I have a Datsun 510 that has a 2.0L engine that was built up by Troy Earmish Racing in 2001 or so. He did a bunch of work to it, including high CR pistons, shaved the head, bigger intake valves, balanced and lightened crank/flywheel.

He mentioned to me that he had actually raised the compression ratio too much, so he put in two headgaskets to compensate. I didn't complain, as the engine ran great, and I later when I did a compression test on it all 4 cylinders were between 205 and 210 pounds.

Due to a mishap with an O-ring, I dumped all the oil on the road and spun the crank bearings. When my regular mechanic (not Troy) put it back together, he only put one head gasket in (like any sane mechanic would do). However, since then it rattles severely at anything over 1/2 throttle.

I'm trying to decide what my tuning opotions are with this motor, but I'd like to find out what the ratio is, if it's possible. Is there a way to find this out without tearing the engine down? Or even if I do tear the engine down?

Any suggestions would be appreciated. And, yes, I searched.

 

no way to tell unless you pull it apart. a comp test may get ya kind close, but way to many variables to pin point it.

 

bummer. Well, thanks for the info.

 

Comp ratio is just that ---- The ratio differance between Head volumn and swept volumn ( volumn from TDC to BDC ) You have to CC the head chamber which is done with the head off and inverted --Determine The CC volumn of the Chamber ---Thats done by adhereing a piece of plexiglass to the head surface --It has a small hole in it which liquid is poured--Generally alchohol from a precise measuring device determaining CC capacity----Thats measurement # 1
Second is swept volumn as above ---Pi R Squared X highth Measurement #2
The plot thickens Flat top pistons ? Dished Pistons ? Pop up Pistons ? That has to be taken into account

Nother words forget it ----Buy pistons that are of the ratio u want ---Keep in mind if u bore oversize it has an affect

 

He still wont know if he picks the pistons because he will be assuming the head volume is stock, he will have to measure the head volume to make sure.

But if the heads are stock then he can assume the CR pistons he picks will end up in that CR.

 

Splitting hairs there---Head Volumn will be stock more or less to a minor degree---If you pick a 8.5:1 piston u will have a CR very close to that. Seldomly is metal taken away or added to the chamber other than some polishing. Metal removal/addition is in the port area which has no effect on CR. Its always hard to second quess others Mods.

 

There is a device called a whistler used by SCCA, NASCAR, etc.. It screws into the spark plug hole and measures dynamic compression. http://www.katechengines.com/corpora...s/whistler.php

If there is a problem @ half throttle I would look elsewhere than cr for the root cause of the issue.

PM me the details of what's going on, see if we can't figure this out for you.

 

old thread but it seems like most shops deck the heads and block during a rebuild so what would the compression ratio be if you ordered a set of 9.5:1 c/r pistons? 10.0:1?

 

It wont change that much---insignificant prob. You could get a ball park by subtracting the cut from the "swept volumn" or chamber volumn and refiguring the cylindrical/chamber displacment and comeing up with the new Ratio