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Cam and vacuum and power disk brakes

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Dano383

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Discussion starter · #1 ·
So just how much manifold vacuum needs to be there for power disk brakes? Throughout the entire RPM range or just idle?

I'm considering cams for my 350 that I'm rebuilding and wanted to visit this question before I pick one. I wanted to ask you guys on here, but then also contact several different cam companies on the issue to see what the contributing factors are. (I'm all about learnin')

My set up:
69 Camaro relatively stock
TH350 Auto (stock, but still need to pick the convertor and rear gears)

350 block
0.060 over (freshly done last week)
3.48" stroke
stock 5.7" rods
Speed Pro hyperutetic flat tops (+6.10cc; 1.563" compressions distance)
Dart Pro 1 heads (180cc intake runners, 64cc chambers) -uncut
Dual Plane Edelbrock Performer (used, not the Air Gap)


Looking for hyd. roller.


Currently looking into the Comp Xtreme Energy 12-433-8
(288 int /294 exh adv. dur.) (0.52 int/ .54 exh lift) 110deg lobe separation
(intake closing at 70deg. ATDC.
http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=161&sb=0

Dynamic compression is around 7.73:1 (10.09:1 static) w/ my deck height and 4.125 x 0.39 head gasket.

Opinions? Suggestions?

Thanks a bunch!
-Dan
 
#5 ·
So just how much manifold vacuum needs to be there for power disk brakes? Throughout the entire RPM range or just idle?
Click to expand...
you just answered your own question....u only have low vac at idle and WOT right
At both those times how often would u pump the brakes to loose the stored vac on the power brake unit?
What speed would u be doing at those low rpms?
I had low vac for many yearts, daily driver.
The only times I noticed was idling out of the petrol pumps, doing so once in a blue moon, touching the brakes sever times before reaching the side of the road..at less than walking speed..hand brakes just they where manual.
 
#6 ·
just finished up putting pwr front. disc brakes on my sons 67. he has same situation, 350 w/cam. first time i tested brakes they worked fine.
pushed pedal again and it was very hard to push. answer, not enuff
vacumn. put a vacumn cannister on--presto, great brakes. note,
don't know specs on his cam, but it has a pretty rough idle sound.
thats my experience and .02.
 
#7 ·
pushed pedal again and it was very hard to push. answer, not enuff
Click to expand...
As I described above...where u atationary or moving at an idle speed?
Because if u where going faster, or slowing down for a cnr or interction u have heap of VAc...And if you drive with an un car vaccuum gauge...whicg All my cars have you will know this.
 
#8 ·
You won't make much vacuum with that cam, probably around 7" at best.
As soon as you blip the throttle, the vacuum will shoot up, as others stated, when you're idling around in a parking lot they will get stiff as if you turn off your modern vehicle with power brakes and push the pedal a few times, they get stiffer and stiffer, but will stop, just over a longer period of braking and much more difficult to depress.
I say, if it becomes a problem, add a canister as others said it has worked, if it gets real bad, just give it some gas while braking to make up for the vacuum you've lost.
When you're braking from highway speeds to a stop light, you should have plenty of power assist.
 
#10 ·
A liittle over simplified...
A pump makes vaccuum..
A engine is a pump..it sucks mixture in...
If u restrict the air /mixture gong in u get a vaccuum...suck.
This is controlled by the butterflies and the valves
When the butterflies are WOT zero to 1.5"
If at real low rpms much depends on ther valves duration and overlap in the cam making the engie very ineffient... As the rpms increase, the time the valves are open reduces and becomes more efficient as it reaches its power rpm range
 
#12 ·
You boys have it backwards. There's more (most) vacuum present at idle and cruise
Click to expand...
?????
Idle most? well as I said above it depends on the cam...high overlap duration creates very little 'pump effect from the engine because of working out of the range of efficiency

Cruise, again like idle depends on the rpms at the time and how far into the effiency range the cam is....Generally a mild cam idle Vac will be a little less than than high way cruise....increase cam highway gets better
But Where Vaccuum realy gets high is engine brake...an engine that doesnt see 18" under engine brake it will see 22" even 26".

