Advice on Engine Vacuum pls

Oldskoolrob

Oldskoolrob

Active Member
Hi! How much vacuum would you expect a standard 3.5 to pull at idle? Trying to figure out if a 6.5 is the right powervalve in the Holley. Cheers :)
 
My fairly healthy 4L runs at 40Kpa. Atmospheric is 100, so he is pulling 60 Kpa at idle. I thinks that's how it works :)
 
I can't remember the vac, its maybe 16 hg ? it is not very high. If the Holley is a 390 then I have one on a 3.9. Dreadful bog from the off, and also coming back on the gas. Made it nearly go away by fitting a 10.5 powervalve and messing with the shooter size and shot timing / volume. Took a long while.
 
I've got a Holley 450 (4160 model). I figure the primaries are smaller than a 350, and once set up the secondaries will only come on if it needs it. I will probably buy a vacuum gauge but as I'm on a shoestring budget and only need it for this reference, I hoped the interwebs might help. I'm surprised you've gone to a 10.5 but I know f-all about carbies and I'm still learning. Current set up is a 6.5 power and 57 jets. I think the pump is (standard) 50cc?
 
60kpa is kind of 17 inches hg so that might be about it -which would dictate an 8 or 8.5 (not sure what sizes they come in)? From what I've read if you up the power valve the jets should also be increased? I forgot that lumpier engines usually pull less vacuum than stock, so now getting my head in the game. I think mine has a slight cam, (I didn't rebuild it).

I do have a 2 barrel manifold but I think the 4 barrel is better?
 
It was years and years ago I was messing with the 390. What is your problem with the 450 ?
I cannot even remember the jets in the 390, I converted it to secondary metering block and fitted jets to the secondaries. It still has a tiny hesitation, but makes the engine rev to bursting, in fact I did burst it once after it was newly built, valve bounce bent a couple of valves and smashed a guide.
They are tricky to get right, you won't like the 2 bbl though.
The trickyness is the reason I went for an Autolite 4100 on the 4.6, a much nicer carb IMHO.
 
No real problem with the 450, I just want to make sure it's tuned to the engine as well as it can be. When I got it the previous owner had done everything to disconnect the secondary circuit which I'm almost finished fixing, and then I want to go through it and make it as good as it can be. I would also like to convert it to a secondary metering block, this one only has a plate, and seeing as it was supposed to be one of a pair on tunnel rams of a much bigger engine I'm not confident that the plate is best suited to my engine. But one thing at a time as funds are tight! It's been a good (but expensive) journey so far with this car, most things I've played with have stayed 'improved' so it's now at a point where I can hit the key and drive it as far as I want so it's time to tune it as best I can. The really funny thing with this car (maybe I'm growing up) is that the more reliable I get it, the less I think about the huge modifications I wanted to do to it at the start.....I'm now just seeing those modifications as a lot of gas money lol. I love driving this old gal, just as the 'survivor' she is.
 
There is always a balance between $ and enjoyment and final end value. I agree that when something is right there is an enormous amount of pleasure to be had. When I think back to when I was younger I was always working on things that didn't really need doing. Its like the old saying - if it ain't broke don't fix it. It is hard sometimes to stop fixing things until they are broke.
I could not begin to tell you how many hours were ploughed into that 390. Nowadays if it coughs a little I just accelerate through it, I am done 'fixing' it.
I never wrote things down back then, I always had it in my head, now I cannot remember things like jets etc. When I next go into the carb I will have to write a spec sheet.
 
Happened to trip across a cheap vacuum gauge ($50 new, got it unopened for $15 with an unused oil pressure gauge), so now I can check and start mucking with the carby. Doing my youtube research is doing my head in, but in a good way. So, in order should I tune Idle-Jets-PV-Secondary Spring-squirter? Is there a particular order to do it in?
 
Do write down settings on everything you have mentioned to log a start point.
You mentioned that there is no real problem with the 450, I always found with a Holley / Rover combo it is easy to make a problem. If the thing does not bog, and the gas mileage is good then maybe just a check over ?
You won't get jetting right without either a session on some rollers or by fitting an AFR gauge, sure you can do some runs and check plug colour, bit hit and miss.
I fitted the 10.5 PV on the advice of Real Steel, the supplier of the new Holley, the bog I had is typical of a 390 apparently and their way of getting over it, and I think with a 25 thou shooter and some fiddling with pump settings.
If you have a nice idle and no bogging, then its really down to if you have a continuous and seamless delivery of power through to WOT.
I used to run without a bonnet on and watch the secondary linkage come in, commonly they do not open until very late. As you no doubt know there is a spring kit available ( I assume you have vac secondaries )
The 4.6 has vac secondaries and the first run on the rollers they did not open until late, and even then did not open fully, the lightest spring cured that, and did it turn into a gas guzzler - No !
Enjoy your playtime :)
 
Cheers! It does bog down a little. I'll make sure I have the current settings logged before I start fiddling :)
 
So for the record I get 20"hg idle in park, 17/18"hg in drive - and about the same holding speed at 50mph and 60mph. Conventional maths states I should be using an #8.5 PV, so I'll start there I guess (#6.5 at the moment).
 
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