Piston Dome and high compression

paz

Active Member
Ok, I said I'd ask this for a friend of mine, who's Swedish cousin is currently planning to build up a decent spec r. This names no sense to
me at all, so apologies if it's nonsense!! Lol.

what he's asked is if he fits forged pistons with a 12.5mm dome, will he be able to run higher compresion so he doesnt have to run as much boost?


What he's got against high boost or what he's specifically trying to achieve I'm afraid I don't know.

Cheers in advance.

Paz
 
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pulsarboby

Guest
yes what hes saying is actually true in some ways!
basically the more boost you run then the more load you are putting on all moving parts mainly being crank, bearings and pistons / rods

by raising the compression you will have a far more responsive engine off boost with no lag, similar to a naturally aspirated lump.
but.....the problem you then have is that when the engine does come on to boost you have a lot higher compression which in turn loads all the parts 'as mentioned above' to extreme, so you have a much higher risk of engine failiure and a high risk of blowing the h/g as well as warped head etc.
in many respects a supercharger is better than a turbo, as the power is linear and progressive rather than one second nothing then the next all the moving parts have huge amounts of stress on them which can ultimately lead to early engine failiure.

this is why people use a 2mm gasket to drop the comp ratio and make the engine safer at high boost levels with less direct load on moving parts and less stress on the head!

its a fine line between getting it right and having a responsive engine and cocking up (as i did with my previous build) and creating a lag monster with loads of midrange to top end grunt but slow getting there (cams also play a major part in this). whats the point in having something that wants to make max power above 7500rpm!!! all youll end up doing is throwing the rockers, thats why for road or circuit car its best to have power at lower to mid end of rev range.
yet that setup would be great for a drag orientated car where midrange is irrespective.

going back to your mate though....he has a point in some ways but in others it defeats the whole point in buying a turbo car if hes not gonna run any boost!
 
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pulsarboby

Guest
why thankyou sam but pulsar stu is my man now so no good trying to butter me up:lol::lol:
 

Trondelond

Active Member
I don't know if compression is the issue, but on rotaries, they usually go for massive turbos and low boost, as these engines are so sensitive to detonation.
It's the same thing as turboconverting say a B16/B18 honda engine, usually people don't run a lot of boost on them with stock internals, so it feels much more N/A-like responsewise, on and off boost.
 

larsa

New Member
Ok, im the cousin from sweden;) the point is that my english cousin missunderstood my question
.I didn't know what meant with "dome" so i talked with a person who builds v8 engines and he said that
a dome; -12,5 will get higer compresion becuse it will be a hill on the piston (easy explain)
And with higher compresion not will be good on a turbo engine ( just like pulsarboy said "engine does come on to boost you have a lot higher compression"),
and that maked me worried becuse higer compresion and boost can make big problems, and i want go for higher boost!

Now know i that the piston dont' have an Hill but a "hole" on the topp of the piston, and will be good for the engine.

I,m enormously sad for my bad English.

But i don,t want you to think that i have my head upp in my ass!;)
/Larsa
 
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pulsarboby

Guest
think i kind of understand that:lol::lol:

yes gtir pistons are concave not convex, so you will be ok
 
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