28 replies to this topic

[ministox man]

Hi,
Are dished pistons any better than flat top pistons in a 998?
Thanks
Rob

[Asphalt]

The flattops are actualy better because they give higher compressionrate (and because of that more power *g*).

The dished ones were used in the low-compression 998 with - i guess - 36HP.
The flattop-pistons came from the A+ engines with 42HP.

A special-variant are the d-top pistons from cooper 998.
They are made for the usage of an 12G295 head.

If you want to use a 12G295 (ccoper-)head on your 998 you should at least use the flattop pistons because of the rquired compression (but even with flattops, the 12G295 must be - ohm - "lowered").

Grretz,
Jan

PS: Anybody please correkt me if i`m wrong

[ministox man]

hi
thanks for that
the reason i ask is that i have heard that the dished pistons swirl the mixture in giving a better fill
just that we run on such tight regs that any extra power is a plus
Rob

[Wil_h]

I have also heard a number of different opinions on what gives the best power, dished of flat. Last I heard it was flat, but I could be wrong.

[Pavel]

If you're going to have the same compression ratio in the end, dished pistons are better.

Basically this is because power seems to increase if you skim the head (therefore skimmed head and dished pistons with 10:1 ratio should give higher power than standard head with flat tops giving 10:1 ratio)

But as been said, opinions about this are divided

[Sprocket]

Visard recons the dish is better, but then he was looking at engines above 100bhp and gaining 2 to 3 bhp. Not much on a standard 998 then.

[Asphalt]

I would suggest to use the flattops.

Simply because the head need`nt to be skimmed(?) that much than with dishes ones.
The advantage of dished pistons is - as far as i think - negligible (on an 998).

What are you going to build?
A street-engine for daily use, or a pedigreed race-machine?!?

On a pure race-engine you might benefit from the dished ones - if every little fragment of HP is needed

You see:
Opinions are realy divided.
So no it`s on you to choose the best setup for your engine with the informations / opinions given.

Greets,
Jan - fan of 998s

PS: I would use what i have... low-budget racing ;-D

[ministox man]

the head is skimmed, its a racer and, unfortunately, we have to run a restrictor plate which goes between the carb and manifold which is only 18mm and they only produce 32-36 bhp because of the size of the oval we race on its flat out in second gear so.......as tesco says "every little helps"

I[/i] would use what i have... low-budget racing ;-D

we have a set of each so thats the only reason im asking

[Pavel]

In that case I'd go with a dished piston set up and skim the head (if feasible) to get a rather high compression ratio (assuming you can get fuel to run this high a ratio). What other mods do you have and are allowed to make incidentally? It's likely there are other ways as well to increase your power that little bit more.

[Sprocket]

irrespective of how much you increase the Volumetric efficiency, A restrictor will only allow a certain amount of air into the engine and therefore only burn the apropriate amount of fuel. You have the size of the restrictor, you should be able to work out the theoretical power figure that would be available if everything else is at ideals. Taking the difference of this theoretical power and actual power, will be the amount of inefficiency still within the engines function and should be improved. If fitting dished pistons improves this then use them. There will be no chance what so ever of producing more power over and above that of the theoretical power of the restrictor, thats the point.

[Leonard]

are you allowed to do anything to the restictor plate to aid gasflow, eg some kind of stubstack, or polish and bevel the edges with changing the diameter of the hole? im not familiar with the rules?

[DANTOO]

I didnt want to make a whole new thread just for a short question... but when it says....

AE 1275cc High compression std piston set +20

or it will say under that +40.....

what do they mean by +20 or +40???

[Leonard]

+0.020 means the cylinders have been bored bored out to 0.020 inches over standard. (20 thou)

[ministox man]

the restrictor plate has to stay totally standard
It has a ministort road/rally cam and a few other little bit like the light pressure plate and the lighter end pulley
jammy_basturd stop using all tose big words you're confusing me!

[Jammy]

When did I get dragged into this?!