What does it mean to have the motor stroked out. Is that like having the block bored out to a larger size.
Changing the crank to get more throw out of the pistons. sort of opposite of boring. Bore x stroke = displacement(for 1 cyl)
It just makes the pistons travel further up and down in the cylinder. There is about a half inch to an inch or so below the current travel of the piston that is not used. A different crank with longer arms will let the piston travel further down the cylinder, using more cubic inches. Usually, you "bore and stroke" adding a little more cubes by making the cylinder a little bigger around as well as letting the piston travel further up and down.
An engine is rated by how much volume it displaces. Therefore you can figure the 'displacement' of a given engine in cubic inches. If you bore the block during a rebuild, the larger piston displaces more volume when it travels. That is why a rebuilt 302 cubic inch engine becomes a 306 cube engine when you clean the block up by boring .030" over stock dimension. The other guys put the stroke pretty well. The crank pins distance of throw determines how far the piston moves to displace air, so if you add a longer throw (stroke) to the crank, the engine gains cubic inch displacement. Common stroker combos: 302 + .030" overbore and .4" additional throw = 347 cubes 302 + .030" and .25" = 331 351 + .030" and .25" = 373 351 + .030" and .35" = 393 351 + .030" and .5" = 408 The factors that become crucial when changing throw distance is the hieght of the piston and length of the rod. You have to keep everything in the same bock. If you add throw without adjusting these two parameters, you will run the top of the piston into the combustion chambers. So you have a choice of shortening the piston or the rod in simple terms. However sometimes you can work out the math on a combo that gives you a longer rod with a still shorter piston. The purpose of that is to keep your rod ratio. A higher rod ratio will make the engine more durable and it will actually soak up more power from the combustion. That is a whole other long discussion though! The easiest stroker on the planet IMO is the 393 though. You can do one of those for little more than the cost of a stock rebuild on a 351w engine. Only because the longer throw crank is the one special part you need. Someone figured out you could put shorter and cheaper 302 pistons on the 351 rods, and extend the throw of the crank to bring the piston back up to the top of the block. This gains good cubes while keeping it really simple. I have seen new cast 393 cranks in the aftermarket for less than 300 bux. It's still stronger than stock, and you save the money you would have spent to machine the surfaces of the old crank. With the savings of the pistons you get the 393 cubes pratically for free.
To calculate cubic inch displacement Bore x Bore x Stroke x .7854 x Number of cylinders = CID So you can see by increasing the bore or stroke you increase the displacement (size) of the engine
I am also a big fan of the 393. I used a Coast kit and the Edelbrock Performer Package with heads and got a 400+ HP motor for less than $3500.
She's ported and relieved and she's stroked and bored ........she'll do a hundred and forty in the top end floored.......... Never mind.
347 seems to be most popular stroker for a 302. 331s are gaining popularity though, because folks are realizing the better rod ratio is good for the engine. At least that is a good reason. The 347 can't get a good rod ratio. This increases wear on the block. Also, most kits put the ring package into the pin. This causes problems with longevity as well and usually creates oil control issues. The kits that do get the rings out of the pin do so by either pushing the rings to the top of the piston, weakening the lands, or they use a shorter rod that really kills the rod ratio. 331 is a wise choice, unless you use an aftermarket 8.7 block. With the aftermarket block, you can use a 3.4" or even 3.5" stroke with decent rods. It would still look like a 302 block to 99% of the folks out there. The only intake that bolts right on to that block is the Vic JR... (last time I looked) Dave
Here is a little exert taken from one of my favorite high performance Ford books, it is referenced at the bottom of the page. I think this is great support for my opinions on rod ratio...... Rod Ratio Theory- “We’re discussing the issue of rad ratio, a subject that has about as many opinions as there are engine builders. The current thinking is that longer rods are always better, but it’s not that simple. Any engine combination is a compromise, and component selection depends on what kind of use the engine is being built for. In the mix of heads, cam, headers, and displacement, there are other components that are higher on the ladder of performance importance that rod ratio. Here’s two opinions on the subject from Coast High Performance and Probe Industries. First, what is a rod ratio? The ratio