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Old 12-19-2009, 07:13 AM
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cant figure out how to go onto previous thread. so heres another question

how much does it matter that we would be running pistons that offer 10:1 compression. when the cam suggests that it needs 11:1.? Is this a stupid question?

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Old 12-19-2009, 08:42 AM
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Welcome to the site! There is no such thing as a stupid question on here. Nobody ever learned anything by not asking questions unless, you stick your hand in fire and ask yourself if it's hot.
I can't answere yours other than there would be more to gain with 11:1's than 10:1's. SQZBOIX
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Old 12-19-2009, 09:17 AM
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Generally, this type of cam will have longer duration which means that the opening time of the valves is sooner, the closing time is later, and the overlap is greater. The longer durations allow for greater air/fuel flow through the engine. The trade off is that it sort of acts like a "leak" in that it will bleed off some of the pressure created by the piston travel. This in turn reduces the "dynamic compression" or "cranking compression." In the end, this reduces the mean effective pressure which is what pushes the piston down, creating torque through the crankshaft. This "allows" you to run a higher static compression ratio. In order to obtain full advantage of the camshaft, the mean effective pressure needs to be at a certain level and due to the cam flow characteristics, a minimum compression ratio is specified to meet that MEP.

Short answer, the 10:1 will work but it will not be optimal. You may even be disappointed in the performance over your present cam. For best results, always match your components. Think of your engine as a team.
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Old 12-19-2009, 11:57 AM
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Would you mind posting the cam specs, the engine and how it'll be driven (ie cruiser, bracketcar, rockcrawler etc). There are a lot of generalities floating around that manufactors like to put in thier paperwork (cover your butt mentality), some applies and some won't. Ther's some talented people on here that'll be happy to help if you give them more info ok? Later, oj
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Old 12-19-2009, 01:10 PM
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Quote:
Originally Posted by wifeseekinginfo
how much does it matter that we would be running pistons that offer 10:1 compression. when the cam suggests that it needs 11:1.? Is this a stupid question?
The cam's long duration results in a late intake closing event. This timing of the intake is there to take advantage of the inertia the incoming mixture develops inside the ports. This becomes a sufficiently large velocity that the intake will continue to flow into the cylinder against the compression forces of the rising piston. But this characteristic is variable with engine speed. At low thru moderate RPMs it doesn't exist strongly enough to overcome the compression of the rising piston which results in some amount of mixture being blown back into the intake and the density of the mixture trapped in the cylinder after the valve closes is less, therefore, the power is less.

To compensate for the lost bottom end power, the compression ratio is increased to force more work out of less mixture. In this case giving up a full ratio could take 10 to 15 percent off the power curve below the torque peak, and possibly a like amount off the top end.

In the range of the top end, the engine starts to run out of breath from not having enough time to intake a full cylinder volume. At this extreme, the high compression ratio comes back to help extend the RPM range by again getting more work out of less mixture.

The middle RPM range at high loads with high compression is the detonation danger point. When things are at the optimum for cylinder filling and the compression ratio high, an engine that is working hard, thus hot but not turning really fast, is subject to detonation. Stiffer gears to get the revs up which reduces the working load, a cooler operating environment, cooler intake air, a richer mixture, reduced timing advance, or greater exhaust restriction are potential solutions to get thru that problem.

In the end you want a Dynamic Compression Ratio (DCR) in the range of 8 or 9 to 1. A quick and dirty calculator of the DCR can be found in many places a decent one is at Kieth Black's site http://kb-silvolite.com/calc.php?action=comp

The DCR is always less than the Static Compression Ratio (SCR) which is the one you calculate from the volumes contained by the cylinder and head.

Bogie
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Old 12-19-2009, 07:28 PM
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Running a Lunati cam...#20202... Duration at ,050 Tappat lift..242,242...Advertised Duration 310,310 Lift at Valve .530,.. Lobe Separation 114, Compression Ratio 10:1. Solid Lift cam. Hot rodding...weekend warrior at an official racetrack.
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Old 12-19-2009, 11:48 PM
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based on the LSA of 114 i dont think that the 10:1 will be a problem, you should bleed off enough cylinder pressure with this cam, to honest it's a bit more LSA then i would want to run on a high duration cam (high out put cam) such as this i would look more for a 108-110 LSA, but this would be less forgiving to the difference in recomended compression ratio and actuall ratio
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Old 12-20-2009, 06:56 AM
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Thanks.....another question....when I get the answers...I feel so knowledgeable ..Say your running 106cc heads..and you lengthen the stroke..are you then raising the compression ratio? What exactly is the compression ratio.....please explain it to me in terms that I can then repeat.For instance if you lengthen the stroke aka a stroker kit and then use bigger head cc aren't you going right back to where you were? it seems lke if you kept the smaller head you would have a smaller space to combust and therefor raise the ratio?????? i might be way out there on this one? :
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Old 12-20-2009, 03:49 PM
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Quote:
Originally Posted by wifeseekinginfo
Thanks.....another question....when I get the answers...I feel so knowledgeable ..Say your running 106cc heads..and you lengthen the stroke..are you then raising the compression ratio? What exactly is the compression ratio.....please explain it to me in terms that I can then repeat.For instance if you lengthen the stroke aka a stroker kit and then use bigger head cc aren't you going right back to where you were? it seems lke if you kept the smaller head you would have a smaller space to combust and therefor raise the ratio?????? i might be way out there on this one? :

