Team Camaro Tech banner
1 - 20 of 22 Posts

·
Premium Member
Joined
·
799 Posts
Discussion Starter · #1 ·
I have a 383 stroker motor in my 1967 Camaro. I am using a SS 750 Quick fuel carb on a Performer RPM AIR GAP intake.

I am currently sharing one vacuum port from the carb for the PCV and brake booster.

My brakes are sluggish still so I am thinking of getting a spacer for the car so I can fit a 90 degree vacuum port under the secondary pump.

I am worried that this may rise the carb too high up though and hit my SS hood.

Any ideas? Any suggestions? I do not want to go with a different hood. What is the best way to measure the clearance for my air cleaner?
 

·
Registered
Joined
·
13,314 Posts
If the cam is not too wild for street, then 1st look at dialing in the O2 and dizzy curve settings...once these are tuned or at least ball park in then vaccuum issues disappear.
u are looking for ball park
timing ball park starting point:
initial between 10 and 14 degs
Idle (initial+VA) 16 to 18 degs... higher if wilder cam
total (intial+cent) 34 to 36 degs all in 3000/3200 rpms
Timing all up (initial+cent+va) no more than 40/42 degrees without dialing in with knock sensor.
eg initial approx 12 22 in cent approx 8 in VA all in no more than 1" above best idle vacuum on initial , no VA attached and mixture screws`adjusted temporarily to best stable idle.
 

·
Premium Member
Joined
·
799 Posts
Discussion Starter · #3 ·
For this motor the recommended idle timing = 12 to 14
Instructions with this motor explicitely state not to hook up the vacuum advance on the ready to run MSD distributor.

total = 32 at 3,200

Idle is about 1300 rpm right now and the car drives just fine.

I am not sure about the O2 and dizzy settings? What are these? This is a very basic carbureted system.
 

·
Registered
Joined
·
13,314 Posts
motor explicitly state not to hook up the vacuum advance on the ready to run MSD distributor.
Well there is no such thing as a "ready to run dizzy....other than in the heads` of marketing propaganda BS deskjockeys..
We have an assme u also have a non egr pollution add on engine correct?
HEI and moden dizzies are set up ball park out of the box for lean egr type engines...and have to be messed aound with put more in the cent curve and take a`lot out of the VA.

Which is why they expiity state refering to a MSD (late model post early mid 70s,hei type dizzie

HEI/ MSD dizzies are NOT an upgrade, all they do is provide a bigger spark because leaner mixtures (egr) require bigger spark, and if one does so on a pionts dizzy the pionts dont last long

Bottom line thu...we spend a lot n money n body work, interoirs, engines then when it comes to dialing the engine in..have little understanding of timing mixtures, cant be bothered, resent finshing the project off and the money...and wonder hy they often dont run as` well as expected, be it idle, power economy etc
 

·
Premium Member
Joined
·
799 Posts
Discussion Starter · #8 ·
The engine builder actually curved the distributor. The engine was dyno tested and broke in with the 750 holly, the MSD distributor and the MSD box. The engine was delivered with everything on it ready to go.
 

·
Registered
Joined
·
8,154 Posts
I ran power brakes, PCV, and transmission vacuum from one port (the one on my carburetor) using a T with multiple branches, While not the optimum set up I never had any issues with any of it.
That was on my old 406 with 250 @ .050, 108 LSA solid roller, timing locked out at 34* and an HP 950 carb.
It makes me think possibly the tune needs to be dialed in a bit closer on your carburetor, I would think you should be able to make enough vacuum to support that from the one 3/8 port. You never know though, all motors are different.
How much vacuum does it pull at idle or in drive?
 

·
Registered
Joined
·
10,515 Posts
What is the best way to measure the clearance for my air cleaner?
Get a big lump of modeling clay and work it til it's soft. Set it on the high point of the air cleaner lid and gently shut the hood. Open the hood back up and measure the thickness of the clay.
 

·
Registered
Joined
·
8,339 Posts
The problem is not from running on one port. You either have a bad check valve in the brake booster or other brake related issue, or just not enough idle vacuum for repeated brake application at idle speed.

