short rod long rod

[Fired Up]

Talking with an engine builder today. He said building a BBC with a longer rod (6.385) and raising the pin location in the piston is better than running a standard rod (6.135) and standard pin location. You keep the same cu in size but it is more efficient. Can someone explain to me why..?

[ARS Marine inc.east]

Slows Down the dwell time the piston is at the top of the bore
thus producing more of a complete Burn of the intake charge
Also less stress on the rod's because of it slowing down the recipocating movement.

[steelcomp]

Slows Down the dwell time the piston is at the top of the bore
thus producing more of a complete Burn of the intake charge
Also less stress on the rod's because of it slowing down the recipocating movement.
Don't you mean it increases the dwell time at TDC???
Another advantage is less rod angle which puts less side loading on the piston, causing less friction and piston rocking, which in turn, keeps the rings squarer in the bore. The rod dosen't "swing" as far, either, and takes less energy to change directions.
There's tons of pro's and con's of rod length ratio, and literally volumes could be written on the subject. Great debates have taken place on this subject.

[Infomaniac]

This is a very good article (http://victorylibrary.com/mopar/rod-tech-c.htm)

[GofastRacer]

Yes it is!.. :cool:

[steelcomp]

right on!!!

[wsuwrhr]

everyone is right on.
problem is no two people will agree on this or other engine building philosophies.
Brian

[Fired Up]

Good article. I ran some of the calculations and there is a small advantage to the slightly longer rod over the short, but each has some factors to consider. Its not just a simple change over. The cam dynamics are probably the most affected. The rod angle is only about 1/2 a degree less with the 6.385 over the 6.135 (18.6 degrees vs 18.05 degrees). Thanks for the info...

[TIMINATOR]

+.400 rods cost the same as + .250 rods and will fit a standard deck block and stroke, and the longer the rod, the lighter the piston(usually). Oh yeah, anybody putting shorter rods in their 460 jetboat motors? I thought not.... The long rod is what allows 440 Mopars and 460 Fords to run as good as they do against the BBChevys superior heads, but shorter rods. That is also one of the main reasons that the flathead Ford hung on as long as it did against the SBChevy. Somebody please post the rod and stroke and ratio of a flathead Ford will ya? I know its great! P.S. build a tall deck Chevy truck block and use a 7.00 rod with a stock stroke, and you get a longer intake runner too! Not that I would do such a thing..... TIMINATOR

[Fired Up]

So you are saying you can run the .400 rod with the STOCK crank and still squeeze into the standard deck height? I'll have to go back to the formula and refigure some things...

[steelcomp]

So you are saying you can run the .400 rod with the STOCK crank and still squeeze into the standard deck height? I'll have to go back to the formula and refigure some things...
1.265" C height. That gives you .77" for a ring stack. Pretty tight.

[Fired Up]

Just to clarify....is the compression height the distance from the TOP of the piston face to the very TOP of the piston dome? Or is it from the TOP of the compression ring to the TOP of the piston dome.....?

[TIMINATOR]

Compression Height is the dimension from the piston "flat" (the part that is even with the top of the block deck), to the CENTER of the piston pin. Most aftermarket piston companys make the +.400 rod, standard deck, piston. TIMINATOR

[Moneypitt]

Sam, the "dome measurement" is usually in volumn, cc s of dome. This is the number used when calculating compression ratios due to displaced/filled area of the combustion chamber cc........Ray

[ARS Marine inc.east]

[QUOTE=steelcomp]Don't you mean it increases the dwell time at TDC???
Steel Yes That is what I was trying to say it was just early in the morning for me.

[steelcomp]

[QUOTE=steelcomp]Don't you mean it increases the dwell time at TDC???
Steel Yes That is what I was trying to say it was just early in the morning for me.
Actually what you said is correct. "Slows down the dwell time"... that means it dwells there longer. It just sounded funny. Sorry.

[ARS Marine inc.east]

Actually what you said is correct. "Slows down the dwell time"... that means it dwells there longer. It just sounded funny. Sorry.
See What Happens when When I read Something FAST
I Thought it was Correct But Looked at it with your point of view and was
Confused Alittle .
:D

[Tahiti350]

Long rod/short rod has been an ongoing controversy that even the prostock racers don't agree on. The biggest issue is the testing to get the whole package to work together. If you build a short rod motor and optomize it on the dyno, then swap out the rod and piston package with one that has the same compression you may lose power and efficiency until you re-optimize the whole thing again. Long rod motors seem to like less spark advance, and seem to tolerate weak (cheap) fuel a little better (anyone had problems with water in the dock fuel?).
I followed this pretty close during my circle car days, ran a dodge 318 against the SBC's and fords- long rod torque out of the corners, but CI hp at the end of the straight. Wanted to switch to a 360 mopar and would have cleaned their clocks.
GaryB> Tahiti350

[TIMINATOR]

Fired up and Steelcomp; read the post, a TALL deck block and a standard stroke crank and 7.00 rod yield a comp ht of 1.200", plenty of room for everything! If you don't mind running oilring supports and having the TOP oil scraper ring below the top of the pin hole. It doesn't hurt anything, mine has been running that way for 3 years now...... and by the way I run PENNZOIL! In everything! TIMINATOR

[steelcomp]

Fired up and Steelcomp; read the post, a TALL deck block and a standard stroke crank and 7.00 rod yield a comp ht of 1.200", plenty of room for everything! If you don't mind running oilring supports and having the TOP oil scraper ring below the top of the pin hole. It doesn't hurt anything, mine has been running that way for 3 years now...... and by the way I run PENNZOIL! In everything! TIMINATOR
Timo...the first part of the post dosen't say anything about a tall deck block...I was referring to your comment regarding putting a +.400 rod in a std. deck block. I didn't say it couldn't be done...I know it has. I've built LOTS of motors with the oil ring in the pin hole. If you're running pin buttons, I don't mind the top oil ring below the hole, but with regular locks, it's a way for oil to get past the oil ring.

[TIMINATOR]

Steelcomp youre right, but you can use a .400 long rod in a std deck block, I have built many for customers. I also don't believe in pin buttons as I don't like the extra reciprocating weight. I have had several BBC motors with the pin bore past the top of the ringland and haven't had any adverse effects. No oil consumption issues, nothing. These were in boats, street/strip and drag cars. Keep in mind that ALL of our engines are 5 step plateau honed and all the hp ones are torque plate honed too, that may make the difference. This is not the kind of thing that you want to trust to a stock/ rebuilder shop. Your results may vary. TIMINATOR

[steelcomp]

Steelcomp youre right, but you can use a .400 long rod in a std deck block, I have built many for customers. I also don't believe in pin buttons as I don't like the extra reciprocating weight. I have had several BBC motors with the pin bore past the top of the ringland and haven't had any adverse effects. No oil consumption issues, nothing. These were in boats, street/strip and drag cars. Keep in mind that ALL of our engines are 5 step plateau honed and all the hp ones are torque plate honed too, that may make the difference. This is not the kind of thing that you want to trust to a stock/ rebuilder shop. Your results may vary. TIMINATOR
OOOohhhh, yeah!!!

[wsuwrhr]

One good thing about a Big Block Mopar is the stock compression height is HUGE. 1.2 or bigger.
Meaning easy to add stroke and still have a pretty long rod without eating into the oil scrapers.
Brian

[AMC-Nut]

Get a motor book by Smokey Yunick, he was the innovator of long usuage. He also was a major player in BBC development. My dad used to have a machine chemical shop right next to his in Daytona.