OK, I'm playing with a hot-rod that is getting a small block Ford engine with some sort of increased stroke. The amount of increased stroke (and so the size of the damn thing) is still up for debate. The engine builder is 'only' interested in going to 408 cid. He says that any more than that gives a rod ratio that is just too small. The 408 has a 4" stroke and a 6" rod (1.5 ratio). I know that is fairly low, but at what point should you actually worry about the side load on the cylinder wall? Other engine builders run a bit of a longer stroke, but with the same rod to get to 427 cid (4.17" stroke, 1.44 ratio). Saying 427 at cruise night sounds a hell of a lot better than saying 408 in my book, but at the same time, I don't want to request the engine builder to do something stupid (especially if he's not into it).
I'm building this motor for high torque (yes, you read that correctly) or more accurately high efficiency at low RPM. The gearbox ratios are set. The rear end ratio is set. A smaller displacement high HP engine that you have to rev the nuts off of won't help me. It not for racing, so it won't see that hard of a life. I doubt if it takes many trips north of 5000 RPM and it'll spend most of it's time under 3000.
So...McG, engineguy, et al., you guys care to chime in on this one? How low is too low of a rod ratio? At what point do you have to worry about the piston skirt/cyl. wall?
Sorry if this has been hashed over before, I looked and couldn't find anything.
Thanks
Rod Ratio (in a completely un-like-me post)
Started by
Fat Boy
, Jul 11 2005 23:18
21 replies to this topic
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#2
bobqzzi
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Posted 12 July 2005 - 00:30
I think your engine builder is right. 1.5 is as low as I would go.
#3
shaun979
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Posted 12 July 2005 - 09:03
Engine speed is a big determinant of side load. Since you are turning low RPM (in fact sub 3000 is considered very low) most of the time, I believe it is safe to go a little below 1.5. 1.44? I wouldn't hesitate at those RPMs. The C5-Rs shift at about 6500 rpm for 24 hours on a 1.5 rod ratio. This is with a Nikasil lined aluminium block. I'm almost sure the SBF has a stronger wall. You might not have Nikasil, but the fact that you're turning such low RPM most of the time outweighs that greatly IMO.
#4
hydra
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Posted 12 July 2005 - 09:15
Personally (and McGuire has echoed this on several occasions) I wouldn't use a rod/stroke of less than 1.6, however Honda have successfully used a 1.5-1.55 ratio on their high-rpm B series engines for a long time, so go figure..
Oh, and using a 4" stroke doesn't mean you can't say its a 427 come cruise night ;)
Oh, and using a 4" stroke doesn't mean you can't say its a 427 come cruise night ;)
#5
Joe Bosworth
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Posted 12 July 2005 - 09:35
The drag racing guys have been documenting the affect of rod length : stroke ratios with dyno tests published for at least 30 years that I have been following this issue.
The final answer is complex and depends on a wide variety of factors including pin offset, combustion chamber design, header tune, and cam grind. I wouldn't want to see you taking any specific advice without a lot more information than what you have posted.
However there are a few things that acn be summarised with confidence:
1. Larger ratios seem to provide more low rev torque than low ratios
2. The best successes are found between ratios of 1.6 and 2.0
3. If you aren't really fully conversant and aren't willing to pay for good advice then stick to somewhere in the lowish to middle of the range I note above.
4. By the same token don't stray out of that range.
Regards
The final answer is complex and depends on a wide variety of factors including pin offset, combustion chamber design, header tune, and cam grind. I wouldn't want to see you taking any specific advice without a lot more information than what you have posted.
However there are a few things that acn be summarised with confidence:
1. Larger ratios seem to provide more low rev torque than low ratios
2. The best successes are found between ratios of 1.6 and 2.0
3. If you aren't really fully conversant and aren't willing to pay for good advice then stick to somewhere in the lowish to middle of the range I note above.
4. By the same token don't stray out of that range.
Regards
#6
shaun979
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Posted 12 July 2005 - 10:24
Originally posted by Joe Bosworth
The drag racing guys have been documenting the affect of rod length : stroke ratios with dyno tests published for at least 30 years that I have been following this issue.
