secondaries not opening all the way, etc.
Your idle might be 7", as you take off with mild throttle, lets say 1/3-1/2, your vacuum will increase linearly. Deceleration on the highway, engine braking will max out your vacuum. 25" maybe. That's with throttle blades shut (large restriction in intake), and large demand of engine (higher RPM, say 3000RPM). High engine demand, large restriction = more vacuum. That's why with a big cam, engine braking helps give more vacuum to your brakes to help you brake

Not sure what timing curve you run, but a MUCH more aggressive one is pretty much needed with that cam. Either lots of base timing and a short mechanical advance, or better yet, locked out at 36* all the time might be a good idea. A starter saver addon to your ignition setup would work to aid in starting it.

I'm hoping you're running vacuum advance on your distributor? An adjustable vacuum can would be needed to set up the vacuum advance to work well with a weak vacuum signal, but an added 10* timing at cruise helps give a bit better efficiency/vacuum/cleaner plugs.

Hope that clears things up.

 

Sonix good answer , and those cam profiles were outdated when they came out in the 80s, stepping up the to lunati voodoo cams or the comp extreme energy cams would make a lot more power and help drivability quite a bit.

 

Thank you for the detailed insight. Just what I was looking to find out and some I was familiar with. Ive been trying to tune the carb with the help of BG Tech over on another thread. So far Im going in the right direction, but am awaiting futher instructions. I had checked out timing and vaccum while doing the carb steps. Which was what sparked these questions. Right now, I have 10-11inHg of vaccum in park at idle with 18* of timing initial and looks like 40* at full adv. I remember giving it a quick curve with the lighter springs but will need to re-check that. Its a MSD Billet that plugs directly to the MSD 6AL. It has no vaccum adv. on it. I need to re-check but I remember the vaccum falling under a load to 6-7inHg when in drive at idle. Also note its an auto tranny and idle is 950RPM at park and 650RPM in gear. After I get done with the carb tune, Ill probably have a higher idle speed. Problem is, if its too high, I get a violent transfer between park and drive gear which cant be good for the tranny. Stall on converter is approx 3200 but I am not confident of that. I need a good 32-3500 stall converter in there. And yes, the Sportsmans ar advetised 200cc. Dist is MSD #85551. What do you guys think on the timing curve, initial and full. I want drivability and some track time. Thank you for being so helpful =)

 

haha, whoa, I guessed damn close for your vacuum

You probably don't need 40* timing at WOT, with that high CR that's a little dicey. ping ping. Shoot for 35-36*. I think your base timing is a bit low, you've probably got the throttle blades open a fair bit to get it to idle. Your off idle is probably a bit mushy? (no EGR with your setup right?)

F-bird always advises this, so i'll pass it along - Just give it a try locked out. Grab some tiny zip ties (temporary) and remove the springs, and carefully zip tie it locked out to full advance. You have to put the "buckle" of the zip tie in a certain spot (down?) so it won't interefere with anything. This will give you 36* timing all the time. Drive that around for a while, and see how you like it. It'll probably have LOTS more midrange and be more fun to drive.

Vacuum advance helps keep your plugs from loading up with crud from highway driving. If it's not an option to add on, then you're kinda stuck. But just a suggestion. If you don't like the locked out timing, I think your distributor has the ability to use little bushings to shorten the mechanical advance travel right? Try 24* base, 12* mech for a total of 36* at WOT. That's actually what I run with only 224/230 on my cam, and it seems to run best this way.

 

Yeah the MSD came with different stop bushings. I think 21* is the stocker and what is still in it. Based on what I see anyway. 40* sounded too high to me too. After I get through the carb tune, I will start to fart around with the timing more. BTW the MSD dist has a built in lock-out which you can do by unhooking and rotating the top plate to a stationary hole. I prefer not to run full advance all the time. Wont that eventually kill the starter without an additional part? I have Autolite #24s in right now but they are temp for tuning the carb right now. They were used for a short time and clean enough to read while I go through the carb tune. I had posted some info back and forth with BG Tech on the thread titled "anyone guess my carb" if you want to see what has been done so far. I am awaiting a reply from him to go futher. 750 is too big but he said could work. Once I get the richness out and keep from fouling the plugs, Ill get a better feel for the timing thing. But I can see that a less total advance and a higher initial is prob. the way to go as you advised. BTW, I looked into the Voodoo cams from Lunati. Seems like they have a narrower power band than this one I have. Mine is advertised around 1500-6000/6500 or something like that. I want low-end drivability but have been impressed with some hi-RPM pulls I have done with this one. I killed a Supra Turbo one time at hwy speed rolling start and want to keep that option open.

