Longer connecting rods?

[Twisted6]

Toss this out there for more brains to help me think it out.

A common upgrade in a SBC V8 is to switch from the 5.7" connecting rods to 6.0" rods, thereby increasing the rod ratio.

Longer Rod Pros:
Less rod angularity
Higher wrist pin location
Helps resist detonation
A lighter reciprocating assembly
Reduced piston rock
Better leverage on the crank for a longer time
Less ignition timing is required
Allow slightly more compression to be used before detonation is a problem
Less average and peak piston velocity
Peak piston velocity is later in the down stroke
Less intake runner volume is needed

Longer Rod Cons:
Closer Piston-to-valve clearances
Makes the engine run a little more cammie at low rpm
Reduces scavenging at low rpm

Shorter Rod Pros:
Increased scavenging effect at low rpm
Helps flow at low valve lifts (a benefit if the heads are ported with this in mind)
Slower piston speeds near BDC
Allows the intake valve to be open longer with less reversion
More piston-to-valve clearance
Can allow for a shorter deck height

Shorter Rod Cons:
More rod angularity
Lower piston pin height (if the deck is not shorter)
Taller and heavier pistons are required (again, if the deck height is not reduced)
More ignition timing is required for peak power

Stock rod ratio on the Vulcan is 1.76, which is surprisingly good. Motors built for endurance racing have ratios usually between 1.9 to 2.2. Chevy also seems to use a big end bore of around 2.2" (Same as the Vulcan) on their rods, if a 5.7" rod were possible that would bring the rod ratio up to 1.81 and if the 6" rod were possible, even better at 1.91

[JP02xlt]

Look at Tom Morana's site

http://www.moranav6racing.com/ Lot of nice stuff, but pricey.

Problems with the stock Vulcan is the connecting rod bolts they are tiny, & stretch easily. A SBC rod if the pin dia is close or made to fit the Vulcan pistons would be a nice upgrade. Although I would also like to see some weights on each rod. You would not want to load up the crank with heavy rods of course you could always go aluminum.

Jp02XLT

[Twisted6]

I am very familiar with Morana's products, one of my friends has the 4.3L stroker kit on his V6 SN-95 Mustang.

I am thinking of this for people who don't want to drop that much dosh at once, at the same time, giving them something better than what Morana offers. No doubt that what Morana offers could be the basis for some of the most bad ass Vulcans, a forged crank that you can't find anywhere else, goes a long way.

With longer rods, the piston dwells longer at TDC, allowing for a more complete combustion, and raising the dynamic compression ratio, you could see a gain as high as .1 (Not a lot in the grand scheme of things, but longer rods are all about the little things adding up).

Not only will the pistons dwell longer at TDC, but piston peak velocity will slow down with longer rods, allowing the intake valve to open another .010" or .015".

Speaking of piston velocity, longer rods also decrease piston acceleration/deceleration from and to TDC, which lowers tensile loading down on the rod, which is the number one reason why rods break in high performance engines. Using a stock stroke SBC 350 as a reference point, that .300" longer rod decreases piston speed by 1189.23 ft/sec/sec at 7,000 RPM.

And since the longer rod moves past TDC slower, less ignition timing is needed for peak power, thus less chance of detonation and the ability to run even higher compression ratios than normally allowed.

There is also less rod angle, which reduces side loading of the piston, which means there is less friction, thus less wear.

Higher wrist pin height moves the center of gravity of the piston higher, which prevents piston rock, to which that aids in ring seal.

So for between $500 and $600 you get a set of rods that are good for at least 500 BHP, have a machinist put in some oil squirters, drop some money on some pistons to fit whatever your needs are (Whipple, turbo or NA) and at the end of the day, you've spent about half of what Morana charges for a stroker kit and, I feel, ended up with so much more than what that kit offers.

Or... you could get some 4.6L rods, 2.23" big end bore, .8645" pin bore and 5.933" length..

[volrath]

hey twisted. what formula did you use to figure this out?

[Twisted6]

Rod ratio is simply, ratio = Rod length / crank stroke.

[volrath]

for the rod speed differance and such?

[JP02xlt]

I totally agree with you Twisted6, I wish I had a little more time to play with my new motor last year, I focused on the intake, heads, valve springs & rocker arms. Maybe other than Morana no one makes upgrade parts for the Vulcan that fits & works as they should. I would have loved to had a couple of months time to build the bottom end now to go with the heads I have, 25 psi would be a cake walk as long as the blower would live at that rpm. When I blew mine, it was a rod bolt that stretched that then took out 3 of the 6 rods. I think with the addition of good pistons & rods this little motor could be made to scream. The stock crank, (even though its cast is a pretty well designed & HD piece, the block has plenty of main support also. Let me know if you play around with the SBC rods, would love to see some fitment of those into the 3.0.

JP02XLT

[ohnoitsjeff]

I think he was just using the example of SBC chevy rods in a SBC application, showing pros and cons. Id look into all different makes and models that offered not only squirters but billet aluminum pieces and check their lengths and if the mounting locations can be adapted at all. Doesnt have to be ford or chevy. Toyota Supra's would be sweet as the aftermarket is one of the best out there but my guess is they offer shorter lengths not longer.

[Twisted6]

Posted by volrath

for the rod speed differance and such?

Piston speed based upon angle is a quite complicated equation. Hope you enjoyed calculus.

A link is much easier to post:

http://www.camotruck.net/rollins/piston.html

A used the Chebby rods as a reference point to demonstrate what such a small difference in length adds up to, as well as a potential contender. There are a few SBC and BBC rods on Summit Racing that meet the bore diameters of the Vulcan's rods.

I do like the Ford modular rods (As found in the 4.6L V8), the big end bore is slightly smaller than the Vulcan's, the crank can be machined down without compromising the crank's integrity (We're talking no more than .010"). Then we have performance replacement 4.6L rods, take the Manley 14042:

- 5.933" center to center diameter
- H-beam
- 602 grams each
- 2.239" big bore diameter
- 0.867" pin bore diameter
- 700 BHP and 7,500 RPM "advertised ratings"

[sdjr]

If you're only machining that much, isn't it easier to make up the size with different rod bearings? I mean if it's all apart, you're changing them anyways, right?

[volrath]

Posted by Twisted6

Piston speed based upon angle is a quite complicated equation. Hope you enjoyed calculus.

ha ha i'm taking math at tech right now lol the math teacher will like this