Hi, Just out of curiosity what if any would be the benefits of a 350 with 5.6 rods and the proper height pistons (taller)? Obviously you would increase the rod speed but would you gain or lose torque/HP?
I'm not exactly sure but I think you would be going in the wrong direction (lengthwise). I believe the longer rod length allows the piston to be at TDC longer. The shorter rod also increases the cylinder wall pressure between the piston and wall. I have a 377" SBC in my 57 that uses 5.7" rods a 350 crank and a 400" block. It has something to do with the connecting rod to stroke ratio. The connecting rod to stroke ratio has a marked effect on the angle at which the rod angles in relationship to the bore center line. The severity of this effect increases as the rod shortens. Longer rods have a reduced effect on piston side loading than do shorter rods. See the APPLIEDSPEED.COM rod/stroke calculator . The rod stroke ratio and the piston pin offset affect several engine dynamics, including piston speed and acceleration, piston dwell at top dead center and bottom dead center, piston side loads, cylinder loading and bearing loads. Many of these elements play roles in engine aspiration, combustion, and wear. Pat
See I was under the impression that the longer rod made for a slower piston speed so it reduced rpm. My thoughts immediately went to the DZ302 with its higher rev capacity. from what I have read it is the piston pin position that determines the rod size necessary. Plus with a shorter rod I would think there would be less slap because it completes the rotation in a smaller radius. Maybe all that would be gained is rpm. I know right away people will mention that an internal balance also increases rev capability. Perhaps clarity could come from knowing the rod size on a Dz302.
DZ 302 has same rod length as every other small block (except the 400) 5.7".......you might be confusing stroke with rod length....
Well yes stroke definitely does affect the tightness of rotation, but I just wonder if anyone has ever used a shorter rod in a 350. Without changing the bore or stroke it seems to me that using a smaller rod could tighten up the rotation and increase piston speed in theory allowing higher rpm. I’ll be honest this is just an intellectual exorcise. It’s Friday so why not
more vacuum at low RPM, which means better throttle response and low end torque (good for street performance and everyday driving). Spark timing can be advanced a few degrees for some additional low speed torque, and the engine is less prone to detonation. 327 pistons with 5.565" 400 rods would be .020 further in hole.
That’s the answer I was looking for thank you. Could domed pistons also be used or would the exact specs you referenced be ideal
Many years ago (don't recall the publication, maybe SuperChevy?) was an article about the "SBC Chevy should have built" I forget the CI's, they used a 6 inch rod, made very good numbers on lower octane pump gas, which they attributed to the piston being at TDC longer (dwell) reducing detonation effects. I'll see if I can find the article. It's here, guess it was HRM, and 6.2 inch rods. Can't paste the link. Google "The 350 Chevy should have built"
you might also look at rod length to stroke ratio....the 302 (5.7/3=1.90) is a lot better than a 350 (5.7/3.48=1.64), which is a lot better than a 400 (5.565/3.75=1.48).
More from https://www.enginebuildermag.com/2016/08/understanding-rod-ratios/ One of the disadvantages of longer rods and a higher rod ratio is that low RPM intake vacuum is reduced somewhat. Reduced air velocity into the engine hurts low speed throttle response and torque, which is not good for everyday driving or street performance, but works well on a high-revving race engine.
I have built SBCs with long and short rods... you have to think of the top of the piston (6" rods and 5.7" rods use different pistons) which moves the same amount and is in the same position regardless of which combination you use (I don't think any piston "stays" at TDC - it de-accelerates as it approaches TDC, reverses direction at TDC and accelerates back down the hole towards the bottom of it's stroke. Yes you can use a short rod on a long rod piston, but that piston top would be way down the hole and you would take a hit on your CR. The other combination would probably result in piston to head clearance issues. My take on the long / short rod discussion is that it makes very little difference to performance - but in the world of "bleeding edge" performance it could make small dynamic differences.
I've never heard of anyone using shorter rods for performance. Lots of dirt track racing guys have used 6" Olds rods in a 350 Chevy when rules said "stock rods" to get that extra torque. I'm unclear on what OP means by "tightening the rotation" with shorter rods. Smaller stroke would make for a tighter rotating circle, but rods are independent of that.
By bleeding edge you mean CR loss because of slap. That is a clearance issue. I have never read about increased slap with a shorter stroke. But since I do not have building exp. I’m not able to say. I do know from what I’ve read slap is an issue with a larger stroke and rod combo. However I wonder if Hypereutectic pistons could solve that..?
