998 More Power
#16
Posted 05 October 2018 - 11:39 AM
Phil.
#17
Posted 05 October 2018 - 12:24 PM
#18
Posted 05 October 2018 - 02:55 PM
Makes little sense though.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 fantasticIdeally 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.
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
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
Posted 10 October 2018 - 04:34 PM
How do you get to those numbers?Dusky, this might help explain why a 998 will always be a revvy motor compared to a 1275....
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 :)
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
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
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 ?
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