cbracerx

As part of my continuing saga to attempt to use RSR 3.8 pistons on my street/track car, I have a new area of interest. I am attempting to measure the deck height now using no gasket on the bottom of the cylinder and a stock length rod. The value I get using the crushed solder method at the extreme edge of the piston is 1.30 mm (done several times with a variey of placements). I am checking this for the obvious reasons - but alos to also make sure that a stock rod length is appropriate. It appears that it is.

Should I take any pains to lower this height closer to 1.00 mm?

Obligatory images:

cbracerx

The pic above with the solder crushed is not representative of the effort - I did this four times including with small pieces right on the edge of the piston.

Geoffrey

You need the deck height clearance to ensure that the piston does not hit the head. With a good steel rod and under 8k, you can get by with as little as .036". The measurement that you have is about what RSR deck height clearance runs. I personally like to be in the .036-.039 range and never go beyond .060" (1.2mm). When you do, you lose engine efficency and make the engine prone to detonation because the you lose the squish/quench area and the combustion mixture is not properly oriented at the spark plug.

At your measurement of 1.3mm it is not ideal, but normal for an RSR type piston. In my engine I am running 1.1mm which is at the outside edge of what I'd normally run. However, I am planning on swapping pistons this winter and did not want to cut the cylinders to match my RSR pistons and will wait for the new ones to ensure proper deck clearance. If I had measured 1.3mm I don't know what decision I would have made at that time.

You have several options, one is to have your Carrillo rods made slightly longer (lowest cost option) to get the proper deck height clearance. Another option is to machine the cylinder. I'd probably opt for the specific rod knowing that if I rebuilt the engine and used a different piston I might have to make some adjustments. I might mock up the new engine with the longer Carrillo rod and when the rods were checked and resized I'd offset the wrist pin bushing to arrive at the correct deck height clearance.

NineMeister

Ideally check all the cylinders first, then have the Carillos made so that the squish height on all 6 is between 0.8-1.0mm, you will benefit in power, response and economy. Don't worry about the compression ratio from such a small change, as Geoffrey pointed out running a tighter squish will actually suppress detonation.

cbracerx

Thanks very much for the info gentlemen - I will wait for the rods to appear (they are stock length so I am stuck with that for now) and see what each cylinder holds in store. Then I can hopefully adjust with eccentric bushing. If not, cylinder machining may be in the cards.

Cupcar

Wouldn't 0.3 mm change in deck height be worth it from a compression ratio change, unless you have already met your compression target?

If I am not mistaken with a 102 mm bore each 0.1 mm of deck height is ~ .82 cc and, assuming a 0.3 mm change, 3 X .82= 2.46 cc which would be a compression ratio change of around 0.4:1 at the ~61 cc combustion chamber volume and 624.3 cc cylinder.

Right? or did I make a mistake.

I think I would want to checke the compression ratio while I was taking measurements as well to see where I was at in that department too.

Somehow changing the rod seem safer-as in reversible- to me than changing the cylinder and I bet Carillo would not charge too much for this change.

cbracerx

I certainly do want to make sure I have the best setup. As I mentioned in the previous post, the adjustment would be made on the rod first, using an eccentric wrist pin bushing. If that is not possible for some reason, then I would consider the cylinder machining.

I certainly do need to check the compression ratio to make the best decision. I guess measuring the volumes of the combustion chamber and piston top are next on the agenda. Anyone have a good thread on that?

Cupcar

Here is how I do it and although probably not professional, it seems to work and produce repeatable results.

You will need:

1. To make a disc of 1/4" Plexiglas that fits inside the cylinder head and cylinder at the maximum diameter of the sealing surface. You may be able to do this with the same disc. This (These) disc(s)will need a hole in the center about 1/4" diameter with a chamfer cut to form a funnel that makes an acute angle with the side opposite the chamfer.

Note: the plane that the disc of plexi seals to must be the mating plane of the cylinder and head for these measurements.

2. A 100 cc Burette and holder from a chemical supply house

3.Some ATF (automatic transmission fluid) or some other light oil. I like the pink color of ATf.

4. Some grease.

Tak a finished head with a spark plug in it and place the disc over the combustion chamber using a little grease to seal the edge. You can see through the Plexi to see if seal is OK. Fill the head with ATF carefully measuring with the burette, and just stopping filling at the bottom of the Plexi. Read through meniscus in the burette at the bottom of the meniscus each reading. This is "head volume".

Next take a piston and fit it inside a cylinder and push the piston down so the top is below the top of the cylinder. Use your trusty Mitotoyo to measure the depth from the top of the cylinder to the shoulder at the perimeter of the piston. Pick an even number to make the math easy something like 30 mm or whatever since you will calculate a volume using this as the height of the cylinder. Take a finger full of grease and seal the gap between the edge of the piston and the cylinder. Now take the Plexiglas disc and cover the cylinder sealing again with some grease and measure the amount of ATF it takes to fill the cylinder with the piston in it.

Now calculate the volume of the cylinder using the depth you measured above (Pi X radius squared X height) and subtract from this number the amount of oil you measured filling the cylinder and this equals the "dome volume" of the piston

Now all you need is the deck height volume and you measure the distance the piston is below the sealing surface with your Mitotoyo using the ledge around the perimeter as a reference. Do another Pi X radius squared X height calculation using the deck height as H.

Now you can calculate the combustion chamber volume = Deck Height Volume + Head Volume - Dome Volume

Compression Ratio= Cylinder Volume + Combustion Chamber Volume / Combustion chamber volume.

Also your "Target Combution Chamber Volume" = Cylinder Volume / Target Compression Ratio - 1

For example, if you wanted an 11:1 compression ratio your Target Combustion Chamber Volume would be 624.3 cc/ 11 - 1 = 62.43 cc

Hope it makes sense

cbracerx

Cupcar - an Excellent procedure, thanks much! I will do this once my new rods arrive.