L-79 timing / idle problem

[Joe C.]

Re: L-79 timing / idle problem

Duke, I thought 2165 compression height is the same as OE for 300HP (1.671). The compression height of the 2166 piston is more (1.675), maybe I got it wrong.

http://www.flatlanderracing.com/trwchevy283_327.html

The 2165's are flat top pistons, 1.61" compression height, with NO valve reliefs.
The OE pistons, although 1.65 compression height, had 4 valve reliefs.

The 2165's may very well deliver same C/R as OE because of the lack of reliefs. The diff in compression height converts to about .840279 cc's (at 4.030 bore). The lack of valve reliefs will more than compensate for this.
I like these pistons. They are FORGED for strength, but using an alloy that expands less, thus allowing very tight skirt clearances. Seems to be a mix of "the best of both worlds".

I would have to look closer at them, but won't count them out without a thorough comparison.
The lack of reliefs would allow smoother flame front travel, and more uniform piston crown temp, and the lower coefficient of expansion, will deliver more consistent compression ratios at varying cylinder temps. The tight bore clearance gives very good oil control, and less blowby. Less piston "rock", less friction, less heat, less power loss....................all with high strength forged construction.

They are less expensive than the conventional forged, with domes, but more expensive than the cast hypereutectics. They sound like a good compromise to me.

There is probably a downside..............maybe these pistons are heavy?
Again, without further investigation it sounds "all good".

Joe

[Duke W.]

Re: L-79 timing / idle problem

http://www.flatlanderracing.com/trwchevy283_327.html

That's a good quick reference chart. Note that the compression ratio has to be specfied in the context of a specific deck height and head gasket thickness, which is not specified in the chart, but my 1991 Speed Pro catalog lists the same CRs and states it is based on nominal block deck height (9.025") and a .038" head gasket.

By the time you increase head chamber volume with some side overhang relieving, chamber smoothing, and a little valve sinking due to grinding the seats, the head chambers pick up one to two cc and you need to reduce gasket thickness to about .025" to get close to 10.5:1 true, which is okay for the LT-1 cam and current unleaded premium, but it might be prudent to keep it at no more than 10.3 for the Duntov and L-79 cams since they have somewhat earlier closing inlet valves than the LT-1 cam.

Duke

[Joe C.]

Re: L-79 timing / idle problem

Even though the 2165 is forged, the recommended installation clearance is only .0015", so there should not be any noticeable slap if properly installed.

I don't recommend this piston for any application. The compression height is a few thou less than OE and it will not provide sufficient compression in most applications.

For a 300 HP rebuild I recommend Federal Mogul or Keith Black flattop hypereutectics that should yield 9.5-9.75 true CR with nominal block deck height, chamber volume, and suitable head gasket selection.

For L-79s I recommend the L2166F domed OE replacement piston, with a target CR range of 10-10.25:1, which should be achieveable as above with suitable head gasket selection.

Duke

..................................... The 2165's are flat top pistons, 1.61" compression height, with NO valve reliefs.
The OE pistons, although 1.65 compression height, had 4 valve reliefs.

The 2165's may very well deliver same C/R as OE because of the lack of reliefs. The diff in compression height converts to about .840279 cc's (at 4.030 bore). The lack of valve reliefs will more than compensate for this.
I like these pistons. They are FORGED for strength, but using an alloy that expands less, thus allowing very tight skirt clearances. Seems to be a mix of "the best of both worlds"........................................... .......

[Duke W.]

Re: L-79 timing / idle problem

According to my printed 1991 Speed Pro catalog the 2165s have four valve reliefs with a net volume -5.4 cc, and the online Speed-Pro catalog shows the same data.

You can look it up here:

http://21cgt.com/FMWebCatalog/default.htm

Type in part number L2165F and select "Speed Pro" brand.

Both vendor and mfgs. catalogs are full of errors, so if there is any doubt, I always check multiple sources until any ambiguity is resolved.

The 2165 is not a specific OE replacement, but an "upgrade" and the lower compression height probably assumes that the block was decked, so it will achieve a moderately high CR if the block is decked .010" and a typical .038" composition gasket is used.

Duke

[Joe C.]

Re: L-79 timing / idle problem

Even though the 2165 is forged, the recommended installation clearance is only .0015", so there should not be any noticeable slap if properly installed.

I don't recommend this piston for any application. The compression height is a few thou less than OE and it will not provide sufficient compression in most applications.

For a 300 HP rebuild I recommend Federal Mogul or Keith Black flattop hypereutectics that should yield 9.5-9.75 true CR with nominal block deck height, chamber volume, and suitable head gasket selection.

For L-79s I recommend the L2166F domed OE replacement piston, with a target CR range of 10-10.25:1, which should be achieveable as above with suitable head gasket selection.

Duke

The diff in compression height calculates to exactly 0.836133 cc's. (@ 4.030" bore).
All other things being equal, this will have negligible effect on C/R.

[Duke W.]

Re: L-79 timing / idle problem

The actual difference of.004" compression height on compression ratio, all other things equal, is approximately 0.11.

For precision engine restorations I recommend a nominal target CR range that covers about 0.25 of a point. Once the actual min/max range for the specific example is determined, I recommend grinding the largest chambers an amount that results in a min/max spread of no more than 0.10 for the specific build.

For example, if the target nominal CR range is 10.25 to 10.50 and the actuals are determined to be 10.35 to 10.55 across all eight cylinders, which is a typical spread, grind and remeasure the highest CR chamber volumes as required and run the new numbers through a CR calculator until the highest is no more than 10.45.

It's all in the details!

Duke

[Rick K.]

Re: L-79 timing / idle problem

Just a quick up-date on my on going noise & vibration problem.

I pulled the dist. again & switched out the points plate, removed the LL Breakerless ignition (which ironically went south) and replaced with standard point set. Also changed the coil. Checked all plug wires with a known good wire.

Installed a B-22 vac. can.

Removed the belts so I could eliminate the water pump/alt./P.S. pump.

Checked the pushrods for damage. Readjusted the valves again to 0 lash + 3/4 turn. I can make the problem more pronounced if I get a valve to tight.

When the engine is cold I still get the vibration which I think is causing the right side pipe to rattle & bang. When I remove the sidepipe cover the noise stops or when the engine warms and/or RPM goes over 1000 RPM the noise & vibration subsides.

The balancer was replaced in 2006 with a #3817173.

I will be installing a spare sidepipe tomorrow in hopes that will help.

[Rick K.]

Re: L-79 timing / idle problem

I replaced the right sidepipe donut with a solid metal style donut & the banging sound disappeared.

The idle is much better after the latest valve adjustment & dist. work that I outlined in yesterdays post. I can still see an occasional 1.5* spike in timing so I will be chasing that problem down.

With these problems resolved I'am ready to point the car North to Carlisle!