piston to deck height
Turbo Mini Forums

what is the main thought on this, in the yellow bible it says to have the pistons flush with the block to try and keep detonation at bay and give extra power but thats on n/a engines does not say about turbo units any difference or should i get the block machined to get the block flush with the top of the pistons

"Can I legally marry people now that I am ordained in the Church of the SubGenius?" aka i know jack sh"t

i also make things

i love coming home to chrome

Yes the same applies on charged engines. But many poeple don't do this as it's an effective way of reducing the static CR. What they don't know is that combustion is not as effective which increases the possibility to ping.. That measn that the lower static CR might not be of any good.
This is in theory anyways.

im no expert, but i would say keep the piston down the bore and reduce the compression ratio. if your yellow bible is Vizards book throw it in the bin, he is often wrong. the early MK1 Golf GTI 1.6 had a flat cylinder head so the chamber was all in the piston and piston to deck height. if your bible is right the engine would not work. a friend has one and it goes better than my 1.8 GTI

i know its not an a series but if the principle is right then it would be right for all engines

Russell

I'd have the pistons flush with the block,
from what I have read, from reading David Vizards, and others words, squish is important!

Team Racing

It is and it isnt....

I have evidence of a 200bhp A-Turbo running with the pistons way down the bore!!!

It shuldn't be right, but it works!

Alex

AlexF

Have a look at the Porsche/VW 2,5 litre that was found in the 944 etc. A HORRIBLE design by all means. They've got flat heads. Also have a look at the VR6 engines. HORRIBLE designs Flat heads AND flat pistons. NO Squish.

AlexF

No I don't. The fact that you know a 200hp engine with pistons down the bore does not mean that power end reliability isn't better with 0 deck height and bigger pistons/combustion chambers.

Alex.

1: You said BHP. In that case, we're talking about the only mini i know with 200bhp - and that's Matt. And his pistons are not down the bore.
2: You throw boost at any POS engine and it'll make power. You keep revving the engine, and throwing more boost at it, it'll make more power.
3: as said - just because it 'works' doesn't mean it works properly, effeciently, or effectivly.
4: I don't FULLY understand every piece of science and reasoning behind squish / quench. BUT I know that plagiarising others who do is a good start. Others being EVERY OEM who's ever built a decent nat-asp performance engine, or decent turbocharged engine. Hell - even the metro turbo had a tighter squish than the nat-asp versions!

Edited by turbodave16v on 21st Apr, 2006.

OK point taken I should have said bananas...

I'll shut up now.

Alex

AlexF

surely running them down the bore cant be that bad? my mates running flat top omega's down the bore and his engines making 140 banana's @ 10psi, thats nearly an extra 50hp over stock from just an extra 3psi. His CR is 8:1 on a standard turbo head, but combined with a T2 and megajolt.

wow i love all this slagging off, it makes my day at work so much more enjoyable!!!!

as i stated before i am no expert but i know more than some and less than others

also just to throw the spanner in the works, why do you all reffer to BHP as a messurement of a cars acceleration when you should use HP as the B reffers to the braking efficiecy required to stop the vehicle.

anyway this should give you something else to slag off.

PS can you waite till Monday so i can read the replies!!!

Shed, you put yourself right in the firing line if you come out with bollocks like that.

Everyone knows that B stands for bananas!

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

so the jist of the thread is that the piston should go to the top of the bore for somthing called squish.

PLEASE explain squish etc in simple words for simple people. also include calculations demonstrating how 5thou down the bore drasticaly reduces efficiency/performance

and yes i know i cant spell to save my life but i cant be arsed with spell check etc. in my job you have an assistant to sort out the letters etc

OT but Turbo Shed could you change your avatar? I get all kinds of errors on both my computers viewing threads that you have written. I cant see anything else than a red cross there anyway.

someone on here has there pistons proud of the block

thanks for changing that. it used to work but then i have not got web space of my own to host pictures on.

any way stop bumping the thread

its all very well explaining that piston to deck height is important but can you explain how it can make any difference.

if say you have a flat top piston the chamber is all in the head and the head gasket. if the chamber in the head is say the same shape as the bore (unlike a mini) then all force from combustion is all evenly spread on the crown of the piston (ish)

if this is such an issue should i run without a head gasket since this would also improve squish?

sorry for being stupid but please explain and show calcs for drop in efficiency for various piston to deck heights

also if i say have a 40cc dish in the piston does this not cause the same problem,

As I understand it, it is nothing to do with "efficiency", it has more to do with flame propogation, and most importantly, avoiding detonation.

but by having the flame in the correct place, the fuel gets burnt more efficiently and hence increased power and economy

well i think thats about it.

its mainly to do with shape i think.

I would definitely have the pistons flush at the deck, not down in the bore. Pro engine builders seems to aim at 0,035-0,040" squish band in the best engines built in USA. Also, they have realised the benefits of smaller chamber, better spark plug location, squish and also dished pistons (dish mirroring the shape of the chamber). The squish helps to make the mixture more turbulent by "squeezing" the mixture away from outer areas of the chamber. This is often the location of pre-ignition or dieseling or whatever one chooses to call it. (Hot spots are another source of pre-ignition) Another benefit of the squish band is said to be the cooling effect of the band but I personally doubt if it has a major influence. By optimising the squish one does not have to compromise with ignition advance. So, with pistons flush with the deck and nromal mini head gasket we are in fact close to the quoted figure above.

If you think this is BS look at the latest Corvette 427 cid small block. They are running 11:1 CR with current low octane fuel fuel. Go back 35 years and the engine ran BIG chamber with high CR and they required really high octane stuff.

Let´s face it, any engine will run despite the spec and may also make respectable power but almost every engine can be bettered. Some with even big margin.

As for David Vizard, he is nowadays a respected name in US engine building scene and writes really good articles in Popular Hot Rodding. What is even better, his articles are based on testing in dyno. Makes for a good reading !

TurboDave, is it soon time for a real engine tuning ? I just took a first spin with my 4,6 litre engined Moggy and it is great FUN... Think about all the great vehicles available over that side of the pond !

so who's going to outline the differences between detonation and pre-ignition... or for that matter anti-ignition.

Alex

AlexF