27 replies to this topic

[unburntfuelinthemorning]

My 998 block is standard size at 2.543".

 

I could probably get away with +0.020" but then I could go for a +0.120" overbore.

 

What would you do and why?

[Steve220]

The smallest bore oversize you can get away with. Insurance should you have an issue with wear later on.

[DeadSquare]

My 998 block is standard size at 2.543".

 

I could probably get away with +0.020" but then I could go for a +0.120" overbore.

 

What would you do and why?

 

 

If I overbored a 998 by ,120", I would move their center line apart and insert liners, so, if I wanted a bigger capacity, I would start with a bigger engine.

[dyshipfakta]

You won’t get noticeably more power so whatever costs the least money to do!

[GraemeC]

Go the full hog - 998 blocks aren't exactly hard to come by.
And I beg to differ that you won't notice the power difference, especially if the rest of the engine is built to suit.

[ACDodd]

The full hog = 1098 crank as well!

And you you do notice the extra torque!

Ac

[unburntfuelinthemorning]

Vizard always liked to max things out.  What are the chances of porosity problems though?

[DeadSquare]

@ AC,   What are the largest pistons available for a 1098 ?

[ACDodd]

68mm (+0.135") Producing 1219cc.

I am building a 1187cc (+0.100") version now.

Ac

[Spider]

Biggest pistons off the shelf I'm aware of for a 998 & 1100 engines is 0.120" oversize.

 

Go the full hog - 998 blocks aren't exactly hard to come by.
And I beg to differ that you won't notice the power difference, especially if the rest of the engine is built to suit.

 

Yeap, but also do something with the head too. It will need to breath.

[unburntfuelinthemorning]

Do large overbores reduce block rigidity or is that not worth worrying about?

[Retroman]

A 998 or 1098 block will go to 68mm with no issues

 

With a 295 or 940 head and a decent cam, power and torque can double over standard

 

More cc is the best VFM [value for money] next is the cylinder head

 

The dyno sheet is a 68mm with a standard 940 head and 270 cam

 

[ACDodd]

I use sonic testing to check bore wall thickness. Not all blocks are equal!

And yes it goes without saying, finish the job with a sensible cam (peak power below 6krpm) and a nice flowed 295 based big valve head and you have a cracking road motor.


Ac

Edited by ACDodd, 02 May 2019 - 12:09 PM.

[G'Racy]

Big valve head, what valve sizes would these be?

[ACDodd]

I would aim for 1275 valve based 295 with 33.2mm inlets and machined down exhausts to 27mm.

Ac