Well this is kinda off honda topic, but it was brought to my attention and I found it intriguing.

Seems in the midwest reneck races they test vacuum to determine what type of iternals you are running. Some rules state that you have to have to pull 16lbs of vacume at 1200rpm. Yet other classes require you to pull 17lbs at 1000rpm. Now would a longer rod and shoter piston height allow that drastic of a change? Just wondering.

 

Yup. Larger displacement pulls more air.

 

Well not exactly, bore x stroke = displacemnet correct. Were not changing the bore of the piston, nore the stroke since the crank would be the same. Just the travel velocity of the piston would be changed. Is this enough to have adverse/positive effects on the vaccum?

 

Changing rod lengh changes stroke.

 

here joo go...... http://victorylibrary.com/mopar/rod-tech-c.htm

 

If you leave the stroke and bore the same then the pin must be higher in the piston to take up for the longer rod.

I say since a shorter rod experiences higher speeds (i don't have it backwards do I) then it would pull more vaccum.

Looks good in my head anyways

 

disregard ANY info you find on this link, you might learn something from them MOPAR boyez!

 

A vacuum test is normally used to determine the type of cam profile used. The only way to determine whether stock rods are being used when required is to do a teardown. Race cams will have little vacuum at 1000 rpm and hardly idle at all, while street cams will have no problem passing the test.

 

The ONLY thing that changes stroke is the crank.

 

huh? that doesn't make any sense.

 

Chrysler/MOPAR did most of the math engineering for the modern combustion engines. Be surprised that some of the info is 40 yeras old.

 

oh i see.

 

if you want to run more vacuum at idle, run richer (i.e stoichiometric) with a progammable ECU etc. Hondas normally run lean at idle. If you don't change bore or stroke, your displacement won't change regardless of rod length.

Chas

 

Chasrex, please explain to me how a richer fuel setting will change the vacuum at idle.............

This assumes at same idle rpm.

Manifold vacuum is a function of valve timing and piston speed + displacement.

Fuel not in the formula.

 

not true, fuel has nothing to do with vaccum which is based on cam rmap rate, engine displacement and other engine components that have an effect on air flow such as valve angle, header primary size, intake manifold size, etc.

greg

 

So I was close