results when the connecting rod length is divided by the stroke. Rod length is measured from the center of the big end to the center of the small end. A longer connecting rod, the theory goes (and tests support), allows the piston longer dwell time (in crank degrees) at TDC, so the expanding gases have a longer period (in crank degrees) to push on the piston. The higher the ratio numerically, the better it is considered. The better the ratio, the more power the engine is theoretically capable of . Unfortunately, engine reality gets a little muddy, particularly if you pursue rod length to the exclusion of all other engine facets. The chart nearby lists the rod ratios for Ford engines and CHP stroker kits. Displacement Length Stroke Rod Ratio 221 flathead stock 7.00 3.75 1.86 292 Y-block, stock 6.324 3.30 1.91 312 Y-block, stock 6.252 3.44 1.81 221 stock 5.115 2.87 1.78 289 stock 5.115 2.87 1.78 302 stock 5.090 3.00 1.69 302 CHP long rod 5.70 3.00 1.90 317 CHP stroker 5.40 3.10 1.74 347 CHP stroker 5.40 3.40 1.58 355 CHP stroker 5.50 3.48 1.58 377 CHP stroker 6.125 3.60 1.70 351W/5.8L stock 5.956 3.50 1.70 351W CHP long rod 6.125 3.50 1.75 408 CHP stroker 6.125 4.00 1.53 426 CHP stroker 6.125 4.17 1.46 435 CHP stroker 6.25 4.20 1.48 351C stock 5.78 3.50 1.65 428 FE stock 6.48 3.98 1.63 4.6L Modular 5.933 3.54 1.67 5.4L Modular 6.657 4.165 1.59 Notice that a long rod with a stock stroke, like the CHP 302 long rod, smartly increases the rod ratio. But also observe that when stroke as well as the rod length increase, ratio declines numerically. Muddiness about rod ratio influence occurs from several points. Consider the 221 Flathead and the 221 Windsor. The flathead has a terrific rod ratio compared to the Windsor. Yet the flathead was factory rated at 85 horsepower (at 3800 rpm) whereas the original 221 was rated at 145 horses (at 4400 rpm). Obviously, there are many reasons why the 221 windsor makes more power, such as more efficient breathing due to its valve-in-heads and lower frictional horsepower loss because of its short stroke. The 292 and 312 Y-Blocks have excellent rod ratios, yet they were never highly regarded as performance engines in their day. Also note that a 428 FE has a so-so ratio, but no one would accuse the 428 of being short on power and torque. Likewise, 426 or 435 Chp strokers have the worst ratios, yet they make serious horsepower. Look at the world class Modular engine rod ratio. Considering the billion or two that Ford spent developing the Modular engine, it’s a poor ratio numerically. But that doesn’t keep a 4.6L four-valve from delivering 600 horsepower with the right parts. The fact is , engines with supposedly poor rod ratios can make excellent power with the right parts combination. According to CHP, displacement is always a trump card. If you compare a long rod 302 (5.700" center-to-center length) with a 347 (5.400" center-to-center length) with displacement being the only difference, the 347 will make more power. As CHP explains, when a customer mashes the pedal on a 347, he feels the power compared to a 302 long rod engine. If CHP wanted to, they could increase their 347 rod length from 5.400" to 5.800" to improve the rod ratio from 1.58 to 1.70. But doing that would compromise piston stability in the cylinder due to the piston’s very short skirt and tight ring pack. These conditions would result from pushing the piston pin further into the piston to accommodate extra rod length. Ultimately, the question may be posed: how much more power should an engine with a good ratio make compared to one with a so-so rod ratio?? Compare a 302 with stock rods (5.090" center-to-center) with a long rod 302 (5.700" center-to-center) and the long rod will have a broader torque curve. But according to CHP, the change in rod length accounts for only about a 2 percent increase in torque. That means 98 percent of the engine’s power is the result of other components and engine dynamics. A second take on ratios is offered by Probe Industries, makers of Windsor performance pistons and engine components. Instead of focusing strictly on the issue of piston dwell, their theory relates to cylinder head airflow. The bigger the head ports are, the shorter the rod should be. One example would be a 351C with four barrel heads. They continue explaining that if you have a 327 cubic inch displacement, with a head that flows, say, an extremely good 320 cfm, they believe that a shorter rod gets air flowing through the port sooner, enhancing cylinder fill in the lower rpm range. Going up in displacement, if you have 400 cubic inches and heads that flow 200 cfm, work on a long rod. This will delay air movement in the port and flatten your torque curve. Along these lines, CHP also observes that long rod engines are especially to racing classes where induction is limited to stock heads, intake manifold or two-barrel carbs. If your engine runs in an extremely high rpm range, increase the rod length. One stellar example