Long Rod Motors (from old site)
GT1guy
Registered User
Posts: 9
(9/11/03 8:10 pm)
Reply Long rod motors
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What are your thoughts on long rod motors? Anybody run them? I know they shorten the time the piston spends at TDC & BDC, but are there any other benefits?
boodlefoof
Unregistered User
(9/12/03 5:36 am)
Reply long rod
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the higher the rod/stroke ratio (the longer the rod) the longer the piston will stay at TDC and BDC.
A long rod has a few advantages. There is less rod angularity, causing less piston-to-sidewall force. Less instantaneous piston speed, meaning less stress on the rod, which can allow for higher rpm potential. The longer dwell time at TDC can in conjunction with a properly set up quench height be made to reduce risk of detonation quite a bit.
On the downside. The longer the r/s ratio, the slower the instantaneous piston speed... this is good in some ways as I said above, but on the other hand it reduces volumetric efficiency, generally meaning less low end torque. Also, if you have a very long rod, the piston pin may have to be moved up to the point that the pin is in the ring land area, which may or may not be a problem.
For a really good article about building a long rod engine, go to www.airflowresearch.com and click on the "article archives." Take a look at "the small block Chevy should have built."
John
www.geocities.com/boodlefoof
GT1guy
Registered User
Posts: 10
(9/12/03 8:25 am)
Reply Re: Long rod motors
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Thanks for the info John. I knew the time spent at TDC/BDC played into the equation, I just had it backwards. I'm planing to build a twin turbo big bore/short stroke motor, using a CN 5" bore space block w/4.750" bore, and a Crower 3.571"stroke crank. This will give me 506cid at a 1.33 bore/stroke ratio(the same as a 302). Between your thoughts and the article, I'm leaning towards longer rods.
Anyone out there ever run long rods in a turbo motor?
XcYZ
Registered User
Posts: 619
(9/12/03 10:52 am)
Reply
Re: Long rod motors
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I'm running 6.200" rods in my tall deck 434. I'll know how it works with turbos in about a year. The motor will have to wait, as I'm doing the chassis stuff this year.
Monty Williams is running the same tall deck Dart Iron Eagle block as I am, but he's using 6.125" rods. Didn't seem to hurt the 1,200 hp that he made. LOL Here's his website...
Monty's 1,200 hp TT 427 small block
Scott
My 69
Lateral-G.net
boodlefoof
Unregistered User
(9/12/03 1:03 pm)
Reply killer!
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that sounds like an awesome build up!
Someday in the distant future (when time and money allow) I plan on building a big bore, short stroke, twin turbo small block. It'll be fun.
Good luck with your build.
John
www.geocities.com/boodlefoof
GT1guy
Registered User
Posts: 12
(9/13/03 10:52 am)
Reply Re: killer!
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Monty has a wicked ride! I'm glad to see Crower parts in his motor b/c that's what I'm planing to run. Thanks Scott. Now, for my next question: How do you determin how long a rod to use? I'm going to be building a BBC, and stock length is 6.138" I believe. But I've seen rods over 7.500" listed. Where do you draw the line?
Kevin
boodlefoof
Unregistered User
(9/15/03 7:05 am)
Reply where to draw the line
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Well, I can't really say where to draw the line. I have heard that a ratio of about 1.75:1 is "perfect for an accelleration engine." Read that somewhere...
That said, the old DZ302 engine had around a 1.9:1 ratio.
I'd personally just stick as long a rod in there as you can without compromising the piston.
John
www.geocities.com/boodlefoof
pdq67
Registered User
Posts: 170
(9/15/03 6:34 pm)
Reply xx
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An infinately long rod engine will approach dwelling equally at both top and bottom and an engine whose rod is the same length as it's stroke will dwell 270 degrees at bottom dead center!!!
Check it out AND most of this stuff really doesn't mean all that much UNLESS the combination is maxxed out to one or the other!! Short rod OR long rod!!!!
It took me some stetches after reading a Roundy-round mag. on this years ago...
pdq67
gmachinz
Unregistered User
(10/11/03 3:05 pm)
Reply Long live long rods!
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I have a 400SB (1976 vintage) and I am using Ford six cylinder rods which measure out at 6.209" long. They are forged, bigger than stock 400 rods, and were only $22 each at a local parts store! I was running a large journal 327 crank but I found a 20/20 DZ302 crank ( 3.00 stroke versus 3.25) I swapped 'em and picked up a little bit on the top end but the lowend response was way better! I figure I need to re-work my Vortec heads again because I really need more airflow-I'm about at the limits of what the Vortecs can give me. The latest combo has given me a best time of 7.61 in the 8th, but any faster than 7.50 and I have to have a 'cage. I don't want to do that. I really love long rod motors-my compression went from 11.25:1 down to 11:1 with changing to a smaller stroke-maybe I'll hit 7.50 if I go to a 6.300 rod??? What do ya'll think? For now, it's gettin' cold 'round here in Iowa so I'll have to wait until spring to get that phase going. Wow, for a turbo engine I imagine a HUGE set of heads are in order for that!! Yikes!
boodlefoof
Unregistered User
(10/15/03 10:36 am)
Reply very cool
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hey Gmachinz,
Sounds like a fun build!
gmachinz
Unregistered User
(10/16/03 5:45 pm)
Reply and we'll have fun, fun, fun...