you can buy a stroker kit, where the throw or stroke of the piston tavel is increased, this is the meassured thength of travel between the piston at bottom dead center(BDC) and top dead center(TDC). they are able to increase stroke by changing where the rod connects to the piston, it is resesed further into the piston skirt. changing to a stroker kit wont nessesaraly increase your compression ratio because the piston will still come up to the same point in the cylinder when it's at TDC but it well travel father down the cylinder to get to BDC. now for the compression ratio and pistons, average flat top piston actually has a -5 to -7cc dish in the top of the piston, called a valve relief. some pistons will have a D shaped dish with anywhere from -12cc to -22cc, you can also buy a O shaped dished piston that doesn't have a very efficent combuston characteristics(i would stay away from the O dish). a larger dish in the piston will lower the compression ratio, and vise versa. you can also by domed pistons where the center/top of the piston actually rises above the the outer edge of the piston, a higher dome will create higher compression ratios. the compression chamber is the area between the piston crown and the "pocket" in the heads", the compression ratio is the ratio between the area in the cylinder when the piston is at BDC vs. the area in the cylinder when the piston is at TDC. i mentioned before that a cam with a 114 LSA will creat less combuston, this happens because the intake valve stays open longer into the pistons compression stroke, thus actually "bleeding off" or loosing some of the compression through the intake valve, this means it is leaking some of it's possible power out though the intake valve.
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Old 12-20-2009, 04:59 PM
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Quote:
Originally Posted by wifeseekinginfo
Thanks.....another question....when I get the answers...I feel so knowledgeable ..Say your running 106cc heads..and you lengthen the stroke..are you then raising the compression ratio? What exactly is the compression ratio.....please explain it to me in terms that I can then repeat.For instance if you lengthen the stroke aka a stroker kit and then use bigger head cc aren't you going right back to where you were? it seems lke if you kept the smaller head you would have a smaller space to combust and therefor raise the ratio?????? i might be way out there on this one? :
You are absolutely correct. If you increase cubic inches but keep the same profile on the piston top(Flat top, dome or dish size), and squeeze it into the same combustion chamber size you will end up with an increase in compression ratio because you have increased the swept volume of the cylinder when you increased the stroke.
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Old 12-20-2009, 07:48 PM
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Ok, here's the deal in a nut shell. I have a 427 ci motor, .60 over, 4.310 bore. There is a forged crank in it with rods which have 3/8's bolts attaching them to the crank. I must confess that I am concerned about the integrety of the bottom end and also the power limitations of the bottom end. Pistons in the engine are TRW forged 2300 +.60. The heads I have are 3964291, 106cc closed chamber rectangle port, 327 cc intake runner LS6 (1970 castings. I'd like to put parts in the bottom end which will take advantage of the flow characteristics of the heads if there is a better bottom end,crank, rods, pistons. Keep in mind ..using a 10:1 compression ratio, I've never raced on a 1/8 mile track before and I have a 1/4 mile strip within 50 miles of my home. That's what I'd like to do someday. I happen to be 52 years old and have never lost the itch. One more thing. Keep in mind that this 427 is brand new, just rebuilt, four hours running time. Someone recomended a XE294H-10 comp cam but I didn't like the way it sounded. So now the heads are back off and I just slid a lunati 20202 in it. I'll probably end up putting it on the shelf too, if I come up with a better lower end combination. I do not wish to waste a good set of heads with a not-up-to-snuff bottom end. Whats your opinion????? idea.
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Old 12-21-2009, 09:04 AM
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why are you worried about the integrety of the bottem end, you mention that it has a forged rank in it, you didn't mention what kind, you didn't mention what kinda rods are in it either, but even a stock gm crank is capable of some impressive power output if propperly balanced. i didn't look up the lunati cam but if you plan on racing this motor then i would recomend a solid cam whether it be flat tappit or a roller, a hydraulic cam and hydraulic lifters dont hold up well to the abuse done at the track
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Old 12-21-2009, 10:55 PM
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If your rods have had ARP rod bolts put in them and then been resized on the big end I wouldn't worry about them to 7200+ rpm in the short stroke 427 with a forged factory crank. You have plenty good pieces, you could make 650 hp and not hurt them. Only extreme over-revving is going to hurt them.

The cam you have isn't bad, a copy of the factory hi-po solid cam. If you are serious about more race use, you could do better with a more modern grind.

One thind I would recommend is getting an "face oiling" solid lifter. These have a laser'ed tiny hole in the lifter face to constantly oil the cam and lifter interface. BBC's are hard on cams, and this is good insurance.

Good prices and good people here www.competitionproducts.com
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