When an engine does have low idle vac, this is where one keeps the low intial...fireup advance, and adds extra advance with the VA.
From memory the 302 had about 8deg intial and an idle around 18 deg...Cent around 36 deg..on cast heads and leaded higher octane fuel
Modern heads allow for less advamce, modern fuel more advance, and lower compressions more advance....So overall on balance a modern engine with higher performance cam/heads et... at idle should require slightly more idle advance (intial+VA) and in the power range less total (cent+ intial)
This will give better idle vac.
Regardles of old or modern or type of engine bore/stroke ratios etc.....one is aiming nor piont of max explosion arounf the 15 to 18 deg ATDC.
Its a balance
you have a short block on the ebgine stand...#1 is top dead center, u push down on it (as if u are the piont of max explosion.)
It will not move or bloody hard
Turn over a few degrees ATDC..it mover easy but still hard
Turn till 1/2 way down the bore...turns the crank real easy
The 15 to 18 deg range is a balance as to having the explosion in a more confined space, more pressure on the piston and having less of an intense explosion and pist move eay well down in the bore.

Do not think the 15 to 18 deg is near enough...thats just the range...when u press down there is a huge change in diffuculty in that range, and u nes that explosion bang on...
 
#13 ·
Steptoe said:
?????
Idle most? well as I said above it depends on the cam...high overlap duration creates very little 'pump effect from the engine because of working out of the range of efficiency

Cruise, again like idle depends on the rpms at the time and how far into the effiency range the cam is....Generally a mild cam idle Vac will be a little less than than high way cruise....increase cam highway gets better
But Where Vaccuum realy gets high is engine brake...an engine that doesnt see 18" under engine brake it will see 22" even 26".

When an engine does have low idle vac, this is where one keeps the low intial...fireup advance, and adds extra advance with the VA.
From memory the 302 had about 8deg intial and an idle around 18 deg...Cent around 36 deg..on cast heads and leaded higher octane fuel
Modern heads allow for less advamce, modern fuel more advance, and lower compressions more advance....So overall on balance a modern engine with higher performance cam/heads et... at idle should require slightly more idle advance (intial+VA) and in the power range less total (cent+ intial)
This will give better idle vac.
Regardles of old or modern or type of engine bore/stroke ratios etc.....one is aiming nor piont of max explosion arounf the 15 to 18 deg ATDC.
Its a balance
you have a short block on the ebgine stand...#1 is top dead center, u push down on it (as if u are the piont of max explosion.)
It will not move or bloody hard
Turn over a few degrees ATDC..it mover easy but still hard
Turn till 1/2 way down the bore...turns the crank real easy
The 15 to 18 deg range is a balance as to having the explosion in a more confined space, more pressure on the piston and having less of an intense explosion and pist move eay well down in the bore.

Do not think the 15 to 18 deg is near enough...thats just the range...when u press down there is a huge change in diffuculty in that range, and u nes that explosion bang on...
Click to expand...
I can't argue for or against any of this because I can't understand it. Also, your engine may be the only Chevy which experiences "max explosion" ATDC (After Top Dead Center).
 
#14 ·
Also, your engine may be the only Chevy which experiences "max explosion" ATDC (After Top Dead Center).
Click to expand...
Think about it All internal combustion enfines have to ....max explostion BTDC and it will push the piston back down too early and reverse the engine..if it doesnt break the crank 1st
To fire TDC like I said a piston TDC is impossible to push down..the engine will not fire.
What I think u are doing is not considering it takes time in msec to fire the spark plug, intialy ignite the mixture and evenrtually have the flame cross and explode the mixture....ATDC !
With an at low rpms the time needed is short...hence timing is closer to tdc
As rpms increase this time has to be 'extended by firing the spark plug earlier,
 
#15 ·
Steps could benefit from taking in a engine builder 101 course..but he does make a valid point.Lighting off the mixture and completing the burn prior to atdc, results in negative work and detonation issues.
 
#16 ·
I think you are way over-camming this engine.

Try to keep the intake duration at or below 220 and the exhaust 230.

Otherwise you will need a loose converter and 3.73 gears minimum.

JMHO. :beers:
 
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#17 ·
I think you are way over-camming this engine.
Try to keep the intake duration at or below 220 and the exhaust 230.
Otherwise you will need a loose converter and 3.73 gears minimum.
Click to expand...
That is the most practical and sensible bit of advice...the biggest mistake most Rodders make early in the their designs is over building a street/ strip engine.
 
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Steptoe
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