CU vs bore increase?

[BIG PSI]

(Posting as DAVE) I was wondering this today while I was in class. If a 2.5L has 153CU and you bore it out .40 over, does that mean that's .40CU more per each cylinder? Meaning 154.6CU? That doesn't sound beneficial at all, maybe I'm confused on the units here.

Thanks,
-Bryan

[ShelGame]

No, the 0.04" over is in inches of bore. The displacement is in cubic inches. To get the displacement, you have to find the area of the bore (3.14159 * (Bore Diameter/2)^2) and mulitply by the stroke and then by the number of cylinders.

[Frank]

0.040" should never be considered for performance IMHO. However when you start going to the .090" over like on my V6 car, then its performance. It will give me almost .13 liters more! In addition, when overboring that much, you can deshroud the valves better and increase VE, or port flow.


Frank

[Dave]

No, the 0.04" over is in inches of bore. The displacement is in cubic inches. To get the displacement, you have to find the area of the bore (3.14159 * (Bore Diameter/2)^2) and mulitply by the stroke and then by the number of cylinders.

Holy crap that's confusing.

Okay so can anybody just tell me how big my motor will be at .04" over bore? I'm terrible in math and I'm even in CP Algebra 2... bleh

[Una]

0.040" should never be considered for performance IMHO. However when you start going to the .090" over like on my V6 car, then its performance. It will give me almost .13 liters more! In addition, when overboring that much, you can deshroud the valves better and increase VE, or port flow.


Frank

.090.. Unf..What do you do for a headgasket at that point? Seems like the fire rings would be inside the combustion chamber..

[Frank]

Actually the stealth guys go .125" over and have headgaskets for this application. their dohc gaskets work on ours

[pooky]

Holy crap that's confusing.

Okay so can anybody just tell me how big my motor will be at .04" over bore? I'm terrible in math and I'm even in CP Algebra 2... bleh

If you have a 3.44" bore, after .040 over bore would be 3.48". CI=bore x bore x stroke x .785 x # of cylinders, or pi/4 x bore2(squared) X # of cylinders.

[JuXsA]

haha... it would give you what... an extra 2.5in? hahaha

[GLHSKEN]

Yep.. Boring these engines does nothing for power... It's purely to clean-up the block. That's why 1st time s/b .020"... the second .040" (those are readily available piston sizes)

[Tony Hanna]

It's amazing how many people in this area talk about an overbore as if it were a performance modification. At that point, my eyes sort of glaze over and I just keep saying uh huh until they go away.
Seriously, I had to spend over an hour on Yahoo with a friend the other night explaining why he would be better off not going 60 over on his already fresh smallblock. I think I finally got him convinced to put the money toward a heads/cam/intake/header combo that would work well together.

[Dave]

So is there ANY benefit from going STD to .020 over?? If not than I might just pull the block out of the other van in our garage and hone it out and put STD size forged pistons in it.

Which brings about another question that I need some clarification on... what exactly is honing? Does it just take off like .0001" of the wall just to clean up any scuffs??

[Tony Hanna]

The only real advantage to a small overbore like that is to remove any damage to the bores and bring them back into round since they will sometimes wear oval and to take bores that are worn past spec to a size where replacement pistons can be obtained. Personally, I don't see any point if the block is within spec. Basically, the only advantage is if the bores are worn enough to need it. Otherwise, the money is better spent elsewhere.

A hone is used to create the proper finish on the bores to help a new set of rings seat properly. It can remove some very shallow imperfections but isn't intended to remove alot of material.
HTH,

[GLHSKEN]

^^^^^ +1. Full agreement

[Frank]

This is very true.

The reason I am going for the massive overbore on the 3.0L is not because of the gain in liters. It is the gain in volumentric flow for my particular head. By going larger, it allows Steve Menagon to clean up the combustion chamber even more to allow for more deshrouding of the valves.


Frank

[turbovanman²]

Another reason for overbore is as stated, to clean up a rough bore but now you can use a torque plate and really make things nice and true,