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Less overlap more vacuum?

General engine tech -- Drag Racing to Circle Track

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3window
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Less overlap more vacuum?

Post by 3window » Fri Oct 04, 2013 9:18 pm

Question for the engine gurus. Would a cam with less overlap but a NARROWER lsa create more vacuum? I'm going from a cam with a lsa of 110 and 30 degrees of overlap to a cam with an lsa of 108 but less overlap. Let me know what you guys think.

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Re: Less overlap more vacuum?

Post by DaveMcLain » Fri Oct 04, 2013 9:39 pm

Yes but in order to do that you would have to lose duration and that could also reduce horsepower.

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Re: Less overlap more vacuum?

Post by justahoby » Fri Oct 04, 2013 9:57 pm

I am no guru, but just to put myself in target position by opening my mouth, and get put in place by gurus, I will participate by answering... :lol:

In general, if you had less overlap of a given duration, (lets label this as mild cam A) and you had the same duration on a tighter lsa giving it more overlap ( labled mild cam b), Cam A with all other variables the same theoretically should have more vacuum...

Now we take your words "cam narrower lsa" and "less overlap" with comparison to Cam a and b, and call this cam C...
Cam C will most likely have in the same engine WITH the same lobe profile ramps...
Smaller duration,
Earlier intake closing,
less lift,
later exhaust opening...

lets say an HT 383 crate tow engine (tiny cam) 196/207@.050 on 112 lsa was changed to 108... I don't think vacuum would drop in such a case, but you could develop enough overlap to create enough of a problem (doubtful).....
take a 235@.05 and move it around from 108lsa to 114 on the same engine, it may mean the difference to running power brakes or not
As I'm approaching 40,I still think I'm 20. What the hell is wrong with me?

3window
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Re: Less overlap more vacuum?

Post by 3window » Fri Oct 04, 2013 11:22 pm

Yes, we're talking about 20 degrees less duration at .050 and on the seat. Severely over cammed on my part. Picking up a little lift on the intake.

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Re: Less overlap more vacuum?

Post by BigBro74 » Sat Oct 05, 2013 9:59 am

so you are saying the first cam (cam A) is 250 duration 30 deg. overlap 110 lsa, I am assuming @.050 because it is causing you vacuum problems and cam "B" you are contemplating is 230 duration , 108 lsa. 14 deg. overlap @ .050? are these solid or hydraulic? what engine is it in? that's the real question. this is a wide question but IDLE vacuum should increase with less overlap. the actual vacuum you see is influenced by many factors, one of the biggest of which is the engines displacement. let me know if i'm in the ballpark, then we can all look more closely at your setup. :D thanks, Jason

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Re: Less overlap more vacuum?

Post by 3window » Sat Oct 05, 2013 11:42 am

BigBro74 you hit it on the head. The cam I have now isn't causing vacuum problems but the new can I'm contemplating has less overlap AND a tighter lsa. It's a sbc 355.

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Re: Less overlap more vacuum?

Post by BigBro74 » Sat Oct 05, 2013 12:01 pm

But what compression /heads/intended use? is it a street car or vacuum rule oval track ? are these grinds hydraulics or solids? if hydraulics then 250 @ .050 is BIG. if solid, not so much..... because of lash, and vice versa on cam "B". a 230@ .050 solid is teeny duration after lash- but still pretty noteworthy if hydraulic. J

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Re: Less overlap more vacuum?

Post by 3window » Sat Oct 05, 2013 12:08 pm

It's a 9.5:1 sbc with Brodix ik200 heads. It's in a 2300 lb street rod and a hydraulic roller. The cam in the engine now is too big as it was original bought for a bigger engine. Trying to move the power where it needs to be.

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Re: Less overlap more vacuum?

Post by swatson454 » Sat Oct 05, 2013 12:32 pm

I don't know what rpm you're looking to turn but 224 @ .050 on a 108 is probably already a little on the big side at 9.5:1. I'm sure it would pick up gobs of torque over what's currently in there, though. My two cents :)


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Re: Less overlap more vacuum?