The final answer is complex and depends on a wide variety of factors including pin offset, combustion chamber design, header tune, and cam grind. I wouldn't want to see you taking any specific advice without a lot more information than what you have posted.
However there are a few things that acn be summarised with confidence:
1. Larger ratios seem to provide more low rev torque than low ratios
2. The best successes are found between ratios of 1.6 and 2.0
3. If you aren't really fully conversant and aren't willing to pay for good advice then stick to somewhere in the lowish to middle of the range I note above.
4. By the same token don't stray out of that range.
Regards
Hello Joe, the difference in Fat Boy's case is that he is not varying rod ratio while holding displacement constant as in points 1 and 2 that you mention. He is varying displacement as well. By raising displacement and lowering rod ratio, his engine can operate at a lower RPM and produce the same volumetric flow as a smaller displacment, higher rod ratio engine.
Personally I would take the the 427 option - especially given the low operating engine speed range. The extra displacement will bring with it low end power/torque.
Kind regards
#7
Engineguy
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Posted 12 July 2005 - 10:59
People place far too much emphasis on magic numbers. First of all, look at what you're actually doing... moving the big end of a 6" long rod out 0.085" (half the stroke increase). The max angle (max wall thrust) changes less than three-quarters of a degree. If the 1.5 ratio motor is as close to the ragged edge regarding wall thrust as your builder implies, he shouldn't be building that.
The Chevrolet 400 small block had a 1.48 rod ratio (5.565" rod, 3.750" stroke). Whatever you may think about GM, they cannot take factors affecting engine life lightly... if that was right at some magical critical rod ratio limit they would not have even considered it for production numbers in the hundreds of thousands. You can be sure that engine had millions of miles of durability testing... two quarts low on oil... oil that has 30,000 miles on it... and a gallon low on coolant... in Arizona, in August... lugging up hills. I'm guessing you won't treat your engine like that.
Secondly, the 427 with a 1.44 rod ratio WILL make more torque (and more power at a given RPM) than the 408 with a 1.50 ratio. You may make very, very slightly less power per cubic inch, but that doesn't concern you... there is no sanctioning body that's going to make you ballast up your weight proportionately to the displacement increase. The biggest difference in piston location between the two engines is about 0.030" and is at a point in the stroke that makes no difference.
The Chevrolet 400 small block had a 1.48 rod ratio (5.565" rod, 3.750" stroke). Whatever you may think about GM, they cannot take factors affecting engine life lightly... if that was right at some magical critical rod ratio limit they would not have even considered it for production numbers in the hundreds of thousands. You can be sure that engine had millions of miles of durability testing... two quarts low on oil... oil that has 30,000 miles on it... and a gallon low on coolant... in Arizona, in August... lugging up hills. I'm guessing you won't treat your engine like that.
Secondly, the 427 with a 1.44 rod ratio WILL make more torque (and more power at a given RPM) than the 408 with a 1.50 ratio. You may make very, very slightly less power per cubic inch, but that doesn't concern you... there is no sanctioning body that's going to make you ballast up your weight proportionately to the displacement increase. The biggest difference in piston location between the two engines is about 0.030" and is at a point in the stroke that makes no difference.
#8
McGuire
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Posted 12 July 2005 - 11:48
There are pros and cons to everything, and one of the drawbacks of the small-block stroker motor is extreme rod angularity. There is an old rule of thumb that for maximum engine life, rod angle should not exceed 18 degrees or so. (Some say 16 or 17, depending. Engineguy said it just right: these are just numbers.) Beyond that cylinder wall life etc. will tend to suffer. With a 408 or 427 Windsor I believe a 6.200" connecting rod will fit, but the max rod angles are still 18.82 and 19.65 degrees respectively. That's out on the far end for a street engine any way you look at it.
What that means is that these will probably not be 100,000+ mile road engines or six-hour race engines...but we already knew that. A 400+ CID small block looks a lot better with a 4.125" bore, which keeps the stroke and cranktrain geometry more reasonable. This is the beauty of current engines like the GM LS1/LS2 and the NASCAR blocks.