What I mean by tightening the Circle is that the the rod is an extension of the stroke. If you shorten the rod you should be bringing it closer to the center of the circle so in theory that should mean a higher speed of the piston less weight from the rod and less overall shake due to a smaller rotation.
Never used a shorter rod in a 350, but I did in a 305. ????? Guys are wondering why a shorter rod, and then in a 305 of all things. This was M-A-N-Y years ago, when I bought a 305 stroker kit for a 305 block. Same deal as the 350 to 383, but uses a 305 block to make a 334. It was when the emissions testing in Washington State was starting to look like California's. The 305 blocks had a large, upside down, "305" casted in the sides of the block, so "they" knew if you swapped to a 350 block right off the bat. Now, we no longer have emissions testing at all! Take a 305 block and bore it .030 over, cut down 400 crank (just the mains), a 400 rod (5.565), and a dished 305 forged piston. You can use a stock style piston, and not have to go with an aftermarket, custom pin height piston, that a 5.7 rod requires. The engine is still in my DD truck, runs really good, and has a freeway sweet spot right at 70 MPH. The only drawback is, I kept the 601 heads with the smaller valves; good for low end, but not much anything else. Very doubtful you can even tell any difference in the rod length. It really only applies with full on race engines, where you want the piston to remain at the top of the stroke for as long as possible (dwell) in order to better fill/exhaust the cylinder and provide for better flame propagation. Like I said, you'll never see the difference. I am Butch/56sedandelivery.
Hey Butch! That is incredible! Thank you for sharing that, like I said this is the kind of answer I am looking for. I can’t believe that they were that strict with emissions testing lol. That’s crazy they would check stuff like that... geez. Do you think you’ll ever swap heads or valves?
My comments were based on a 350 cu inch SBC using either a 5.7" or a 6.0" rod (would require an aftermarket piston). Strokers... different ball game - big difference on a stroked motor is the angle of the rod at 3 o'clock and at 9 o'clock. To build a 383, you usually have to clearance the block, and you need to modify your pan. A 302, 327, 350 all use the same 4.0" block with different cranks (Main and Rod Journal Bearings differed from Year to Year) and all of them used a 5.7" Rod.
im still not entirely sure what you are saying. but I wonder if you are referencing that too small of rods could also pull the piston too far down and have the same effect as too big of rods?
I actually have a different truck with a 454/TH400 for towing; the stroker motor is just an everyday truck. I actually just sold my "good" heads; more like gave them away, but it was looking like I'd never get around to using them. Made one HAMBER happy. The CR with this stroker, with the 53 CC chamber heads, plus the extra stroke, pushes it into premium fuel range and retarding the timing some. And to think, I almost went with true flat top pistons, instead of the dished pistons! The California emissions people used to use a mirror under the vehicle, to check for the "305" on the blocks (blocks can be re-stamped on the pad easy enough, and there were millions more of 305's made in those times than the 350's); it was just a matter of time before they started that here in Washington. BUT, they decided the emissions were such, with so many NEWER cars/trucks on the road now, that the program was no longer needed (imagine that, a State bureaucracy actually using their heads for a change). I't was only around the big cities also, so really unfair. If it had included a true vehicle safety inspection, I'd have been behind that. Besides, all it takes is one VW Microbus, with "Save the Planet" stickers, and die-hard hippies inside, blowing smoke out the exhaust, to cancel the effects of statewide testing (seen it many times). I am Butch/56sedandelivery.