is the rarefied engine building atmosphere of Winston Cup racing, where every part and part relationship, including rod ratio, is a critical consideration. But don’t forget that in any given race class with a displacement limit, the engine builders build their engines as close as possible to the limit. Probe also lists additional points influencing rod ratio considerations. The include cam grind, track characteristics, car weight and traction characteristics, etc. Not that most of these are race related. Ultimately, for street use, rod ratio us way down on the list of engine things to be concerned about, particularly if you have just started building engines or racing. There are more critical areas to focus on. This would include assembling a well matched induction system, exhaust and cam to produce maximum power. With an EFI application, fuel and ignition mapping would be more productive to producing power gains. When studying engine dyno sheets for torque numbers, Volumetric Efficiency, Brake Specific Fuel Consumption numbers and exhaust gas temperatures are areas to investigate for improvement before ever worrying about rod ratio. Probe closes its comments with this observation. Talk to professional engine builders and parts manufacturers. They build, dyno and race engines constantly, so they’re always testing and comparing combinations. If they can fine tune a particularly potent rod ratio, you can bet they’ll let their customers know. Summed up, the saying “You can’t beat cubic inches” is unlikely to ever be replaced by “You can’t beat a numerically high rod ratio.”“ HP Books, Ford Windsor Small Block Performance, The Berkley Publishing Group, Isaac Martin
Interesting quotes. I noticed that the list does not include the 331 or the 393, so the article is a tad dated. The information is still relevent though.
It is more important to consider what your'e doing. If you want the last bit of HP (I mean a dry sump big bore 9000+ RPM engine) you're going to look at rod ratios. For the rest of us, we're going to look at cubic inches. And...while I'm on my soap box, I personally am totaly tired of seeing "oil control" bought up when the subject of strokers comes up. You have to sit back and think about it. How many of you folks on here don't have an oil leak or two? How many of you have NEWER cars that burn a little oil? I do...every car I've ever had has leaked or burned a little. SO WHAT! If you want a perfectly smooth running, non leaking engine that doesn't burn a couple drops between oil changes, you're better off buying a NEW car..with warranty...and you can go aggravate your dealership about a new engine when you find that it burns a 1/4 quart in 3000 miles. Chances are you aren't going to get a new engine, in fact, it is commonly noted that if the engine in question doesn't use 1 quart in 2000 miles, it is considered normal operation and 99% of the time the manufacturers won't replace/rebuild it. If you want a hot rod, build it the way you want it. Forget the oil control issue...sooner or later, the dang thing's going to grenade anyway if you're making any sort of power, why worry about a little oil loss? We used to be into go-karts. We weren't famous, rich, or even well-known for that matter. But, a little secret we used to do to the 5HP briggs was to use a .010" oversize bore (maybe as much as .012") with a STANDARD piston and filed .010" over rings. Many times there was close to .020" clearance...but they rattled a little when started, burned a little oil (even smoked a little at times) but I was consistently lapping the 2nd place finisher in hundreds of feature races. To this day, I don't ever think I've seen so much frustration among racers. Everything was 100% legal and we were torn down piece by piece almost every weekend. Moral of the story....looser is better and don't get caught up in the oil control issue. Build it how you want it built...347, 355, 363, whatever. Put the dang thing together and go beat on it. You only live ONCE!
Jeez. Couldn't you sand-bag just a little to let the rest of the field have a glimmer of hope? And, btw, your comments are appreciated (as well as the posters before). Always a good bench-racing topic.
I know that we are completely off topic with rod ratio, but I have one more comment... Or at least reiteration: I look at rod ratio as a longevetiy issue as I never intend to race. Maybe test and tune once in a while just to keep the blood flowing. Everything else is street for as long as the engine lasts. No theory here, just fact... A longer rod has less angularity, therefore less friction and loading on already thin cylinder walls. This means your engine lasts longer if everything else is the same. Also, fact: Longer rods put less pressure on the bottom end by slowing the piston. Less G-force and accelleration. This will allow less chance for something to break. Personal theory: Less friction in the engine should add fuel economy as well. Less heat to deal with too. Dave
