484 purple cam/Low vacuum

[Bill Monk]

440 6 bbl, 4 speed. car has the 484 Mopar purple cam and runs like a scalded dog.

The question is on the low vacuum. Timing is at 36 btc all in, checked for vacuum leaks and found none but the best I can get out of it at idle is about 7" of steady vacuum. I don't run power brakes so that's not an issue but is that normal for this cam? I know this cam doesn't give me a lot of vacuum but that seems really low

 

[sdconcepts]

whats your initial timing set at?

 

[Bill Monk]

I'm pretty sure it's at 14

 

[sdconcepts]

how much compression? you can go higher on your initial as long as cranking and starting doesn't become a problem. then just relimit the total advance based on your new initial. 7 mmHg is pretty low but not uncommon with that cam.

 

[Bill Monk]

I haven't run a compression test but I've played with the timing and 7 is as much as I can get period and she is advanced as far as I can go without starting issues

 

[ddpsnake]

THAT is normal,hell i had the 274 purple in my 360 .It had 9'' if you have a holley you will want to change your power valve to 4 not the stock 6.5

 

[wyrmrider]

7-Ive seen 5 that cam has medium (compared to long stock) ramps
enjoy
what gears
do run the compression test
what rpm does the cam kick in?
as post above says keep tuning
any pinging?
if compression is not over the top you can get a cam with shorter duration and more lift

 

[Bill Monk]

7-Ive seen 5 that cam has medium (compared to long stock) ramps
enjoy
what gears
do run the compression test
what rpm does the cam kick in?
as post above says keep tuning
any pinging?
if compression is not over the top you can get a cam with shorter duration and more lift

3.55s in the Dana
Kicks in hard above 2500 rpm
No pinging but I'm cutting some 112 race gas in a tank full of 93

 

[Bill Monk]

THAT is normal,hell i had the 274 purple in my 360 .It had 9'' if you have a holley you will want to change your power valve to 4 not the stock 6.5

running a six pack so yeah it has 3 holleys. I'm not having any issues with the center carb coming in. In fact the only issue I'm seeing is the reading on the vacuum gauge

 

[Cranky]

Did you degree in the cam or just dot it? Generally, advancing the cam will increase vac a bit and will produce your torque at a lower rpm and smooth the idle some. With the older style 28/484 I would always advance them around 3 or 4 degrees. Yeah, they are usually ground with some added advance but....

 

[Ranger16]

Maybe that's just normal for that cam.
My car had a hot Purple cam in it when I bought it (can't recall which one) and it only had 3-4" of vacuum. Made for some hard starts.

 

[Cranky]

It's been years since I built any engines for others and don't remember the vacuum numbers on them but vacuum can also vary depending on the rest of the build. I do remember they ran very good right off of idle and they didn't have much lope....much to the dismay of one guy. 'Axed' him if he just wanted to have a nice sounding car or wanted it to perform at it's best. He didn't say much more about the idle quality after it was broke in and running good. A buddy of mine has a car that doesn't do much until 3000 rpm and then it runs like a bat out of hell. Even though he knew I had a degree wheel, he just set it up using the dot method and has no idea where the cam is set. I'm figuring it's 4 or more degrees retarded lol

 

[Bill Monk]

Did you degree in the cam or just dot it? Generally, advancing the cam will increase vac a bit and will produce your torque at a lower rpm and smooth the idle some. With the older style 28/484 I would always advance them around 3 or 4 degrees. Yeah, they are usually ground with some added advance but....