You are confusing "piston speed" which is measured in feet per minute(FPM) with crankshaft revolutions per minute(RPM)....they are not the same thing. Rod length has nothing to do with piston speed, piston speed is entirely dependent on length of stroke x RPM Say we use a 3.25 stroke(327 chevy) and 7000 rpm...piston is traveling 6.5" for every crankshaft revolution(3.25" up, then 3.25" down), so the math is: 6.5" traveled x 7000 rpm gives us 45,500" traveled in one minute...convert to feet is 3791.6 FPM 3.48" stroke(350 Chevy) @7000 rpm is 4060 FPM piston speed 3.75"stroke(400 Chevy, 383 stroker chevy)@ 7000 rpm is 4375 FPM 3.00" stroke(283, 302 Chevy)@ 7000 rpm is 3500 FPM For any given RPM, shorter stroke is slower piston speed, longer stroke is higher piston speed. The only two impacts rod length will have is: #1- velocity approaching and leaving TDC and BDC will be different for any given stroke, the shorter the rod the less dwell time the piston will have at TDC/BDC for any given stroke. The short rod can show some benefits on an engine with poor cylinder heads as the increased velocity away from TDC will pull harder vacuum on the intake port helping to increase cylinder fill, and the reduced velocity up from BDC allows a little more time for ram effect to pack the cylinder before the piston starts back up the bore. This is a very small amount though, hard to quantify even on a dyno. #2 the short rods increase angularity to the bore at 1/2 stroke mid travel, which can greatly increase bore wear since it side loads the piston harder....one of the big problems the 400 SBC and the early stroker 383's done with the short 5.565" long 400 rods share. I can't think of a single good reason, other than the cheap 305 to 334" stroker or a real tiny budget used parts 383 stroker or stock 400 SBC extreme budget build to bother with using the weaker 5.565" stock rod It is weaker than a stock 5.7" rod because of the deeper cut in the rod to lower the head of the rod bolt for cam clearance. You'll find that for an aftermarket performance rod, you'll find maybe one or two companies even bothering to offer a better than stock 5.565" rod....virtually nobody wants them, no market for them. I don't know where in the hell you would find a 5.6" SBC rod....it would have to be custom made and sure seems like money foolishly spent to me.
Lol the Hippies, yeah I had 2.56s with a stock 350 and TH350 getting 21 mpg. Seems like trying to say one engine is better than another is only a tip of the iceberg. Is there some kind of loophole for classic car owners in California now or does no one follow the rules/own the guzzlers?
@56sedandelivery Hey Butch, when was this with the mirrors to the block? I had a '78 F Body came with a 305, in '81 or '82 I put in a 350, never had issues every other year. They did look things over pretty good, one place made me leave off the property to flip my aircleaner lid over...mercy.
Holy!! Thank you for your reply! Okay so you cleared up a few things for me. I didn’t realize those strokers that used 5.565 rods were the ones I thought of when I made the connection between slap and the 383. but then following what you say there is less FPS piston travel so less friction which is cancelled by the angle. However that is compared to a very large 3.75 stroke to the shorter 3.48 which is less of an angle during BDC rotation. However I do see your point that it is a lot of effort for something that will make little difference.
Perhaps it would be easier to think of 3 engines - 302 (Bore = 4.0" Stroke = 3.0") , 327 (Bore = 4.0" Stroke = 3.25") and 350 (Bore = 4.0" Stroke = 3.48") which in fact at one time or another were all using the same block. They also used the same heads. Let's assume they were all designed so that the piston at TDC was at the top of the deck. Because they all used the same rods (5.7" long), the only possible thing to make these work was where the piston pin was in relation to the top of the piston. When Ross designs a piston for a 350 SBC, they can provide one for a stock engine (5.7" Rod) or one for a 6.0" Rod by changing the the location of the piston pin relative to the top of the Piston. Two engines built with this as the only difference would have virtually the same dynamics (except on the track), and probably very similar power curves. About 15 years ago, the SBC community was all ablaze when it came to Rod length - the only guys who really took notice were the Oval Track guys as they saw some durability and Fuel Mileage opportunities for an engine running 7000 RPM for 500 Miles.
Yeah that’s a really good analogy that equates really well. By adjusting pin position you can really run any rod. I was trying to get at exactly what you mentioned, high rpm and just overall performance. Changing the rods shouldn’t do anything but affect piston speed and rotation characteristics. For me I still think the DZ 302 and 70 LT-1 cams are pretty hard to beat. Probably still capable of outdoing Ford’s flat plane V8. So I just wonder sometimes how can you create something that can rev higher without sacrificing cubes.
If you want decent revs and more cubes you could look at the 302's big brother, the 377. For a street small block engine in a hot rod, I like the 5.7 rod 406. I internally balance it and can run any flywheel or dampener needed. The 5.7 rod keeps the wrist pin out of the oil ring groove. The 6.0 rod works well in a race package where it's not an issue getting into the ring groove.
406? Is that the 400 SBC? That would be cool however I live in Canada, Manitoba to be specific. People here horde the crap out of engines and cars so the prices are exponentially inflated. Like a 70 Challenger, Charger, or Cuda you are looking at people wanting 120000$ dollars at least lol. It’s so stupid. Even my 350 that I have rods, pistons a crank, mains, cam and lifters for a rebuild I’ve still been quoted 2500$ plus for rebuild. It’s not even like I’d need major parts just gaskets and labour but still that much it’s crazy. So for now stuff like that has to wait.
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