8cylinders>4

http://www.summitracing.com/parts/ml...make/chevrolet

Right now I got my motor out getting freshened up with the stock crank, my heads are at AI getting their 200cc top end package

My motor is at the machine shop getting checked out and due to 150,000 hard miles she is getting punched .030 over and am in need of a piston and ring set with a set of 6 inch rods.

I was specifically looking at the the compression height at 1.25" from my calculations after the block gets straightened up this will leave these pistons in the hole about .020 -.025" and with a .026" head gasket will give me a quench of about .046 to .051" which is safe but is it too safe? This setup would give me about 11.5-11.6:1 compression as well.

Anybody have any advice, opinions? In the interest of time i'd like to get these ordered asap, any and all advice welcome!

redline9570

I am apposed to the 6" rod since it moves up the wrist pin and makes the piston less stable in the bore. You can have the deck zeroed to give you better quench.
Rod Lengths/Ratios: Much ado about almost nothing.

Why do people change connecting rod lengths or alter their rod length to stroke ratios? I know why, they think they are changing them. They expect to gain (usually based upon the hype of some magazine article or the sales pitch of someone in the parts business) Torque or Horsepower here or there in rather significant "chunks". Well, they will experience some gains and losses here or there in torque and or H.P., but unfortunately these "chunks" everyone talks about are more like "chips".

To hear the hype about running a longer Rod and making more Torque @ low to mid RPM or mid to high RPM (yes, it is, believe it or not actually pitched both ways) you'd think that there must be a tremendous potential for gain, otherwise, why would anyone even bother? Good question. Let's begin with the basics. The manufacture's (Chevy, Ford, Chrysler etc.) employ automotive engineers and designers to do their best (especially today) in creating engine packages that are both powerful and efficient. They of course, must also consider longevity, for what good would come form designing an engine with say 5% more power at a price of one half the life factor? Obviously none. You usually don't get something for nothing - everything usually has its price. For example: I can design a cam with tremendous high RPM/H.P. potential, but it would be silly of me (not to mention the height of arrogance) to criticize the engineer who designed the stock camshaft. For this engine when I know how poorly this cam would perform at the lower operating RPM range in which this engineer was concerned with as his design objective!

Yet, I read of and hear about people who do this all the time with Rod lengths. They actually speak of the automotive engine designer responsible for running "such a short Rod" as a "stupid SOB." Well, folks I am here to tell you that those who spew such garbage should be ashamed of themselves - and not just because the original designer had different design criteria and objectives. I may shock some of you, but in your wildest dreams you are never going to achieve the level of power increase by changing your connecting rod lengths that you would, say in increasing compression ratio, cam duration or cylinder head flow capacity. To illustrate my point, take a look at the chart below. I have illustrated the crank angles and relative piston positions of today's most popular racing engine, the 3.48" stroke small block 350 V8 Chevy in standard 5.7", 6.00", 6.125" and 6.250" long rod lengths in 5 degree increments. Notice the infinitesimal (look it up in the dictionary) change in piston position for a given crank angle with the 4 different length rods. Not much here folks, but "oh, there must be a big difference in piston velocity, right?" Wrong! Again it's a marginal difference (check the source yourself - its performance calculator).

To hear all this hype about rod lengths I'm sure you were prepared for a nice 30, 40, or 50 HP increase, weren't you? Well its more like a 5-7 HP increase at best, and guess what? It comes at a price. The longer the rod, the closer your wrist pin boss will be to your ring lands. In extreme situations, 6.125" & 6.250" lengths for example, both ring and piston life are affected. The rings get a double whammy affect. First, with the pin boss crowding the rings, the normally designed space between the lands must be reduced to accommodate the higher wrist pin boss. Second, the rings wobble more and lose the seal of their fine edge as the piston rocks. A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC. This is another area where people often get the information backwards.

In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!

Now to the subject of rod to stroke ratios. People are always looking for the "magic number" here - as if like Pythagoras they could possibly discover a mathematical relationship which would secure them a place in history. Rod to stroke ratios are for the most part the naturally occurring result of other engine design criteria. In other-words, much like with ignition timing (spark advance) they are what they are. In regards to the later, the actual number is not as important as finding the right point for a given engine. Why worry for example that a Chrysler "hemi" needs less spark advance that a Chevrolet "wedge" combustion chamber? The number in and of itself is not important and it is much the same with rod to stroke ratios. Unless you want to completely redesign the engine (including your block deck height etc.) leave your rod lengths alone. Let's not forget after all, most of us are not racing at the Indy 500 but rather are hot rodding stock blocks.

Only professional engine builders who have exhausted every other possible avenue of performance should ever consider a rod length change and even they should exercise care so as not to get caught up in the hype.


http://www.iskycams.com/techtips.php#2005

lt1-xjs

A great piston choice, pair them up with the Scat 6" rod with the 7/16" bolts and ARP the mains.

RamAir95TA

I would try to pull your quench down tighter to .035"-.040". Better flame travel and turbulence resulting in more horsepower. Keep in mind with a tighter quench the accuracy of the tune will be much more important because richer mixtures burn slower while lean mixtures burn much faster.

Tune, tune, tune, and you'll appreciate the results.

For the rod length go ahead and run the 6". I've had good results with them in the past.

And I'm kind of tired of the article cutting and pasting. Sharing some real-world experience once in awhile would be nice.

MachinistOne

Yep.

Real world I've run them both ways hundreds of times, long rod takes weight out of the rotating assembly which means less stress on the parts at high rpm, increases resistance to detonation, reduces rod angularity, etc, etc

There's many benefits to running a 6.0" rod. Enough so that the GM engineers went to a 6.098" rod with the Gen III engine family which go 200k + easily with those unstable short skirt pistons.

8cylinders>4

So for the street driven lt1 guys. With these pistons, and AI's 54 cc chambers and .026 head gaskets would it be wise to tell the machinist to leave the pistons in the hole about .015" giving me a quench of ~.041" and a compression ratio of about 11.74:1? would this be too tight? sometimes in my area its hard to get gas better than 91?

or would it be better to go with their 56cc chamber, have the pistons .010 in the hole, for a quench of .036" and a compression ratio at 11.6:1? Would this quench be cutting things a little close for a street driven, m6 car that will see more than 6500 on occasion?

RamAir95TA

I think your first paragraph would be more ideal.

MachinistOne

Detonation is a motor killer, don't run the risk if you can't get better than 91.

Skim cut the block, use the larger chamber and get the quench in the 11.5 range max.

Those are good pistons, I use them regularly. Get the Scat Pro I rods like were mentioned, best rod for the money, but your machinist will need to break them down and stretch the bolts properly and possibly hone the big end a little to put them on size, same goes for the pin end, they're typically a little tight out of the box.