SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: JMS-1 on October 23, 2009, 11:26:02 AM
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first off, sorry about all the stupid questions but i figure it doesn't hurt to ask, lets say i was building a standard 4.00'' stroke BBC (454) with stock length rods. the rod ratio would be 1.53-1. if i was to use a tall deck block with longer rods the ratio would be 1.63-1. if everything else was the same on the two, what would be the performance gains with the longer rods? the higher the ratio the better but, what exactly will this do? any imput would be great ,thanks..
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Probably other things too....but the way I understand it with the 1.53 ratio, the piston has a tendency to scrub side to side more compared to the the same crank with a longer rod. Maybe Barry or someone else smarter than me will chime in.
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Piston speed comes into play....I am currently looking for the graphic I have explaining it....Most people think that piston speed is the same for any engine at the same RPM...not the case at all...I need to check my other 'puter :thumbup:
More later. :sly:
GT
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i know this is a technical question , i was just trying to figure out what way to go with my winter project and if it would be worth messing with the tall block or save it for something else,
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I think a larger ratio gets you a longer "hang" time during the combustion cycle, thereby getting more energy released. But I think a larger ratio also equals more sideways "push" on the cylinder walls which isn't desirable. GM has to work to acheive the much more desirable ratios that Ford and Mopars enjoy.
Treat this information with caution, cuz I know just enough to be really dangerous to other people's projects LOL
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Long rods and short pistons (from dome to bottom of skirt) with a high pin location will net slower overall piston velocity at the top and a better use of the combustion for making HP....But more prone to detination. Higher RPM? >5K + :-\
Lower the pin and use a shorter rod and the piston speed (velocity) at the top or TDC increases and is desired for more torque(which is what a jet pump seeks) often times less prone to detination. Lower RPM? <5.5K :-\
Since this is not something easily tested on a dyno unless you build two engines, the info is not perfect.
The side loading is normally an issue because of the construction of the piston, shorter rods require taller pistons so "skirt slap" becomes an issue (especially with added cylinder wall clearances). Conversly, the short piston height of the long rod dealio has nothing to push against, maybe more of an issue, but I'm undecided.
As far as "the larger the better" rod ratio's, I will respecfully disagree. In the world of streetable HP, sure, but for a torque hungry jet boat, not totally true.
Hope I didn't start another torque vs. HP debate.....I don't have the energy. :thumbdown:
I have no idea where I have the graphic of the long vs. short explanation, I will keep checking....
GT
Forgot to add...short rod = need good wall thickness and beefier rods and pins.... ;) Less of an issue with a longer rod.
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thanks for the info, so there are a lot of variables that come in to play with this and it would depend on what the engine is being built for. this will be going in a v-drive , so unless i misunderstood, it might benefit to use the tall block and longer rod, but not get carried away with it? off the subject a little bit but do any of you use a desktop dyno program, or anything similar? how accurate are they? are they worth the money?
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Completely agree that longer ratio is better on the street. That is where my tiny bit of experience is. Guess I hadn't considered boat needs would be much different, and really, are they THAT much different?
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Completely agree that longer ratio is better on the street. That is where my tiny bit of experience is. Guess I hadn't considered boat needs would be much different, and really, are they THAT much different?
From what I remember, it had more effect on torque than HP...My answer would be "not where I run em'" ;D
GT
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this is why i ask questions on this forum, you guy's are really helpful and the discussion doesn't turn into a snowballing argument that gets out of control like the ''other'' site. i am sure there will be more questions but, again thanks..
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Why would anybody ever argue about the importance of HP vs. Torque ??? :screwy:
And IKXnay on the "snowballing" . . . it gets the Vegas guys all worked up n shit