Jump to content


Photo
- - - - -

998 More Power


  • Please log in to reply
20 replies to this topic

#16 Turbo Phil

Turbo Phil

    Up Into Fourth

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPip
  • 2,418 posts
  • Location: Cumbria
  • Local Club: Cumbria Classic Mini Club

Posted 05 October 2018 - 11:39 AM

The 202 head in standard form is barely any better than the standard 998 head. Though with some work it can be good. Even though it has a larger inlet valve, the massively shrouded chamber pretty much kills off any improvement in flow so this needs attention first.

Phil.

#17 germini30

germini30

    Super Mini Mad

  • TMF+ Member
  • PipPipPipPipPipPip
  • 658 posts

Posted 05 October 2018 - 12:24 PM

Turbo Phil. Can you supply a modified head for a 998

#18 RooBoonix

RooBoonix

    One Carb Or Two?

  • TMF+ Member
  • PipPipPipPipPipPipPip
  • 1,224 posts
  • Location: Essex

Posted 05 October 2018 - 02:55 PM

Ideally you need a 12G295 head, suitably gas-flowed, but 295 castings are becoming hard to find these days.
Alternatively, ask 'TurboPhil' on here, to do you a nice gas-flowed head with slightly larger valves and a 10:1 CR. Then add a better inlet manifold and an improved carb. Fit a better cam, such as an Evo001 (great in a 998).
Lighten the flywheel and fit either a 3.44:1 or a 3.76:1 FDR.
Get the engine balanced and rev it right round to about 6000 rpm.
That's what can be done to a 998 if you so wish.

Everytime I drive my Dad's 998 (standard) I can't believe how much happier it is to rev compared to my SPI, I happily rev my SPI to 5000-5500rpm and whilst it is definitely faster in doing so I feel like the 998 is happier getting to these revs. I can't wait to build a 998 engine for my dad's car with a flowed head and a better cam, I think it will be fantastic
Makes little sense though.
I never got why people think a 998 is capable of more revs.
Bore stroke ratio for a 1275 is better than that of a 998. (0.847 vs 0.868 for 1275 )A lighter flywheel would be better for your cause,cheaper and easier :)

Maybe it's the thick carpets he's got that's giving a placebo effect then :-D

#19 Retroman

Retroman

    One Carb Or Two?

  • Traders
  • PipPipPipPipPipPipPip
  • 967 posts
  • Location: Sunny Sabden
  • Local Club: CDMC

Posted 10 October 2018 - 11:34 AM


Makes little sense though.
I never got why people think a 998 is capable of more revs.
Bore stroke ratio for a 1275 is better than that of a 998. (0.847 vs 0.868 for 1275 )A lighter flywheel would be better for your cause,cheaper and easier :)

 

 

Dusky, this might help explain why a 998 will always be a revvy motor compared to a 1275....

 

It makes perfect sense as the 998 is a 5mm shorter stroke and carries rods and pistons which weigh a lot less

 

Both make for a motor capable of more revs without destroying itself.

 

The overlap on the crank journals on shorter stroke engines makes them stronger and gives less flex.

 

Most 998 engines also have a crank made from EN16T which makes them pretty much bullet proof.

 

The bore / stroke ratio is not greatly different on a 998 to a 1275 ( 1.18 and 1.12 respectively )

 

A bore / stroke ratio less than 1 means the bore is bigger than the stroke (over-square)

 

Its not until you get to an over-square ratio (bore is bigger than the stroke) like the 970 at 0.88 that things can really rev.

 

This means for the cc's you can get bigger valve area for better breathing at high revs plus a shorter stroke means a stronger crank

 

The Cosworth DFV engine won many F1 races with a very over-square ratio of 0.76 

 

The fastest Mini in the world used a 970 S engine...146 MPH

 

https://en.wikipedia...iki/Project_64_(Mini_Cooper)

 

A 970 S engine is just short stroke 1275 and will  always  rev higher as will a 998



#20 Dusky

Dusky

    Crazy About Mini's

  • Members
  • PipPipPipPipPipPipPipPipPipPip
  • 5,322 posts
  • Location: Belgium

Posted 10 October 2018 - 04:34 PM


Makes little sense though.
I never got why people think a 998 is capable of more revs.
Bore stroke ratio for a 1275 is better than that of a 998. (0.847 vs 0.868 for 1275 )A lighter flywheel would be better for your cause,cheaper and easier :)

Dusky, this might help explain why a 998 will always be a revvy motor compared to a 1275....

It makes perfect sense as the 998 is a 5mm shorter stroke and carries rods and pistons which weigh a lot less

Both make for a motor capable of more revs without destroying itself.

The overlap on the crank journals on shorter stroke engines makes them stronger and gives less flex.

Most 998 engines also have a crank made from EN16T which makes them pretty much bullet proof.

The bore / stroke ratio is not greatly different on a 998 to a 1275 ( 1.18 and 1.12 respectively )

A bore / stroke ratio less than 1 means the bore is bigger than the stroke (over-square)

Its not until you get to an over-square ratio (bore is bigger than the stroke) like the 970 at 0.88 that things can really rev.

This means for the cc's you can get bigger valve area for better breathing at high revs plus a shorter stroke means a stronger crank

The Cosworth DFV engine won many F1 races with a very over-square ratio of 0.76

The fastest Mini in the world used a 970 S engine...146 MPH

https://en.wikipedia...iki/Project_64_(Mini_Cooper)

A 970 S engine is just short stroke 1275 and will always rev higher as will a 998
How do you get to those numbers?

Bore/stroke ratio bigger than one is whats called oversquare in my books. (Just a question)

In standard form I wholeheartedly agree that a 998 revs more freely. But I see a lot of people talking about 998 screamers,wich, with the 1275 being more square doesnt really hold up for me. With the current lightweight aftermarket rods etc though. :)
Then again, these days im working on chevy v8's, very long strokes and they still rev to 8K ?

Edited by Dusky, 10 October 2018 - 04:35 PM.


#21 Retroman

Retroman

    One Carb Or Two?

  • Traders
  • PipPipPipPipPipPipPip
  • 967 posts
  • Location: Sunny Sabden
  • Local Club: CDMC

Posted 10 October 2018 - 10:24 PM

Apologies the figures are wrong, but a stroke less than bore size is over-square, a 970 S is 1.14

 

A 998 will and has to rev more than a 1275 to make the same sort of power, and less torque 

 

but will take the revs better as its shorter stroke

 

The weight of the conrods and pistons on a 1275 don't help and for the same revs a 1275 has a higher piston speed 

 

there is probably not enough difference in the bore / stroke ratio on standard 998 v 1275 to have much effect

 

My experience of chevy v8's is in F1 stockcars, and on the short tracks with low diffs they go like stink

 

Most small and big block chevys are over-square are they not ?






1 user(s) are reading this topic

0 members, 1 guests, 0 anonymous users