200 crank in a 250 block

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I want to increase the rod/crank ratio in my 250. I was wondering if a 200 crank is a direct replacement for a 250 crank? Will the 250 timing chain/gears work, or does the crank need to be worked on?
Thanks,
Ken
 
8) even if the cranks use the same bearings, which i think they do, you are dropping the stroke by .8" and doing it in a block that is 1.5" taller. that would make the rod/stroke ratio very large, to the point of being too large for best power. a better choice would be using a 221 argie crank in the 250 block. you still get the better rod ratio, with out it being too large.
 
Wouldn't he be still better off with the long rod 250 pistons from ACL, and a set of Oz 200 rods?
 
Unless you're downsizing to meet the displacement requirements of a specific class, it's almost impossible to gain performance by reducing displacement, especially by 20%. The small gain in efficiency from an improved rod/stroke ratio just isn't worth the cost and effort.
 
Ok, here's what I have:
stock rod ratio is: 1.504
a 351M rod 6.58 Lg and rod bore of 2.3107 r/s ratio of 1.683 Journals need work.
a 292 Y-block rod, 6.324 and rod bore of 2.188 r/s ratio of 1.610 Journals need work.
a 300 i-6 rod, 6.2097 and a rod bore of 2.1232 r/s ratio of 1.588 No journal work needed.

I want a ratio of 1.9 to 2.2.
rbohm, How do I get a argie 221 crank? If you can get your hands on one, let me know, I'll send you a check. What is the stroke of the crank?

Thanks,
Ken
 
The 221 has a 3.46" stroke. From memory, its crank snout is smaller, so you'd need to sleeve down for the front main oil seal. Also the flywheel bolt PCD is as per the 200 - smaller than the 250/302.
 
From KB website. There is more, this is just a "clip"

Rod Ratio vs. Intake Efficiency
An “nâ€￾ value of 1.75 is considered “idealâ€￾ by some respected engine builders, if the breathing is optimized for the design. Except for purpose-built racing engines, most other projects are compromises where 1.75 may not produce the best results. There will be instances where the choice of stroke or rod has not been made, but the intake pieces (carburetor, manifold, and head) have been selected. Some discretion exists here for making the rod and/or stroke choice compatible with the existing intake. The “nâ€￾ value can be used to compensate for less-than-perfect match of intake parts to motor size & speed. The reverse is also possible: the lower end is done, but there are still choices for the top end. Again, the “nâ€￾ value can be used as a correction factor to better “matchâ€￾ the intake to the lower end.
The comments in the following table are not fixed rules, but general tendencies, and may be helpful in limiting the range of choices to those more likely to produce acceptable results. Rather than specify which variable will be changed in the lower end, “nâ€￾ values will be used. Low “nâ€￾ numbers (1.45 - 1.75) are produced by short rods in relation to the stroke. High “nâ€￾ numbers (1.75 - 2.1) are produced by long rods in relation to the stroke.
 
StrangeRangers quotes are totally spot on. Drag-200-stang used a 200 Ford crank in a US 250 block in his turbo pro-streeter, so you'd be advised to check his posts. He used alloy rods, and it made all the power you'd ever want... 450 hp or so.


Mathematically, from four independent sources, the optimum cubic inch for a Ford I6 is a 56 thou over bored 250, destroked to 227 cubic inches, and a 1.8:1 rod ratio. Rule of thumb is that whenever you improve the rod ratio, you get half the improvement in power. Confirmed by Stan's TRD Formula Toyota engine, Engine Analyser programs, Ford Australias X-flow 200 veres 250 figures, and David Vizards 1435 veres 1559 cc dyno engine, where this very factor was examined in the early 1980's. Frictional losses due to side thrust component were calculated on a Super Flow dyno.

Basically, if it could be done, a 19% improvement in rod ratio on just a 250 engine would yield a 9.5% pro rata power benefit, and a lessor percentage in torque.

In practice, even a shorter piston shorter than 0.875" compression height limits the best rod ratio increase to less than 1.65:1. The shallowest commerical I6 piston is the 1.04" Iapel or Wiseco blank. With some kind of 6.475" costom rod, 1.65:1 is it, and the power gain is half the 9% improvement in rod ratio. So your 250 would behave like an engine 4.5% bigger, or 11 cubic inches better for power.

