4k Headwork For 5k Block

[philbey]

What's the service interval on the dirt bike though mick (the actual one, not the Mick one)

 

Most dirties recommend 500-1000 service!

[rockinrolla]

so in regards to the head and block query, if I used the 8cc 5k pistons and the 3k bp head the compression ratio should be ok as the 3k had flat tops at 10:1, so hopefully I should get around 9.5:1 may be, and a mild camm with the twin sprinter carbies should be ok for todays shit fuel :abuse: :P :D ;)?????????

Edited June 3, 2010 by rockinrolla

[Taz_Rx]

so in regards to the head and block query, if I used the 8cc 5k pistons and the 3k bp head the compression ratio should be ok as the 3k had flat tops at 10:1, so hopefully I should get around 9.5:1 may be, and a mild camm with the twin sprinter carbies should be ok for todays shit fuel :abuse: :P :D ;)?????????

 

Nah mate. 3k's had flat top piston...... ie 0cc of dish in the piston compared to the 8cc with the "O-dish" 5k pistons. Hence you'd end up with more like 8.5:1 CR.

[Felix]

Nah mate. 3k's had flat top piston...... ie 0cc of dish in the piston compared to the 8cc with the "O-dish" 5k pistons. Hence you'd end up with more like 8.5:1 CR.

 

:abuse: Don't think so.

 

Taz, you are not accounting for the difference in cylinder volumes. 3k=291.5cc, 5k=372cc.

 

If you pulled a head off a 10:1 3k, and threw it on a 5k with 8cc pistons, you'd end up with around 10:1. Same head on a 5k with flat tops would give around 12.5:1.

 

I used Dyno2000 which makes it quick and easy to work out total needed combustion chamber volume to achieve a desired compression ratio.

[Taz_Rx]

Taz, you are not accounting for the difference in cylinder volumes. 3k=291.5cc, 5k=372cc.

 

No I didn't.

 

A 3kbp head has a bigger combustion chamber (in the head) than a 5k head right?

Stock 5k CR is 9.5:1 IIRC?

 

So how does the CR go UP when the combustion chamber volume in increased!?

[Felix]

Stock 5k has 21cc pistons. Not 8cc which the dude was talking about using.

Edited June 4, 2010 by Felix

[Taz_Rx]

Oops, true.

 

Ok so lets assume the 8cc give 10.5:1

 

provided this A 3kbp head has a bigger combustion chamber (in the head) than a 5k head right? is right.....

 

So how does the CR go stay the same when the combustion chamber volume in increased!?

 

I'm not trying to be argumentative, I'm honestly interested!? The only part of the bottom end I'm taking into consideration is my assumptions* is the dish (or lack of) in the piston.

 

*Assumptions are the mother of all f-ups!! :abuse:

Edited June 4, 2010 by Taz_Rx

[philbey]

Does the pin-crown distance stay the same 3K-5K. Deck height to crown?

[rockinrolla]

yes, as far as i can see. :abuse: :P :D ;)

I have read all the info and now understand i think, increasing the volume of the cylinder is increasing the amount of air and fuel to be compressed which in turn ups the psi, but the 8cc dish will compensate for the increase which should bring the cr back to around 9.5 or 10:1, does that sound about right.

Edited June 4, 2010 by rockinrolla

[Felix]

yes, as far as i can see. :abuse: :P :D ;)

I have read all the info and now understand i think, increasing the volume of the cylinder is increasing the amount of air and fuel to be compressed which in turn ups the psi, but the 8cc dish will compensate for the increase which should bring the cr back to around 9.5 or 10:1, does that sound about right.

 

Kinda.

 

Compression Ratio = (Cylinder volume + combustion chamber volume) / combustion chamber volume.

[rockinrolla]

Kinda.

 

Compression Ratio = (Cylinder volume + combustion chamber volume) / combustion chamber volume.

got it, thanks guys :) :laff: :lolcry: