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Anyway to calculate vacuum from camshaft overlap?

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Old 03-04-2017, 01:01 PM   #1
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Default Anyway to calculate vacuum from camshaft overlap?

As the title says... is there any rule of thumb way to do this?

I know rings, compression, engine size, and intake manifold design all play a role in vacuum... but the cam is still the biggest variable in how much vacuum you'll pull.

Is there a way to calculate this?

On my two cams, I've run in this motor, one with 5.5 degrees of overlap idled at 850 I was around 58kPa @ 850 or 13" hg. The one with 16 degrees saw 70kPa @ 950 or 9.5" hg.

I'm trying to figure what a cam with 22 degrees of overlap would look like on a 408. And what one with 10 degrees would like like.
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Old 03-07-2017, 10:12 AM   #2
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Default EAP Model

Hi, I use EAP to do this type of work.

I will do this for free, just need your specs.

Lance
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Old 03-14-2017, 11:36 PM   #3
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Hi Lance,

What specs do you need?

The 408 would be 4.03" bore, 4" stroke, 2cc flat tops, -.010" out of the hole, .040" gaskets 4.130" bore, 67cc chambers. Rings gapped .018/.020.

Cam specs:

@ .006" 293/306
@ .050" 244/256
@ .200" 166/173
Lift: .627/.615

114 LSA 111 ICL

Spring Seat Pressure 170lbs @ 1.750" height, 440lbs @ 1.200"

Heads flow:

0.100 69 55
0.200 146 122
0.300 237 199
0.400 294 250
0.500 336 277
0.550 348 284
0.600 353 290

FAST LSXR 102 Intake Manifold
Kooks 1-7/8" Primaries, 24-28" length, 3" collector 6" long, 3" dual exhaust

Last edited by JakeFusion; 03-14-2017 at 11:52 PM.
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Old 03-15-2017, 11:54 AM   #4
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Wow! Jake those are some incredible exhaust #s, can you tell
me about those heads? and with 84%-82% E/I Ratio why
the 12* exhaust split? 300 N2O shot?
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Old 03-15-2017, 12:55 PM   #5
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It's a nitrous exhaust port flowed with a pipe. So somewhat inflated. But no NA - see the ring gaps.

The nitrous port is being used with the 1-7/8" headers to create a lot of velocity.
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Old 03-16-2017, 11:40 AM   #6
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Default Eap @ 23 cfm 15 hp

Hi Jake, there where some choices :

Auto Trans in gear.
Spark Advance set for best HP
Air door set to 23 CFM.

Results :
500 = 2.9 Hg
600 = 3.1
700 = 4.2
800 = 5.3
900 = 6.4
1000=7.5
1100=8.3
1200=9.7

YES your chosen camshaft has AGGRESSIVE lobes.

Lance
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Old 03-16-2017, 02:10 PM   #7
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Thanks, Lance.

Those numbers are terrible. LOL

Supercharger it is.
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Old 03-16-2017, 03:08 PM   #8
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Jake if you want to run a nasty cam with low vacuum - have you considered an electric vacuum pump & cannister as a band aid?

I had a stupid 24x/25x cam in my 72 vette once that was good for like 2-4 inches of vacuum at idle.
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Old 03-16-2017, 04:36 PM   #9
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I was trying to see how bad it would be to aim for a 550+rwhp 408 vs just putting the Procharger on my current setup (which sees 13.5" of vacuum @ 850). So the choice is pretty clear now. Any less cam and it wouldn't be worth the upgrade. Not when the Procharger will make significantly more power and drive better.
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