SQUISH HEIGHT

Hey guys/Andre,

Whats y’all take on squish height on forced induction engines? There are lots of varying opinions on the internet with regards to this. I know it’s very beneficial to have it set correctly on a normally aspirated engine but is it that critical on a forced induction motor? On some of my builds 4g63, 4g64 and 3S GTE I get heights of .048” - .055” and EJ25 around .060”, guess it can be set closer to the industry standard of 40 thou if there are benefits to be had. Andre may have noticed on the dyno changes to HP, temperatures, anti knock properties ect. So what are the advantages and disadvantages of moving in either directions with the squish height on FI engines?

One of the many deficiencies faced in my region is the lack of a proper machine shops so adjustments would have to be made through varying the head gasket thickness.

Another area that need some discussion in the varying of torque to achieve desired bearing clearances. Some builders claim it’s acceptable to alter torque within 10% on either side of spec to get the clearance you’re looking for. Again with lack of proper machine shops to align hone bores, cut or polish a crankshaft to achieve clearances desired we resort to unorthodox practices.

Lastly, is it really that critical to do measurements or set bearing clearances at the set standard of 68 degrees Fahrenheit (within reason of course)? Where I am living our temperature is generally at 85 F. I conducted a comparison test where I took some measurements on a cast iron block crank bores, crankshaft journals and bearing clearances at 85 F then put everything in an air conditioners room overnight at 68 F, what I found was that the measuring tools were all off the zero reference mark but once reset to zero the results yielded within 1 to 2 thou of the previous measurements. If iron and steel expands or contracts at approximately.000007” per 1* F then both tools and parts measured should all change proportionally if exposed to the same temperature. Aluminium may yield a bit more difference. The dial bore gauge is very sensitive to temperature changes, even your finger temperature after prolong use can cause changes that’s why I think it’s necessary to reference the mic regularly during use to ensure it’s still at zero.

Hoped I explained myself well enough to invoke some discussion, thanks in advance.

With regards to your comment about adjusting bolt torque to adjust the bearing clearnaces. This seams to be common in my location on the EJ20/25, on the main bearings clearances, when using upgraded fasteners, like ARP. I asked around, and this is what most of the guys doing Subarus in my area do.

I was faced with this dilemma recently on my engine, and instead opted for a line hone, so i could use full torque on the bolts. Thinking this would be best. But the shop botched it because they forgot to tighten up all the bolts. They had a second go at it, and botched that. And now my block might be poo-poo.

If your part of the world is like mine, lacking any competant, skilled and caring machine shops. Then I would probably opt for adjusting bolt torque instead of paying some idiot machine shop operator to destroy your expensive engine. It sounds dodgy, but it is probably the lessor of 2 evils. Unless you happen to be graced by having a skilled machine shop in your area, who can actualy do sonething without butchering it. In my experience, this is not common.

As for temperature affecting precise measurements. Yes. This is a thing. So much so that if i want to take a precise or critical measurement now, i wait until the temperature is approx 20°c. I keep a laser thermometer handy and ensure the part is at temp before measuring. Also very mindfull of not heating a mic or bore gage up in my hand by holding it too long. Because this will effect precise measurements aswell.

Earlier in the year (winter), i had to wait until afternoon for the parts to warm up. Now that summer is coming, i wait to do my measurement early in the morning or late evening.

If your measurements are all landing in tolerance, then it will probably be ok with a small temp changes. (Thats one of the reasons why there is a tolerance.) But if you want to be precise as you possibly can, then you need to consider temperatures.

Yes Bram I feel your pain but at least you’re still fortunate to have lots of incompetent machine shops to choose from we have none here that does cylinder boring, honing, alignments, crankshaft cutting or even polishing.

With respect to the measurements yes if you want to confirm the manufacturers numbers yes you have everything at 20*C but my point was for clearances temperature shouldn’t make much difference if the components are of similar material. So for instance we set the main bearing clearance at 2 thou at 20* C then we run the motor and it gets up to 125*C the clearance remains basically the same because both crank, bore and bearings expands at pretty much the same rate.

On Aluminium blocks on the other hand we set our clearances on the mains in reference to what we want it to be at temperature. So if we wanted .0028” at 125*C operating temp then we set it at .0015 at 20* C because the metals expand at different rates.

There are multiple opinions around squish clearance and also squish pads in general. The general aim is to help move the fuel/air charge towards the centrally located spark plug and thus reduce the chances of detonation occurring. In some engines however the squish design has been linked to the engines being very knock sensitive. The theory is that unburnt fuel/air charge can be trapped in the crevice volume above the top ring where it isn't ignited by the flame front and instead auto ignites due the the increased heat in the combustion chamber during the combustion process. A popular technique is to modify the squish pads on the head and angling them into the centre of the combustion chamber. this is referred to as slant squish. While the quality of the video is terrible, you can see the effect of this in a video from mazworx here: https://www.youtube.com/watch?v=4nK-zG0hM7k. This retains the advantages of squish in promoting mixture motion towards the centre of the combustion chamber while reducing the chances of mixture becoming trapped in the crevice volume.

With the 4G63 drag engines I built I would run zero deck height and we ran a 1.2 mm HKS MLS gasket. this would probably be considered quite generous by many. I'd probably suggest maintaining at least 0.030-0.040" though to prevent any chance of piston to cylinder head contact.

To answer your other questions, I'm not personally an advocate of adjusting the fastener torque however I know that it is a technique many use and within reason it can be effective. I would just be mindful that the clamp achieved from the fastener will obviously be effected. The clearances you are measuring will also be affected by ambient temperature although you are correct in saying that this becomes much more obvious with aluminium as the components will grow more as they heat up.