Compression

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Old 04-18-09, 03:53 PM
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Question Compression

How many pounds of compression should a motor that is 10.0:1 have when its brand new?
 
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Old 04-18-09, 04:29 PM
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Are we discussing a rebuilt PERFORMANCE motor?

Not many factory vehicles are built with that kind of compression due to Fuel Quality concerns.

Now to answer your Q.... It all depends on The "Duration " and "Overlap" of the camshaft. A cam with a longer duration and a long overlap, will keep the valves open longer, thus "Bleeding " off Compression. This makes no difference to performance, but does make it impossible to give an Exact number on a gauge....

Rule of thumb, as long as all cylinders are within 10 pounds of each other, And there are no driveability concerns...I wouldnt sweat it....
 
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Old 04-18-09, 05:13 PM
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Its going to be used as a race engine. The rules state that the engine can not have more than 175lbs on any cylinder when cranked over 5 times. But as for the overlap and duration issue with the cam the rules also have a vacuum rule which says it must pull 12in of vacuum through the intake at 1200rpm. I am at a loss on these 2 items. I have always raced in a class that had no requirements so I need a good starting point or some suggestions. Thanks
 
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Old 04-18-09, 05:25 PM
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Ahh Thats a different animal.......

Ok....Has this motor been built yet? What is it??? and ..the all important question of the day........What do you expect out of it?

175 should be do-able, since as I mentioned, you are looking for "Dynamic" compression, rather than the "Static" or Design compression.(I think I may have those reversed--But No matter , you get the IDEA)...

The vacuum, can be dialed in if necessary with Ignition timing adjustments...Naturally aspirated..or Juiced???? it will make a difference.
 
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Old 04-18-09, 05:46 PM
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The motor hasnt been built yet. Im not sure what cam to get or cc on the heads. I have the block at the machine shop right now getting bored and decked. The rotating assembly does have 2 valve relief flat top pistons thats where Im gettting the 10:1 compression with 76cc heads. I can flat mill the heads down and obviosly want to get as much compression as I can while not going over 175lbs per cylinder and get my vacuum still. WOW I am not wanting much am I.
 
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Old 04-18-09, 06:14 PM
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There are other areas where you can make up the "Limits" placed upon you by the powers that be....

Big or Small Block? Type of vehicle...Weight? I add that , simply because I have seen Chrysler Newyorkers that turn mid 11's, and a six cylinder Dodge Omni, that turned low 10's......Neither of which would have passed a sanctioned Tech inspection the way they were set up. So the Limits combined with what you try to accomplish, will go a long way toward your goal.There is no sense in building a Small block streeter to push a C -body Dodge.. but A big block in a Vega is overkill,,,,,

Do they limit gearing? If this is a STRICTLY RACE motor, Never to see the streets..The skys the limit, as long as you get past TECH.
 
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Old 04-18-09, 06:21 PM
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its a small block that will never see the street. So the skys the limit.
 
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Old 04-18-09, 06:35 PM
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This motor is going to be raced on a dirt track. I can set the car up to hook up all the motor I can get.
 
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Old 04-18-09, 06:38 PM
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Mid size car? Auto or stick? There are virtually zillions of Cams out there, some work ,others wont come close, ...Then there is the right one for your combo.....

Years ago...I saw a DOS program for windows....I believe it was called CAMQUEST.....Type in your info and it would spit out a cam spec card......It worked pretty well, Might be worth the HUNT.

Bigger isnt always better, and a Bit on the conservative side usually ends up stronger than trying to force it......Just to avoid the car that either wont hook until your halfway up the track, or the car that launches like a Rocketship, then falls on its face when it hooks.If you have guidelines to meet, By all means do so, But dont build it based solely on meeting guidelines.It isnt necessarily a Bad thing to "Back Off A Bit", If it makes the Car and The "Judges" happy.
 
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