# new displacement after overbore

Discussion in 'Members' Lounge' started by reckedracing, Dec 7, 2004.

1. ### reckedracingTTIWWOPVIP

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how would you calculate the new displacement of a motor if its been bored?

for example

xr250 249cc, bored.040 over?

2. ### Slammed90LudeSenior Member

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well, displacement is cylinder volume at BDC minus cylinder volume at TDC

you really need to know the volume of the combustion chamber to figure it out- if you know the originial bore and stroke you can find the volume of the cylinder at TDC

[(bore/2)^2*pi*stroke+combustion chamber]-[volume at TDC]

assuming that everything but TDC volume is known, you'll find a value for that- then just plug the new value for bore into the same formula

3. ### Battle PopeNew Member

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I think the quadratic formula exploded.

4. ### reckedracingTTIWWOPVIP

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how about a simplier way...? lol

just looking for approx figures here...

5. ### pissedoffsolRETIRED

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go to c-speedracing.com and use their caculator

6. ### reckedracingTTIWWOPVIP

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http://www.bgsoflex.com/displacement.html

1988 xr250

bore = 73
stroke = 59.5

overbore = .040 approx = 1mm

stock = 249cc
overbore = 255.9cc

hmm, i figured it would be a bigger diffrence, guess i was wrong

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Those have nothing to do with the volume of a cylinder.

8. ### hcivic.comSenior Member

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just water test it

9. ### tabSuper Moderator

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I just use the desktop dyno for most engines. They calculate that my Olds 455, bored .o60" over, is ~468 ci.

A normal bore will not gain you all that much displacement.

10. ### hcivic.comSenior Member

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tab you payd for dt dyno?

11. ### tabSuper Moderator

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not really........I DL'd it. It does a pretty good job of calculating what changes will give you the most. If you know the flow rates on your parts, you can input that data. It's all fine and good, but none of us have a flowbench.

It also does not account for VTEC, or changes in cam profile midway through the rpm range.

Not good for a Honda, but great for an old Chevy, Ford, Oldsmobile, etc.

12. ### reckedracingTTIWWOPVIP

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hmm, didn't think of that, but searching with google for specs on motor, and then sae to metric, and then a calculator is a bit easier...

so tab, what else do you use the desktop dyno for?

13. ### tabSuper Moderator

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A few things, although I haven't used it in a while.

I have a 1968 Olds Toronado(FWD V-8 ). I bought Arias 12:1 pistons that were bored .060 over. I wanted to know how much extra that would give me, and what would best compliment those parts for the buck. That program told me that my oversize pistons would raise my displacement to 468 ci.

You can change carb sizes, go single or dual plane manifold, create your own head profile if you have the flow bench results, change compression, engine size, bore, stroke, cam profile, roller rockers, roller cams.

Basically, there are a ton of things you can easily click to find results.

I found that my engine would produce over 550 tq and about 570 hp by using a good carb, roller cam, roller rockers, decent heads, and a single plane manifold.

It really helps if you have specs for your parts. There are templates to get you close on most V-8 applications, but that program isn't the best for imports IMHO.

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