25 replies to this topic

[DeadSquare]

In theory yes the more capacity you have the more work can be done.

However there comes a point where the breathing capacity of the head and valve train cannot feed the needs of the bigger capcity, the result is higher rpm horsepower suffers more than peak torque.
I machined a little 850 a few years back and stretched it to 1150cc. This engine want from stock 33hp to 71.5bhp. in ordert o get that far it needed a 12g295 head fitted with. 1275inlet valves and twin hs2 carbs. That being said the original 850 block went on to produce 72lbft of torque up from the original 44lbft!
Capacity wins here everytime.
For fast road applications think that 68 to 70lbft per litre is possible. The more litres you have the more grunt you have.

Now for the overlooked bit.
Most used engines are well past there best ring seal conditions so people don't realise that a simple rebore will restore a lot of lost power and torque. So a simple 0.020" rebore and new pistons can feel like twice the engine! The point is it's not the increase in capacity that gave the massive increase in performance. Is the restoration of the lost seal.

Ac

 

 

 

1150,  well done !..............I was pleased to bore an 848 to 999

[ACDodd]

The secret is to fit either the 998 or the 1098 crank as in the example above. That used the 3.3" stroke together with a 0.125" overbore get to 1150cc.

Ac

[DeadSquare]

The secret is to fit either the 998 or the 1098 crank as in the example above. That used the 3.3" stroke together with a 0.125" overbore get to 1150cc.

Ac

 

 

Ah.  I misunderstood your use of the word "Stretch".

 

Silly me, thinking about it, plus .060" didn't get a 1071 out to 1100.

[Curley]

What's the phrase: there's no replacement for more displacement

[MiniMadRacer]

One other small factor to be aware of is if you increase the bore without any other mods, you will up the Compression Ratio which will "usually" give you more power also. I say usually because there is an optimum Compression Ration depending on CAM, head fuel available etc, going beyond which will not give you more power but may "grenade" your engine pretty quickly. 

[luismx123]

so in theory, boring a 998 to 1275 would give it more power as you'd have a higher CR right? of course the stock 998 heads aren't capable of the flow but like AC said before, his 850 rebored had more bhp and torque than a 1275 albeit upgraded head/cam etc but with less CC's

[Ethel]

He used a 1098 crank, that's the longest stroke A Series. 

[ACDodd]

I have 998 that is now a 1.2 litre well (1187cc to be exact. 1098 crank and bored +0.100" oversize.(for sale!) These little 'big' engines make great road engines giving lots of torque all below 6krpm.

Ac

[mini-mad-mark]

AC - is it better to use a 998 A+ block or a 1098 A block for such a build?  I have access to either - the 1098 A came with the car and I can get hold of a 998 A+ cheaply (I hope)  

Also hoping to get a 12G295 from the same source (again not too expensive I hope!); Is that better than a 12G940 for this type of build?

 

I was thinking of a similar spec for my project car but I am 6 months (at least!) from needing an engine  - unfortunate as you already have one for sale but it would be sitting for too long before i could use it

 

Regards

[DeadSquare]

so in theory, boring a 998 to 1275 would give it more power as you'd have a higher CR right? of course the stock 998 heads aren't capable of the flow but like AC said before, his 850 rebored had more bhp and torque than a 1275 albeit upgraded head/cam etc but with less CC's

 

so, in theory, boring a 998 to 1275 would give more power................

 

No.

 

Instead of having four O's for bores, there would be two conjoined 8's and the pistons would have flat sides rubbing against each other...............

[ACDodd]

The 998 and 1098 blocks are the same thing, except some real early 1098's that had thin walls. Use the latest block you have.

Ac