27 replies to this topic

[unburntfuelinthemorning]

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

 

You can certainly see where that comes on cam!  How does it drive below 2700?  Is it still useable if a little soft?

[Cooperman]

A sensible view is that a 998 can be over-bored as far as you like and that will result in much improved torque which is what the smaller engines normally lack. Replacement blocks are easy to come by for not a lot of money.

With the 1275 engines it is best to always just go to the next size up because if you go 'massive' the bores must be offset which can be expensive in the first place, and then when a further re-bore is necessary it will be necessary to fit special sleeves or the block might well be scrap. That is very expensive. You will not notice the difference between 1330 and 1380 unless you are racing and the quality of engine build is more important. I always go to 1330 in increments, then fit the original standard sleeves to get back to 1275. The only reason for going to 1380 is for bar-room bragging rights and because you have plenty of spare 1275 blocks available (who has these days?).

[unburntfuelinthemorning]

You can get something like +0.135" pistons for the 998.  Do you not need to offset bore at all even for these?

[ACDodd]

You offset bore and convert the block to take the 1275 head gasket. Shown below is sonic wall testing, and then plug and relocate water ways to suit the gasket.

Ac

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Edited by ACDodd, 03 May 2019 - 01:49 PM.

[unburntfuelinthemorning]

So what is the maximum size you can overbore a 998 without having to offset bore and to retain the 998 head gasket?

[DeadSquare]

You work it out.  Measure a head gasket;  it has to sit on something, so that is the theoretical minimum.

 

For reliability, I like to leave no less than .025" either side of the gasket, but I have taken it down to .010" .

[luismx123]

doesnt swapping the rocker to something like a 1:1.4 or 1.5 increase power similarly to a new cam?
Im really interested in this thread as I think the coming winter will see a new head/rocker combo (if needed), vizarded hs4/hif38. Not wanting to remove the block for that stuff yet. Slow and steady wins the race ;)

[DeadSquare]

People used to come to me and ask  "What will it do if I change this or do that" ?

 

My answer each time was "About 5%", whether it was or not, and about 10% for changing the cam;  then, at the end I would explain that if you add up the 5%'s for a bigger carb, better manifold, polished head, raised compression, bigger valves and free-flow exhaust, it adds up to 30%, however, you probably get 40% because they work in unison. BUT, if the correct cam is now fitted, the 10% for the cam doesn't just bring it up to 40%, it suddenly boosts it to 80%.

 

Of course, if you do a bit by bit 5% improvement each time, you don't really notice the big increase in power that comes when it is all done together.

Edited by DeadSquare, 03 May 2019 - 04:23 PM.

[hhhh]

doesnt swapping the rocker to something like a 1:1.4 or 1.5 increase power similarly to a new cam?
Im really interested in this thread as I think the coming winter will see a new head/rocker combo (if needed), vizarded hs4/hif38. Not wanting to remove the block for that stuff yet. Slow and steady wins the race ;)

 

If you go all out to 1220 or thereabouts, the 1.4 or 1.5 rockers may be a good idea, but otherwise stick with 1.3.

[luismx123]

 

doesnt swapping the rocker to something like a 1:1.4 or 1.5 increase power similarly to a new cam?
Im really interested in this thread as I think the coming winter will see a new head/rocker combo (if needed), vizarded hs4/hif38. Not wanting to remove the block for that stuff yet. Slow and steady wins the race ;)

 

If you go all out to 1220 or thereabouts, the 1.4 or 1.5 rockers may be a good idea, but otherwise stick with 1.3.

 

1220? cc/bore? 

[DeadSquare]

 

 

doesnt swapping the rocker to something like a 1:1.4 or 1.5 increase power similarly to a new cam?
Im really interested in this thread as I think the coming winter will see a new head/rocker combo (if needed), vizarded hs4/hif38. Not wanting to remove the block for that stuff yet. Slow and steady wins the race ;)

 

If you go all out to 1220 or thereabouts, the 1.4 or 1.5 rockers may be a good idea, but otherwise stick with 1.3.

 

1220? cc/bore? 

 

answer = 71.38829mm

[DeadSquare]

68mm (+0.135") Producing 1219cc.

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

Ac

 

Oselli engineering used to offer a 1200 engine, presumably a 1098 conversion.

 

I never found out what pistons the were using. 

[unburntfuelinthemorning]

So what is the maximum size you can overbore a 998 without having to offset bore and to retain the 998 head gasket?

Anyone used +0.060" pistons?  Did you have to offset bore?

Anyone used +0.080" pistons?  Did you have to offset bore?

Anyone used +0.100"............