I just ordered my engine kit the other day and I'm getting a kit with 350 .125 dome pistons and 6" rods! I've heard that 6" rods give you a lot more torque! What are the advantages and disadvantages (if any) of going with these 6" rods?? TIA!


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1986 Trans AM
305 TPI
Well over 200,000+ miles (speedo/odometer non-funtional! Odometer reads 142,000)
4 Wheel Discs
9 bolt Borg Warner Rear (2.77's....oh joy) :P
Completely Stock
Current Project: upgrade to a 355 TPI, 6" rods, 9.5:1 Compression, Stock TPI ported, 24#/hr LTI injectors, Hedman Shorty Headers, SBC Stock Casting heads pocket ported, Comp Cams XE262-14 TPI Cam, Ed Wright Fast Chip, Holley AFPR, TPIS airfoil, MSD wires, MSD 6-AL Box, proform hot coil!!

 

They won't give you torque on the bottom end. It a street engine that doesn't see 6000 rpm very often, they don't have any advantage. They help a high revving engine stay together--better rod angle. Fairly common around here to see a circle track engine run the long rods, they see 8000+ rpm lap after lap. They do add a few horses in the upper rpms on these circle track engines, but it's not usually done for power.

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82 camaro--original steering wheel, brake/gas pedals, seats--everything else modified
82camaro

[This message has been edited by 82camaro (edited May 27, 2001).]

 

Thanks for the reply! I had heard that it would broaden the RPM range for the HP and TQ Curve a little! I know the stock TPI intake won't rev past 5500 but I plan on doing some major port work to the intake so I'm planning on hitting 6000 easy! Hopefully this 6" setup will give me a little bit extra advantage at the strip!

 

it will give more power all the way through the band. peak numbers will go up, but more of a flat torque curve is what you'll get. better yet is more allowed compression with them. one of the best issues is the less weight of the pistons. it allow's quicker revving, smoother (less vibration), and higher revving capability.

 

From my DeskTop Dyno2000.

Question: Everyone talks about how longer rods make more power. Why isn’t rod
length one of the choices in the pull-down menus?
Answer: Tests we have performed with the Dyno2000 show that rod length has
virtually no affect on power. We realize that many actual dyno tests have shown
power increases, but our simulation tests tell us that the power, when found,
probably has little to do with piston dwell at TDC (and the associated thermodynamic
effects) or changes in rod angularity on the crank pin. The measured
power differences are most likely due to a reduction of friction on the cylinder-wall
from changes in side-loading on the piston. This can vary with bore finish,
ring stability, piston shape, the frictional properties of the lubricant, etc. These
variabilities are highly unpredictable. Some development, after all, can only be
done in the real world on a engine dynamometer.

[This message has been edited by AJ_92RS (edited May 27, 2001).]

 

Real Answer: Desktop Dyno is an inexpensive dyno simulation (cheap) that does not have the programing it takes to show the difference in rod lengths.

 

Rod length has alot to do with top end power. If you are building up say a stock 400 ci motor to rev to 7,500rpm and select the proper cam, heads, ect...

A such a high RPM the stock rods will rock in the bore unseating the rings and causing high RPM blowby and this most definatly has to reduce power output. If the motor with these short rods will stay together at this rpm anyway since this really lays a beating on the piston skirts as well.

 

They can provide higher HP, but I don't think you can globally apply any numbers, conditions vary too much.

Anyone remember the hotrod article called the 350 chevy should have built. They took a 400 block, and used a 327 crank with some ford 6 cylinder rods that were about 6.12 inch. The motor had AFR190 heads (with some lame valve sizes 2.00/1.56 designed to keep the chamber small) They got it to run on pump gas and it had 11 to 1 compression and a comp hydroller with pretty short duration (215 at 050). The motor made about 415 hp (at like 6000) and 420 ft/lbs torque. These numbers are not any better than a standard 350 with aluminum heads. Long rods will not really show much of an improvement in the 4000-6000 rpm range. And while they will reduce piston weight, it is not worth the trouble if you have to compress the rings into a small compression height. Often times, the oil control ring will need a support in the piston pin area because the pin has been moved up so high.

 

BadSS,

Did you read the rest of the answer? It says it all. It's just an explanation of why rod lengths DO make more power. They even said a REAL dyno is the ONLY way to find out. I posted it to show that longer rods do help, not that they don't. No I don't have $2k to spend on a program that takes in all of the variables know to man. But this just gives a brief overview of why they do help. That's all.

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92 RS w/t-tops 305 TBI Auto.
170K miles and don't burn a drop o'oil
-K&N Truck filter #1500 w, open ele.
air cleaner
-Dynomax 2 1/2" cat-back
-B&M TransPak
-Jet Stage2 Chip
-180* T-Stat w/ 185* Fan Switch
-JVC CD--Alpine speakers & 4ch. amp
Rockford 2ch. to 2 MTX 12" subs

 

Dyno2000 may be a cheap program but if it you model your motor accurately, it's predictions are amazingly close to real world results.

