What's Your Head Volume?

[viejopatron]

I did a poor-man volume measurement on a 3.0 head and got 100 milliliters. The head is vintage 1980, stock valves and modified with a second spark plug. I'm wondering if my 100 ml value is in the ballpark. Please speak up with your measurements. Thanks.

[ChrisBennet]

A stock SC runs around 90cc.
-Chris

[kstylianos]

Chris is right.

I had .35mm milled from my 82 SC heads and they measured ~87.5cc's

[tom1394racing]

Compression Ratio Calculation

Bore (mm)
95 95
Stroke (mm)
70.4 70.4
Deck Ht (mm)
1.55 1.7
Head CC
84.5 83.75
Jo Block Ht (mm)
54.89 54.88
Jo Block to Dome (mm)
61.06 61.27
Jo Block to Deck (mm)
77.75 78.25
Depth H2 (to dome mm)
6.17 6.39
Dome Ht (mm)
16.69 16.98
Cylinder CC
121.5 124.2
Theorectical Volume
162.04 165.65

V1 - Swept Volume
499 499
V2 - Deck Ht Volume
11 12
V3 - Cylinder head volume
84.5 83.75
V4 - Piston Dome Volume
40.54 41.45

Comp ratio (0.25mm shim)
10.08 10.18

[tom1394racing]

Sorry guys....I pushed the submit button on the previous post before I asked my question.

I recently completed all of the CR checks on my 3.0 SC based RSR clone motor and got a combustion chamber volume of 84 cc for my twin plug heads. Although this seems low relative to the numbers posted above, my CR calculated out to be 10.1:1. This is about what I expected since I am using new 3.0 Mahle RSR pistons and cylinders. A combustion chamber volume of 88-90 cc would give much lower CR.

Can one or two of you experts take a look at my numbers and let me know if you see anything that seems weird?

[emcon5]

The heads on my 82 measured at 90cc.

Tom

[ChrisBennet]

Hi Tom,
I'm not an expert but as a fellow "student" we can compare notes until the "professors" get here.

Your deck height (1.55, 1.7mm) seems about 50% bigger than what I'd expect. Maybe you're base gaskets aren't compressed flat or something?

Here are some deck heights I've measured:
1.27,1.09mm: Stock 964, biggest deck height I ever measured
.97,.92mm: 3.2SS w/JE's (from 3.0L)
.85mm: 2.8L using 66mm crank and 90 bore with 2.4L? heads
1.12mm: 3.4L w/JE's (from 3.2)

Your 3.0 head volume (~84cc) seems small, esp for twin plugs. A stock 3.0 is "supposed" to be around 90cc. The last ones I measured were 87.8cc after some cleanup and edge chamfering.

-Chris

[Carrera3.5L]

Sorry, I have 3.2L heads but as a data point they measured 91.8ml twin-plugged (I didn't measure them stock, oops!) and 88.8ml after .020" was milled to gain back some compression.

Geez, your deck height does seem conservative, .8mm - 1.2mm are what some of the accomplished and professional builders out here are running on STREET motors.

Ralph

[tom1394racing]

I am getting a little worried about some of these measurements as my engine is already sealed up and ready for carbs and exhaust.

Deck Ht - I measured the deck ht two different ways on cylinders #1 and 4 and got similar results. First I used my dial calipers resting on the lip of the cylinder and measured down to the outer rim of the piston (RSR type) where it was flat. I was able to ensure the caliper was perpendicular to the piston top by resting the edge of the caliper on the lip of the cylinder. I made 4 measurements each at 90 deg intervals around the piston top and got from 1.7 to 2.2 mm on cylinder #1 and from 1.4 to 1.8 mm on cylinder #4. I then used the solder method on both cylinders. Measuring the ends of the compressed solder, I got 1.56 mm on cylinder #1 and 1.70 mm on cylinder #4. Given the recommendation in Waynes book for 1.25 - 1.5 mm, I figured I was on the safe side and proceeded to measure the CR. I used the deck ht volume calculated from the solder deck ht in the CR calculation.

