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bent valve woes


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well i finally got my Z running last week, what did i forget to do you ask ? properly adjust the sole rocker i replaced an over tightened rocker i assume resulted in some valve float or a delay in the exhaust valve closing thus the up and coming piston struck the valve and bent it. for now i will be replacing this valve and pushrod alone, i need my Z to be somewhat mobile over the winter..... what things should a newbie engine "builder" remember when putting a head back on ? if something like this has been asked before (probably not because most everyone here knows alot more than i) just say so and ill use that beloved search function :) also, is a nicer set of fresh 2.02 heads worth it if my motor has less that 2000 miles on it ? my motor is a 71 350 all stock with the 882 heads on it (stock too) -pete

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Guest greimann

Be sure to use a new head gasket. They are only good for one squish. Also be sure to follow the book on rocker adjustment and head bolt torque. A set of 2.02 heads would be a good investment for future improvements, but would provide little or even negative power gain on a completely stock motor.

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thank you sir thats exactly what i wanted to hear.... would you do a cam on a stock setup ? im looking at the summit 1105 because of the nice torque curve ive seen on DD2000 and the xe 274h-10 im really new to all of this so if these are really extreme cams or something just say so and ill do my own research. thanks

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Guest greimann

The Summit 1105 is too much cam for a low compression motor. You would need at least flat top pistons and 64cc heads (~10.5:1 compression) and intake + headers to get the cam to work.

 

A better chioce would be the 1102. That cam is esentially a copy of the Edelbrock Performer, which is good for low RPM torque and suitable for low compression.

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well i wanted to keep the compression down so i dont have to worry about any knock, this is going to be a daily driven toy, i already have a nice weiand split single plane, and 86 corvette tubular manifolds, but i will be fabbing my own headers once i get this thing actually driving.... why is the high compression essential for the big cam ?

 

i was just playing with DD2000 while reading some things about matching cams and heads, and why 2.02s advantage is minimal.... but im confused... this site said that instead of running 2.02 heads youd be better off having larger exhaust valves installed, which would be similar to running a cam with more exhaust lift.... is this true or did i mix it up? when i punched this into DD2000 i got the power back that i lost from the transition from the 1105 to the 1102 and now its at a much nicer RPM. what im basically going for is something fun to drive that has a fairly linear (read straight line) power curve. also one last question before i get annoying, why does volumetric efficiency go up with a bigger cam ? and is there an ideal vom.eff. ??? thanks bro your always there for me with these things

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Don't forget to re-torque the bolts after you've driven around a bit. check them after 50 miles, or about an hour of running time.

 

Larger cams have higher volumetric efficiency at higer RPM's, but lower VE at low RPM's. Torque is a good indicator of VE. At the torque peak, generally the engine is operating at the higest VE.

 

Any engine combo has different VE at different RPMs, high-torque motors generally have the highest VE, but it occours at lower RPM.

 

There is no Ideal VE, greater than 100% is possible, but only for a limited RPM range. The trick is to get the broadest RPM range you can, thereby providing a broad range or RPM at which the motor produces good power. Roller lifters can do this by increasing the rate of lift and closing, allowing the valves to open farther in less time. Higher ratio rockers also do this, but at the cost of inreasing the stress on the valvetrain. If the heads aren't built to handle the stress, problems can result.

 

Oh yeah, why is higer comp needed for a larger cam? Because generally the VE decreases with greater valve duration, and higer compression offsets this effect somewhat.

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Guest greimann

Basically a cam with a lot of overlap bleeds off cylinder pressure at low RPM. You need high static compression to make up for that otherwise the motor is a real dog at low RPM cruise. As RPM rises to the cam's "sweet spot" Cylinder filling is maximized and volumetric effeciency is optimized.

 

A milder cam "traps" more air at low RPM and doesn't need a high static compression ratio to be efficient. Low RPM torque makes a street drivien car very fun to drive. It is easy to tune and gets better fuel mileage. Don't get goo goo eyed by a big cam horsepower numbers. Street cars are much more suited to a cam that can pin you back as soon as you mash it coming out of the corner.

 

Be careful what you see on Desktop Dyno. When you punch in the cam numbers, do you have the actual cam card timing numbers? These are the degrees of the valve events that are not listed in the advertising text. You need IVO, IVC, EVO, and EVC figures to accurately model the cam behavoir.

 

Lots and lots to read about engine component combinations. I would suggest you go to Amazon.com and pick up some books by David Vizard.

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ahhhhaaa that makes soo much more sense now.... i found in playing with dd200 that enlarging the exhaust valve will create the same power that was lost going from the 1105 to the 1102, is this because the larger exhaust valve flows more for the same amount of duration than the smaller? does this also mean that you can get away with running less overlap ????

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