Here is a comparisone betwwen Dyno2000 predictions and the actual results of my motor on a SuperFlow engine dyno:

---Dyno2000--------SuperFlow
RPM---HP---TQ-----HP---TQ
2000--164--430----166--432
2500--229--481----232--483
3000--286--501----287--502
3500--354--530----355--530
4000--420--551----423--555
4500--488--569----490--570
5000--547--574----551--579
5500--596--569----598--571
6000--624--546----620--543
6500--642--518----642--519
7000--644--483----621--466 (Rev Limiter was retarding timing)

I also tried to more sophisticated and much more expensive Engine Analyzer Pro, and it was no more accurate than Dyno2000 was. But then again, an accuracy of over 99% for a 650hp motor is pretty darned good.

SuperFlow Dyno printout

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1982 Corvette Tremec TKO
<A HREF="http://www.geocities.com/monty_williams/434specifications.html" TARGET=_blank>SBC 434 700hp/600tq
</A>

 

yea, I think what monty (and aj) said is pretty true. The formulas that are utilized in DD2000 are no less complex than another simulator. The fundamental difference is that the higher dollar sims have more variables to adjust.

[This message has been edited by jcb999 (edited May 31, 2001).]

 

Test conclusions for longer rod ratios vary from test to test. Some say a big increase of 8-20HP and some say more. On the other hand, some tests conclude only a 1HP increase to none at all. From what I can see from many tests done on longer rod ratios, the one advantage they all have in common are that angular thrust wear against the cylinder walls is reduced.
Longer rods due allow the piston to maintain longer dwell times at TDC. But on the down side, dwell time at BDC is a lot longer also. Instantaneous piston velocity is slowed down also, which makes the motor more sensitive to the size ports you have on your heads at lower RPM speeds. What this means is that, say a faster port of 180cc can slow down to a port of say 200cc. General rule of thumb is that longer rods improve higher RPM HP while shorter rods increase lower RPM torque.
Looking at your post, I can see that you are using dome pistons. One thing about domed pistons is that the dome can get in the way of proper flame propagation leading to a more inefficient combustion process. Flattop or even better, reverse dome (dished) pistons allow better flame travel across the piston face resulting in a better combustion process. Instead of going with a domed piston to increase compression, I would go with a cylinder head with a smaller combustion chamber over flattop or dished pistons.


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N/A 406, TPIS Miniram, 58mm TB, 30lbs injectors, Custom Ground Elgin Cam, Ported TFS heads, SLP 1-3/4", & DFI computer.
11.70@117mph

 

Sorry to step on toes, but the formulas utilized in Desktop Dyno are less complex than the standard version of Performance Trends Engine Analyzer program and do not come close to their Pro Version. It seems like Monty has compared the differences at least on one engine. However, I have run simulation programs on a number of engines I’ve built and had dynoed (or customers have had dynoed). For what it’s worth, I’ve been building engines and tuning cars for over 21 years and have used simulation software since 1992. While no newcomer to simulation software use, I admit I have limited experience with Desktop Dyno. However, I did use it enough in comparison to real dyno runs to see first hand that it does not have the amount of programming needed to properly simulate low overlap cams (small cams). Desktop will get you close on the peak values and is less accurate the lower the rpm is from peak. At peak torque and above the figures are pretty close even for the small cams. However,,, the larger the cam the more accurate the Desktop is at the lower rpm scales. I don’t want to knock Desktop too much, but you guys using it to optimize the camshafts, especially the smaller “street” cams, are going to be in for a rude awakening. Plus I’ve consistently found errors or conflicts to my personal (know to be accurate and true) data in the drop down boxes in all the simulation software I’ve used. The old saying is so true, good in, good out. I’ll go out on a limb here and open myself up to be slapped around by Monty. I’d bet the cam used in his engine is at least 240 degrees at .050,, maybe at high as 250 on the intake (depending on the lobe spread),, and he probably manually input data for the heads also. Like I said,, I don’t want to pick on the Desktop program too much, but it’s not all knowing as AJ pointed out

Edited - Monty I just checked out your site and see that the cam is a 256/264 at .050 (112 spread), and it appears that the 4500 numbers posted above is a typo and should be 470 HP and 548 TQ based on the dyno sheet scanned in. I couldn't double check the rest because you don't have the real dyno numbers under 4400 rpm scanned in.

