what should vacuum be - 350 chevy, 9.3 CR, 236 @ .050 - Hot Rod Forum : Hotrodders Bulletin Board
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Old 06-24-2012, 09:00 PM
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what should vacuum be - 350 chevy, 9.3 CR, 236 @ .050

Im trying to slowly get my engine tuned just right.

With a 350 chevy
9.3 CR
236 duration @ .050 cam on 108LSA
base timing in the neighborhood of 20 degrees
what should me Vacuum be at idle.. at .. 1500rpm..?

trying to figure if I have a vacuum leak, and also if my vacuum isnt right, to adjust how rich /lean my carb should be..
please help, im new to the concept here


thanks

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Old 06-24-2012, 09:50 PM
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recurve the distributor to allow 24 to 26deg base timing at idle.

A short limited 10deg advance curve allows this. (26 +10 ='s 36 total WOT advance)
It should have about 6" vac at idle when idling in gear.
800-900rpm in gear.
A stock torque converter will not work. Get a 10" 3500 stall converter.

Reset the carbs throttle position at idle for .020" fuel transfer slot exposure at idle.
Pri and sec.
Use a 4.5" rated power valve.
The more idle timing you give it the nicer it will idle and the better the throttle response at low rpm.

May even like the distributor advance curve simply locked out.
34 to 36deg locked. You are on the border line for this.
give the motor all the idle timing it wants to idle clean.
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Old 06-24-2012, 11:36 PM
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Quote:
Originally Posted by F-BIRD'88
recurve the distributor to allow 24 to 26deg base timing at idle.

A short limited 10deg advance curve allows this. (26 +10 ='s 36 total WOT advance)
It should have about 6" vac at idle when idling in gear.
800-900rpm in gear.
A stock torque converter will not work. Get a 10" 3500 stall converter.

Reset the carbs throttle position at idle for .020" fuel transfer slot exposure at idle.
Pri and sec.
Use a 4.5" rated power valve.
The more idle timing you give it the nicer it will idle and the better the throttle response at low rpm.

May even like the distributor advance curve simply locked out.
34 to 36deg locked. You are on the border line for this.
give the motor all the idle timing it wants to idle clean.
Thanks again Fbird for your advice.
This motor loved the high base timing u suggested, but at 3rd or 4th gear, when id back of the gas, I was getting massive detonation. I have a 3,000 stall already.. is that not high enough and causing the detonation?


Quote:
Originally Posted by F-BIRD'88
Reset the carbs throttle position at idle for .020" fuel transfer slot exposure at idle.
Pri and sec.
Use a 4.5" rated power valve..
Ill give this info to my carb guy so he can set my carb up proper
its a rochester Qjet
and yea I noticed it idles pretty ****ty with low timing. THe higher timing, it sounds much better. I only changed it back to low initial because of the detonation.

is 6" vacuum in gear enough for my power brakes? or do I need a pump..?
thanks Fbird
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Old 06-25-2012, 01:35 AM
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Detonation ='s too much timing at max advance.

You cannot just crank in timing. You have to modify the distributor curve to limit the advance travel. 10deg. stock is 20-22deg.

limiting the travel stop to 10deg allows a high idle timing while avoiding excessive WOT max advance. WOT max advance should be 34-36deg .

Further a stock vac advance can will not work.
way too much vac advance, again can cause detonation

Qjet does not use a power valve. Uses a metering rod power piston and spring.
A light tension spring is required.

You have already been told in many previous posts how and why to set up the distributor properly.
But you are not willing to do the work.
Soon you will damage this motor from excessive spark advance @WOT.

This engine will not work on crap gas either.
Find yourself a pro that can tune your car correctly.

This cam is all wrong for the th700r4 OD trans and 3.55 gears and mild converter..

Last edited by F-BIRD'88; 06-25-2012 at 02:03 AM.
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Old 06-25-2012, 06:31 AM
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ocean-angler/f-bird'88---question

i have a 383 / 700r4 with the same cam spec. except 110°lsa.
i have 31" tires with 4:56 gears.
presently running a 12" 2500 stall, should i also have a 10" 3500 ?
oh, i have the 4150 holley 770 street avenger / 4.5 pv
my timming is as f-bird'88 suggested.


not to hi-jack this thread , i hope .

thanks ROGER
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