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Discussion Starter · #1 ·
I retarded my cam using the retard keyway on the timing set I have. By doing this I can maximize the rocker arm ratio 1.73in./1.80ex. I was watching the assembly as I rotated it by hand and noticed that at the beginning of the compression stroke the intake valve is still closing for a little bit. I can imagine, all put together, the engine blowing some of the fuel back out through the intake. Can that be a problem or am I okay? I have a supercharger I plan on using also. This is my first ground up engine build. I'm building a low compression 408w with 2v Cleveland heads.
 

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what is the duration of your cam? what is your overlap? What is the distance between lobe centers?
How much did you retard it and what do you expect from it?

Retarding the cam will take away the bottom end and move the HP curve into a higher RPM range. Is that what you want to do?

On a cam with any overlap (and that includes stock) the intake valve is always open for a short time at the beginning of the compression stroke. The "ram" effect from the column of air and fuel moving at high velocity into the cylinder will keep filling the cylinder as the piston begins its upstroke to compress the gasses. (note that the piston moves very little in the first 20 degrees of crank rotation from BDC). You will probably be fine with the retarded cam. What you are doing, though, is very unusual. Normally cams are selected with too much duration and people have to advance the cam to gain some low end and in doing so they give up a little top end RPM which isn't usually a problem because they don't intend to run that engine at 6800 or 7000 RPM.
 

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You lose FAR more by retarding the cam than you gain by going with maximum rocker ratio.

It will kill off the low and midrange power, and not really do much for the top end.

Detroit did this in the 70's and 80's to reduce NOX emissions. It also killed power. It was amazing what a swap to a 'regular' timing chain and gears would do for power in some of those vehicles. Totally sucktacular power.

DON'T DO IT.

I you feel the NEED to turn 7000 rpm.... and actually make power there.... install a camshaft designed to make power there........ and also be sure to buy all the other parts required to efficiently make power at that engine speed.

My advice... It sounds like you have a decent combination. Put it together and enjoy it. You can start saving and dreaming about, hotter build after you have this one running. Don't make the mistake of trying to force your engine into doing what the combination wasn't designed for. You'll be very disappointed.

Good Luck!

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302 4-speed 289 heads, 10.63 @ 129.3
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<font size=-1>[ This Message was edited by: n2omike on 10/1/06 11:09pm ]</font>
 

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retarting a cam works out about ONCE IN A LIFE TIME.It can be used by serious class racers that go from track to track, to adjust for bad track conditions. but it is not used to make more HP.Also, this idea of using high ratio rocker arms is not a sure thing too. I have had many(25) engines on the dyno where the owner wanted to see the results of a rocker change, The higher ratio only works out about 35 to 50 percent of the time. the gains are usually 5 to 10 HP. you can even tell you did anything. the other 50 percent of the time, there was NO gain, or, a loss of power. It has a lot to do with the cam and heads you have to work with. Most of the time, it is a WASTE of money. JOE SHERMAN RACING
 

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Discussion Starter · #8 ·
Wow guys, thanks for all the input. Here's what I have. I didn't retard the cam to go with bigger rocker am ratio. I retarded so I would have better piston to valve clearance. I have set the timing at retarded 3, 4, and 5 degrees and 5 dgrees gave me better clearance on the intake valve. It does allow me to run the bigger rocker ratio. Performancewise, I can live with losing a little horsepower. I'm not building a horsepower queen. Horsepower looks good on paper. At 8.8:1 compression I'm not going to be making much HP anyway. ll be depending on the supercharger to make up for lost compression. I just want the engine to run well.

Well, I am willing to cut some of the pistons. I have Probe -22 dished SRS pistons. The clearancing is on the flat part above the valve reliefs. How deep can I cut?
 

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On 2006-10-01 13:46, latamud wrote:
Well, I am willing to cut some of the pistons. I have Probe -22 dished SRS pistons. The clearancing is on the flat part above the valve reliefs. How deep can I cut?
I can't tell you how deep you can cut, but I can help you find the deck thickness of your probe pistons (which I suppose is just as important.)

I hope you have the part #:

http://www.probeindustries.com/Pistons/Piston%20Specifications/PistonSpecs.htm

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393W NA, 2.78 Toploader 4 Spd, 9" 3:70 Posi
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<font size=-1>[ This Message was edited by: dennis111 on 10/2/06 4:54am ]</font>
 

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Discussion Starter · #10 ·
Thanks. 12360-030 gives me .180 deck thickness. I measured the deck thickness just from the deck to the bottom of the dish and get a rough .170". I'm going out on a limb here but guessing probe's advertised deck thickness if of the thinnest point. The flat top pistons have .350 deck thickness. The part I would need to cut is the high part above the dish. I won't have to cut more than the dish depth so I should be okay.

Thanks everyone.
 

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On 2006-10-01 13:46, latamud wrote:
At 8.8:1 compression I'm not going to be making much HP anyway.
Retarding the cam will kill off what little cylinder pressure you've got with that compression ratio.

Cylinder pressure is what makes NOX emissions... and horsepower. The OEM's retarded camshafts to kill emissions early on. They also killed power. You will too.
 

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I agree retarding the cam is not the answer.

Although it does bring the peaks up in the rpm range, it kills low end vacuum due to the late intake valve events. With a relatively short duration cam like yours, the intake valve opens so late it cant get a good cylinder fill

In addition and even MORE important in this case, the wide LSA plus the retarded timing pushes the exhaust centerline so late, that it can make scavenging ineffecient and make a motor very fussy with a full exhaust

Basically you'll fight very low rpm bucking due to reversion and a poor signal to the carb for two reasons

1 - intake valve timing lowers vacuum
2 - the cylinder wont have the exhaust velocity it needs and will have a contaminated charge when it goes to the intake stroke again

Now, if its race only, it may not be noticed, but its only a bandaid, its not the right way to do it and it wont be efficient with that much LSA. You'd be MUCH better off street or strip with a similar cam grind, but with a 108 LSA to increase overlap, degreed in to 105 or 106 ICL.


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70 Sportsroof, 427 FE/489 cid, 10.4:1,Stage 2 Edelbrock heads, RPM intake, 1000 Holley HP, TKO-600, 3.70's, A/C
71 F100 4x4 shortbed 390 hot rod
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<font size=-1>[ This Message was edited by: My427stang on 10/2/06 7:45pm ]</font>
 
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