All this said, with good parts and assembly engine life should not be a problem with either stroke.... as long as we are realistic about our expectations with the package. A 400+ CID small block is what it is, and I wouldn't anticipate any difference between the 408 and 427 in service life due to the slightly longer stroke. ...maybe the engine builder knows something about certain critical loadings with this application, or maybe if this is just his r/l ratio theories talking.
What that means is that these will probably not be 100,000+ mile road engines or six-hour race engines...but we already knew that. A 400+ CID small block looks a lot better with a 4.125" bore, which keeps the stroke and cranktrain geometry more reasonable. This is the beauty of current engines like the GM LS1/LS2 and the NASCAR blocks.
All this said, with good parts and assembly engine life should not be a problem with either stroke.... as long as we are realistic about our expectations with the package. A 400+ CID small block is what it is, and I wouldn't anticipate any difference between the 408 and 427 in service life due to the slightly longer stroke. ...maybe the engine builder knows something about certain critical loadings with this application, or maybe if this is just his r/l ratio theories talking.
#9
McGuire
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Posted 12 July 2005 - 14:02
The small block, big displacement stroker motor used be an all-around bad idea. Good aftermarket cranks were not available at reasonable prices, so the combination depended on offset grinding and/or welding or metal-spraying the rod journals on the original crank...or even adapting a crank from another engine. One SB Chevy stroker combination involved butchering up a Studebaker V8 crank to fit. True story. Same for connecting rods...you either threw down the large $$$ for Carillos or adapted Buick rods or something to get the correct pin height to match the longer crankpin offset.
But nowadays all the good parts can be had right over the phone at decent prices...including blocks with raised camshaft and spread oil pan rails. The other thing that makes the SB stroker work is the wide availability of aftermarket cylinder heads with sufficient intake port volume for these large displacements. Of course they won't flow as much as a good BB head but for a nice street package or a lightweight race car they work very well indeed.
Of course when all is said and done, there was a reason the manufacturers built large and small case engines. When we ask ourselves which will work better all around and last longer, a 427 small block or a 427 big block, we all know the answer. The big block has the correct bore spacing, deck height, bearing diameters, coolant jacket volumes etc and so forth right from birth. With the small block 427 we are essentially putting seven liters of engine in a five liter bag. We can make it work, but like everything else it involves some compromises.
But nowadays all the good parts can be had right over the phone at decent prices...including blocks with raised camshaft and spread oil pan rails. The other thing that makes the SB stroker work is the wide availability of aftermarket cylinder heads with sufficient intake port volume for these large displacements. Of course they won't flow as much as a good BB head but for a nice street package or a lightweight race car they work very well indeed.
Of course when all is said and done, there was a reason the manufacturers built large and small case engines. When we ask ourselves which will work better all around and last longer, a 427 small block or a 427 big block, we all know the answer. The big block has the correct bore spacing, deck height, bearing diameters, coolant jacket volumes etc and so forth right from birth. With the small block 427 we are essentially putting seven liters of engine in a five liter bag. We can make it work, but like everything else it involves some compromises.
#10
McGuire
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Posted 12 July 2005 - 14:19
Originally posted by Engineguy
The Chevrolet 400 small block had a 1.48 rod ratio (5.565" rod, 3.750" stroke). Whatever you may think about GM, they cannot take factors affecting engine life lightly... if that was right at some magical critical rod ratio limit they would not have even considered it for production numbers in the hundreds of thousands. You can be sure that engine had millions of miles of durability testing... two quarts low on oil... oil that has 30,000 miles on it... and a gallon low on coolant... in Arizona, in August... lugging up hills. I'm guessing you won't treat your engine like that.
All very true. But that said, there is no question that the 400 CID SB Chevy never had the service life of the 350. Two problems there: the compromised cranktrain geometry was harder on the pistons, rings, and cylinder walls. And the siamesed bores required to get out to 4.125" caused some cooling issues, which in turn led to some oiling problems, as the center cylinders tended toward coking the valve guides and piston rings.
