Dome Volume?

[126coupe]

Does anyone in their notes know the dome volume for the Mahle 2.5L racing pistons? Like these.

Thanks in advance.

[Carrera3.5L]

I thought I might but looking in my notes all I have are cylinder heights (85.6mm), pin diameter & length (22x58) and ring thickness' (1.5, 1.5, 4.0)...all junk food info, no meat & potatoes to share...

Call Pete at Andial, I'm sure Dieter probably has some info that he can relay to you (especially since you bought the set from them)

Ralph

[126coupe]

I saw Dieters partner today, nice guy, picked his brain for about 40 Minutes. Bought some piston rings. I posed the question "dome volume on my 2.5L" he pulled out a notebook w the info. 19.38 cc, same as my measurement. Mahle does exaggerate their CR. They came up w the same CR that i did 9.8:1 on their 10.5:1 pistons. apparantley the Cr changes when the engine is hot. MMMMMMMMMMMMm

[Henry Schmidt]

Something is not right.

Doing the calculations:
66 mm stroke
89 mm bore
1.0 mm deck
68 cc head chamber ( 2.2 to 2.7 heads are usually close)
19.38 cc dome
Compression ratio = 8.49 ?

Perhaps you meant dome height = 19.38 mm ?

To make 10.5 to 1 compression in a 2.5 engine you would need 31 cc piston dome.

[126coupe]

I agree something is not right, I am taking it apart tomorrow and I will re-measure and re-cc everything.

[matty74]

Did you guys get any confirmation that the Mahles are 9.8:1 on a 2.7 dome?

[126coupe]

Originally posted by matty74
Did you guys get any confirmation that the Mahles are 9.8:1 on a 2.7 dome?

I only have the info on a 2.5 10.5:1 CR

[Henry Schmidt]

The inconsistent compression ratios can be directly attributed to the varying chamber sizes associated with the 2.2-2.7 heads.
Stock unmolested heads can measure as little as 67cc and as much as 73cc.

Using the same piston with these differences in heads can give you a .7 point compression change.
Simply stated 10.5:1 could equal 9.8:1.

[snowman]

Assemble the engine, using plenty of grease on the rings. Put a piston at tdc with valves closed. On an engine stand, fill the hole via the empty spark plug hole with water and antifreeze solution until you just reach the bottom of the spark plug hole. record this volume. Now add this volume to the displacement of a single cylinder and divide it by the measured volume. This is the EXACT CR, no BS, NO fudging. Turn over the engine to dump the water/antifreeze mix.

50 cc syringes’ from an animal supply house (feed and tackle shop) are ideal for this.

[126coupe]

Originally posted by snowman
Assemble the engine, using plenty of grease on the rings. Put a piston at tdc with valves closed. On an engine stand, fill the hole via the empty spark plug hole with water and antifreeze solution until you just reach the bottom of the spark plug hole. record this volume. Now add this volume to the displacement of a single cylinder and divide it by the measured volume. This is the EXACT CR, no BS, NO fudging. Turn over the engine to dump the water/antifreeze mix.

50 cc syringes’ from an animal supply house (feed and tackle shop) are ideal for this.

so the volume of a single cylinder on my 89mm x 66mm is:
8.9 x 8.9 x 6.6 x .7854 = 410cc's correct?

Animal supply house? OK

[Jeff Alton]

Make sure you have the sparkplug hole perfectly level...........


Cheers

[2.7RACER]

Is it possible the dome volume is 29.38cc vs 19.38cc?
29.38cc works.

[snowman]

Originally posted by Minkoff
so the volume of a single cylinder on my 89mm x 66mm is:
8.9 x 8.9 x 6.6 x .7854 = 410cc's correct?

Animal supply house? OK

Your in so CA so I know for sure that you can go to a place called the Feed Barn. They sell syringes for horses that will hold 50 cc, be careful not to stick yourself, the needle is very sharp.

If you have a 2.4 liter engine, that’s 2.4/6 or 0.4 L per hole, ie 400cc.

Say you measure 45cc with the method I described. 400+45=445 take 445/45=9.888 That’s your exact comp ratio. No fancy calcs, just one simple measurement.

All the other info is usefull for calculating the compression ratio before you actually assemble the engine, and if your careful, they are exact, but the final measure should be done like I described, a double check, extreemly foolproof and accurate.

If your truly anal you can measure the 400cc as well. off by 4 cc, so what, thats only 1% or 0.1 for the comp ratio.

[Henry Schmidt]

Measure everything. CC the heads, measure the piston dome and calculate compression then dimension the engine the best you can. After you assemble the engine , measuring will just tell you if you did it right.
Check valve pockets and cam lift.
Do it right before you assemble it and your experience will be more satisfying.
Who knows, you might get it right the first time.

[2.7RACER]

Let's not forget this is a 89mm bore x 66mm stroke equals 410.6 cc's per cylinder.
Or the "2.4" liter motor actually is 2341cc's. Or 390.25cc per cylinder.
Sorry I'm anal this morning, got woke up by my smoke alarm at 4:30am. Just needed 9v battery.
Jack's method is simple and correct.

[126coupe]

I used the "Syringe" method and came up w these calcs:

8.9 x 8.9 x 6.6 x .7854 / 40 cc's = 10.27:1 CR

I will double check all the measurements this weekend.

[snowman]

Looks like you need race gas or at least 50% 91 octane + 110 octane race gas (mix will give you over 104 octane, just fine for a 10:1 TO 11:1 engine.) Your measure of 10.27:1 is very reasonable for an advertized 10.5:1 piston. Might not sound high, but with todays gas... well you cannot run pump gas, and unless your water cooled and have the very latest engine and ignition and fuel controls, and head designs, you are stuck with race gas. Might as well gone to 12:1 for really good torque and power.