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I have a sbf, 13.8:1 cr .080" quench, was wondering what is a good cranking compression should be, and whats the theory behind it.
 

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165-180 psi with a N20 Cam Anything over 200psi the tunning window gets smaller

X2

I have a 12.5:1 sbc with a nitrous cam and it cranks 205psi, the cam is advanced which makes it worse but its has its best ET's that way. I have to pull alot of timing in comparison to most, 11-12* for a 150 hit 15-16* on a 250 hit, to keep it from detonating
 

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The nitrous guy
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You can have 200psi of cranking compression, but the more you spray, the smaller the window gets, as far as tuning. Another thing to consider, is that if an engine has a lot of cranking compression, that translates into a lot of overlap. With both valves open a long time, a good bit of the charge, gets sucked straight thru the port and out the exhaust. This is why nitrous motors like cams with wider LSA numbers. You have to keep the charge in the cylinder to burn it. Heads play a factor as well. A very efficient, fast burn head, like some of the SBF stuff, does not need cylinder pressure to work well. I have some LS based motors with cranking compression in the 130s, Power Mad says his spread port BBC is 150....efficient heads. Pumping pound numbers that low in a large chamber, conventional headed motor, would likely result in a chamber that did not burn well. It is all about the combination

Monte
 

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Discussion Starter #7
Thanks Monte, once the motor is built, I will play with the cam timing to get the cranking compression to where its easier to tune.
 

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by no means do I mean that 150 lbs is ideal cranking compression.
just telling what I like and has worked for me ,and hope it helps someone ..
 

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How can overlap have any bearing on cranking pressure? , take two identical camshafts, one on a 104 LSA the other on a 110 , install the 110 on a 104 intake centreline & cranking pressure will be pretty much identical .............as far as nitrous being shot out the tail pipes on overlap, this has more to do with exhaust scavenging rather than lobe sep/overlap#s.

Big inch motors use wide lobe seps for a reason , nitrous not being one of them......just correcting you Monte as i know you do the same for me.

:p
 

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my cam is small to me .. g lift -in-.869 ex-.840
lsa 116.0
icl 113.0
duration-in @ .50 -288 ex @50-312
comp 4/7 swap
in a 632 with big dukes # 1803 ,and a ray franks 2x4 with pro 1050s
 

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My cranking compression on our 15* EPD headed 665 averages 175#, as does our 18* Big Duke 1802 headed 582b (slightly lower, say 165#). The difference is that our "lil" 582 has a 14.8:1 Diamond piston, while the 665 has a very small dome. While having a flatter dome piston, the 665 has a smaller cc area, and is timing sensitive, and this is the reason we do not want the cranking compression to be too high to make tuning too narrow. Our camshafts on both are similar to the stats cited by Powermad. Most relevant point to me was made by Monte, i.e. it is the "combo" that makes things work best as one.

Good luck.
 

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The nitrous guy
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How can overlap have any bearing on cranking pressure? , take two identical camshafts, one on a 104 LSA the other on a 110 , install the 110 on a 104 intake centreline & cranking pressure will be pretty much identical .............as far as nitrous being shot out the tail pipes on overlap, this has more to do with exhaust scavenging rather than lobe sep/overlap#s.

Big inch motors use wide lobe seps for a reason , nitrous not being one of them......just correcting you Monte as i know you do the same for me.

:p
Sorry, but I don't agree with what you said. In your example the cams can't be identical, the events are different when you move the LSA numbers, plus, the 110 cam installed on 104, still has the same overlap and LSA, you just advanced the events to happen earlier, so of course it pumps the same. Overlap, means both valves are open at the same time. This was done in years past, because the heads sucked and needed help pulling the charge in. If both valves are open longer at the same time, some charge is pulled through. There is a lot to cam profiles, I agree, but a big motor generally uses a wider LSA for nitrous use, than it does for N/A. All about event timing and cylinder pressure

Monte
 

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Another thing to consider, is that if an engine has a lot of cranking compression, that translates into a lot of overlap.
Are you saying the "5th cycle" is in play during a cranking compression test? Or, is this just a rule of thumb based on the fact most tight lobe center (high overlap) cams close the intake valve earlier, thus trapping more swept volume.
 

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The nitrous guy
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This is just a general statement. You can make anything, be anything you want, if you grind it that way, but USUALLY, with your average run of the mill, kind of standard cam profile, if it has a lot of cranking psi, that generally means it has a shorter LSA angle and generally high amount of overlap. Lets face it, you close the LSA, the lobes are closer together, you get more overlap. So all I am doing, is telling a guy, that if he screws his compression tester in and spins his motor over and it is 200 psi plus, chances are very good that his unknown cam has a short LSA and a lot of overlap and would not be the best choice to load up with spray.

Monte
 

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if it has a lot of cranking psi, that generally means it has a shorter LSA angle and generally high amount of overlap. .

Monte
Overlap has no bearing whatsoever on cranking PSI , you've said this before & i just thought it was a typo error .....an engine with high cranking pressure does'nt necesarily mean it's ground on a tight lobe sep , could be of a short duration with a wide lobe sep , many variables to consider , a 220/220 110lsa installed straight up in a 600" motor will net some good cranking psi :rolleyes: , notice overlap is @ zero.
 

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Very interesting, and way over my head...LOL. My 665 has a 116 LSA & when we first bought the motor "fresh" & did a cranking compression check on it, the cranking compression was 200# to 210#. We thought this was due to several factors, i.e. combustion chamber of the Brodix 15* EPD heads, Head Gasket size (I assume thin), Camshaft, and other unknown factors. We subsequently changed the Head Gasket to a larger one, and degreed the CAM differently (don't ask me to explain...LOL), and we now have what we feel is a more manageable cranking compression of closer to 175# in all cylinders. We thought this was wise as this motor has 3 n20 stages available, capable of 750 HP.

The more you learn, the more you realize you don't know...LOL.
 
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