Compression ratio

What are some opinions on compression ratios
What is a good compression to stay at for power
And reliability I am currently building a wd 45 engine
With a 4 1/8 bore by 5 1/8 stroke I am going to be around
11.5 to 1 my thinking is it will have enough compression
To make good power and torque but still be pretty reliable

wi50 what kind of honest hp do you think I will
Get out of the combo I am running and should
Get a cam done for it

I was planning on using the 45 rods is there something wrong with them

Thanks for the advice I will round up some gleaner rods

All the same length, but if you notice, the newer the rod, the casting number keeps changing. I'm sure they are stronger with every generation, and would bet if you start weighing them, you'll find a little more beef in a newer rod. I've seen bent rods before at overhaul time on some of those old engines, just from a carb that ran over and pull starting the engine to get it to fire.

One of the engines I'm working on is built like you are talking about. 4 1/8" bore and 5 1/8" stroke. Stock rods, trimmed pistons to get the compression ratio in check. It was built by someone down south who will remain un named, but the engine is a mess. It's way off balance, the cam timing is way off, the crank is bent a little and the rods are hitting the block.

Don't take this as an insult, I'm just trying to maybe point you in a better direction if you are still in a position to go a different direction. Why are you wanting to build this combination? You already have the parts? If you already have the parts or got a deal on something go for it, it's not a bad combination at all but you can work just as hard, spend just slightly more money and wind up with something a bit better or easier to use in the future with different parts should you want to change something. With what you want to do you wind up with a combination of parts that it's hard to use over or change in the future. You spend the same money on the crankshaft to stroke it to 5 1/8" as you do to stroke it to any other dimension. The Allis rods are weak, the 2.375" crankpin isn't common, the rods are large and bulky on the bottom and with the straight cap they do not clear well. They will however work in your wd45 block without much work.

What I'm saying is that it is cheap and easy to do the stock parts and 5 1/8" stroke but when you're done you wind up with a 273cube engine with fairley weak rods and cast pistons.

The engine I'm working on, I have to grind the crank anyway, it's bent. It's not much more money to offset grind it again and run a different rod, so I cut snap ring groves in the pistons to hold the wrist pin in with snap rings instead of the clamp on the stock rods. I took a set of Buda rods and made bushings for them. The crank is 5.125" stroke with a 2.375" crankpin, offset ground that to 5.4" stroke with a 2" crankpin for the Buda rod. I had to trim a few CC out of the piston bowls to make the compression ratio come out, I was going for about 10.75 to 1 with the new combination.

So it only cost about $200 more to offset grind the crank than it was going to cost to grind it anyway. Buda rods are pretty cheap, say $25 each, our salvage yard is full of them. The piston work isn't rocket science and it's easy to make rod bushings, I can cut a set of pistons in just minutes. A little money to size the rod bores, pin bores and balance them, I just didn't have to spend but maybe $400 total in parts and machine shop labor over what it was going to cost to fix it anyway.

I'm not the biggest fan of the Buda rods, but they are a nice little rod for this type of engine, much stronger than the 201-226 rods. It works in this situation. If starting from scratch a guy could have went to about 5.5" or 5.6" stroke and it would have all worked out fine with the material the pistons have to remove yet. I wasn't starting from scratch so I had to take what I had to work with.

The Farmall H rods are strong, cheap and common, the offset cap makes them clear fairley well, at 5.4-5.5 stroke I typically use them with a Chevy 400 type piston. I just bore an old set of 4 1/8" liners .out .030" to get a true and fresh bore and there's a multitude of cheap and common off the shelf pistons that work for that combination. Going to the 8" long rod keeps the angle better and isn't hard on parts, and it works well with larger bore pistons that are also common should you bore the block and put in say a 4.5 to 4.625" bore in the future. You can find some strong and light Chevy pistons cheap, the rings are lower tension, thin, not so many of them and the rings are located high on the piston, all little things that help the engine live longer and make a little more power compared to those 226 AC pistons.

What I'm saying is if you're starting from scratch, it may be wise to go a little different direction and you'll have a package that is easier to work with both now and in the future while still keeping the costs down. If you already have the parts and are married to the project, then what you want to do is fine.

And to answer your question as to what kind of power the 4 1/8" X 5 1/8" will make, it'll make as much power as you can give it air to do so. I can't say, it'll make 50 hp if you don't feed it and it can make 75 if you give it enough air..... maybe it'll make 52 on one dyno and 80 on the next one. I don't care much for the dyno numbers, it may make 70 hp but if the cam isn't timed right it'll lay down quick and one with an honest 50 hp across the range will beat up on it bad.

Yes, you should have a cam ground. Make sure it gets indexed or timed to the crank properly. I typically set the engines up with an intake centerline of 104* ATDC. On these type of engines I typically go for a cam with 200 to 204 degrees duration @ .050" lift on the intake. The exhaust is nice to have a little less duration, 192 to 196 degrees @ .050 and I try for a lobe separation angle of 106 degrees on these mid sized engines. Depending on who you have grind your cam, specs may be slightly different but these will get you pretty close to perfect and you'll have something that works well in all situations. I've got a few engines that I've ran longer duration on, wider lobe separation angle, but it depends on what I'm doing for the induction package, engine size and speed.




Edited by wi50 - 23 Feb 2014 at 12:33pm

So, you've proved both our points on the limits of the A-C connecting rod beam. I stand by my claims of, the "newer" the rod, the better. I also stand by the correct ignition timing and really good fuel octane, so there are no detonation problems. Your set-up is dependent upon this style of rod, so without more work and expense, I'd go with it. You could have your newer Gleaner rods shot-peened, and that would add some toughness to the beams.   There is also a way to heat-treat the rods by putting them in your wifes oven and heating them to a specific temp for a period of time and then dunking them into a bucket of oil to toughen them. The beam area is where the weakness is. You'll also notice the later model conn rods use a Gr 8 bolt on the piston pin end.

wi50 can you show pics of the inside of your heads after it has the port work done

Roman, give me a call