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Long rod 302

18K views 51 replies 22 participants last post by  Helmantel 
#1 ·
I'm looking for info on a long rod 302. I'm shooting for a 1.76 r/s ratio. I was thinking of using 351 rods to keep the price down. But from my limited searching so far it's looking like no matter which way I go it'll cost a few dollars. BTW it'd be a budget motor. Maybe gt40 heads, and flat-tops.
 
#4 ·
rod ratio has an effect on both power produced and longevity of the motor. long rod will tend to want to rev higher therefore more power. puts the pin higher in the piston and side thrust on the pistons and bore are reduced, therefore wear & friction are reduced.

_________________
Dave Shea
S/ST 1818 S/P 1018 H/R 1564

<font size=-1>[ This Message was edited by: igo1090 on 4/3/05 11:13pm ]</font>
 
#5 ·
when you change the rod length/stroke ratio you change the powerband. longer rods usually mean a wider power band. you can also use smaller ports to make the same power, which translates to better fuel efficiency as well. also longer rods allow an engine to rev higher, and a bit quicker as well. long rod engines do have drawbacks though as often times you need aftermarket pistons to make them work. one other thing, long rod engines have less stress on cylinder walls, especially the thrust side. the reason long rod engines do th ethings they do is, the piston has spends more time around tdc, and has more consistant acceleration and deceleration than short rod engines do. for the most part though the difference is nominal untill you make large changes in the rod length/stroke ratio. one more thing, in order to get the best output from ANY engine, you have to build the engine as a system, rather than a bunch of parts put together. long rod engines like smaller ports, wider lobe centers, more lift and less duration, for the most part. short rod engines like more port, tighter lobe centers, more duration and lift. the range is narrow though in either case, though the long rod engine is more forgiving.
 
#8 ·
On 2005-04-03 11:18, kechke wrote:
A long stroke 302 needs a block with increased deck height such as the M-6010-S302 8.700" "tall" deck block.

bull, i have seen 302's taken out to 363ci. it used a 3.50" stroke crank from a 351c(yes it was modified), with the stock 8.2 deck hieght.
 
#9 ·
On 2005-04-03 16:28, 85mike wrote:
Rod ratio means nothing. Zip.
Suit on!

also bull. a change in rod ratio has shown an increase in power, and a change in the power band. HOWEVER to get hte best results you have to build a long rod AS a long rod engine.
 
#10 ·
I understand what the r/s ratio has to do with the rest of an engine as a system. I just need to know how I can get it done. The cheaper the better too. So could it be done with 5.4" rods, and custom pistons? It's a more part specific question.
 
#11 ·
350 chevy pistons and boss 302/standard 289 rods is the cheapest way. Nowdays, finding 5.4 rods with Ford big ends is a bit challenging.....and then, pistons aren't exactly found at Autozone either.

If it were me, I wouldn't bother. The ONLY time I'd make a true stock stroke length "long" rod motor would be if I was building an FE that originally came with short rods and I put long rods in it.

Ironically, the 351M has an excellent rod/stroke ratio. If rod length made such a huge difference, how come it didn't help those slugs?
 
#16 ·
saying rod ratios dont affect power (tq more specifically) is like saying making a wrench longer doesn't give you any advantage. Rod ratio has everything to do with the lever arm length about the fulcrum.

As for parts, it would seem that if the new piston has a shorter skirt, you could potentially lose the small benefit of a long rod to piston rock and incomplete sealing.
 
#17 ·
On 2005-04-03 20:47, mavman wrote:
Ironically, the 351M has an excellent rod/stroke ratio. If rod length made such a huge difference, how come it didn't help those slugs?

because the engine was a slug to begin with. second it really didnt have that great a rod/stroke ratio, what it did have have was a very heavy piston with a very tall compression hieght.
 
#19 ·
I agree with 85Mike, the rod ratio is something to consider, but it is not the only thing to consider, and the rod has nothing to do with the stroke of the engine, so a longer rod will not give you a bigger lever. On brand X engines, I have built short rod engines that are not supposed to run worth a darn, but they do!! I think the overall combination is more important than stuffing the longest rod into an engine.
 
#20 ·
Small increments in rod length aren't going to do much.

If you actually take the time to measure the differences in angularity between even a half inch of rod length, you'll see there's not much of a difference.

SIGNIFICANTLY increasing rod length (enough to make a real difference) would require a super tall block and the rods that weigh a metric ton. Small increments in rod length are simply not all that significant.
 
#23 ·
On 2005-04-03 23:32, rbohm wrote:
On 2005-04-03 20:47, mavman wrote:
Ironically, the 351M has an excellent rod/stroke ratio. If rod length made such a huge difference, how come it didn't help those slugs?

second it really didnt have that great a rod/stroke ratio

It had one of the best rod/stroke ratios of most american V8 engines made. 6.58" rods, 3.5" stroke, that's 1.88.
 
#24 ·
n20mike has it right. He and I always seem to be on the same page with things...kudos Mike

Small increments in rod length do very little to piston motion. I wrote an excel worksheet a few years ago to look at this phenomena for the specific purpose of seeing just how much of a difference it would make to use 5.155 rods vs 5.4 rods in a 331 stroker. The difference was astonishingly small. The same is true here for a 5.09 rod vs a 5.4 rod 302. Here are two graphs to show the difference. The first graph looks almost like one line. You can barely make out two lines in the center part of the graph. Look at how small the so called "longer dwell" is at TDC...squat. The second graph shows the absolute difference in piston motion between the two rod lengths. Notice how it peaks at .013 at 90 degrees. Its showing that the 5.4 rodded piston is .013 higher in the bore at 90 degrees crank angle. That's 13 thousanths of an inch!

By the way, the rod angle is one degree better which reduces the side loading force by 6%, but the shorter rod piston has a larger skirt which distributes the side loading better.

What does a long rod do...it does hold the piston at TDC longer, but this is tiny for a rod that is .310" longer, the piston accelerates a bit faster from 0 degrees to 180 degrees, it reduces the rod angle, and most importantly it allows the usage of a smaller lighter piston which reduces friction. This is where the real benefit is...less weight and less friction. The G force on the rod is reduced due to the lighter piston allowing for more rpm.

Rods have to be a lot longer to show a big difference in piston motion.

Ignore the fact that the graphs say 5.155 vs 5.4 (that was there for my 331 analysis).






_________________
Tracy Blackford: Corona, Ca
'65 FB Mustang 331 with H-beam 289 rods, KB 322's, 282S cam, fully preped 351W heads. Built C4 and 3.50 9" posi


<font size=-1>[ This Message was edited by: blkfrd on 4/4/05 11:59pm ]</font>
 
#25 ·
For the money, you'll see larger gains by sroking it. If you're on a budget, but are going to spend the money on rods & pistons (and are worried about rod ratio), throw in a 3.25" stroked crank as well.

Oh yeah, the 351 has different journal sizing, that's why those rods will not work.
 
#26 ·
On 2005-04-04 07:48, mavman wrote:
On 2005-04-03 23:32, rbohm wrote:
On 2005-04-03 20:47, mavman wrote:
Ironically, the 351M has an excellent rod/stroke ratio. If rod length made such a huge difference, how come it didn't help those slugs?

second it really didnt have that great a rod/stroke ratio

It had one of the best rod/stroke ratios of most american V8 engines made. 6.58" rods, 3.5" stroke, that's 1.88.

i stand corrected on this point. however, the 351m had the wrong cam, and combustion chamber design to take advantage of the rod/stroke ratio it had.
 
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