Treated well the 350 could go 250K miles even in a dump truck. The 400, not so much. But can you buid a decent street performance engine from either the 350 or the 400? Sure, just depends on what you are trying to do.
#11
Joe Bosworth
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Posted 13 July 2005 - 01:32
Perhaps the main thrust of my discussion on rod length:stroke ratios above was open for confusion.
The thought that stroke length chosen while stroking a small block should be limited by the rod length is a bit crazy. In reality, if one is doing a bore and stroking job then not only is a new crank going to be suppied but so are pistons and rings.
If one is willing to spend that amount of money, surely new rods are going into the package. In this day and age rods of differing lenngths are available in about 1/8 inch increments in a variety of strengths. Virtually any ratio can be provided for any stroke length. Deck height and reasonable piston/ring configuration become the limitations.
Adding another 20 odd CI to a 400 CI block/crank will indeed add 5% torque capability. Optimising rod length will likely add about 2%. Optimising valve and port size and cam selection will add far more torque where you want it than 2 or 5%.
I come back to where I was with my original reply. Identify what your objectives are and then optimise the package as a whole to meet that. There is too little information provided to give good answers to the thread.
May be a fully knowledgeable engine builder should be consulted.
Regards
The thought that stroke length chosen while stroking a small block should be limited by the rod length is a bit crazy. In reality, if one is doing a bore and stroking job then not only is a new crank going to be suppied but so are pistons and rings.
If one is willing to spend that amount of money, surely new rods are going into the package. In this day and age rods of differing lenngths are available in about 1/8 inch increments in a variety of strengths. Virtually any ratio can be provided for any stroke length. Deck height and reasonable piston/ring configuration become the limitations.
Adding another 20 odd CI to a 400 CI block/crank will indeed add 5% torque capability. Optimising rod length will likely add about 2%. Optimising valve and port size and cam selection will add far more torque where you want it than 2 or 5%.
I come back to where I was with my original reply. Identify what your objectives are and then optimise the package as a whole to meet that. There is too little information provided to give good answers to the thread.
May be a fully knowledgeable engine builder should be consulted.
Regards
#12
McGuire
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Posted 13 July 2005 - 04:28
Unless there is some criticality in loadings that is not apparent, there will be very little difference (none, essentially) in cylinder wall or piston/ring life between the 408 and the 427 versions. As Engineguy noted, the difference in crankpin offset is only .085".
About this much --->
Frankly, neither engine is going to be stellar in the wear department as the cranktrain geometry is compromised in order to fit a 4" plus stroke in a dinky cylinder case. But then maximum service life is not the goal here.
As for differences in output or tuning characteristics due to anything other than the extra 19 CID, they will be too small to measure, even on a very good dyno. Out in the world they won't exist. Street combinations are just not that sensitive to bore/stroke ratio and rod length. A street engine that peaky would suck anyway.
Here are the actual piston position vs velocity and position vs acceleration for the two combinations at 6500 rpm:
About this much --->
Frankly, neither engine is going to be stellar in the wear department as the cranktrain geometry is compromised in order to fit a 4" plus stroke in a dinky cylinder case. But then maximum service life is not the goal here.
As for differences in output or tuning characteristics due to anything other than the extra 19 CID, they will be too small to measure, even on a very good dyno. Out in the world they won't exist. Street combinations are just not that sensitive to bore/stroke ratio and rod length. A street engine that peaky would suck anyway.
Here are the actual piston position vs velocity and position vs acceleration for the two combinations at 6500 rpm:
#13
Ray Bell
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Posted 14 July 2005 - 23:19
Joe touched on it and nobody has developed it at all... what about pin offset?
With small numbers like this needed to make everyone happy, ie. a little bit less angularity and the engine builder will okay the plan, why not take this direction?
Another thing that I'm not clear on... what deck height block is it? I know there's about an inch difference between the original small Ford deck and the deck introduced to cope with enlargement to 351 cubes.
Isn't that the difference between what's known as the 'Windsor' and 'Cleveland' blocks?
