Hey guys, I have a question about idle rpm and vavuum on a stock 70 455. I have q-jet from Every Day performance, a small body HEI setup per the power timing thread, and a Edelbrock Performer intake. Vacuum advance is set to manifold source. In idle in gear with vacuum advance connected, I have set 650 rpm by adjusting the idle mixture screws for max rpm and vacuum. In park that gives me about 840 rpm. However, the manifold vacuum I get is only about 14 in gear and 18.5 in park. I am concerned that my power booster is not getting enough vavuum, and that I might have a vacuum leak, though I have done lots of searching and found nothing in terms of a leak. Does my vacuum look too low for an old tired engine? I have seen conflicting posts about whats is adequate, and they don't always specify in or out of gear. Also, some sources say a booster needs 15 in and others say 18 in. Thanks!
The power piston is controlled by the spring underneath it. The spring is trying to push the piston upwards which lifts the primary rods out out of the jets, while the vacuum is trying to pull the power piston back down. The vacuum and the spring are working against eachother. So the spring needs to correspond with the idle vacuum in which the camshaft profile has a big part of dictating. 14" of manifold vacuum for a stock 70 engine is quite a bit lower than it should be. You probably already knew all of that. Lifter preload, valve sealing and internal leaks are just a few things that affect idle vacuum.
The 14" was in gear at 650 rpm, so that is indeed to low? Is there a good guide so tracking down valve/internal leaks?
Lower than optimum. What is your altitude? Before you go there, consider ignition timing. Wouldn't surprise me to see you have excessive timing advance at idle--or not enough. Once you're assured that the timing is where it needs to be, you'd look at cranking compression test results, leakdown test results, a cylinder-balance test, perhaps verify cam chain slack, and go from there.
^^^ What he said. There is fudge room on factory timing. Dampeners slip, too. One thing I see often is a bit too much throttle opening from retarded timing, causes exactly that. Chucking some air into a cylinder at TDC will tell you where any leaks are. Tailpipe, carb or valve cover.
Your above the threshold.. the 11" GM booster needs approx 10" of vacuum to function normally, 12" is that "cushion soft" feel. Lots of factors to consider here, one thing that jumps out is the use of manifold vacuum source for the vacuum advance. I assume you have a stock torque converter, but even so, that much loss of rpm indicates that your probably not completly out of the advance curve at 850 rpm. What springs are in the distrib.. if it's the medium tension silver ones that I have seen Dave use over the years, you should be fine there, I would suggest re-testing with ported vacuum as a source for your advance can. JW
Altitude is 1000' MSL. Total timing is 32, initial is about 12, with vacuum advance should be about 20. Jim, what do you mean by not out of the advance curve? Do you mean the mechanical or vacuum?
Mechanical. Do this simple test. Warm the engine up to normal operating temp- disconnect vacuum advance Set idle to 850rpm in park. Observe timing now slow the engine down, via the carb baseplate screw, and watch the timing mark... it has to stay steady. If it does not, put a heavier set of springs in the distrib. What is happening is if your still in the mechanical advance curve at 850 rpm, the load of putting it in gear drops that rpm... if the timing does not stay stable, but drops due to the lower rpm, then you get large difference between in and out of gear idle speeds. With the vacuum can on manifold vacuum, this would tend to inflate this issue. Try ported vacuum with the can. your within the testing specs at 1000 ft... that motor should be 17-21". I see 18" a lot on well worn Buick 455's. That's at 750 rpm, stock base timing, all stock engines. So your a couple inches low maybe, but I would not worry about it, if everything else is fine.
Ah NOW I am tracking! ou: That makes sense, I will check that out tomorrow. She is definitely worn. From the looks of the receipts I received from the last owner, the last rebuild was in 90's, and who knows what was done! She seems to run well enough but one of your 470s has been on my RADAR for a while. Are you still cranking those out? Thanks for all the help!
Yup, you have low vacuum considering the altitude. 1. Have you verified "Zero" on the damper at true TDC? 2. Total "all in" at what rpm? 3. You've restricted the vacuum advance to only 8 degrees? Are we sure that isn't 8 distributor degrees, 16 crankshaft degrees? 8 distributor degrees is not very much. Jim has already asked when the timing curve starts, and I'd have probably forgotten about that.
Sure built 6 of them last year.. and the blower motor that is on here is fundamentally a 470 with .015 bigger pistons, a bit deeper dish, and a girdle. JW
I verified true zero by using the paper towel wad method in the #1 spark plug hole. Total all in by 2,500 RPM. I didn't personally restrict the vacuum advance, but the guy who did has been building distributors a long time. He set it up exactly like specified in Larry's How to Power Time your Buick thread. That should be crank shaft degrees, since we talk about adding the vacuum advance to the total or initial advance to get the cruise or idle advance. The distributor runs at half the RPM of the crank shaft, so 8 distributors degrees would actually be alot! Am I getting that right? Jim, that big old supercharged motor sounds like a dang Merlin. Excellent work!
I don't understand how that verifies true TDC. 8 distributor degrees would be 16 crankshaft degrees. The vacuum advance below started out at 25 crankshaft degrees, I restricted it to about half that much.
You got me on the TDC, I was thinking you were asking about if I had indexed the distributor properly, which the paper towel method would do (just a way to find the compression stroke). It has been a while since I installed the distributor when I first got the car, but I don't think I did a very scientific analysis other than using a straw or something to see where TDC was for the piston in relation to the balancer. Thanks for the help.
Yup, sounds like Dave's favorite curve. 12* initial, 20* mechanical, all in by 2500 RPM or less. 8* vacuum advance for a cruise timing of 40*. I would check your vacuum reading with another gauge. The commonly bought gauges seem to vary greatly. I use a good Autometer gauge.
Larry, that would be it. Do you have that auto metet gauge actually installed in the car, or do you just use it by hand?
Jim, I did your test, and sure enough I do have a little advance in at 850 and I have the silver springs as well. I don't have the rest of the spring set so I guess I will have to source some somerwhere else since Crane doesn't sell the springs separate from what I can find. Thanks for the tip.
So, just a little update: I was messing around with the engine the other day, and found that my carb base gasket has a small leak in the front. I had checked multiple times for vacuum leaks, but never found anything. I finally used the water and spray bottle method, and sure enough if I saturate the front of the carb base the leak seals and my RPM decreases by 40-50 rpm. I am sure that is not helping things! I could not find any more leaks after plugging everything else. I also installed a new 11" booster from Right Stuff Detailing. I went ahead and removed my old one just to look at it since it was looking pretty ratty, and I had a bit of a hard pedal. Upon inspection, the input rod had a lot of sloppy play to it. I am pretty sure it had ingested some brake fluid at one point as well. Sure enough, after installing the new one, the pedal feels much better and I develop more brake pressure. Oddly enough, it does seem to take a minute or so for the booster to 'charge up.' I would assume that is more fallout from the low vacuum? I still think there is room for improvement, but when I get the new carb gasket and those springs from Jim that should help out even more! I still need to figure out the whole internal leak test... I heard that you can pug the dipstick and all vacuum sources and then feel for pressure over crankcase vent?
