engine and drivetrain Compression Ratios and Cams for 1973 Tii

[SBriggs]

I am trying to work out what the compression ratio is for my 1973 Tii (US spec). The engine is stripped and there are nearly new grand piano pistons (I believe this is what they are called) with 88.97 stamped on the top, I assume this is the diameter. The distance between the top of the top of the piston pin to the upper part of the raised part of the piston is 35.9 mm and the distance between the bottom part of the skirt to the top of the piston is 84.7 mm. The cylinder head is 128.53 mm as opposed to the standard 129.0 mm. I have read that 10:1 is a good ratio for a standard road car with high octane fuel. The only modifications are gas flowing the head, full balancing and a JB Racing aluminum flywheel. Any advice would be appreciated.

SB

P.S. I also have a standard cam, should I look to a 294 Schrick (or any other manufacturer for that matter)?

[c.d.iesel]

USA tii stuff from BMW Repair Manual :

BORE 89MM (3.50")

STROKE 80MM (3.15")

CYLINDER HEAD HEIGHT(original 129 (+/-0.1)MM (5.0787")

-= MUST NOT REMOVE MORE THAN 0.5 MM (0.0197") =-

COMPRESSION RATIO 2002 tii

9.5:1 EURO - USA VERSION 9.0:1

Compression ratio 9.5:1 and modified combustion chamber

for BMW 2002 tii from Ch. No. 2 710 840, and from BMW RHD 2002 tii

from Ch. No. 2 750 846, and from BMW Touring 2002 tii

Ch. No. 3 423 221. This can be seen by the code " E21 " cast

into the cylinder head on the intake side.

Prior to the Ch. No.'s stated above the compression ratio was 10 :1 .

[john_a]

You've provided some measurements, but not the important ones needed for your answer.

The two piston features that really matter are:

Compression Height (CH) this is the distance from the pin to the top of the piston, not the top of the dome. Typically it is from the centerline of the pin, but it will be easier to measure from the top of the pin as you had, and then add one-half the pin diameter. Note that the BMW service manual references height from the top of pin in their description, and then you have to add 0.433" to find conventional CH.

Dome Height, measured from flat top of piston to top of dome.

Do these pistons you are looking at have a Mahle maker's mark on them?

What cylinder head is this, E12? That's what is expected with piano top pistons.

It's likely these are a common Mahle piston for the E12 head. I know you gave the sum of CH + Dome, and if there was nothing else to go on we could guess from that, but please confirm the above questions to make it easier to answer without speculation.

[SBriggs]

Thanks for this. I think I have established that the MC measurement is 31.1 mm. I hope this is correct, explaining what I want over the phone via a third party in Polish may have lead to a mistake! If you can tell me fom this what the compression ratio is I would be very grateful. Regards SB

[SBriggs]

I meant to say the CH measurement. I also have the E12 head.

[john_a]

Thanks for this. I think I have established that the MC measurement is 31.1 mm. I hope this is correct, explaining what I want over the phone via a third party in Polish may have lead to a mistake! If you can tell me fom this what the compression ratio is I would be very grateful. Regards SB

Still need to perform math to answer the questions I posted.

Add 11 mm (1/2 pin dia) to 31.1 that's CH = 42.1 mm

Then subtract the CH from your 35.9 + 11 and the dome = 4.8 mm

This looks a lot like the 9.0:1 piston shown on the chart on this site's FAQ

http://www.bmw2002faq.com/content/view/46/32/

Your skirt length & dome height measurement are just a little bit off, but I think you probably have what's shown on the chart.

84.6 skirt vs your 84.7 mm

4.5 dome vs your 4.8 mm

9:1 vs your approx 9.4:1 due to your reduction in chamber size

Mike Macartney's book tells us that 0.5 mm shaved off the E12 head reduces the chamber enought to raise compression ratio by 0.435 points, so the best guess would be you have achieved 9.435:1 with the stock 1.5 mm thick head gasket (avoid the 1.8 mm gasket, and consider getting the Mahle # 082 02 01 first oversize 9.5:1 E12 pistons when you are ready to take the bore to 89.265 mm.)

With your shaved head you'd have near the 10:1 you're looking for.

[SBriggs]

Thanks. Sorry to be so slow, but are you saying that I am already at 10.0:1 or do I still need to save 0.5 mm off the head? Also you say my pistons are probably the 9.0:1 but they look like the 10.0:1 or the 8.6:1 shown on the link you supplied. The measurement I have for the piston is 84.7 mm (close to the 84.7 mm in the diagram) so maybe I already have the 10.0:1 pistons, is that a possibility?

[john_a]

Thanks. Sorry to be so slow, but are you saying that I am already at 10.0:1 or do I still need to save 0.5 mm off the head? Also you say my pistons are probably the 9.0:1 but they look like the 10.0:1 or the 8.6:1 shown on the link you supplied. The measurement I have for the piston is 84.7 mm (close to the 84.7 mm in the diagram) so maybe I already have the 10.0:1 pistons, is that a possibility?

If you say your pistons looks like the 10:1 or the 8.6:1 dome shape on the chart, then that changes everything. Those are called "bathtub" domes, not the "grand piano" dome like the 9:1 or 9.5 shown on the chart.

I did not think you had those, because you had mentioned "piano" and confirmed they're being used with an E12 head.

The bathtub shape is not supposed to be used with the E12 head, not sure how much of a mis-match it is. Those bathtubs were made for the 121 & 121t & 121ti head castings, and they also work well with the E21 casting head.

In my example above, where I thought you had 9:1 E12 piano tops, I said with your already reduced size chamber (you stated the head thickness was about 0.5 mm less than original) you would have had 9.4:1 ratio.

Again, with that skimmed head you already have, changing to the 9.5:1 E12 pistons would have resulted in nearly 10:1 ratio.

BUT, if you really have the bathtub 10:1 pistons, your already shaved head might produce something like 10.4:1 with your existing parts, if they truly fit together. Not sure how the dome volume compares, and that will change the CR. Are you sure the head is E12 and the pistons are bathtub domes?

Look closely at the specs & the pictures of dome shapes compared to what you have, read the posts & you will figure out what parts you have, and what the measurements you took are close to representing.

[TobyB]

Actually, if the 'dome' step is 4mm,

then you're probably going to be at 9 1/2 to 1.

I don't say 9.5, because you really need to cc your chambers

to get it down to the single decimal point....

With a 60 cc chamber, then you'd be right up to 10:1.

But I bet you're more like 65cc...

Just guessing...

t

[Guest Anonymous]

Sir John:

Maybe I am way off base here (I usually am) but the FAQ artikel, as comprehensive as it seems, may be missing one or two optional "piano-topped" piston exemplars. I might be confusing this with a diagram used for e3 and e9's but I thought there were at least 3 if not 4 different factory versions of the piano tops. Isn't there an 8.8:1 and/or a 9.3:1 for US markets? I just remember beating my head against the wall years ago when a machinist showed me several variations found in a tear down M30 I brought him. Could be wrong, but my head is still bruised!

[SBriggs]

All

Thank you very much for your help.

SB