High Compression Pistons

[StealthWyvern]

compression wont be a problem....even at 12:1...regular pump fuel will deal with it, with suitably backed out timing. Problem with the fuel isn't octane on highly strung NA Motors normally...its usually piston velocities outrunning the flame front of the fuel (i.e. pump fuel doesn't burn nearly fast enough....)....

Just curious but kind off subject but not really but how can bikes have such high rpm limits over cars and not have the piston out run the flame front as you put it? I don't knwo jack about this which is why im asking.

 

[LordWorm]

Just curious but kind off subject but not really but how can bikes have such high rpm limits over cars and not have the piston out run the flame front as you put it? I don't knwo jack about this which is why im asking.

quite simple really....its all a question of mean piston speed....

mean piston speed is a function of stroke.... heres the formula:

S x 2 x RPS

where S is stroke in METERS
and RPS is revolutions per *second*

to get this... we go...

(stroke / 1000) x 2 x (RPM / 60)

that equates to mean (average) piston speed in metres per second....you can then work out feet per second from there if you like.

Essentially, the shorter the stroke, the lower the mean piston speed - thus the higher the engine can rev without the pistons exceeding the speed of the flame front.

piston acceleration plays a part also - you can push the limits by working out what the piston speed is at and before 30 degrees ATDC etc..

on a 92 mm stroke, the mean piston speed is approx 21.5m/s (or 70.5ft/s) @ 7000rpm
on a high revving bike motor with a stroke of 53.6mm (such as the yamaha R1), the mean piston speed is 12.5m/s (41ft/s) at the same speed...
infact, to get the 21.5m/s, the R1 would need to rev to a little over 12000rpm...

in short, the piston is having to cover less distance in the same time, thus is traveling slower (at a given engine speed), thus doesn't outrun the flame front......and the flipside is, that it can rev HIGHER, because its got more headroom

If you can't drop the stroke enough to get the piston speed lower than the burn speed of the fuel - your only real option is to chose a fuel that burns faster.... so we get into race gas territory...

Bikes also generally have very light rotating assemblies, fantastic rod:stroke ratios and are built massively "over square" (bore is larger than stroke) - which all means less wear and tear at high engine speeds......(fuel burn speed is only one part of the consideration when thinking about building a spastically high revving engine.... obviously you need to be careful the thing doesn't tear itself to pieces)

Hope that answers your question

 

[jimmysuite]

would it be possible to install a thicker head gasket to simulate a longer block? Also, would it be feasable to put some sort of shim under the block to space it a little further from the crank? Even if you could only get a few mm it might help with piston design so you would have to eat into the ring recesses.

I tried browsing to find answers but not much light was shed on the subject. (probably means its a dumb idea but...)

Did anyone ever find out if the crank from the FP and FS are a direct swap?

 

[Captain KRM P5]

would it be possible to install a thicker head gasket to simulate a longer block? Also, would it be feasable to put some sort of shim under the block to space it a little further from the crank? Even if you could only get a few mm it might help with piston design so you would have to eat into the ring recesses.

I tried browsing to find answers but not much light was shed on the subject. (probably means its a dumb idea but...)

Did anyone ever find out if the crank from the FP and FS are a direct swap?

cranks will not swap. been there done that. there is a thicker head gasket from cometic.

 

[LordWorm]

cranks will not swap. been there done that. there is a thicker head gasket from cometic.

cranks will not swap or cranks will not swap without custom rods? whats the problem with the swappage? will the crank (minus rods etc) physically bolt in?


jimmy: thicker gasket wont simulate a longer block, unless you have the piston clearing the deck...

you could (in theory at least), build a plate to lift the head, then bore and sleeve through the plate.... essentially raising the deck height of the block...

 

[Maffydan]

Hi Guys need some assistance in locating a set of standard piston and rings for my FS 2.0L mazda protege....however I want to continue run normal aspirated therefore I need the high compression pistons. Any assistance will be appreciated....

 

[twilightprotege]

easier to just buy my uber NA set: http://www.mazdas247.com/forum/show...The-I-m-taking-my-Protege5-Back-to-Stock-Sale

 

[Maffydan]

Saw the pics....I'm interested in the piston set.....davealewin@gmail.com.