Comp Ratio - Check My Calcs Please

[danm54]

I think I've worked this out right but I'd appreciate it if someone could go through it and check as I've not done this before so I'm relying on an online calculator!

First off its a water cooled 1100 with 77mm (3.031'') flat top pistons with valve cut outs and a 60mm (2.362") stroke.

I've cc'd No.1 chamber on the head and a piston in No.1 cylinder set 1mm down the bore.

Combustion chamber held 24.4 ml which if I'm right is 24.4cc?

Piston 1mm down held 6.5ml so 6.5cc

77mm bore 1mm deep is 4.66cc

6.5cc minus 4.66cc = 1.84cc which is my piston volume.

Piston deck height is 0.533mm

Cometic do gaskets in multiple thicknesses so I've worked it out with 2 different ones. A 0.018" and 0.040"

So with all that, it came out at
9.38:1 with the .040" gasket
10.05:1 with the .018" gasket


I'd like to dish the pistons a little if possible so assuming the above is right I'd go with the thinner gasket.
Also, the .5mm piston deck height is about spot on right?


Hopefully I've not missed anything.
Thanks in advance

[CHR15]

your correct, if you want, pm me your email address, i made an excel sheet that you drop all your values into and it works everything out, so you can change things one at a time and see what it does.

[danm54]

Cheers Chris, appreciate that.

[wombat258]

Machining the piston crowns can weaken them, and is generally not a good idea. Add extra base gaskets to get the compression to where you want it.

[danm54]

I spoke to Roger Upperton about the possibility of machining them when he done the rest of the work.

He never measured them or made any promises but if I work out the volume of dish I'd like he said he'd measure up and see what could safely be done.
A dished piston would in my mind make a proper job but it may not be needed or sensible.

He was also going to put a small rad around the edge and take a bit off around the valve cut outs to prevent hot spots anyway so I'll work out the volume I need and give him a call.

Cheers



Edit, they are MTC pistons so they've got more meat than stock ones anyway.

[arttu]

I have dished Wiseco pistons in my engine. If I recall correctly the dish is about 1mm deep and the crown thickness is now about 5mm in the middle. No problems by this far.

I think your squish band clearance is getting slightly on tight side with thinner head gasket. 0.5mm deck height and 0.018" gasket would result 0.96mm clearance which might be slightly risky already on mechanical aspect. Probably it's still ok but better to check from someone who knows this particular engine type well. But in addition it's quite common belief that boosted engines like slightly larger squish band clearances than NA engines. So personally I would aim to 1.2-1.5mm range.

[danm54]

Thanks Arttu, didn't know boosted likes a bigger squish band.

If I use a .027 head gasket and have a dish volume of 1.96cc it'll work out at 9.24:1 ratio with just over 1.2mm squish band.

The dish volume is 50mm dia by 1mm deep. With the other bits being removed it may not need to be quite that big but haven't allowed for the dish edges to taper up as I don't know how to work that out!

[katana]

MTC use essentially the same piston blanks for their flat top and for their turbo dished pistons - only difference is the flat tops weigh more. MTC have the reputation for producing the strongest crowned pistons - still melt with loads of nitrous though! LOL.

[wombat258]

I have found that the minimum squish you can run on 1100W engines without the piston hitting the head is 1.4mm. The consequence of the pistons hitting the head is damaged rod bearings, followed soon after by one of the rods exiting the block. My preference to reduce the comp would be deshrouding the combustion chambers (good flow gains as well), followed by adding base gaskets. Pistons flex and crack easily enough normally aspirated, so removing material from the crown WILL shorten their life with a turbo. I have rarely seen turbo pistons for the WC's, so I doubt that the blanks cater for thicker crowns. I have used JE, Wiseco and MTC, and they were close to minimum crown thickness out of the box.