how much cam before losing vacuum? [Archive] - Team Camaro Tech

: how much cam before losing vacuum?


JayBird
Sep 20th, 00, 05:11 AM
I have a motor that has these cam specs. 242 duration at .50" and 540 lift. I have power brakes and I need to know if this is too much cam or will it work with or without using a vacuum canister? I can change the cam, but if it can work without, that saves me $.

Thanks,



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Jay 69 SS-350

sr71bb
Sep 20th, 00, 05:33 AM
Jaybird,

YES that is too much cam without using either an additional vaccum can or an electric vacuum pump mpbrakes.com (http://mpbrakes.com) .

The magic figure is around 230 degrees duration at .050. That is when you start to lose a substancial amount of necessary engine vaccum. Valve lift can play a role too, but cam duration is by far the biggest factor. Power brakes generally require between 14 and 17 inches of vaccum at idle to work properly. With my cam on my BIG BLOCK I pull only 11 inches at idle with a CAM that has 236 degrees duration on the Intake side (.610 lift).

I will be using a electric vaccuum pump from Master Power Brakes to supply the needed vaccum. This pump supplies 21 inches of vaccum and has an auto-switch that activates it with it SENSES low vaccum. This pump will feed BOTH my power brakes and my rally sport vaccum canister so that my headlights will open properly.

On a personal note, my prior setup WITHOUT the benefit of the electric vacuum pump almost solely contributed to minor accident when I was unable to stop (no power assist, not enough vacuum!!!) in time to avoid a collision (BUMMER!!).

Oh well, hope this helps.


[This message has been edited by sr71bb (edited 09-20-2000).]

JayBird
Sep 20th, 00, 06:04 AM
Thanks. That helps alot. It may be possible to keep that cam and get the pump.

Thanks again!

427TRI
Sep 20th, 00, 07:08 AM
JayBird, incomplete answer, but may be correct. Duration at 0.050 is only one parameter needed to determine vacuum at idle, other is lobe separation angle ( LSA) , and of course, cubic inches.

But with 242 in a 350, your LSA should be 114 or > to allow normal vacuum operation. Judging by that lift figure your LSA is less than 114, but check that spec before you buy something you dont need.

with that duration, your engine starts work at 3000-3200 and runs to 6500, hope your stall/gearing is set up for that.

[This message has been edited by 427TRI (edited 09-20-2000).]

[This message has been edited by 427TRI (edited 09-20-2000).]

sr71bb
Sep 20th, 00, 01:33 PM
427,

You kill me sometimes. Exactly which part of my answer was incomplete???? All other factors are basically irrelevant if your idle vaccumm (in gear for an auto) is less than what is required to operate your power brakes. The other factors such a lobe seperation, duration, lift are the variables that AFFECT vaccum so what did I miss???

reference:
http://mpbrakes.com/mp27.htm


[This message has been edited by sr71bb (edited 09-20-2000).]

Mark W. Winning
Sep 21st, 00, 04:12 PM
Vacuum is a funny thing. My truck runs a flat tappet solid cam with a much higher lift than your and my power brake work fine. I was always under the impression that duration had more of an effect than just the lift, but I am not sure.

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Mark
1992 Firebird 355/Six Speed
1991 RS 350 / 700-R4
1987 Toyota Pickup 383 / 500 + HP 10.963 @ 119.95 Slicks / 11.997 @ 114.23 Radials
http://personal.lig.bellsouth.net/~racer383/

pdq67
Sep 21st, 00, 05:29 PM
I'm an "arm chair" hot rodder and have been studying cams for quite a while once I got computer literate enough to operate a copy of Desktop Dyno and later Dyno2000. Sure, I've ran the old -097 Duntov, the -151 and RV 280/230,.480" type cams on the street in the past but never really knew why.

I was picking one of the cam manufacturers Tech's brains and he mentioned by chance that stroke played more of a roll in cam selection then bore. This got me to thinking and when I checked out the intake valve closing point, lo and behold all things being equal, like stock length rods and bores adjusted to hold cubic inches the same between a stroker and a bored engine.

The stroker causes the piston to be down in the hole deeper at the intake valve closing point, thus trapping more charge and creating more cylinder pressure. This is why a stroked engine with stock length rods makes a cam that seems rough in a bored engine of equal parts buildup and cubic inches smoother.

This means that you can run more duration in a stroked engine size for size then you can in just a bored engine.

Now this isn't saying that valve shrouding due to a smaller bore doesn't matter or that internal engine friction doesn't enter into all this at high rpm. Of course they do to a certain extent. It just means that if you limit rpm to say a street range, they don't matter all that much and it's stroke that is controlling power production more than bore in the same cubic inch sized engines.
Now a lot of this cam/c.r./torque/hp/rpm/gearing B.S. is beginning to make since.

Now, consider cam generated vacuum and this might get interesting. pdq67

PS. Ed Staffel's book really turned the light bulb on for me so grab a copy and read it.

JayBird
Sep 22nd, 00, 03:05 AM
Thanks for the info..I'll grab a copy of that book. My motor is a 350 bored 30 over with lightweight 6 1/4" rods, very light weisco pistons with custom cut valve reliefs, 62cc bowtie cast iron heads with 2.02 intake and 1.55 exhaust with cam mentioned above and crower 1.7 rockers on intake and 1.6 on exhaust. I was thinking about a performer rpm intake. This motor will be matched up with 75o carb, 3.73 gears and muncie 4-speed.

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Jay 69 SS-350

sr71bb
Sep 22nd, 00, 08:57 AM
Jaybird,

Be careful about the RPM Performer intake. It is a great intake and I have one but hood clearance with my stock SS hood was a problem and I had to get a Cowl Hood because the RPM Performer manifold is a high-rise design.

I would search past posts about this. There is alot of good info here on that.

Crash
Sep 22nd, 00, 10:05 AM
It seems that although theory can get you close, what you actually end up with may be slightly different.350 with a Summit 292 cam which came out to 244@050 with a Torker intake should have caused me vacuum problems but never did. Always has good brakes.

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74 Camaro
Canada, eh!

JayBird
Sep 22nd, 00, 01:39 PM
sr71bb, I put on a cowl hood so I should be fine, but thanks for that info. My cam is made by Crower and its a solid camshaft. I am going to leave the cam in and try a vacuum canister. Its cheaper to try the canister. If it doesn't work, i'll change the cam around 230 at .50 and 515-525 lift. That's the specs of the hot cam by GM.

Thanks,



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Jay 69 SS-350

sr71bb
Sep 22nd, 00, 02:47 PM
Sounds like a plan!!!!