Ok so as a "forum reader" I'm gonna chime in here.
This whole discussion about rod length and piston choices all comes down to one thing and one thing only. Micro-Tuning.
I firmly believe that a longer rod will build horsepower because even though the average piston speed will remain the same, piston speed at TDC and BDC will be slower and the piston will dwell longer (stop moving) at TDC and BDC, simple mathematical calculations and geometry will prove this to be true. At TDC this will create more time for cylinder pressure to increase before the power stroke actually begins. It's only logical. Also although I have never heard this said, I believe too that since the dwell time at BDC will last longer, there will be more time for momentum (or pressure in a forced induction app.) to bring the intake charge in just before the compression stroke starts.
Keep in mind though, I could be wrong there due to the fact that in the dead middle of the stroke the piston will move signifigantly faster (remember for avg. piston speed to remain the same, if the piston is moving slower near top and bottom dead centers, other areas of piston will move faster). Take away from that what you will, I am not an engineer, I'm just a teenager.
NOW TO THE OP, who seems to have dissapeared. You really shouldn't waste the time or money pursuing this idea and here is why. RustyRatRod, has repeatedly said that slower piston speeds will be better for a street engine. What he hasn't said, is the reason why. If the piston dwells longer at TDC of the power stroke, pressures will rise more.
Higher pressure = higher temperatures = detonation. So if you choose to run 198 rods, you will either have to pay more and run higher octane fuel, or pull ignition timing out, or turn the boost down OR all three. Is this really a comprimise you want to make after spending close to $700 in pistons and rods?
Here is my suggestion to you. If you are really after getting a quicker revving engine to spool the turbo do this build.
- Find a cast crank block. (1975-1982 blocks)
- Run a cast crank (they weigh ~24 lbs. less than a forged crank)
- Run 225 rods
- Run a good set of cast or hypertectic(spelling?) pistons
- Use arp crank main studs, rod studs, and head studs
- Pay super close attention to ring gaps
- Send your cam, or a cam core to Oregon cam grinders and let them help you pick a good turbo grind.
- If budget allows, head work. Open and polish the ports, smooth all sharp bends, bigger valves.
Honestly, I don't think your engine should be anymore potent then it is now, young guns and fast cars on the street causes problems. Instead of going faster why not focus on some other areas? Are your brakes big enough to support your power? (hint, think disc brakes up front and ten inch drums out back). What you got for a tranny? A manual VB 904 and a converter that stalls at the appropriate rpm will help alot. Please tell me you aren't running a 7-1/4" rear, at least use an 8-1/4" with a gear ratio in the 2's range, believe it or not gears over a 2.76 ratio will slow a turbo motor down.
Basically it all comes down to what me and RustyRatRod talked on the phone about last week. Alot of V8 guys have no idea where to stop and get obsessed with micro tuning and all this little expensive crap that doesn't really make big and/or favorable changes when considering what the car will be used for.
Slant guys make use of micro tuning because we don't have the cubic inches to hide the missmatch of parts so we have to be spot on to be competitive. But here is the thing, the last statement in italics applies to naturally asperated engines. When you start pushing NOS in your slant or running a turbo or a supercharger, you are band-aiding the missmatch of parts and therefore micro tuning becomes largely unimportant in the grand scheme of things.
Basically what I am saying is,
if you are on a budget, you should spend money wisely and dumping money on these rods and pistons will be a waste of time and money.
And one last thing, before anyone calls me out on running a cast crank turbo motor, go over to ss.org, ask and discover how many cast crank engines are living well into the 300-400 h.p. ranges.
Keep in mind, this post is exclusively centered around street cars. If you are running at the track (100% drag car, no street use), everything I said goes out the window. Micro tuning becomes important and you want to extract every h.p. you can irregardless of cost.
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