Thread: camshaft for rebuild in 85 chevy truck?

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Originally Posted by Gilligan2591

after rebuild should be about 9.72:1 compression (advertised)

Written properly, this would read....after rebuild, should be about 9.72:1 static compression ratio.
The word "advertised" applies to camshaft duration, not static compression ratio.

Originally Posted by Gilligan2591

with flatheads

This should be written as flat-top pistons. The word "flathead" refers to an engine design made famous by Ford Motor Company and also to a tribe of Indians in Montana.

Originally Posted by Gilligan2591

and 64cc edel heads

You're on this forum asking for help. That means that full disclosure will be necessary. We have no idea what heads you have chosen, therefore cannot comment further on your combination. You should post here everything that Edelbrock has to say about them or provide us with a link so we can see what you want to use so that we can help you....or not.

Originally Posted by Gilligan2591

(decided to bail on the factory "smog heads", that do NOT have 193cc runners)

Where did that number come from? I'm unaware of any such runner size on Edelbrock or any other heads.

Originally Posted by Gilligan2591

but i could lower CR with dished pistons?

Yes, you can lower static compression ratio and dynamic compression ratio by using a larger combustion chamber or by using pistons that have a dish, as opposed to being flat-top or domed. Use caution when selecting a piston though. Standard 350 Chevy pistons have a compression height of 1.560" (measured from the centerline of the piston wrist pin to the crown of the piston). Rebuilder pistons have a lesser compression height of 1.540" (or some other height shorter than 1.560") to compensate for cutting the block decks (resulting in a shorter block deck height). If you use these cheapo rebuilder pistons in a block that has not been decked, the piston will be further down in the bore at top dead center than the correct 1.560" compression height piston. This will make it very difficult to arrive at a close "squish" figure and could make the motor more detonation-prone, depending on cylinder pressure and fuel quality. Use caution when selecting the crown configuration of the piston. Many pistons have a shallow dish in the middle, with very little raised area (the actual piston crown) around the perimeter of the piston and will offer insufficient flat area to mate up with the underside of the cylinder head with the piston at TDC to create turbulence in the air/fuel mixture. Here's an example of a "D-cup" piston. See the large flat area on the piston crown on the left side of the photo?
http://www.kb-silvolite.com/kb_car/p...tails&P_id=154
That's the part of the piston that will mate up to the underside of the cylinder head to "squish" the air/fuel mixture out of that area as the piston comes to top dead center to fire the spark plug. This squishing of the mixture "jets" it across the chamber, homogenizing the mixture and eliminating rich and lean pockets of mixture that may not burn when the plug fires. This squishing of the mixture will allow the use of a lesser grade of fuel without detonation than if there were little or no squish at all. A squish of 0.035" to 0.045" is considered by many builders as optimal. Squish is the sum of the piston deck height (how far the piston is down in the bore with the piston at top dead center) added to the gasket thickness. Here's an example of a dished piston that has no squish pad and is made in the rebuilder compression height of 1.540".......
http://www.summitracing.com/parts/sl...make/chevrolet

With aluminum heads, you will want to use the head gasket that is recommended by the manufacturer of the heads. Most head gaskets have a fire ring built into them and if the ring has not been pre-flattened at the time of manufacture, then the ring can brinell the aluminum heads and they would have to be re-surfaced in order to be used again in another build. At any rate, the correct gasket will probably be on the order of 0.039" compressed thickness, so you would want to cut the block decks for zero piston deck height in order to achieve a 0.039" squish, for instance.

Originally Posted by Gilligan2591

to much cam? not big on comp XE? im not starting this build till janruary and am still doing LOTS of research so by all means i am open to suggestions.

Here's where most guys get into trouble on their first build. Too much cam. The camshaft is not a stand-alone piece. It is one of a number of components that must be matched-up in order to present an orchestrated result. You must take into consideration what you will do with the truck THE MAJORITY OF THE TIME, as well as the transmission used, the torque converter used, the rear end gear ratio, the weight of the vehicle and the desired operating range of the final product. Those big tires will change the overall gear ratio, going the wrong way for a performance camshaft. The more cam you use, the numerically-higher you will need the rear end gears to be and you have the double consideration of big tires in the equation. You will need more gear for the cam and also more gear again for the tires. In other words, if you decided you could use a 3.73 gear with standard size tires, you would need to move up to 4.11 or 4.56 gears with big tires. You have to understand that any cam you install that is hotter than stock will move the power band up in the rpm range and you must change gears to allow it to work in the new rpm range that it was designed for. A hot cam with a stock gear will be a dog and you'll hate driving the truck. Depending on cam timing, you may also have to use a torque converter that stalls at a higher rpm than stock, so keep that in mind. If, for instance, you were to choose a cam that makes power from 3000 to 6500, there would be little power from the motor until revs would reach 3000, so you'd need a looser converter to allow the motor to rev past the rpm's where the cam begins to make power. If you used a stock converter with such a cam, the truck would be a dog until you reached 3000 rpm's and it would be the same way at every stop light. Can you see how this works?

I am not big on XE cams or any other cams with snappy valve action for everyday driving. It's just some "high-helix, double-throw-down" bullshit that the marketing guys have come up with to sell more cams to unsuspecting hot rodders in my opinion. There is a mechanical limit to the speed at which a lifter can be lifted by a camshaft lobe and these guys are right up against the wall with the numbers. This is insanity for a street-driver. If you were in an engine shoot-out against other competitors and needed a few more hp to win the event, then you could justify quick ramps. But on a street-driver motor, it just means less longevity and more chance for failure compared to a camshaft with gentler ramps. To compare the ramp speed of one cam to another, deduct the 0.050" duration figure from the advertised duration figure. Comp uses a 44 degree difference on their Fosdick XE cams, which is probably at or near the mechanical limit. The name for this difference was coined by Harvey Crane many years ago as "hydraulic intensity". Here's a Melling shaft for instance that is cut on the intake lobe at 204/278, for a hydraulic intensity of 74 degrees. Can you see how this would be much, much easier on the cam and lifters, by raising the tappet much more slowly?
http://www.summitracing.com/parts/me...make/chevrolet
Try to understand that the average hot rodder does not need radical hydraulic intensity. Also please understand that the pin point of contact between the lifter and the cam lobe on a flat tappet camshaft generates in the area of 250,000 lbs of pressure per square inch, so everything you can do to help these pieces survive will be to your advantage.

Originally Posted by Gilligan2591

BUT those edel heads are going to seriously up my rebuild cost, so lets keep it reasonable too.

Nobody said you have to use expensive aluminum heads. There is nothing wrong with cast iron heads, particularly for a street-driver, and they will come at a somewhat reduced cost compared to aluminum.

To do a proper rebuild, you'll want to align-bore or align-hone the main bearing bore in the block, cut the block decks to square by registering off the main bearing bore and to the block deck height you want to use with your particular stack of parts, then bore and hone the block for the piston type and ring material using a deck plate.

One last thought. If I were you, I'd seriously consider going with a roller cam. There are just too many hoops to jump through with a flat tappet cam in today's world.

Oops, one last-last thought. In your opening post, you made mention of mileage. Please be advised that if it's mileage you're looking for, find a 305 long block with 416 heads, install a bone-stock cam, stock intake manifold mounting a Q-Jet and sell those tires in favor of a set of 27/28" tires.