Typical vacuum for a 400 ?? - Pontiac GTO Forum
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post #1 of 6 (permalink) Old 12-10-2017, 08:11 AM Thread Starter
 
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Typical vacuum for a 400 ??

I have set my timing and the car feels strong: initial timing 14 IIRC and total timing is 34/35. My problem is I then checked my vacuum at two locations and am only getting like 13" at idle. The car doe not sound like it has a cam in it, but based upon the heads (low compression 14s) and how it pulls, I am going to guess it has something special.

My question is what do these cars typically see for vacuum ? Thanks

Chris

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #2 of 6 (permalink) Old 12-10-2017, 09:24 AM
 
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very common to measure 16.5-18" of vac @ 1000-1200 rpm in a rel stock cam in a 400. The exception, a RA4 cam without Rhoades lifters, will see low vacuum numbers. by your earlier description of performance, the '68 400 most likely has some type of quick ramp speed cam, a modest XE or Voodoo grind. IF the engine had better flowing even stock big valve heads with proper valves springs, the extra port flow along with better suited cam would pull even harder up top.
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post #3 of 6 (permalink) Old 12-10-2017, 08:33 PM
 
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Chris,
I run a Comp Cams XE284, 507"/.510" and 284/296 deg duration in my 74' 400 with stock rebuilt#62 heads, 1.50" roller rockers. I measure about 6 to 8" vacumn at 850-900 rpm idle. Pretty rough idle hard to measure on my gauge as its bouncing around a lot but definitely below 10".
chuck
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post #4 of 6 (permalink) Old 12-10-2017, 09:04 PM Thread Starter
 
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Originally Posted by Pinion head View Post
very common to measure 16.5-18" of vac @ 1000-1200 rpm in a rel stock cam in a 400. The exception, a RA4 cam without Rhoades lifters, will see low vacuum numbers. by your earlier description of performance, the '68 400 most likely has some type of quick ramp speed cam, a modest XE or Voodoo grind. IF the engine had better flowing even stock big valve heads with proper valves springs, the extra port flow along with better suited cam would pull even harder up top.
Thanks! The idle is super smooth, such that it really has me guessing if it could have an aftermarket cam.

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Originally Posted by clevelandpartsguy View Post
Chris,
I run a Comp Cams XE284, 507"/.510" and 284/296 deg duration in my 74' 400 with stock rebuilt#62 heads, 1.50" roller rockers. I measure about 6 to 8" vacumn at 850-900 rpm idle. Pretty rough idle hard to measure on my gauge as its bouncing around a lot but definitely below 10".
chuck
Thanks! If I can't get more vacuum, I will likely purchase an electric vacuum pump.

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
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post #5 of 6 (permalink) Old 12-11-2017, 09:45 AM
 
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Chris,
yes I've been thinking of getting an electric vacuum pump and installing it this winter, obviously my power brakes are really hard to brake at low RPM. . let us know if you find a good pump and how it turns out, and ill do the same.

Chuck
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post #6 of 6 (permalink) Old 12-11-2017, 11:39 AM Thread Starter
 
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I will ! There are several options and kits available....I will likely go with an OEM quality (AC Delco or Hella) unit for better reliability, but Summit and SSBC both have "kits" with relays and switches already packaged together.

I will be utilizing brakes from an LS1 and will record pressure to brake booster and line pressure for anyone that is interested.

Chris

Quote:
Originally Posted by clevelandpartsguy View Post
Chris,
yes I've been thinking of getting an electric vacuum pump and installing it this winter, obviously my power brakes are really hard to brake at low RPM. . let us know if you find a good pump and how it turns out, and ill do the same.

Chuck

1965 GTO Convertible 3 speed tri-power drum brakes originally

Current setup:
400 cu in with tri-power
4 speed Muncie
cij911 is offline  
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