i like boost

Quote:For a NA, mild street engine, there will be no problem whatsoever, but for a blown engine in competition mode I would be taking a second [& third] look at it.
The reason for the exercise is to boost torque & if I am successful, it will allow me to operate at lower rpm's generally across the board & this should translate into less stress & heat. Initial target is to be able to cruise at 3,000 rpm @ 100 Kph in top gear. In a lightweight like a KB10, I think it should be achieveable.

Edit, per Tapprts request.
The reply was directed straight at A14Force & he is aware of the engine that I will be building for my coupe, but basicly its an overbored A13 short deck block with a stroker crank. To make it work I have a set of 'replacement' A15 pistons. These have the bowl centralised in the crown of the piston while the originals have an offset bowl. This offset gives a wider flat surface area on one side & this is cleverly placed under the squish area of the head to maximise this feature.
The pistons with the central bowl have less flat surface under the squish part of the head, however, with the stroker crank, it's necessary to machine a small amount from the top of the pistons to bring the crown down flush with the block as it stands proud at TDC even though they have a lower crown height than all other A series pistons. As you rightly concluded, any machining will result in the crown of the piston, that band around the bowl, becoming wider & presenting a greater surface to the underside of the head, & this goes some way towards reducing the difference between the original A15 pistons squish area, & that of the replacement pistons.

Additionally, the distance from the crown of the piston down to the top ring is not particularly great in these Datsun engines & removing too much material from the crown can weaken the top of the piston. It then doesn't take very much detonation to cause a failure of this part of the piston, & he was asking about that.

A14Force has been through much of this with me before & [hopefully] understood what I was writing about.
If you scroll backwards to my previous post you will see the pistons in question. This modification works just fine in a normally aspirated engine, but I suspect that it might be a bit risky in a blown engine.

Hey datsik,
lets have that article about building the knock indicator.

won't be an issue on those pistons, and you can get away with that sort of thing on even fairly hot atmo engines. I think I took about .090" off my old standard cast L18 pistons to get the CR I wanted.

D/Man, I 'spose .020 isn't much, but how much meat will that leave above the top ring? Probably wont create a problem, but with some senarios I looked at this was potentially an issue.

the squish discussion is fairly irrelevant to rallycar's turbo engine. much more important to get the CR right than to worry too much about squish.

besides, with EFI the mixtures are (well, should be!) alot better and more consistent than old carby engines anyway and squish is not as important.

squish was a technique used to help control detonation in high CR engines in the olden days :P Well, high CR for the fuel used and combustion chamber design and material used. It is of less importance in lower CR engines.

modern engine designes with do not use squish to any real degree with their pent-roof chambers. The fuel injected in such a fine mist under high pressure provides a very good even combustible mix, so squich is of little advantage.

If you decrease the squish 'area' there is a lesser volume of gas that is being trapped between the piston & the head to jet out into the combustion chamber to promote swirl. It's the degree of swirl that helps to control detonation. The squish area provides that high speed jet of gas into the combustion chamber to give you the swirl.
The higher the speed of this jet of gas, & the greater the volume of it, the better, within sensible limits.
The speed of the jet is to some degree controled by the gap between the piston & the head, & the volume by the total surface area of the effective squish area.
A15's seem to have had offset bowls in order to provide an increased squish area.

The squish area isn't just the flat part of the combustion chamber that is close to the piston, it's the area where the flat part of the piston & the head come together & if this is just a band around the perimiter of the bowl, then the squish area is just the part of this band thats under the relevant part of the head.
The closer the piston comes to the head, the higher the speed of the ejected gas & the more turbulant the swirl.
Some engines had pistons that protruded above the deck in order to reduce the gap above them at TDC. That's back in the days of fairly thick copper asbestos head gaskets.

Squish area & swirl have nothing to do with compression ratio, but if the squish area is reduced in order to reduce the compression ratio, then swirl characteristics are changed & not necessarily for the better.


Both of these are A15 pistons. The used one on the left is a factory item while the new one is 'replacement' I will need to machine the crown of the new pistons by about half a mill or so & this will leave me with slightly more compression & a wider bowl preimeter. This will hopefully increase the total squish area of this piston up to about the same as the stock piston.

Having more squish improves the burn, but to compensate the increase in CR, you could either take more cc's out of the chamber in the head or have a deeper dish. From memory that why some pistons have offset dishes is to take advantage of squish.

after skimming over that, i'm more confused! decreasing the "squish area" reduces detonation as it reduces the compression ratio.

the problem 1200rc faces is that there is way too much to upgrade if you are doing it properly - valves, valve springs blah blah blah.

keep it simple (standard) and efficient.

Thats the best description of why it's like that I've seen D/man.