Post by 3window » Sat Oct 05, 2013 12:49 pm

The cam in the engine now is 248 @ .050. WAY too big. It's on a 110lsa. The engine is fuel injected and runs pretty good now. I know EFI tends to make people think you need to widen the lsa, but right now with the 110 and 30 degrees overlap, the engine runs pretty strong. The new cam will have less overlap even at 108 should make more vacuum.

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Re: Less overlap more vacuum?

Post by swatson454 » Sat Oct 05, 2013 12:53 pm

I agree 100% I think it will definitely be a worth-while change.


Shawn
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Re: Less overlap more vacuum?

Post by 3window » Sat Oct 05, 2013 3:03 pm

Has anyone run into ptv problems running .560 lift at 226 @.050 on a 108? Right now I'm at .540 lift with 248 @ .050 on a 110 and I'm good. This is on the intake. I know valve timing events have everything to do with it. Just trying to get an idea. Thanks.

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Re: Less overlap more vacuum?

Post by swatson454 » Sat Oct 05, 2013 4:37 pm

My last 383 had a 9 inch deck, .038 head gasket and a solid roller that was .600 lift, 242 @ .050 on a 106. My cam hit the rods long before the valves hit the pistons :D

In my case, no clearance needed.


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Re: Less overlap more vacuum?

Post by justahoby » Sat Oct 05, 2013 6:26 pm

BigBro74 wrote:so you are saying the first cam (cam A) is 250 duration 30 deg. overlap 110 lsa, I am assuming @.050 because it is causing you vacuum problems and cam "B" you are contemplating is 230 duration , 108 lsa. 14 deg. overlap @ .050? are these solid or hydraulic? what engine is it in? that's the real question. this is a wide question but IDLE vacuum should increase with less overlap. the actual vacuum you see is influenced by many factors, one of the biggest of which is the engines displacement. let me know if i'm in the ballpark, then we can all look more closely at your setup. :D thanks, Jason
Wrong, cam A and B were said mild cams in the same engine.. same duration different overlaps.


The examples I gave later were of a different view( hence not within the same paragraph for those who obviously seem to write to everyone in the same paragraph), not related to cam a and b.: different durations instead.

If I was to increase duration, on the same LSA overlap WILL increase.

And yes, we refer to vacuum 99% of the time at Idle...

I could be wrong but 230@ .050 and lets say for shits 280 at the seat at 108 lsa would no way I hell be 14 degrees overlap
Often overlap is not even really considered at 0.050., but in total crank degrees... (lsa is in cam degrees)

but whatever dude,

point was at a tiny duration cam moving lobe separation wouldn't change a ding dong of vacuum at idle with a cam that has brigs and straton duration vs if you stuff a considerable larger cam overlap would be much more a consideration and fall below certain limits.

And less overlap, with a tighter lobe separation means you most likely will have to have less duration.
As I'm approaching 40,I still think I'm 20. What the hell is wrong with me?

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Re: Less overlap more vacuum?

Post by justahoby » Sat Oct 05, 2013 6:48 pm

3window wrote:The cam in the engine now is 248 @ .050. WAY too big. It's on a 110lsa. The engine is fuel injected and runs pretty good now. I know EFI tends to make people think you need to widen the lsa, but right now with the 110 and 30 degrees overlap, the engine runs pretty strong. The new cam will have less overlap even at 108 should make more vacuum.
less duration with less overlap, should have more vacuum, if that's what you want.
In this case I would say overlap would tell you more than not here.
20 degrees less duration already puts these cams apart in a category...
as well as requiring different compression ratios at optimum ect.

30 degrees overlap is not a whole ton either.
I have run 45 with power brakes at 220 @.050 in a 350 cu in chevy,

Now this was not fuel injected, and about 9:1 compression, but was about 16" of vacuum.

Generally( I say this as I speak for only my experience not yours or any others) either cam you stated is not recommended in FACTORY electronic fuel injected as well as the lsa angle , as you stated people said is optimal at 122 ( which you obviously see it alone has little to do with overlap.... ( you didn't state factory EFI or not, or any specs),

but if its a runnin fine at start up and idle with that big cam, and you KNOW you need a smaller cam, try it..
As I'm approaching 40,I still think I'm 20. What the hell is wrong with me?

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