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Boodlefoof, I went with this combo after seeing it in Car Craft, so I can't really take any credit for it. They went with a 327 crank as well. I went with Vortec heads instead of smog era ones and I have no went with a noticeably faster rotating DZ crank. I too went converted to a hydraulic roller valvetrain with a rev kit. I have to say that I really don't have problems running high RPM's. I hear many people make the claim that 6000 R's are the limit for a hydraulic roller motor but I can take it to 7-7,500 at any time. What I like about long rod motors is how fast they "come on"....espescially in a big car. With the relatively quick rotating assembly, I'd like to combine it with a turbo sometime in the future. I bet there'd be very little time waiting for the turbo to spool up. I'm looking into various ignition systems right now and I plan on building another long rod motor this winter but I am still looking for something to put it in....
boodlefoof
Unregistered User
(10/22/03 7:17 am)
Reply someday...
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Yeah, turbos would work great on that kind of engine. Someday... I'm going to put one in a Datsun 240Z. Or, my '68 Camaro.
gmachinz
Unregistered User
(10/22/03 9:38 am)
Reply RE:someday...
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Hey, I love Camaros but around here it's difficult to find a clean one. I am more into G-bodies. Right now I'm looking at scoring an '87 SS Monte Carlo that suffered a small (yeah, right-we'll see) electrical harness fire underhood. It didn't scorch anything...supposedly it was all contained in the main wire loom along the firewall. The car is totally dead and won't start. Perfect candidate for a total tear down to me!! After seeing everyone's projects on this board, I am undecided between an LS1/6spd or another long rod w/centrifical charger and a stout 4L80E. If I go with the 'charger I'll HAVE to have a manual though, I think that's for sure. Now, where in the hell did I lay that extra $15,000 to get started?!? (yeah, right!)
walapus
Registered User
Posts: 71
(10/24/03 10:12 pm)
Reply Long rod motors
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There was along discussion about this subject on an engineering site not too long ago. The overview was this:
The increased "dwell" is bunk. If you do the math, the change in time "near" TDC is miniscule ( forget exactly how they defined near, I think it was within 0.100" of TDC). Changes were fractions of degrees, too small to make any difference.
the trends most engine builders experience do exist, they are not placebo effect, but it has more to do with where peak piston speeds are occuring relative to peak cylinder pressures and valve events if I remember correctly. I'll try to dig up some of the text, it's very informative
walapus
Registered User
Posts: 72
(10/24/03 10:26 pm)
Reply excerpts
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"Most of my opinion is based upon looking at what happens when you change R/S ratio. For my examples, I assume a stock 350 Chevy, 4.000 bore and 3.480 stroke with zero wrist pin offset. Assume stock rod of 5.700 inches (1.638 R/S) and a 8.700 inch rod (2.5). The first thing Smokey said is that a piston dwells longer at TDC with a large R/S ratio. Let’s look at the numbers. I assume dwell at TDC occurs while the piston is within 0.200” of TDC displacement.
Down R/S Ratio
in 1.638 2.5
0.200 335 334
0.150 339 339
0.100 342 342
0.050 347 347
0.050 372 372
0.100 377 377
0.150 381 381
0.200 384 385
The super long R/S ratio only gets a 2 crank angle degree (CAD) advantage over a stock R/S ratio. In time, that’s a only a 0.045 msec advantage at 8000 rpm. Not much. When it comes to allowing more time for combustion to occur, the difference between these two drastically different R/S ratios is not much, hundredths of a millisecond.
Now, let’s look at what I think are the most important parameters R/S ratios change, combination unit force and “how hard the piston sucks on the intake port.” Combination unit force is the percentage of combustion pressure force that causes a torque on the crankshaft. More torque on the crankshaft is the goal of anything to increase power and torque of an engine.
Combination unit force is determined by the geometry of the slider/crank assembly. If you keep stroke constant, combination unit force is driven by R/S ratio. So, if our goal is to increase power and torque, the higher the combination unit force, the more power and torque we’ll make. However, you also have to remember most torque transferred to the crank occurs during the first 20 to 25* after TDC. I think it’s more important to have a good combination unit force during the 20 to 25* after TDC than to have a good number later in the power stroke, say 75 to 100* after TDC. Looking at the example below, a shorter R/S ratio is much better at extracting power from the combustion chamber force than a longer ratio WHEN IT COUNTS. The peak values increase with increasing R/S ratios, but the peak occurs when virtually zero cylinder pressure is available to push on the piston.
