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914World.com _ 914World Garage _ CR/Interferience question
Posted by: r_towle Nov 15 2005, 06:35 PM
On a stock US 2.0 liter with the dished pistons...
If the head gasket is removed...no cylinder base gasket is installed.
What is the new compression ratio...circa???
Do the valves have a chance of hitting the top of the piston...
I have not doe this, so I dont know what it looks like yet...just want to see if someone knows already...
Rich
Posted by: Aaron Cox Nov 15 2005, 06:36 PM
rich -
im in the same boat.
pulling the heads to remove head gasket. i have a 94mm euro piston..... and a big cam.
not sure how much deck i can lose to up the c/r...
Posted by: SLITS Nov 15 2005, 06:47 PM
Engineers.....sheeeeesh
http://www.csgnetwork.com/compcalc.html
Posted by: Aaron Cox Nov 15 2005, 06:51 PM
i dont know my deck height.... yay. more stuff to play with 
Posted by: Mark Henry Nov 15 2005, 06:51 PM
Measure measure MEASURE!!!!!
I've seen some stick right out of the bore.
Posted by: Dr. Roger Nov 15 2005, 07:00 PM
i've seen those valve to piston clearances get to almost nothing on factory cars. ford probe for one using interference engines. no problems as long as the timing belts are good. a non issue for 914's.
just bolt it all up and hand crank it slowly.
if it goes round and round you're probably alright.
just don't plan redlining it or my opinion becomes null and void. 
Posted by: r_towle Nov 15 2005, 07:02 PM
so your saying that stock pistons stick out of the cylinder...are you also saying that they are all different ....not by a small bt, but by alot??
Rich
Posted by: Mark Henry Nov 15 2005, 07:05 PM
Good way to smoke an engine.
You need at least .040....but what is your CR?
Too much and it won't last long...too little and you leave power on the table.
Posted by: Mark Henry Nov 15 2005, 07:10 PM
| QUOTE (r_towle @ Nov 15 2005, 09:02 PM) |
| so your saying that stock pistons stick out of the cylinder...are you also saying that they are all different ....not by a small bt, but by alot?? Rich |
Not all, but I have seen it...I've also seen the aftermath.
Yes they can be different, out by .050 is a mile to a machinist.
Posted by: SLITS Nov 15 2005, 07:15 PM
If your really hot....
bring pistion to absolute tdc...lay straight edge across cylinder and measure down to edge of piston w/ sliding veniers. Subtract thickness of straightedge and you have the deck height. (make sure cylinders are secured). Do for each cylinder.
or
put modeling clay on edge of piston(s)....bolt heads back on....roll engine 360 degrees...take heads off and measure thickness of clay. Gives you p to h.
Posted by: Mark Henry Nov 15 2005, 07:15 PM
BTW if you take out the head gasket most likely you need shims anyways.
Measure it.
Is your engine worth a guess?
Posted by: nebreitling Nov 15 2005, 07:35 PM
you definitely want to cc your heads and (re)set your deck height. i know it's ghetto, but i actually found it easier and more reliable to use a straight edge and feeler gauges (at TDC) to figure deck height than the depth gauge on the micro caliper-things that i have. definitely fab/find some spacers and torque down those cylinders when measuring.
it can be kind of time consuming, but if you have a wide variety of base shims at hand, it's not at all hard to get all cylinders the same spec (to the thou). in any case, i wouldn't want more than a couple thou out on each side for head sealing issues. i believe i used hylomar to seal the cylinder bases, seems to have worked well.
Posted by: r_towle Nov 15 2005, 07:38 PM
| QUOTE (SLITS @ Nov 15 2005, 08:15 PM) |
| If your really hot.... bring pistion to absolute tdc...lay straight edge across cylinder and measure down to edge of piston w/ sliding veniers. Subtract thickness of straightedge and you have the deck height. (make sure cylinders are secured). Do for each cylinder. or put modeling clay on edge of piston(s)....bolt heads back on....roll engine 360 degrees...take heads off and measure thickness of clay. Gives you p to h. |
thank you...i went out and realized that i could not accurately measure it...
I was thinking modeling clay or build a 1/2 inch plate....I like the clay method better....
I like the old school tricks..
Rich
Posted by: r_towle Nov 15 2005, 07:45 PM
Let me ask a detailed question.
Does the CR in any way affect the DJet FI system?
Rich
Posted by: SLITS Nov 15 2005, 07:55 PM
I also would coat the head and clay with a little motor oil so that the clay doesn't stick and pull to give you a false reading.
I would cut through the squashed clay with a razor blade to measure the squash in the middle with a set of sliding veniers.
Good luck.....
edit: I wouldn't think that CR would have a detrimental effect since it is not subject to the pressurization in the cylinder.....it only injects into the airstream flowing into the cylinder.
Posted by: Aaron Cox Nov 15 2005, 07:56 PM
so. ron, ill bring my motor by, and we can do this 
Posted by: SLITS Nov 15 2005, 07:57 PM
| QUOTE (Aaron Cox @ Nov 15 2005, 06:56 PM) |
| so. ron, ill bring my motor by, and we can do this |
Not a problem.............
I guess............
Posted by: jhadler Nov 15 2005, 08:49 PM
| QUOTE (r_towle @ Nov 15 2005, 05:38 PM) | ||
|
One word... Plastiguage!
Neat stuff. A PITA to use for air cooled engines, but pretty good stuff in general.
-Josh2
Posted by: type47 Nov 15 2005, 09:28 PM
CR is about 7.6:1 + or - alot. stock CR was 7.6 according to factory specs
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