moper
Well-Known Member
I figured I'd post this as a new thread, with the discussions of "stroker build or not" coming out. http://www.fordmuscle.com/archives/2003/09/stroker/index2.shtml
I know, I know..It's a Ford site..But the info is very good. Interestingly, the Ford 302/5.0L block is not know to be of great material, and can be split in two down the cam bore by making more than 600 hp in a factory block. Mopar blocks are known to be of better material. I say this because the rod ratio has been brought out as a premature wear item in the other post. The 347" ford stroker engine has the same rod ratio (ok, 1.58 for Ford w/5.4" rod and 3.4"stroke, vs 1.53 for a 416 w/6.123 rod and 4"stroke)and has proven very reliable over the years. In factory blocks. Some with super/turbo chargers or NOS systems. Another interesting read is this..I'll post the link to the full article. It's by Stahl headers.. http://www.stahlheaders.com/Lit_Rod Length.htm
"II. Short Rod
A. Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cyl head from 10-o ATDC to 90-o ATDC the early part of the stroke, but will not suck as hard from 90-o to BDC as a long rod. Will require a better cyl head than long rod to produce same peak HP. Short rod may work better for a IR or Tuned runner system that would probably have more inertia cyl filling than a short runner system as piston passes BDC. Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod.
B. Compression Stroke -- Piston moves slower from BDC to 90-o BTDC; faster from 90-o BTDC to TDC than long rod. Thus, with same ign timing short rod will create less cyl compression for any given crank angle from 90-o BTDC to 90-o ATDC except at TDC. As piston comes down, it will have moved further; thus, from a "time" standpoint, the short rod may be less prone to detonation and may permit higher comp ratios. Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes--ie. may permit longer intake lobe and/or later intake closing than a long rod.
C. Power Stroke -- Short rod exerts more force to the crank pin at any crank angle that counts ie.--20-o ATDC to 70-o ATDC. Also side loads cyl walls more than long rod. Will probably be more critical of piston design and cyl wall rigidity.
D. Exhaust Stroke -- Stroke starts anywhere from 80-o to 110-o BBDC in race engines due to exhaust valve opening. Permits earlier exhaust opening due to cyl pressure/force being delivered to crank pin sooner with short rod. Requires a better exhaust port as it will not pump like a long rod. Short rod has less pumping loss ABDC up to 90-o BTDC and has more pumping loss from 90-o BTDC as it approaches TDC, and may cause more reversion.
Another spot in regard to side loading/losses...
"Empirical experience; however, indicates that the mechanical stress is less with the longer rod length. There are two reasons for these results. Probably the primary reason for these results is that the profile of the instantaneous velocity of the piston changes with rod length. The longer rod allows the piston to come to a stop at the top of the bore and accelerate away much more slowly than a short rod engine. This slower motion translates into a lower instantaneous velocity and hence lower stresses on the piston. Another strong effect on mechanical stress levels is the angle of the connecting rod with the bore centerline during the engine cycle. The smaller the centerline angle, the less the side loading on the cylinder wall. The longer rod will have less centerline angle for the same crank angle than the shorter rod and therefore has lower side loadings.
Interesting note tho..This article adds that any rod ratio below 1.60 " is a design flaw..." lol. Both these examples fall into that catagory, where a typical 340 has a ratio of 1.85...
Be warned tho, the Stahl article was a bit heady, and not designed to prove or disprove, just to show the effects of both long and short on power in respect to exh port flow in competition engines (read as 600rpm by his definition). I suggest a read for anyone who's thinking about stroking a Mopar of any size.
I know, I know..It's a Ford site..But the info is very good. Interestingly, the Ford 302/5.0L block is not know to be of great material, and can be split in two down the cam bore by making more than 600 hp in a factory block. Mopar blocks are known to be of better material. I say this because the rod ratio has been brought out as a premature wear item in the other post. The 347" ford stroker engine has the same rod ratio (ok, 1.58 for Ford w/5.4" rod and 3.4"stroke, vs 1.53 for a 416 w/6.123 rod and 4"stroke)and has proven very reliable over the years. In factory blocks. Some with super/turbo chargers or NOS systems. Another interesting read is this..I'll post the link to the full article. It's by Stahl headers.. http://www.stahlheaders.com/Lit_Rod Length.htm
"II. Short Rod
A. Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cyl head from 10-o ATDC to 90-o ATDC the early part of the stroke, but will not suck as hard from 90-o to BDC as a long rod. Will require a better cyl head than long rod to produce same peak HP. Short rod may work better for a IR or Tuned runner system that would probably have more inertia cyl filling than a short runner system as piston passes BDC. Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod.
B. Compression Stroke -- Piston moves slower from BDC to 90-o BTDC; faster from 90-o BTDC to TDC than long rod. Thus, with same ign timing short rod will create less cyl compression for any given crank angle from 90-o BTDC to 90-o ATDC except at TDC. As piston comes down, it will have moved further; thus, from a "time" standpoint, the short rod may be less prone to detonation and may permit higher comp ratios. Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes--ie. may permit longer intake lobe and/or later intake closing than a long rod.
C. Power Stroke -- Short rod exerts more force to the crank pin at any crank angle that counts ie.--20-o ATDC to 70-o ATDC. Also side loads cyl walls more than long rod. Will probably be more critical of piston design and cyl wall rigidity.
D. Exhaust Stroke -- Stroke starts anywhere from 80-o to 110-o BBDC in race engines due to exhaust valve opening. Permits earlier exhaust opening due to cyl pressure/force being delivered to crank pin sooner with short rod. Requires a better exhaust port as it will not pump like a long rod. Short rod has less pumping loss ABDC up to 90-o BTDC and has more pumping loss from 90-o BTDC as it approaches TDC, and may cause more reversion.
Another spot in regard to side loading/losses...
"Empirical experience; however, indicates that the mechanical stress is less with the longer rod length. There are two reasons for these results. Probably the primary reason for these results is that the profile of the instantaneous velocity of the piston changes with rod length. The longer rod allows the piston to come to a stop at the top of the bore and accelerate away much more slowly than a short rod engine. This slower motion translates into a lower instantaneous velocity and hence lower stresses on the piston. Another strong effect on mechanical stress levels is the angle of the connecting rod with the bore centerline during the engine cycle. The smaller the centerline angle, the less the side loading on the cylinder wall. The longer rod will have less centerline angle for the same crank angle than the shorter rod and therefore has lower side loadings.
Interesting note tho..This article adds that any rod ratio below 1.60 " is a design flaw..." lol. Both these examples fall into that catagory, where a typical 340 has a ratio of 1.85...
Be warned tho, the Stahl article was a bit heady, and not designed to prove or disprove, just to show the effects of both long and short on power in respect to exh port flow in competition engines (read as 600rpm by his definition). I suggest a read for anyone who's thinking about stroking a Mopar of any size.
