Benefits from longer rods on mild BBC?

[Robert1320]

Conn Rod, no piston design.

Race motor.

If the rod is long enough to allow the piston to miss the crankshaft, you have the right rod.

I am more concerned with the correct piston than rod.

Is there enough top ring land?

Is the dome/top thick enough?

Is there enough 1-2 ring spacing?

Can I keep the oil ring out of the piston pin hole?

Is the pin located correctly to allow rod to dome clearance?

Does the piston have a fully support skirt?

These come to mind before I think about a Connecting Rod.

 

[Anonymous]

Soooo, to change the heads, you need a different length rod?
I am so stupid I never thought of that.

You know I gave you the benefit of the doubt many times but now you are proving how little you know and how much you like to start trouble with your out of context quotes and one line vague answers.

Here is some rod ratio 101 for you. The length of the rod changes how much the piston dwells at certain points and how it accelerates at different points. So then it directly correlates to the chambers/cam/runners/manifold/plenum/carbs. For a person who is such an advocate of longer rods you really don't know what changes do you? You probably think its for cylinder wall scuffing, (barely) but wrong again. better cylinder filling? only if the above are sized correctly. One more thing while the piston is stopped at the longer dwell its not pushing on the crank is it? Does it matter? Yes it does. Can you, Goob check any of this? have you checked any? have you ever built a engine from scratch (i don't mean assemble either) and done numerous dyno pulls/race track testing changing and optimizing the combination to see? Have you ever ran your engine with different rod lengths to see? I did.

You compare rod lengths on stock engines as your argument. Maybe they put bigger counterweights on the crank for a smoother running engine so the rod had to be increased? maybe they did it for quieter engine noise (not exhaust noise), maybe it was for emissions. Who knows why the engineers did what they did, but unless they post here your argument doesn't have any data to back up blanket statements.

Bottom line, make the piston combination number one on the list and hook the rod to the crank no matter how long or short it is and 99 percent of the engines will be happy.

I'm still curious if you are such and engine wizard why do you bracket race and why don't you have your own engine building business?Maybe you need do some research before you post YOUR magazine theories. In the future if you want to make a personal attack on me shoot me an email or call MY business and talk to me. leave it off the site.

 

[454RAT]

i have also always been told longer rod = longer dwell at top dead center. if you look at the new vette motor 427. i think it has a longer rod ???? just my .02 but looking at motors with longer rods they always seem to perform well and ususally better than the shorter rod motors. again just my conclusion.

 

[Goob]

The differences in rod length only matters in relation to how well it works with the chamber/runner/plenum/ carbs. How fast it accelerates to and from is what *pulls* on the cylinder head and how it helps with the combustion process.The secondary reason is its ability to push the crank.

How can you say it has nothing to do with heads?

Let me type this real slow, and stay on the topic for you....

To break it down, the potential improvement of the combustion process from a longer rod would be ONLY that the piston dwells at TDC and BDC longer, allowing in theory, less ignition timing advance to complete the combustion process while the rod is still at a desirable angle to apply the power to the crankshaft.

Changing how it "pulls" air because of the increased piston speed is meaningless unless you adjust and/or tailor the ramp speed and lift of the camshaft to maximize that extra piston speed. Meaning the quicker and more aggressive the cam lobe, the more it can benefit during the piston "sweep".

If you are not considering the camshaft profile, you're missing the boat, you can change rod lengths and cylinder heads until you are blue in the face and it won't make a bit of difference. The camshaft profile determines the airflow potential.

Everyone knows that each combustion chamber profile, shape, and size will affect the combustion characteristics, tuning requirements, and power output, but it has nothing to do with the rod length.

Once again, there are only four areas of the entire cycle that the rod length does not matter, TDC, 90* ATDC, BDC, and 90* ABDC, period!

 

[MalibuRacing]

This thread is locked. This has been beat to death. If you'd like to research this issue further, then Google it.....