Fast Life seems to know as far as rod ratios go, the reason you see some dyno pulls showing more benefit from a rod swap than the other is the combination was more suited for one rod than the other. What I’m saying is, there is little difference between the distance traveled per degree of crank rotation and the dwell at TDC and BDC is different when comparing shorter and longer rods. However while these differences are small, they are enough that a cam swap (duration and or lobe spread depending on the difference) should be utilized to optimize the difference. It’s also enough difference that the efficiency of induction system,, head flow, cam, intake, and carb size comes into play. A dyno comparison where only the rod length is changed seems fair enough at face value, but that may not be the case. Folks that know how to manipulate the results could try to trick you into thinking what they want you to think,, or buy. Not saying all comparisons are corrupted, I know the ones I’ve done are real word enough and in most street/strip applications the stock rod length for a SBC is fine. Most folks would be better off applying the money difference to the heads than “aftermarket” rods, especially in “budget” applications. However, over the years the cost difference is becoming less and less. While I’ve run and use 5.565 rods in 400s, I do prefer bumping up to the 5.7 rods in all but the tightest budget builds. If I sound like I’m knocking longer rods, I’m not. Based on tear downs I’ve seen that a longer rod engine does appear to have less overall cylinder wear and they exhibit less engine noise, so there is no question there is a benefit there,, and that can not help but to free a few frictional horses,, as well as long rod theories having merit based in fact. So,, long story short,, my personal opinion is there is no need to loose any sleep about what rod length to run. However, run the longest rod you can afford without compromising the ring package (and that depends on application), and be done with it, whether it be the stock 5.565 rod or not.


[This message has been edited by BadSS (edited June 01, 2001).]

 

By all means, don't think that I believe that the DD2000 is all knowing. For $40 it's great, especially for guys like me that have little or no experience, designing a combo. It keeps them, and myself from putting AFR 220 heads under a dual plane, TBI intake that's being feed by a 194-224 @.050" dur. cam. So too me it's a great program. That and it's cheap entertainment. I fiddle fart with it all the time, just to see all my different options.

Oh, and BTW, FYI BadSS, the head flows are available for the DD2000 on MotionSoftware.com, for anyone who wants 'em. They have from AFR to World heads on there. Bench flows too, not manufactures claims. So yea, they are available, and yes they help. And you didn't step on toes. Just toe nails. JK.

[This message has been edited by AJ_92RS (edited May 31, 2001).]

 

thanks for the info bad. Its good to know that DD2000 has some specific areas that it performs less accurately than others (sims and dynos)

I personally would not expect an inexpensive sim (under $200) to do anything other than make SWAGs at peak numbers, but thats just me.

I have looked at the EngineAnalyzer programs and the method they seem to use for head flow coefficents seems extremely non-intuitive. (granted, the pro is much better)

In defense of monty, I have also seen DD compared to real dynos (peak numbers) and it does do pretty well. Same old story though, garbage in...


[This message has been edited by jcb999 (edited June 01, 2001).]

 

BAdSS,

I wasn't trying to be argumentative. I was just saying that if you model your motor accurately, it's predictions are amazingly close to real world dyno results. For the cost, around $40, it has the potential to be a very useful tool.

We tried 4 different custom Comp mech roller cams in my motor on the dyno. 1 of them was based on what Dyno2000 predicted would be a good cam for what we wanted. It ended up being kinda an odd-ball cam in the sense that the duraiton was 242/254. As you know, most split duration cams are typically in the 6-8 degree lobe difference range. It performed as predicted by Dyno2000. However, based on some other factors we decided that we could run a larger cam and still have excellent idle and throttle response, and ordered the 256/264 cam we ended up with, and which was used on the dyno I linked to.

I'm about to put a 383 on the SuperFlow next week that I built for a friend. We're using the Comp hyd roller XR288HR cam, 236/242. We ran it throught Dyno2000 and we'll see how it compares.

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1982 Corvette Tremec TKO
<A HREF="http://www.geocities.com/monty_williams/434specifications.html" TARGET=_blank>SBC 434 700hp/600tq
</A>

[This message has been edited by Monty (edited June 01, 2001).]

 

Please accept my apologies to all if I seem confrontational. It was and is not my intent. I should not have put cheap in parenthesis – huh? With the right expertise and information, these simulations (all of them) can be extremely accurate – within reason. All the simulations I’ve used,, and can afford,, have some sort of shortcoming. Some have more, some have less and it usually depends on the amount of money spent on the program. I’ve arrived at my own conclusions over the last 8 or 9 years using dyno software, and could go on and on. However, I won’t, and probably should have posted the following link without much, if any comment. That’s because what I’d have to say would not deviate much from what David Vizard has to say about the programs, based on his analysis in the following link.

http://www.motortecmag.com/current/products/index.html

Have a good one!!

 

OK. In my own defense (not that it's a big one) I stated what the purpose of the program is. Well I just read the article BadSS suggested (one of about 10 I've read) and it says exactly what I was (and all the other articles were) saying. Even the article said that dynos also vary by a few horses. But it was designed to be "cheap" (sorry, just had to ) and give approx. #'s. But for guys like me that have no engine designing experience, and know nobody who knows how, that program is great. If I guessed how many marbles are in the jar, and was off by only 10, wouldn't that be pretty good? Especially if I never played that game before? That's the same scenario really. That's all I was saying.

Yea, it might be off by 10-15 HP in general. But don't you think John Lingenfelter would be off by that much if he tried to guess the hp of my motor that I am building for my car? Even after I told him all the general info he would probably say "220 to 230hp". That's all this program does. It's a heck of a lot better than my guess would be, and that's all I need. Enough said. I'll be in my room. HEHE!!!

[This message has been edited by AJ_92RS (edited June 01, 2001).]