Combustion Chamber Volume - I used a 4" diam plexglass plate with a mm burette and repeated the measurement several times on cylinder heads #1 and #4. I got 84.5 cc on # 1 and 83.75 cc on #4. I can't imagine that my heads could really be in the 88-90 cc range, unless I was doing something terribly wrong in my measurements.

When I calculated my CR using this data, I got 10.08 for #1 and 10.18 for # 4. This seemed about right so so I proceeded to install the heads, cam towers, time the cams and check piston to valve clearance. Minimum piston to valve clearance was at 2.25mm for my exhaust and 1.80mm for my intakes.

If my deck ht was really 1.0 mm, this would push my CR to 11.1...too high.

If my deck ht was 1.0 and my head volume was 88 cc, my CR would be 10.3. This is about right for this configuration.

I've been pretty careful in making all my measurements and am reluctant to teardown and remeasure. What do you expertes think??

[1meansc]

Hi Guys,
My Euro spec SC, with twin plug and 0.5 mm milled off the face gave a volume of 89 ml.

I would also keep the deck height as close to Wayne's specified, to reduce the chance of detonation in the relatively large piston to head clearance.
If this value is kept closer to 1.0 mm, it keeps the effective combustion chamber smaller.

[Steve@Rennsport]

Tom:

IMHO, you really need to confirm your CR measurements and calculations. There is a LOT at stake here,........

Lastly, deck heights greater than 1.0 mm (.040) really increase the possibility of detonation, especially with these 95mm and larger bore engines. These are relatively lazy combustion chambers with little swirl and its those end gasses left at the periphery of the piston crowns at TDC that are the first to spontaneously ignite.

[tom1394racing]

Thanks for all the input.

Sounds like the heads and cam towers need to come off so that I can recheck the deck ht and CR. From all the data posted, here it sound like I should be targeting deck hts around 1 mm and a head volume of around 88-90 cc.

Can I remove the heads, cam tower, rockers and cam as a unit to simplify the disassembly and to avoid disturbing the seal between the cam towers and heads and cam cover plate? I could then do both the solder and caliper deck ht checks on all 6 cylinders. I will probably redo the head volume and dome ht checks on all 6 cylinders as well.

Wiil this work?

[ChrisBennet]

I think I've removed the cam carrier+heads&chain box as a unit (yes, the chainbox) before. You get creative when you forget to put in the air deflectors...
-Chris

[tom1394racing]

I went ahead and tore down the LHS tonight. Wouldn't have enjoyed my day at the ALMS races at Lime Rock tomorrow if I hadn't done it.

I removed the cams towers, heads and chain box as one unit with the cams and rockers still installed. This time I used my dial indicator to ensure that the pistons were really at TDC. I measured all three of the deck hts using my digital calipers as a depth gage at the piston edge. The pistons have a nice flat lip that matched the width of my caliper tip.

The numbers are much closer to what Chris and others have measured. Results are:

#1 1.25 mm Ave highest reading 1.26 mm/lowest 1.10 mm
#2 1.10 mm Ave highest reading 1.16 mm/lowest 1.05 mm
#3 1.17 mm Ave highest reading 1.39 mm/lowest 1.18 mm

These results seem pretty reliable to me. I don't think I'll get better data with the solder method.

Later this weekend I'll redo the head volume measurement on all 3 heads to see if I come up something more in the 88-90 cc range and I'll redo my CR calculation.

[tom1394racing]

Well....I remeasured my head volume last night and again this morning.

I did it using both windshield washer fluid (for the color) and then used a mix of WD40 and motor oil. I also have used both a 25 ml burette I borrowed and 10 ml syringe I got from the local pharmacy. I did it on cylinders #1 and #3 and am still getting 84 cc.

These heads supposedly have had little work done other than the twin plug mod and "cleaning up the ports". They are supposed to be 3.0 SC heads (the P/N stamped is 930.104.329.3R). I bought them from Auto Associates.