“How hard the piston sucks on the intake port” is also important, and I have to get a better way to describe it. The plot below shows the change in cylinder volume with respect to crank angle. This helps to determine the volumetric efficiency of the engine. But, you can’t just look at how hard it sucks, but when does it suck in relationship to valve lift. A shorter R/S sucks harder, but it’s peak is roughly 70 degrees after TDC, the longer R/S doesn’t suck as hard, but they suck when the intake valve is open more. Translated, it’s easier to get more mass flow and higher VE when the R/S ratio is higher.
So, we have two parameters that want two different R/S ratios. To take advantage of the cylinder pressure you have, you want a short R/S ratio. To get more mass flow and higher VE, you want larger R/S. It’s all a trade off.
Does this analysis solve much? No. I don’t think an ideal R/S ratio exists for all engines. It’s all dependent on the rest of the engine. If an engine is under valved for its displacement, it may benefit with a longer R/S to enable more air and fuel at higher rpm. If airflow is sufficient for the desired speed, a shorter R/S may give more power by utilizing the cylinder pressure you already have. It’s all a trade off and compromise.
On the short deck pro stock engines, I agree with your conclusions. Plus, I’ve spoken to some Comp guys who run old pro stock engines, and he concurs. The reason for short decks is to get better airflow mainly with a side benefit of stiffer pushrod.
On F1 R/S ratios, I looked at paper I have. On the 1989 Honda F1 engines running 2.5 bar of boost. It’s R/S ratio is roughly 2.28. But, I would imagine the engines of today are more concerned about a physically smaller engine to get better aerodynamics rather than a longer R/S ratio. "
walapus
Registered User
Posts: 73
(10/24/03 10:28 pm)
Reply and some additions/rebuttals
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"Rod to stroke ratio changes where the stroke happens. You must have the cam on a tight lobe seperation angle. If it's a wide LSA, it's likely the reason it has a wide curve if you can say it has one. Rod ratio's of 1.45 make very explosive torque as do tight LSA's. Rod ratios effect the rate of change of volume. Tight ratios also need bigger intake ports to make high RPM power. They tend to make the curve very peaky. This is why they respond well to wide LSA's if a guy wants a wide band and has a hard time getting it to the ground."
"
If we look at the crank/rod/piston relationship vs. rod length/stroke, we find that a "short rod" allows the angle of the rod to a straight line between the main and rod journals to be 90 deg. when the piston is higher (more pressure). At that point it is easiest for the rod to rotate the crank. I *think* this is what you are getting at in the your paragraph.
Granted the psi on the piston will be well past the peak, but I suspect that 20-25 deg. ATDC may be a little early-the rod still doesn't have much leverage on the crank even tho the PSI is near peak. Someone has probably worked this out, and I'd like to see it.
The biggest problem with high RPM engines and short rods is the effect on piston acceleration-Ford sure found this out some years ago."
they say it better than I can!
GT1guy
Registered User
Posts: 14
(10/25/03 9:54 am)
Reply Re: found this info
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I found this kind of interesting.
www.grapeaperacing.com/Gr...length.cfm
Where it says :Exhaust Gas Scavenging
A longer rod is moving slower at TDC, which reduces the speed of the exhaust gasses during the overlap period. This reduces the scavenging effect at low rpm and reduces low rpm power slightly (makes the engine run more cammie). A short rod on the other hand moves faster past TDC and increases the scavenging effect and help low rpm power.
How would this effect a turbo motor? Would this raise the boost threshold ?
Kevin
Edited by: GT1guy at: 10/31/03 3:20 pm
gmachinz
Unregistered User
(10/31/03 6:15 pm)
Reply RE:Re: found this info
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It is a very good article on comparing the two. But, keep in mine that comparison was done with two diff. rod lengths using the same 3.48 stroke. What I did (to offset the cons) is run first a large journal 327 crank which is 3.25 stroke, then I changed to an orig. DZ302 large journal crank which has a 3.00 stroke-(my rods are 6.209). So piston speed is turned back up while still keeping the benefits of a longer rod. The article made a small boo-boo I think by saying longer rods can allow you to run as much as "a full compression point higher" because of reduced detonation. The author says you can go from 9.5 with a 5.7 rod to 9.6 with a 6.0 rod. I think its more like 10-10.50. I have put togther SB's that run as high 11.5 to 1 on 89 pump gas with no detonation @ 30 degrees total advance. Piston speed is incredible and sidewall loading is all but diminished with a 6.209 rod. Volume for me is not as important as velocity of the air moving. This is partly why long rods prefer high RPM's and the broad range in which they operate leave a lot of room for error such as miss-shifts, etc. I firmly believe that getting as much air moving as possible is the key. Strokers do the same thing but at a lower RPM range and they don't have nearly the same torque/HP range. I don't understand why anyone would build a 383 for example if the goals are to reliably see 160+mph UNLESS one goes with a 6-spd. of course. I guess its two schools of thought on this one. Strokers are easy to build but I still feel longer rods are the better design. I'd like to see a long rod/stroker engine shootout in Popular Hot Rodding or something.
