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Old 01-30-2009, 06:32 PM
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Need some technical theory

It's been a long time since I've had to know some of this so please bear with me.

I read somewhere that a wider lobe separation is good for higher compression engines because it will bleed off some cranking pressure and ward off detonation.

I also read that a narrower lobe separation is better for LOW compression engines because it creates more cylinder pressure and, therefore, more power.

Now, someone please correct me if I'm wrong. Wouldn't a narrower lobe sep such as 106 or 108 degrees create more overlap and therefore bleed off what little compression you have? Wouldnt a narrow lobe sep LOWER your cranking compression?

Wouldnt a wider lobe sep (112 to 114)give less overlap and therefore INCREASE cranking compression?

What am I missing here?

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Old 01-30-2009, 07:02 PM
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Don't believe everything you read!

You are confusing cranking compression and making power. Cranking compression is an indication as to whether an engine will ping or detonate at low RPMs, but not an indication of high RPM power. In general, very high compression engines make more power with a wider LCA. Lower compression makes more power with a tighter LCA>

Actually, at a given duration and centerline, a wide LCA gives worse dynamic compression at low speeds, since compression begins when the intake valve closes, and this occurs later with a larger LCA. Overlap is not a factor.

thnx, jack vines
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Old 01-30-2009, 07:09 PM
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Overlap does not bleed off cylinder pressure.

At low rpm, overlap increases the exhaust delution in the intake manifold and has much the same effect as EGR.

At higher rpm the cam timing is in phase with the exhaust system pulsing creating a scavedging effect and exhaust delution is no longer and issue.
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Old 01-30-2009, 07:15 PM
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Hmmmmm, so what I read was true. It just sounded backwards to me. I was thinking that tighter sep would mean more overlap which would bleed off low rpm compression.

Here is why I am asking. I have a parts counter rebuilt SBC 350 in my S10. The heads are 416s, the cam is just a smooth idling stock type replcement (I would assume) and I have 175 psi cranking pressure. I have no way of knowing what pistons are in there or how far they are in the hole, dush size, none of that. I'm only going by what I DO know. So, I want to put a cam in there for the street, want it to sound mean and make the most power I can for the money I spend. At this time, cam and lifters is about it, I have 4.10 geard with a TH475 kitted and a 2300 rpm converter. Should I go tighter or wider? From what you're telling me I am thinking I should go tighter.
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Old 01-30-2009, 07:27 PM
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http://www.wighat.com/fcr3/engine.htm

Low compression ratio motors need to get the intake valve shut relatively early to begin compression or the cylinder pressure will be low.

Try this one. It's about as big as you want to go with a 2300 stall converter.
Degree it in to verify the cam timing. Move it if nessessary. Install so that when checked at .050" tappet lift the intake closes at 34 ABDC.
The cam will effectively be "5 degrees advanced" when installed like this.

The distributor will need to be recurved to allow some increased base idle timing. It will have the sound you want and will perform well for you.
Throwing on some high perf valve springs would be nice.
http://store.summitracing.com/partde...8&autoview=sku

http://www.iskycams.com/timingchart....g_chart_id=165

If you were to go to a higher stall converter (10" 3500stall) you could go bigger on the cam.

But if you're going to stay with the converter you got, don;t get all carried away.

Last edited by F-BIRD'88; 01-30-2009 at 07:50 PM.
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Old 01-30-2009, 07:56 PM
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So, as a baseline for me, this cam would normally be straight up,according to the specs, and you'd want to advance it 5* from as ground?
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Old 01-30-2009, 08:04 PM
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Quote:
Originally Posted by Greg T
So, as a baseline for me, this cam would normally be straight up,according to the specs, and you'd want to advance it 5* from as ground?
All (or most) the Isky cam cards cam timing seem to show the cam in a straight up position. (even if the cam is made with a bit of advance at the factory) You need to degree it in a see for your self. You want the cam slightly advanced in the motor to enhance torque, especially when the thing is short on compression ratio.
Getting the intake closing timing point is critical. If the timing does not reflect the cam is in fact slightly advance (33 to 35 degree intake closing point in this case) when you check it, you have to adjust the installed cam timing yourself to get it right.
It's worth getting it right. Details matter.
There are other production tolerances that can and will stack up to effect the actual installed cam phaseing. if you want max performance, check it and move it as nessessary.
Get yourself a copy of Comp cams free Cam selection program. Punch in the isky cam card specs and see the effect that advancing the cam has on the power curve.
http://www.compcams.com/CAMQUEST/

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Old 01-30-2009, 08:10 PM
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Okay, thats easy enough. Appreciate the assistance. So, with the given info you would classify my engine as a lower compression piece? I've read many different opinions on that too. Some say 175 cranking pressure needs premium fuel while others say it is relatively low.

You know, I have Cam Quest but have not yet been able to modify the cam specs for whatever reason. I'll try again.
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Old 01-30-2009, 08:23 PM
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If you want a high performance vehicle use a high performance fuel.
Yes you'll probabily need premimum gas.
If you wnat to make power you need to yank those 416 heads off and port them. Then you'll see what pistons are in there.
Cams do not work in a vacuum by themselves.
the need head flow, good intake and exhaust sytem and compression.
If your motor has dished piston, Reinstall the 416 heads with a shim style head gasket to max compression ratio. Your cr will be about 9.5:1.
More is better up to about 10.5:1 on 92 octane.
use cooler heat range high performance spark plugs in the 416's Champion RV8C or Autolite #144 works well. stock R44 or R45 is too hot.
Don't forget to recurve the distributor advance curve.
initial 14 to 22 total 32 to 36 at max advance.
vacuum advance should be around +10 +15 degrees at high cruise.
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Old 01-30-2009, 08:30 PM
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Was seriously considering the thin gaskets for a year now. Distributor is already done with 20 initial and 36 total mechanical @ 3000 with can hooked to man vac and adding another 12*. Actually runs really well for what it is. Just want to get what I can out of it for the money cuz I gotta sell my camaro to finance the 550 ponies going in later.

Thanx a million!
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Old 01-31-2009, 08:52 AM
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AHA!! Apparently CamQuest's free download is not the same as the $12.00 version on Cd. That is why I can't alter the cam timings in my program.
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Old 01-31-2009, 12:28 PM
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Here is a good read.
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