On the Aussie 200, which is a destroked 250, loosing 50 cubic inches is offest by a rod lenght increase from 5.88 to 6.27"...this improves the rod ratio by about 34%. The specific power per cubic inch goes up 17%. A stock 250 has 130 hp at 3800 rpm , the 25% smaller 200 has 121 hp at 4400 rpm. Technically, it should be only about 105 hp based on the 25% loss in capacity, but its only 9 hp off the 250. So destroking 25% with a longer rod is actually going to give an 8% loss in peak power. The bad news is that torque tumbles from 230 lb-ft at 2000 rpm to 177 lb-ft at 2400 rpm, a 29% loss, more than the capacity drop. If the car is geared to suit, the torque loss is irrelevent, as quarter mile times are dependaent of start line thrust and peak power...torque is never a factor when gearing is optimized.

If you run the engine analyser, the optimum engine size for ideal specific power is a 250 block with a 221 crank. Use a rod longer by 0.225 (6.105"), which would yield a 1.76:1 ratio, up from 1.50:1. There is no power loss for this optimized combination, but the rpm peak goes up about 200 to 400 rpm, and again, you loose torque. You gain in smoothness and rev range, and get a minor Specific power and brake specific fuel consumption improvement. One of the torsional vibration periods is moved up higher. On 250 Ford sixes, the rpm at which this happens is around 5300 rpm depending on combination. Reducing the stroke raises it up proportional to the stroke decrease.




Getting back to the US 200 crank into the US 250.

There are four things to do, all of them big jobs for a US 250 to US 200 crank conversion.

First, the US 200 crank is sleaved to suit the bigger US 250 timing gear and balancer,
Then the main bearing caps are fitted with either another set of scaped and linished US 200 F770 bearing as spacers, pined to the stock US 250 bearings with 1/8" steel roll pins or brass dowels

Optionally, the 200 crank needs its seven main bearings enlarged by a long run welds made in a downhand postion, and then remachined to stock 250 sizes.

Lastly, the 200 crank needs its flange long run welded up to the 3.622" daimeter from the stock 3.4". Its not a biggie.

Then you just use the big bell 164 tooth flexplate of flywheel, which has the smaller 200 cranks 2.75" bolt spacing. The 250 uses the 3" bolt centres sizes, so you just add a 1981 to 1984 3.3 plate, and your good to go.

I've done a 221 to 250 Aussie engine conversion. The cost was 1700 bucks back in 2001, which includes the pistons, rods and crank welding. I used forged Chevy 229/305 pistons, our 6.27" 3.3 Aussie rods, and main bearings welded up on the stock 221 crank. The crank flange where the flywheel meets still has to be welded up 0.220 to suit the block, and then machined back to the 250 size. A spacer won't work. I've tried making one up, and then looked at the other otions of a special crank seal, but they don't seal properly.
 
Thanks xecute,
I have read a couple of your posted authors, also Smokey Yuicks Power Secretes, which he states: "Historically, the designers of American V8 combustion engines have viewedrod ratios of about 1.65:1 to 1.75:1 as ideal for all around engine performance. .......
Then he goes on to state: All of the dynometer testing I have done indicates that a racing engine always produces more torque and horsepower when the rod ratio is increased beyond 1.75. In fact, in any conventional engine with a stroke length of approximately 2.5 to 3.5 inches, I believe there will be a notable increase in performance as the rod ratio is increased-all the way to ratios as high as 3.0:1!"

The other articles I have read is an article in an Engines Magazine: The truth about rod lengths, short vs long. Without copying the whole article, the tests were done at Air Flow Research Race Engines. They made over 250 pulls on a small block Chevy, 3.00 stroke, 5.50 and 6.50 rods, 9.020 block. They compared All-Pro aluminum heads and cast-iron heads, Holley 4412 2 barrel and a Holley 4779 4 barrel.
Long story short, there results were: Long rod, iron head and 2 barrel engine made more hps and torque than the short rod, aluminum head and 4 barrel set up. Long rod engines work better with restrictive intake/head combinations, which the log head falls into that catagory.