It seems that the head volume is about 4-8 cc too low. My plan is to take them back to the AA and have them verify the cc measurement.

If they are really 84 cc my compression will be too high at around 11:1. What options would I have at that point to bring CR back down to 10.5??

[beepbeep]

OT:

Many winters ago, I was doing some C programming in Borland C++ (for DOS!)

Somehow, I found interesting commando that let me play tones trough PC-speaker. Help-file for this particular commando included a small sample that played "4Hz sound" trough PC-speaker.

This code sample in help-file contained interesting info.

It explained that "you cannot actually hear 4Hz" and had a short story about chicken farm in Australia that suddenly suffered from massive chicken death after a nearby factory started their production.

Finally, boffins concluded that problem lied in fact that chickens had their head volume tuned to 4Hz sound emanating from a neighboring factory. So poor chicken had their heads resonating at 4Hz and subsequently died...

Now that was a interesting story... I don't know why I typed it but at least it had to do with head-volume...hihihi

[Steve@Rennsport]

Originally posted by tom1394racing
It seems that the head volume is about 4-8 cc too low. My plan is to take them back to the AA and have them verify the cc measurement.

If they are really 84 cc my compression will be too high at around 11:1. What options would I have at that point to bring CR back down to 10.5??

Hi Tom:

If you need more chamber volume, I'd start by machining larger plug reliefs in the heads. That will pick up about 2-4cc.

After that, the squish bands need to be milled to "deepen" the chambers a bit. Thats a delicate operation and needs to be done by someone who specializes in 911 heads and has done this before.

[tom1394racing]

Well......

I repeated all of the compression measurments on cylinders 1,2 & 3 this weekend. I verified that my deck ht was really in the 1.1 to 1.2 mm range rather than the 1.5-1.7 mm range from my original measurements. I had not had the pistons at TDC when I had measured them the first time. I also verified that my head volumes really are at 84 cc and my dome volumes are between 39.4 and 40.4 cc. The head and dome volumes repeat my original measurements.

Bottom line seems to be that the reduction in deck ht from around 1.6 mm to 1.1 mm will boost my compression from my original number of 10.1:1 to a higher CR of 10.3-10.5:1. This is right where I had targeted the motor.

I still don't know why my heads measure at 84 cc as opposed to 90 cc like stock SC heads. Before I reassemble I will be verifying all of this with the experts at Auto Associates.




Cylinder #1 Cylinder #2 Cylinder #3
Caliper deck Ht1 (mm) 1.18 1.05 1.16
Caliper deck Ht2 (mm) 1.24 1.06 1.19
Caliper deck Ht3 (mm) 1.32 1.16 1.26
Caliper deck Ht4 (mm) 1.19 1.14 1.1


Compression Ratio Calculation

Bore (mm) 95.00 95.00 95
Stroke (mm) 70.40 70.40 70.4
Ave Caliper Deck Ht (mm) 1.23 1.10 1.18
Head CC 84.00 83.80 84.00
Jo Block Ht (mm) 40.30 40.30 40.30
Jo Block to Dome (mm) 42.12 43.02 46.16
Cylinder Lip to Deck (mm) 18.96 19.51 22.90
Depth H2 (to dome mm) 1.82 2.72 5.86
Dome Ht (mm) 17.14 16.79 17.04
Cylinder CC 95.00 99.00 122.00
Theorectical Volume 134.39 138.29 162.32

V1 - Swept Volume 499.01 499.01 499.01
V2 - Deck Ht Volume 8.74 7.81 8.35
V3 - Cylinder head volume 84.00 83.80 84.00
V4 - Piston Dome Volume 39.39 39.29 40.32

Comp ratio (0.25mm shim) 10.35 10.54 10.59

[ChrisBennet]

Sounds like you're at a good deck height and the compression ratio that you wanted. I think you'll sleep better too.
-Chris

[tom1394racing]

Just to complete the story....

I spoke with Scott at AA today. He confirmed that the head volume on my heads should be at 84 cc. They had been machined to the 84 cc volume to make them work with the RSR pistons.