[Fot] TR4 Compression Ratio Calculation Question

[Duncan Charlton]

Steve,

I seem to remember seeing an online calculator to figure dynamic compression ratio.  It needed stroke, rod length and the crank degree figure at the point of intake valve closing.  However I measured it the “easy” way, just measuring piston rise every 2 degrees from BDC on a partially disassembled engine (using a dial indicator) and comparing those figures to my camshaft specs (I’d checked those using a degree wheel and dial indicator before disassembly).  Once I had a figure for the amount of stroke remaining after the intake valve closed it was simple to calculate dynamic CR, which I was trying to get just under 9:1.

To get unswept volume I did a test assembly of an engine (just one cylinder needed valves and piston w/ rings for this test), put the piston at TDC and tilted it so that the plug hole was the high point, and then measured the amount of fluid needed to fill the “overhead” space.  I used water with a bit of alcohol in it which gave lower surface tension than plain water, filling the cylinder using a calibrated burette bought through Amazon.  Very light oil might be better if you don’t want to have to remove the rings, etc. afterward to remove the water but I was concerned what the oil would do to the plastic burette.

To determine solely the chamber volume, I used the old tried-and-true method.  I cut a round piece of 1/4” plexiglas with a small hole for liquid to enter and a smaller one next to it for air to exhaust.  Put the head upside down on the bench (tilt it enough for the air to get out of the exhaust hole in the plexiglas) with the valves inserted.  I used some vaseline to seal the plexiglas to the head (and to better seal the valves since they were old and worn) and let the liquid pour in, noting the number of milliliters (1 ml = 1cc) that had dripped in when the chamber is full.

To answer part of Steve’s question, according to TeriAnn’s Triumph web page a stock high-port TR head is 3.30” thick.

Duncan Charlton
Elgin, Texas USA



> On Sep 23, 2016, at 10:14 PM, Steve Yott <tr4 at wi.rr.com> wrote:
> 
> Steve,
> In order to calculate the combustion chamber volume desired you need also know the bore, stroke, head gasket diameter and thickness, piston to liner clearance, depth of the top ring and deck clearance of the piston (or pop up if you are running any).  These all make up the total volume which you need to calculate the chamber volume you need for the static compression ratio you desire.  Also when doing a chamber volume test don't forget that the valve head thickness will affect this measurement.
> 
> There are a good number of web sites out there that will do all these calculations for you and give you a required chamber volume for the desired compression ratio.  There are various head castings out there so I would not take the thickness into account except for a rough idea.
> 
> This all creates a "static" compression ratio and will not mean much when the dynamics of the cam and valve overlap come into play and create the "dynamic" compression ratio.... wish there were a good method of calculating that!
> 
> Steve Yott
> 
> 
> 
> -----Original Message-----
> From: Fot [mailto:fot-bounces at autox.team.net] On Behalf Of Stephen Borowski
> Sent: Friday, September 23, 2016 6:45 PM
> To: FOT Triumph <fot at autox.team.net>
> Subject: [Fot] TR4 Compression Ratio Calculation Question
> 
> Hello All,
> Does any one know the "chamber volumes" vs "compression ratio” for the TR4 high port head. What should the head thickness measure?
> 
> All comments welcome!
> 
> Thxs. Steve
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