Just wondering if we need 18 lbs of vacuum or will 10 do it. The Blueprint 350 341 hp engine has 18 and the 350 390 hp and up only has 10. Will I have to run any kind of vacuum booster if I go with an engine that only has 10?

 

Do you have a link to the 390 engine you’re looking at? The one I’m seeing on Blueprints website says 12 inches at idle, which people run with no issues.



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With my 383 I had 10" of vacuum and no problems.

 

I just looked again and can't find where I saw the data that said 10 inches of vacuum on 390 hp BP engines and up. Looking tonight like I said I can't find that info, in fact they are 12 or 13 inches of vacuum. Has anyone had vacuum issues like headlights or brakes? I wonder because some people are going to the electric headlights, which might be because of loss of vacuum when using the bigger cams and separation of 110 deg or less. I don't want to choose an engine and then have to put a vac booster on or change headlights to electric. I'm asking because I'm sure there are guys out there that know better than I do so educate me please.

 

You'll be fine. Guys change over to electric headlight doors because they don't know where to get quality replacement parts or they just don't understand how the vacuum systems work.
Silly I know.

 

12 is about minimum for decent brakes.

 

There is a 383 410 hp with 14.5 vacuum. Which is the closest to the 350 341 hp with 18

 

JMO: 195cc intake port volume on a 350ci/341hp motor is probably limiting both intake charge velocity and manifold vacuum; velocity matters.

 

With only 10" of vacuum, you need to be aware of fuel metering signal to the idle and transition circuits of the carb you are using. The idle circuit discharges fuel from an orifice below the throttle plates as a result of manifold vacuum - not as a result of airflow through the venturi and the Bernoulli Effect. If you change the manifold vacuum from 18" to 10", your carb will lean out grossly, and it may not have adequate flow capability through the idle circuit to have adjustability range to maintain idle mixture at the poor manifold vacuum spec. Some carbs are designed to handle this. Others are not. You will also have problems with ignition vacuum advance - you may not be able to meet the "2-inch rule" requirement for vacuum advance, which will have a significant change requirement on your ignition timing specs in order to get the engine to idle acceptably.

Lars

 

Lars makes some good points. For a street driven engine. Regardless of displacement. Cam selection that pulls down vacuum that low isn't really the best choice. I run a fair bit of cam for a street driven car. Idle vacuum is 14 -14.5 inches.
Seriously, a Cam more rowdy than this becomes a real pain in traffic. As well as the issues mentioned above.

 

Exactly. A lot of this is driven by camshaft LSA. Get it too tight in a street engine without enough compression, and drivability and tunability will suffer... Those "Thump-Yo-*****" cams are typical of this...

 

And the young guys just love that crap. And there horrible to drive at low speed on the esplanade (beach front) or parking lots or just slow and heavy traffic. Just terrible.
But they idle with a thump, thump, thump!
Cams for *******.

 

All excellent points. Follow the advice now so you don't have to start another post later on how to fix it.

 

I agree completely with the above posts.

I ran a large 254* solid cam in my LT-1 for 27 years.
(A 240* hydraulic would perform the same.)
It idled at 900 with only 10.7" of vacuum.
If I idled it any lower, the engine made so little power, using the PS would stall the engine, in a parking lot.
It was the DZ302 or 63 Fuelie 30-30 cam with 114 LC.
Honestly it was kind of a pain on the street.
The low end TQ was pretty light, and it did not come on very strong til 2700rpm plus.
Which meant 1500+ to come off a stop light.
I raced it a lot, and it would rev hard to 6700 rpm, so that is why I left it in there.

But I never had trouble with the power brakes.
And I autocrossed it and used the brakes hard.
Past idle it has a lot more vacuum than that, and the brakes work fine.
The booster stores a couple brakes worth of power.
The only time I ever even noticed was putting around in a parking lot, and using the brakes a lot.
It would actually change the way the engine idled.
I even ran the valve lash loose, at .040", which killed some duration, but actually made it faster, and drive better.

For a street cam, you would be well advised to run 230* or less, and have 12-14-15" of vacuum, at idle.
And a "normal" 110-112* LC.
It will run better, everywhere , except maybe 6300 rpm plus.

Oh and changing the idle a 100rpm, can add or subtract an inch or two of vacuum, with a cam like that.
Which may be why you are getting different opinions on that.

The Blueprint 350-390 has a much smaller cam than I had, and much better heads.
So similar HP, just at a little lower rpm. Say 6200 tops.
Yep dyno curve says the party is over by 6 thou.
I'll bet the idle vacuum is 12" -14".
But it only has a a 9.2CR, that is kind of light.
That cam would like 10CR much better, it would run much better.
They are being very conservative.
That thing might even run on regular gas.
In a light car like a C3 10:1 would be much better.
It will make the throttle much more responsive.

 

I run an automatic trans so upping the idle is not a good idea for my setup. I'm really looking at the 350 390 hp with 14.5 vac. All the HP I'll need to cruse with and it does have a nice idle non rump cam with 115 deg. I like that combo for me and thanks everyone your input really dialed me into what I need. Oh and yes they say it will run on 87.

 

Yes I think you'll be very happy with that.

 

FWIW my 427 small block has about 11 inches of vacuum and I don't have problems with vacuum operated accessories. I would definitely run a little less camshaft if I were running an automatic.

 

Yeah you will be real happy with that one!
It has a cam much like a 350-350HP L46, one of my favorite all around SBCs.
Great power from idle to 6000 rpm redline.
A little less compression to run regular gas.
And much more modern better flowing heads than that old motor to up the HP by 40, with zero driveability drawback.
You could easily drop the idle to 700 ish or so for an auto.