I am trying to get a quote for destroking a 250 crank from 3.91 to 3.0 add 240 I-6 rods, 6.7947 in and 305 KB835 pistons. This would get me a rod/stroke ratio 2.2656. I also think this will make the engine very efficient. Yes, I'll lose low end torque, but that can be made up for in gearing.
Now, my back ground is I'm a truck driver, not an automotive engineer.

Ken
 
Well, after playing with the DynoSims Engine Simulator software, a 3.00 stroke is not going to happen. Something about the RPM peak around 8500 rpm, and a single row timming chain. :shock: I bet it would have sounded great, at 8500 rpm. Well, at least once. :roll:
I am back to the reality of 3.91 inch stroke, and a long rod with a piston with a 1.09 comp height. I am looking for a 272 Y block rod. The lengh is 6.324, but the big end is 2.188. I am talking to my engine builder to crank tips. (And cost.) That get's me a 1.617 r/s ratio. What are your thoughts?
Ken
 
I am building a 347 stroker for my 86 mustang and I have an article that claims 421 hp at the wheels n/a.They are using a 3.4 crank with 5.315 rods = 1.56 ratio,they spun it to 6400 but didnt make any more power past 6000 ,so how important is the ratio on a street engine that only spins to 6000? :? and is the power difference worth all that extra time and money :hmmm:

I'm using 5.4 rods and 3.40 crank with a supercharger,so power is not going to be a problem.I think that more power can be had by using lighter weight internal parts and reducing internal friction ,that includes ring drag and crankshaft windage,I think thats going to make more of a difference than trying to correct a rod ratio. Thats just my opinion.
 
8) rod ratio has an effect on more than just hp at high rpm. a short rod engine sees pressure spikes as the piston accelerates up or down in the cylinder since the piston speed is not as consistent as a long rod engine. also a long rod engine has more dwell time at the top and bottom of the pistons travel, and this also has an effect on cylinder filling and port size. a long rod engine requires less port area than a short rod engine to make the same power. on top of that a long rod engine puts less pressure on the cylinder walls than a short rod engine does, thus extending the life of the engine.

how much these differences are depends on how much difference there is in rod length, but most are somewhat small overall. you have to take a systems approach when building an engine, and rod ratio is a part of that system.
 
I am hoping to build an efficient 250. Both power and gas mileage. The engine's final build will be a ~9.5 comp ratio, ported head and 2 32-36 webers. I have a head that was cut apart to see the ports on both sides, int and exh. When I get back in town, I plan to take a few pictures of the head and post them. We are also looking at the largest valves that can fit that doesn't restrict air flow, along the combustion chamber wall.
My hps goal is 250, 1hps/cid, if I achieve that, I will be happy.
Thanks,
Ken
 
ken - would be really interested in the photos of the port sections so let us all know when they are posted .

the australian chrysler 6's in the seventies used to get 302 HP @ 5600 on their E49 motors at 265 ci . the E49 was limited edition race spec homologation motor from the chrysler factory.
think they only made 149 cars ( should have been 200 but some skulduggery with build numbers saw them signed off as built)


bore was 3.9" and stroke 3.68"
3 x 45mm side draught webbers and 308 deg cam

and something like 2" inlets and 1.7" exhaust valves.
execute can chime in here

point of all this is that 250hp should be posible but valve size available in the ford may be a factor.


brett
 
the 265 was a 3.91 bore with a 3.68 stroke thats why they had larger valves.
no reason why you cant get 1 for 1 but may need a 2v head
 
checked up valve sizes were 1.96" and 1.6".

info here : http://home.iprimus.com.au/lsd265/hemi_6.htm

interestingly CHI who do the new ford alloy head cstings do an alloy chrysler 6 head :

http://www.chiheads.com/6cyl_alloy_hemi.php

the oz ford 250 with 2v head was rated at 170hp (gross) so with a fair bit of work 250 hp should be possible (net or gross i aint sure).

there was a 2v manifold for 3 off side draught webers last week on oz ebay - went for $185 aust.


but i guess the way is to get the new alloy head and start from there.

brett
melbourne
 
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