OK guys -

Been a little hesitant to throw this out, but here goes.

My 69 440 HP was initially rebuilt in 1982 by Mansur Machine (Some of you older guys may know that name).

Anyway, pertinent info
comp. 10.85:1 (advertised)
stock heads ported / proper 3 angle valve job
Edelbrock Torker / 750 vac followed by 850 DP
Isky cam and full valve train 280 duration, 465 lift (single pattern)
(Don't know spec at 50. cam card long gone and company doesn't make this kind of stuff any more) 1.5:1 rockers.
3000 stall converter

Here's the thing, the cam is fairly mild, yet the engine has a nice lope, and has pushed a 4,200 pound car into the high 12s @ 108 mph. (with street tires and a 3.23 rear). I'm going to assume the engine is fairly healthy.

The thing is it holds only 10 -11" of vacuum in gear and about 14" in neutral at operating temp.

So, the question is why the low vac? Even after a carb change and engine rebuilds (same specs) the vac readings have identical over a 30 year span. Yes, different vac gauges were tried, same readings, and no, there are no leaks.

I just see / read about much "heavier" cams with higher vacuums.

I know what can cause a low vac reading, but that usually equates to a less than efficient cam or configuration.

Probably should just sit back and enjoy it, but curious if you have any thoughts.

Thanks

Archer

 

IMO that is NOT a mild cam. if the cam requires that stall converter it is not a mild cam. to push a 4,200 lb car to 12.s with a 3.32 gear. you are doing something right. totally respectable.

 

The 10-11" is not terrible and the 14 is good.

Have you fooled with ignition timing to bring it up?

 

Duration & Profiles were different..... Idle quality suffered quit a bit with the old stuff. Not that we didn't like that.....

Now Days... If you run something like a Lunati VooDoo Cam you don't get the lope, But you get better idle quality and better vacuum!!!
CompCams Thumper is rite there also....

I like the old 3/4 race style cams.... Must just be the lope..

 

Guys -

Thanks.

Coronet -

The timing is optimally set by both vacuum and rpm. (10 initial / 34 total), about the most I can handle on pump gas, given the compression.
The difference in vac between idle and in gear corresponds to about a 200 rpm shift. If I'm in gear and slightly open throttle to make up for the rpm drop, the vac reading goes up as well.

Bob -

I know that profiles have changed a lot over the years, but there was always something a little freaky about this engine when it left Mansur's shop. Not sure if it will still turn those times after the last rebuild.

BTW - IIRC (and that means I may be off) the advertised rpm power band for the cam was about 3000 - 6000 (+/-). What we did was try to match all the components to "kick in" around the same time (rpm). When you flashed to 3K, you kinda knew it.

Yes, I am assuming we got the Isky 1.5:1 rockers. They may have swapped in the 1.6:1 rockers which would put me around a 282 duration and a .496" lift (their numbers). That might answer the question. I don't recall seeing any numbers of the rockers, last time I had the valve over off.

The guys at Isky are pretty cool, but this cam is 30 something years old, and I guess their records fell into the same void mine did.

Archer

 

I'm not sure, but I think that's a fairly hairy cam for a stock bore/stroke B/RB. I've always combatted poor vacuum with a quick stab of the throttle when approaching stoplights to get the brakes to work.

 

Many cams I have had never showed lift on the stamping, usually a part number or model represented by duration, which dosn't say much.

I would have a dial indicator on it in 10 minutes to confirm lift which should in turn confirm duration as the stamping. Best idea I can come up with.

 

Archer,

It funny you happen to bring this subject up. I was just on the phone to an old friend that I had built a Brand-X motor for last week... He was going on and on about a motor I had done for him some 20yr's back... What he wanted was the numbers for that cam. He loved that cam. Out of all the other ones that he had tried, they where not what he was looking for....lol

Long story Short.... I had an old used one laying on the shelf at the house. I'll send it out to American Cams to have it re-profiled for his motor...
And I'll send it to him for Christmas....

 

1............As stated, that is NOT a "mild cam."

2............If you can't get more that 10 initial into it with that compression and pump gas then that is the best you'll do

3..........I don't think those numbers are that bad


4...........WHY are you worrying about it? What is the REAL question?

Off idle launch? Boggy at bottom end? Power brakes? What?

 

440 -

I've been running the car that way since 1982.
I'm not worried about it, kinda like it actually.

The real question is "why?".

I don't believe the specs I gave are "that" far from the stock cam.
The car/engine performs well off the line, the brakes are fine, and I'm certainly happy with it.

Sometimes we need to know something to get a better result, sometimes we tend to over complicate things and sometimes curiosity gets the better of us.

As a said, with those specs a low vacuum typically means a hot cam or a problem, but I'm not seeing any, and that's where the "why" comes from.

Usually, I'd leave well enough alone, but figured it might make an interesting discussion.

Thanks

Archer

 

I think the question was why would it have lower Vacuum?
It's not like it's a 292* or 300* cam...

 

Have you got it timed that way because that's the curve in the dist (ending up with total)

or because it won't start well with more

or because it pings badly on bottom end?

 

My son's new "modern" cam is 290/300 adv. duration .496/.520" lift idles at 11 inches of manifold vacuum 650rpm 18 degrees of initial timing.

 

Coronet -

That was sorta my point, your son's cam should be "hotter" than mine, but we're running almost the same idle rpm (I'm about 700 in gear/900 in N) and vac. I do have less initial timing though.

As I said, it's nothing I'm worried about.
Just curious if any one knew anything about the old Isky cams.

Archer

 

Most of the old Isky stuff I remember was on a 108* lobe center...
Those of today are like 110*.. I think there slowing the closing down for the valve.

 

Bob -

Maybe I'm wrong, but the slightly tighter lobe sep should increase the vac.

Yes, a slower closing valve would decrease the vac, helping gases get out. Making the cam act "bigger"?

Wish I new the values at 50. That plus the lobe sep should tell the story.
However, I'm sure Mr. Mansur (the guy who did the original rebuild) had a few tricks up his sleeve. He kinda knew what he was doing.

Some of the younger guys might want to google Pierre Mansur

Archer

 

1..........Differences in timing curve and initial timing

2..........We have no idea where this cam is "degreed in"

 

True..440

 

Tighter LSA = more overlap, less vacuum.

Wider LSA = less overlap, higher vacuum.