Any time an engine builder sells a street engine and says not to use vacuum advance, it's because either they don't know how to tune one for it OR to protect themselves if you install an improperly tuned setup and detonate the engine. Also I've seen lots of crate engines built and dyoned with an HEI (any brand) and vacuum advance disconnected, mainly because dyno tuning only sets the wide open timing curve and every HEI I've seen regardless of maker has a can with over 20 degrees of advance in it which is not correct for a typical performance timing curve. Heck, my 400 was originally dyno tuned with such an HEI but I installed a limiter plate in the vacuum can and adjusted its rate.
 

·
Premium Member
Joined
·
799 Posts
Discussion Starter · #14 ·
Engine is 383 CI stroker

10.3:1 comperssion
3.75 stroke

Rocker ratio 1.6

CAM duration 227-241
CAM Lift 547-531

Dart 200 heads
Springs PBM 3425

IGN timing: 33

I have not measured the vacuum yet. I can do that tonight.
SS 750 Quick fuel carb
 

·
Registered
Joined
·
8,339 Posts
That's within kissing distance of my 400, a little bigger cam though. I get around 15" idle vacuum at 14 degrees initial timing plus 10 degrees vacuum advance. 30 degrees total mechanical advance. I'm going to guess your vacuum is going to be around 10-12" which should be plenty to run an 11" booster although repeatedly hitting it at idle may show up as a hard pedal. Pop your booster check valve out and see if you can blow through the side that the hose connects to (like blowing into the booster). You shouldn't be able to.
 

·
Premium Member
Joined
·
799 Posts
Discussion Starter · #16 ·
I have that engine married up to a Tremec T56 6SPD Magnum and 3:73:1 gears.

I just had magna flow exhaust installed. Is it worth the few extra bucks to get an X-pipe? I was told it does not help that much with a carburetor fed engine.

I know the power brake booster works correctly. I just pulled out the old motor 2 weeks ago. The old engine had 2 vacuum ports. I was using the one on the intake runner for the brakes.

I do get the hard pedal feel for sure. Almost like having manual brakes.
 

·
Registered
Joined
·
8,339 Posts
Is it worth the few extra bucks to get an X-pipe? I was told it does not help that much with a carburetor fed engine.

That....is ridiculous.

So right now do you have a few good brake pumps and then it goes hard only when idling around?
 

·
Premium Member
Joined
·
799 Posts
Discussion Starter · #18 ·
My brake booster is in perfect shape. I had it professionally rebuilt a few years ago. It was working perfectly 2 weeks ago before I swapped out the old motor for this new one.

My car is idling at about 1000 RPM. I could probably bring it down to 900 since I am running with a manual transmission.

I am going to add the adaptor plate tonight and see if I have clearance to put in the second vacuum line. If that fails, I will buy a vacuum canister.

BTW - my new motor is mated up with a Tremec T56 magnum six speed with close ration gears. I am also running a 12 bolt rear end with 3.73:1 gears.
 

·
Registered
Joined
·
13,314 Posts
I'd check vacuum first, that added port may not make any difference if vacuum is low for some reason.
yep and adding a spacer doesnt help.
I ran power brakes, PCV, and transmission vacuum from one port (the one on my carburetor) using a T with multiple branches, While not the optimum set up I never had any issues with any of it.
Think about it...the engine draws a vaccuum...goes everywhere, so long as` there are no leaks it reaches it total lack of nothing, and what is in (or not is lol) in the booster is trapped there till u press on the brakes.. and even then takes 2 or 3 presses to 'use it up...
Which is why the above comment is also correct... thu it is best practice to separate...and saying that there are many multi ported fittings for manifolds that have brake + lots of others.

12" vaccuum AT idle is more than sufficient... because above idle the vaccuum increases.. so if u run out of vaccuum at idle.. u are either rolling the car or stopped...Think about it
And when u are going to use the brakes u have the car moving.. engine brake on huge vaccuum...
So unless u are idleing across a petrol station for court and use the brakes more than 2 times it doesnt matter... and if u do on that real occasion hit the brakes more than 2x u are just using manual brakes... at less than walking speed...

The isse of idle vaccuum, regardless of tuned in whqat ever, unless u have a`wild cam, too wild for street... the vaccuum is a`timing/ mixture issue... assuming correct PVC and no massive vaccuum leaks...Period
 
1 - 20 of 22 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top