With small numbers like this needed to make everyone happy, ie. a little bit less angularity and the engine builder will okay the plan, why not take this direction?
Another thing that I'm not clear on... what deck height block is it? I know there's about an inch difference between the original small Ford deck and the deck introduced to cope with enlargement to 351 cubes.
Isn't that the difference between what's known as the 'Windsor' and 'Cleveland' blocks?
#14
Engineguy
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Posted 15 July 2005 - 00:38
Originally posted by Ray Bell
Another thing that I'm not clear on... what deck height block is it? I know there's about an inch difference between the original small Ford deck and the deck introduced to cope with enlargement to 351 cubes.
Isn't that the difference between what's known as the 'Windsor' and 'Cleveland' blocks?
Can't be the low deck... it runs 5" rods with only a 3" stroke.
There are actually FOUR deck heights, not two (and five rod lengths)... and Windsor vs. Cleveland does not determine deck height:
Windsor low deck 289....... 8.206" deck ht 5.150" rod length
Windsor low deck 302....... 8.206" deck ht 5.090" rod length
Cleveland low deck 302..... 8.206" deck ht 5.090" rod length
Boss low deck 302............. 8.206" deck ht 5.090" rod length
Windsor mid deck 351....... 9.500" deck ht 5.954" rod length
Cleveland mid deck 351..... 9.200" deck ht 5.780" rod length
Boss mid deck 351............. 9.200" deck ht 5.780" rod length
Cleveland high deck 351... 10.297" deck ht 6.580" rod length
Ford
#15
Catalina Park
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Posted 15 July 2005 - 10:28
The Cleveland engines in Australia used the same block for the two different capacity engines (302 and 351) The rod lengths were...
302C rod = 6.03"
351C rod = 5.78"
The trick for the 351 was to use the 302 rod with special pistons with higher pins to reduce the side loading.
302C rod = 6.03"
351C rod = 5.78"
The trick for the 351 was to use the 302 rod with special pistons with higher pins to reduce the side loading.
#16
Ray Bell
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Posted 15 July 2005 - 10:52
So now let's ask Fat Boy the big question...
Which block is he playing with?
Which block is he playing with?
#17
Joe Bosworth
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Posted 15 July 2005 - 12:56
Now we are getting to the nub of the rod length question posed by Fat Boy.
Cat Pk is getting sensible, you pick your piston to suit the rod length you want . (And can get under the deck height.)
Ray has picked up on the issue that Fat Boy has simply provided too little information to make any sense of this thread. Not only what block but what vehicle, what do you really want to do with it., what gears and much more.
We all probably should bale out of this discussion until more info is forthcoming.
Regards
Cat Pk is getting sensible, you pick your piston to suit the rod length you want . (And can get under the deck height.)
Ray has picked up on the issue that Fat Boy has simply provided too little information to make any sense of this thread. Not only what block but what vehicle, what do you really want to do with it., what gears and much more.
We all probably should bale out of this discussion until more info is forthcoming.
Regards
#18
Engineguy
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Posted 15 July 2005 - 14:01
Originally posted by Joe Bosworth
Cat Pk is getting sensible, you pick your piston to suit the rod length you want . (And can get under the deck height.)
The assumption, I believe (well, at least on my part), is that there is no room left to move the pin up for a longer rod... if there was, they'd have done it, and this topic wouldn't have started.
#19
shaun979
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Posted 15 July 2005 - 14:31
Originally posted by Joe Bosworth
you pick your piston to suit the rod length you want . (And can get under the deck height.)
Another school of thought is that you optimize your ring package and piston, and then fit the rod into the space, knowing stroke, compression height, and block height.
Regards
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#20
McGuire
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Posted 15 July 2005 - 16:04
Originally posted by Joe Bosworth
Ray has picked up on the issue that Fat Boy has simply provided too little information to make any sense of this thread. Not only what block but what vehicle, what do you really want to do with it., what gears and much more.
FB just wanted to know our views on the effect of a slightly longer stroke on cylinder wall life.