I have no idea brother, I didn't build this engine, it was in the car when I bought it

 

[condor74]

The idle vacuum is related to how far you can keep the throttle closed while the engine is idling. Large cams with lots of overlap require the throttle opened further at idle to not stall. This can be overcome by advancing the ignition timing. With the advanced ignition timing the idle goes up and you can close the throttle some to gain back manifold vacuum. Although sometimes, as mentioned already, if you advance the timing It will kick back against the starter .
There are a couple solution if you want to run more timing at idle. Lighter advance springs are one. I have a small block that at idle has 30 advance of timing. It has very light springs and basically goes to full advanc at idle speeds. At cranking speeds the timing is quite a bit less.
Another option is if you are running an aftermarket ignition box such as an MSD 7AL. You can set it up to retard while cranking. Just a couple ideas

 

[Cranky]

I have no idea brother, I didn't build this engine, it was in the car when I bought it

Well, if it's lazy until 2500 and ignition timing, carb tuning etc doesn't help, the cam might be straight up or worse. The older 484 cams still ran pretty decent off idle iirc. It's kind of a hassle to check it but I've seen minor cam timing changes show nice results in ET's at the track.

 

[Cranky]

The idle vacuum is related to how far you can keep the throttle closed while the engine is idling. Large cams with lots of overlap require the throttle opened further at idle to not stall. This can be overcome by advancing the ignition timing. With the advanced ignition timing the idle goes up and you can close the throttle some to gain back manifold vacuum. Although sometimes, as mentioned already, if you advance the timing It will kick back against the starter .
There are a couple solution if you want to run more timing at idle. Lighter advance springs are one. I have a small block that at idle has 30 advance of timing. It has very light springs and basically goes to full advanc at idle speeds. At cranking speeds the timing is quite a bit less.
Another option is if you are running an aftermarket ignition box such as an MSD 7AL. You can set it up to retard while cranking. Just a couple ideas

What was your compression on the small block? Seems like the lower the compression is on any engine, the more timing they like. Had more than one engine that liked 50 total!! First time running into that, it really made me doubt my timing light until using 2 others to just make sure lol

 

[Bill Monk]

The idle vacuum is related to how far you can keep the throttle closed while the engine is idling. Large cams with lots of overlap require the throttle opened further at idle to not stall. This can be overcome by advancing the ignition timing. With the advanced ignition timing the idle goes up and you can close the throttle some to gain back manifold vacuum. Although sometimes, as mentioned already, if you advance the timing It will kick back against the starter .
There are a couple solution if you want to run more timing at idle. Lighter advance springs are one. I have a small block that at idle has 30 advance of timing. It has very light springs and basically goes to full advanc at idle speeds. At cranking speeds the timing is quite a bit less.
Another option is if you are running an aftermarket ignition box such as an MSD 7AL. You can set it up to retard while cranking. Just a couple ideas

I run vacuum advance with manifold vacuum for that reason. The timing is more retarded at start up but then immediately advances

 

[Bill Monk]

Well, if it's lazy until 2500 and ignition timing, carb tuning etc doesn't help, the cam might be straight up or worse. The older 484 cams still ran pretty decent off idle iirc. It's kind of a hassle to check it but I've seen minor cam timing changes show nice results in ET's at the track.

not so much lazy, it runs well off idle and at cruise it just really hits a sweet spot from 2500 on. Maybe I'm overthinking this, if it ain't broke don't fix it ya know

 

[condor74]

What was your compression on the small block? Seems like the lower the compression is on any engine, the more timing they like. Had more than one engine that liked 50 total!! First time running into that, it really made me doubt my timing light until using 2 others to just make sure lol

It is a 10.5 to 1 340. It likes a lot of initial timing. 34 degrees total. Advancing It more didn’t make the car go any faster. Retarding it just made it harder to idle. It is a bracket car so street drivability isn’t the concern. I just noticed at 30 to 34 degrees at idle it didn’t kick against the starter. Probably only 10 or so advanced while cranking.

 

[SlinktRR]

normal. I could only get 9-10" with timing and carb adjustments, idle at 1000rpm and advanced as much as possible without the starter kicking back. Distributor mech advance recurved with lighter springs. powerband doesn't start until 2500rpm with that cam so a 3000 stall converter and 3.55+ gears needed to really enjoy the car. i also had 8:1 compression so higher total was required (40 degrees total).