3.2ss in process

gestalt1 · 01-22-2006, 05:42 PM

well i got the case together - i'm surprised how everything is working out. it's a 3.2 short stroke with mahle pistons and cylinders. i installed a .5mm copper shim and the cylinder with piston to check what the deck ht is at - only about .65mm. i will add another .5mm shim to bring it up to 1.2mm. the case was from a 140k mile engine and needed a little machining so i was not surprised with the need for thicker shims.

i decided to solid model in the computer the cumbustion chamber, piston top and cylinder and use the software calculate the measurements for the compression ratio. i know the piston dome volume for the rsr 98mm pistons is 29.2cc, and my cad work actually matched this so i feel fairly confident about the volume measurements. i did not model the exact profile of the valve seat and spark plugs but the approximation is fairly close. the heads are twin plug and the groove for the sealing ring was fly cut off and a larger chamfer added so the cc measurement ended up being 77.5cc, not the usual 90cc for sc heads. this worried me, i was told the pistons were 9.5:1cr but they look like the rsr type with 10.2:1 cr. after calculating the deck ht the cr ended up being 10.16:1. if i increase the deck ht to 1.5mm the cr drops to 9.9:1. it looks like i lucked out, usually when you buy parts from different people they rarely go together the way you think. i'm trying not to get to excited - there must be a problem ahead!
-matt

jpnovak · 01-22-2006, 06:53 PM

Nice modeling. I have the same set of PC's sitting in a box waiting to go together next summer.

btw, who set up your heads for the new PC set?

gestalt1 · 01-23-2006, 05:59 AM

i got the heads off ebay, they were rebuilt by competition engineering, i had ollie's ad the twin plug. i was going to use carrera heads but these came along and were cheaper than the work done to them cost. i had to install the valves - what a pain. i first bought the cheapo spring compressor and realized i did not want the spring to pop out and hit me in the head. the rented big compressor was only a little better. i reccomend the porsche bolt on style compressors, or even better - have the machine shop do it.

ChrisBennet · ChrisBennet's Garage

Did you physically CC everything or just model it? I've said this before but when it comes to motors my motto is Trust No One, Measure Everything.
-Chris

kstylianos · 12:44 PM

Quote:

Originally posted by ChrisBennet
Did you physically CC everything or just model it? I've said this before but when it comes to motors my motto is Trust No One, Measure Everything.
-Chris

I agree with Chris here.....measure everything and calculate your static CR. Increasing DH to lower compression (thinking it will decrease your chance of detonation) is not really a good rule of thumb. Increasing DH will also decrease squish and introduce a larger pocket at the outside of the combustion which is where detonation like to occuer first. Like many of the experts here say; Get your DH as close to 1mm, calculate your CR, and machine you piston tops (valve pockets) to bring your CR back to 'safe'.

gestalt1 · 01-23-2006, 01:53 PM

from what i've read the deck ht should be between .8mm and 1.2mm. i was not planning on actually measuring the volume the traditional way but i think it might be a good idea now. i think i will do my valve clearence check first however - if the valves are to close then i know piston machining is required. the other problem is that the chamfer on the heads is a little (.5mm) over the size of the cylinder. i posted about this and got a 50/50 opinion on this. i feel that since there is a small chamfer on the cylinder top that a small groove is created anyway and i should be ok. i was hopeing the computer model would be good enough but measure twice, cut once as they say.

gestalt1 · 01-23-2006, 02:51 PM

i just re-calculated with 1mm deck ht and i get 10.5:1 cr, this is still safe for twin plug and 93 octane, right?. what do SC heads normally cc at? they are refered to being 90cc but if they were i would only have 8.77:1cr.

cstreit · cstreit's Garage

I've got a 3.0L running 10.5:1 with twin plug and have been racing it with our Midwestern 93 octane fro 5 years. I do run less total advance because of the twin plugging. If memory serves me it's 29 degrees at 3000 RPM and increases to 32 at 7500RPM.