Social-skills tip for technical people: when someone asks you what time it is, they are not really asking you how to build a clock.
#21
Fat Boy
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Posted 18 July 2005 - 16:41
OK, I'm back in the mix on this one.
I've got a little more info which may/may not change things.
1st, it's a Cleveland engine, and the deck height is at 9.2". The deck height makes things tricky, but the heads on these engines helps things out, so it's a give/take. None of the rod lengths that we're discussing are stock. The piston/ring package the builder uses allows him to use a longer rod for a given application, to a point (as Shaun alluded to). The crankshaft in any of these flavors is essentially 'off the shelf', so we're not re-inventing the wheel on that part of the deal. It seems I screwed up some of the info I originally gave out. With the 4.0" stroke, he is able to use a 6.2" rod and with the 4.17 he can only use a 6.0" one. So in rod/stroke ratio terms, the long stroke engine goes wrong on 2 counts (longer stroke and shorter rod). Just so you don't have to pull the calculator, we're talking about ratios of 1.55 vs. 1.44.
I have to think that McG is right, and the extra displacement is going to make a much bigger impact than the better cranktrain geometry. Having said that, I'm not racing a 5000 lb car, so I'm looking to have some sort of reliability. Because of this constraint, I'm limiting myself to maybe 5500 RPM or so, and the vast majority of the time, I'll be pussy-footing around at a pace that a 4-cylinder would suffice for.
The graphs that McG posted show that at 6500 RPM, the piston accels really aren't that bad, so if I keep things 1000 RPM less than that when raising hell, it should be no big deal at all. I would say that if there is a certain powerband that I'm trying to optimise, it would be from 3500-5000. Having said that, it will have reasonable gears in it, so it'll have to be able to live at 2700 RPM running down the highway or cruising into the donut shop early Saturday morning at 1500 RPM. If I had to throw some pistons at the thing in 25k or 30k miles (i.e. years), that'd be no real tragedy. What I don't want is something that is apt to eat a piston and cut itself in 1/2 with 5000 miles on it.
So, that's where I stand. Any other questions I can answer that might clear things up?
I've got a little more info which may/may not change things.
1st, it's a Cleveland engine, and the deck height is at 9.2". The deck height makes things tricky, but the heads on these engines helps things out, so it's a give/take. None of the rod lengths that we're discussing are stock. The piston/ring package the builder uses allows him to use a longer rod for a given application, to a point (as Shaun alluded to). The crankshaft in any of these flavors is essentially 'off the shelf', so we're not re-inventing the wheel on that part of the deal. It seems I screwed up some of the info I originally gave out. With the 4.0" stroke, he is able to use a 6.2" rod and with the 4.17 he can only use a 6.0" one. So in rod/stroke ratio terms, the long stroke engine goes wrong on 2 counts (longer stroke and shorter rod). Just so you don't have to pull the calculator, we're talking about ratios of 1.55 vs. 1.44.
I have to think that McG is right, and the extra displacement is going to make a much bigger impact than the better cranktrain geometry. Having said that, I'm not racing a 5000 lb car, so I'm looking to have some sort of reliability. Because of this constraint, I'm limiting myself to maybe 5500 RPM or so, and the vast majority of the time, I'll be pussy-footing around at a pace that a 4-cylinder would suffice for.
The graphs that McG posted show that at 6500 RPM, the piston accels really aren't that bad, so if I keep things 1000 RPM less than that when raising hell, it should be no big deal at all. I would say that if there is a certain powerband that I'm trying to optimise, it would be from 3500-5000. Having said that, it will have reasonable gears in it, so it'll have to be able to live at 2700 RPM running down the highway or cruising into the donut shop early Saturday morning at 1500 RPM. If I had to throw some pistons at the thing in 25k or 30k miles (i.e. years), that'd be no real tragedy. What I don't want is something that is apt to eat a piston and cut itself in 1/2 with 5000 miles on it.
So, that's where I stand. Any other questions I can answer that might clear things up?
#22
Ray Bell
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Posted 18 July 2005 - 23:09
What about a 6.1 rod... such things are available, are they not?
