we used an ACL head shim on the A14 turbo we had (stock bottom end). the shim is fitted to the block with sealant (Hylomar spray can recommended) and the head gasket goes above that as normal.

I don't think you will have any problems with a conventional head gasket even at 1bar, IF the tune is right. detonation can blow the fire rings. Who cares when a head gasket is $20 and takes 30min to replace?

the E15 turbo pistons are a good idea, but ACL no longer make them. I looked into that a few years ago. So you would have to buy an E15T and use secondhand pistons (hopefully they'll be in useable condition). But if you did that, you're probably better off using the whole E15 motor...

From my experience, I doubt a stock cast piston is ever going to live to a grand old age in a turbo A series with any sort of power. But the A series is cheap and easy to rebuild... Forgies would be the go, but it comes down to the budget, and you'll get many sets of cast pistons for the price of a forged set! rather than being too worried about finding a hyperutectic cast piston, I would be looking for one that has drilled oil return holes rather than a long groove, the thickest headland, and the strongest casting around the gudgeon pin. But the drilled oil returns are the most important of those factors.

we used water injection on the A14 turbo early on in our tuning to help control detonation, and it definitely had big benefits. They can be simple and cheap to setup if you are not too squeamish to inject the water before the turbo. on your fmic setup, that won't work. It would be much more cost effective to setup a water spray for the IC in your case, and that can reap good benefits too.

I wasn't going to even mention getting rid of the twins and going the single route - I'm pretty sure you already know my opinion on that

yeah, i'm on the old gte rid of those bloody turbos bandwagon too. if you just paid $300 to get one rebuilt and you're willing to do it for another then that's $600 that could have been used on a more effiecient and appropriate turbo for what you are wanting to achieve.

if you go forged pistons you're up for a grand, but so far you haven't had any piston probs, even with big boost pressure. rings may be just old, and compression can't be too much of a problem if you've been hammering 14psi into it.

reuild a motor to stock, at least you know the bores will be honed and new rings are doing the sealing.

save the money from the forgies to buy a fully programmable computer and 'sort out' your turbo issues, whether it be runnning them within their efficiency range or replacing them with bigger thems or an it.

the a15et(t) has plenty of potential!

Quote:
The advantages of 'squish' seem to have been discovered by an engineer named 'Ricardo' back in the 1920's What he found, in side valve engines, is that if the flat top pistons had zero deck height, & the portion of the head that covered HALF of the bore before opening up to the valve/combustion chamber was dead flat with the gasket face, then the piston & head were separated with only the gasket thickness.

When the piston rose to TDC, the gas that was trapped between the piston & this flat portion of the head was 'squished' & it jetted out into the combustion chamber at high speed. It 'swirled' around the combustion chamber & promoted rapid combustion of the burning fuel charge, rather than simply having a relatively static compressed charge that has a slower moving flame front burning across the fuel charge, creating a rising pressure wave in front of it untill the remaining [unburned] charge reached a point of self ignition & it spontaneously combusts [explodes] resulting in what we call detonation [ping]

These Ricardo cylinder heads allowed compression ratios to increase from about 3.5 to 1 up to as much as 6.5 to 1 in the space of only a few years & this was staggering when you remember that the fuel of the day was little better than Kerosene & Tetra Ethyl Lead in fuel was still over twenty years off.
It didn't take engineers long to realise that 'high swirl' was critical to detonation control in all combustion chamber shapes, so a good squish area became a vital part of combustion chamber design.

Now lets look at our Datsuns
If you lower the deck height of the piston in the bore, and/or raise the height of the head above the block with a shim, you reduce the efficiency of the squish area & you reduce the intensity of the resulting swirl. This then increases the possibility of detonation.

If you increase the diameter of the bowl, & thereby reduce the width of the band at the top of the piston, you decrease its ability to create a jet of gas from the squish area & thereby reduce the swirl, which results in a higher probability of detonation at any given degree of ignition advance.

So even in a blown engine, the higher the level of 'swirl' the better, even at lower compression ratios as this then allows more ignition advance, & up to a point, that's where the power is.

The amount of 'squish' need not be directly related to compression ratio.

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

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.

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.

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.

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.

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.

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.