Thanks Ted:



The vacuum reading is at idle 700 rpm.



I opened the valves up wider however the clatter got louder than I wanted to hear. On my other car that has the same Isky E4 I found .017 was the best for clatter, vacuum to a good idle. Less than .017 lash had a terrible rough idle. Isky specs say set .015 lash.



On another note for to get good performance and idle I have had to advance the distributor lots (MSD) advance set at 18 degrees vacuum line unhooked. Running blue stop with heavy silver and blue springs. If I reduce the initial advance it is very sluggish at low end, heavy idle and if I advance more will ping. Total advance exceeds 40 degrees.



I am going to see if I can borrow a wheel this weekend and advance the cam as it seems to be the culprit.



P.S. Just re-read your post Ted on opening up the valves and checking vacuum. Will do that before tearing front down



thanks marv

What rpm is the 17”Hg vacuum reading being taken?

 

To reitterate what John says, advancing the camshaft will increase the intake manifold vacuum at idle.  The torque band will also come in at a lower rpm but how much lower depends upon the amount of advance.  Stated in more detail, advancing the camshaft closes the exhaust valve earlier during the overlap cycle which minimizes an internal engine vacuum leak and provides a net gain to the overall vacuum readings.  Advancing the camshaft also closes the intake valve earlier after bottom dead center which increases cranking compression and this in turn increases the torque being produced by the engine.

 

As Pete mentions, your cranking compression readings are low for your particular combination.  Without degreeing in your camshaft with a degree wheel, it’s questionable exactly where your camshaft is installed but sounds like it’s on the retarded side now based on the compression readings.   Loosening up the lash to 0.022”-0.024” and rechecking manifold vacuum should give you an idea of how much more vacuum is available by advancing the camshaft.  I’ve experimented with as much as 0.025” lash on the E4 camshaft in my 272.  Loosening the valve lash reduces the amount of overlap between the intake and exhaust valves and basically reduces the amount of vacuum leak through the exhaust port at the expense of exhaust scavenging at higher rpms.  I have the Isky E4 in a 272 with 8.0:1 cr and it pumps out 155 psi cranking compression with ECG heads and that’s with the E4 camshaft installed at 104° intake lobe centerline (cam is ground on 108° lobe centers).  Your 312 should be pumping at least as much if everything else is equal.

Lorena, Texas (South of Waco)

Yes there is a check valve at the booster.



I have not pulled the hose off after running the engine to see if still holding. The booster is new however still can be at fault.

Marv:

Is there a check valve in the vacuum hose connection at the booster?  I think there should be. If there is, and it is holding, then the booster may be losing vacuum internally while sitting at a light. 

John - "The Hoosier Hurricane"

The brakes work good when traveling, actually effortlessly. When coming to a light and engine drops to an idle the pedal does get heavier.



John: that info on improving idle vacuum is what I was needing as it will give me a light brake pedal at an idle. I was afraid I might be going to a heavier pedal at idle.



When I picked this Isky E4 cam it was the mildest Isky cam and the closest to the original 57 cam I could find so should work if I advance it and lower the power band, as well, as others have said advancing will improve the compression. I don't want to go to that B8A replacement camshaft as they are pathetic.



With the info I have gotten I think I should be close. Looks like this weekend has a project.

thanks everyone, anymore comments is appreciated.

marv

I think that cam should have the power start much lower. 2800 to 3000 almost makes me think its retarded, or maybe even the grind is incorrect. The compression pressure might also indicate that but it could also be that the rings haven't seated yet. Invest in a degree wheel.


Lawrenceville, GA

Marv:

I'm sure you already realize that you have too much cam for towing purposes, since you want to advance it.  Just think about the brake booster situation.  When you are using the brakes, the throttle is closed and the engine is revving.  Vacuum is highest then.  Cam position probably has no effect under those conditions.  As stated above, vacuum should improve slightly at idle with 4 degrees advance.

John - "The Hoosier Hurricane"

marvh

   I will guess and it is only a guess that advancing the cam will give more vacuum. You can check by adjusting the valve lash tighter and looser and see what that does for you. Also I think your compression at 135 PSI is too low. I would expect 155/165 at least.

I do not want to reduce the engine vacuum by changing the cam advance as 17" is just enough for the brakes to operate properly. Any decrease in vacuum and the brakes will be heavy to push.



When I hook onto the trailer the old 292 had better low end power, however once rolling this 312 is no comparison to the stock 292. I would like to get the power band lower to about ~2200-2500 rpm. Now it is about 2800-3000 rpm when the engine comes on strong. The torque converter is still the stock 62 cruise-o-matic solid case converter. I don't want to change that unless necessary

marv

I'm not sure how advancing the cam will affect vacuum.



17inhg should work fine. Do they not seem to be working right? Do you feel like you need more bottom end?


Lawrenceville, GA