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By CK - 11 Years Ago
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I am curious if any one has experimented with connecting rod ratio's in the Y block to gain optimum Piston Dwell.
Having fitted a Cam which is encouraging the engine to operate much different to original, I would think a longer rod would increase dwell to ensure complete combustion at higher engine speeds. More torque! Though piston speeds also becomes altered which works well for compression yet can create problems of detonation post ignition. I believe old smokey can be quoted saying "stuff a rod as long as you can get into it". For example my 292 has a ratio of about 1.9:1 and a windsor 302 has a ratio of 2;1 .However the windsor has a larger bore stroke ratio as well.
So with all the options of rods and pistons available I found a set of Flat head rod at 7" with a 2" rod journal, H-beam type. I could then stroke the 292 crank to 3.4" or 3.5" at 2:1 or more. Does anyone know of sets which would fit without grinding the journal down as far? |
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By Ted - 11 Years Ago
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See if this link helps. http://www.eatonbalancing.com/blog/2013/01/31/y-block-585hp-without-a-supercharger-or-other-form-of-power-adder/ As a general statement, shorter rods assist in making more torque while longer rods help the horsepower attributes. In a circle track application, the short rod engines are stronger out of the corners while the long rod engines are better at the top end of the straights. In another racing venue, the engines that are making 3HP/CI are running as short a rod as possible but these engines work in a very narrow rpm range. The camshaft is a big player in all this as the cam that works for a short rod engine will likely need some tweaking to work as well in a long rod engine or vice versa.
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By CK - 11 Years Ago
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Ahhh ! So then would the longer rod be better for HP due to greater induction at higher rev's? HP takes over at 5250rpm and these heads struggle to rev, so working towards a shorter rod would be appropriate?How does this read?Torque is strength Power is the amount of work done in time And the HP is the repetition of torque-power Therefore I should be stroking the engine in place of increasing the bore? Yet if I increase the stroke by .200" I only gain 18cu yet if I bore it .200" a gain of 44cu is achieved. I guess this is the difference of HP and TorqueTherefore a 3.5" stroke would work ok with a 7" rod? This would be what you'd say HP ratio? Is 2" big end to small? Something I have to ask is if a head is flowing 200cfm, will it perform differently on a smaller cube engine 292 apposed 312-340? Windsor guys talk about 400+HP with such flow. Is this because the rod ratio and bore/stroke ratio? Sorry about all the questions, but I have many more. |
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By charliemccraney - 11 Years Ago
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CK,
Ted can explain things better than I, but some thoughts.
If your engine is built for the street, then you simply do not need it to produce power much above 5000rpm. You will never use it and it will become less enjoyable for the street. It sounds cool to say you have 600hp at 7500rpm, but that will be lousy on the street. I'd rather have something like 350 at 5000 with a solid torque curve.
I assume your bore and stroke increases are hypothetical if we are talking about a 292. The maximum stroke you will get with a pristine crank and a 2.00" diameter journals is 3.488 (+.188) and a bore increase of .200" might be possible but is likely unrealistic so the actual gains in displacement for your engine with bore and stroke increases are yet to be determined.. Further, the 3.50" stroke with a 7" rod will leave .992" for compression height which leaves about 1/2" for the rings which I do not believe is possible without more custom parts ($$$) and may simply not be possible.
Regarding the head flow and displacement of an engine, everything else being equal, a smaller engine will peak higher than a larger engine, so you will have different performance on a 292 vs a 312-340. How much different will depend on just how radical the engine is.
Everything you do to an engine will affect just how much power and torque is produced and where they will peak. It's not just one thing and I would not get stuck on the rod ratio. As long as you don't get too wild with the cam, just about anything you do to the engine will have positive results. People usually seem to go overboard on the cam.
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By pegleg - 11 Years Ago
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Very simply, and to add to what Charlie's trying to tell you, Work at getting the most TORQUE you can in the 2500 to 5000 Rpm range and you won't care what the HP number is. Torque moves the car, HP impresses people who have no clue.
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By aussiebill - 11 Years Ago
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pegleg (5/28/2014)
Very simply, and to add to what Charlie's trying to tell you, Work at getting the most TORQUE you can in the 2500 to 5000 Rpm range and you won't care what the HP number is. Torque moves the car, HP impresses people who have no clue.
Couldnt have summed it up better!
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By CK - 11 Years Ago
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Yet a HP engine is a smoother ride. And I agree a mid range engine will perform quite well as I used to race around MX on Yamaha's and there were few I could not beat, but bikes like the Honda would do as Ted says run away in the straights. The Cam I have fitted is a 280-107 Clive Cams. 2500-5800rpm As you'll know its a pitchy torque cam "107" and "280" my Holley Terminator considers this as a entry level street and strip Profile. The heads I've ported 113's as you may remember in the post 5752-113 On that If anyone has a set of Mummert Heads they'd like to part with for the right price please let me know!!!!! Maybe a swap could be considered? Getting back to the rod ratio, 1.8:1 - 1.9:1 would better suit the Bore x Stroke ratio then? 1" compression height Pistons are available. and I'm currently at 296cu
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By CK - 11 Years Ago
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charliemccraney (5/28/2014)
CK,
Regarding the head flow and displacement of an engine, everything else being equal, a smaller engine will peak higher than a larger engine, so you will have different performance on a 292 vs a 312-340. How much different will depend on just how radical the engine is.
Thanks, this is how I understood it too.
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By Ted - 11 Years Ago
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There seems to be too much emphasis laid on the rod/stroke ratio. When the ratio gets less than 1.5, then there is some additional scuffing taking place at the lower ends of the cylinder walls. Piston design and cylinder wall clearances can be altered to help that. Beyond that, camshaft selection in regards to lobe centerline and duration and matching those relationships to the particular exhaust system being used are much bigger players than the rod stroke ratio. It’s all about managing the piston dwell time at TDC both at the compression and overlap strokes and this is much more manageable by manipulating the camshaft rather than altering the connecting rod length. You also have to keep in mind that having a rod that is too long makes the combustion process inefficient in that the connecting rod ends up having insufficient angle just after TDC to help push the crankshaft around.
Where I had picked up 45+ horsepower in shortening the connecting rod 0.450” and increasing the lobe centerline of the camshaft 4°, the camshaft change was the biggest part of that horsepower gain. On that particular combination, the piston compression height was changed from 1.000” to 1.450”. Shortening the rod was done because it was relatively easy to do so in that particular instance and reducing the dwell time at TDC as a result was conducive to the final results being sought after. Even without the rod length change, there was still a 40+ HP change attributed to just the camshaft.
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By CK - 11 Years Ago
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Ok, then that sums up my short motor. 296 for now.
I still think it's great that an engine can be geared by a connecting rod !
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