 

hmm, no. The advertised powerband is pure marketing BS. Trust me, a similar voodoo (the one with 233* intake duration or thereabouts, 60104?) will run circles around what you have now. It'll be faster, more vacuum, and better low end. Worth $200 for cam and lifters? I think so, the amount you save in wasted gas will be worth that in about 1 seasons. 1 oil changes of kms lets say. ( I can prove this with numbers if you like)

Yea, it's harder on your starter to run the timing locked out. I just said try it temporarily. If it works noticeably better, you can get that little part to save your starter. (a starter saver - the name is self explanatory).
F-bird'88 showed how to wire up your own version cheaply.

Or run a reasonable amount of base, and a short mech. I really think locked out would give the best performance, but a close 2nd place would be the 24ish* base and 12ish* mech advance.

I'm not a BG or Holley tuner, so I won't dig into your other thread. But I think doing your timing first might be a good idea. Once you set it up right, you'll have to go back and do the carb. Timing will effect what you need from you carb, but carb won't effect what you need with your timing. Know what I mean?

 

OK..I will try to find those MSD bushings and set the dist. timing tomorrow. The carb tuning was to improve the over-rich conditioon which was fouling the plugs. I am working on getting the PCV operating. Ive had two breathers and no PCV. I heard this is causing the plug oiling and I have oil burn off fumes from the breathers when motor gets hotter...like after running a while. I just found out my vibration problem was from a driveshaft and am about to elimnate that issue. I have a tranny problem now too. I think the 2-4 band is gone. While I have some time and its kinda down, I want to eliviate the carb, timing and PCV issues. It would be a while B4 I could get another cam, so I must make the best of what I got. THX for the input and anymore is welcome =)

 

I found out that I had replaced the bushing with a smaller one which caused the wider advance from base the full. I put one in it that allows me to run the highest initial without going too high on full. I have 35* set full and the smallest rate 18* bushing in it which starts at 17* initial now. Vaccum is at 12.5-13inHg now and idle seems better. It still needs carb tuning which I am working on. I think my idle needs to be higher....any input on that? Oh I also changed to a little bit slower advance curve since initial and full are closer. But ill have to wait to get the tranny working properly B4 I give it some road test to determine best setting.

 

wait, before you said you had 18* initial and 11" vac, now you have less initial, only 17*, and 13" vac? I'm guessing you cleared up the idle mixture and idle speed?

Are you serious, that smallest mechanical advance you can set is 18* travel? I figured that'd be in the middle of the road. I really think the car would work better with closer to 24* timing at idle. Or, if it's easy to lock it out, do so. *Just to test it out*. If it runs a lot better, then you can figure out a way to go in the middle of the road (17->36, so, ~25*?) and use 11* mechanical travel or something.

You should have all your mechanical advance in by 3000RPM. That tends to run the best.

Idle around 750RPM in gear should be fine. I don't drive an auto, so that's not my specialty. Xpndbl3 might have some advice, I think he's ran a cam in that size range before...

 

My setup is 900 out of gear and 750 in gear and I don't have any issues with it trying to die out at stoplights. I was sure MSD sold a bushing that would limit advance to 12 degrees, so 24 initial and 36 total. Maybe check with them on that. Use a light spring and a medium spring or else the dual light springs might not be strong enough to close the advance at idle and cause the idle rpm to be twitchy.

 

It had the 2 heavy springs in it OEM from MSD. I had put the 2 light springs in it at one time. I was ahead of you in putting the light silver and med blue combo in on the mech. advance. I will fart around some more with it.