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I have read and heard speculation about the true compression ratio of Motronic pistons that are nominally 10:1, so today Rich Hanning and I cc'd my old motor with motronics with his un-cut head. We did two cylinders and we got 9.6 and 9.7 for them. The 9.7 is most likely the better number as we took extra effort to get the motor exactly level for that one. So if you are planning to shave a head to get a certain compression ratio with a set of motronic pistons you will now be able to calculate the correct amount to remove.
 

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Comp Ratio

Ed, what dome volume and head volume did you measure? From my somewhat cryptic notes ( I need to keep better records) I recorded a motronic dome volume of 39.7cc and a head volume of 87cc which have me a CR of 9.8 with a 1.5 mm head gasket.

Regards,
Don
 

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Here is a thought I've had for many years that may be usefull or perhaps not. Years ago (mid-60's) I worked with a lot of 1600 heads. At that time, I noticed that all heads NEW were not the same height. Close, but not the same. As these heads became race engine heads, I cc'd a lot of chambers, and all the NEW 1600 heads that were different heights, had chambers that cc'd close to the same. What Alfa was doing AT THAT TIME was making chamber volumes close to the same, regardless of head height, to give the SAME compression with the SAME cast pistons of the day. Ok, so what? I do not know if 1750, or 2L heads were built in the same way. Never saw enough NEW ones to measure all that again. It may be that the 10.1 Motronic pistons are 10.1 in some standardized chamber 2L heads, but not in ALL 2L heads.
Just a thought from times past.
 

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On the picture you see on the left Motronic Piston complete with rings 622 Gramm,ORGINAL MOTRONIC piston for ALFETTA !
right ,Spider Serie 4 piston kpl. mit Ringe u. Bolzen 628 Gramm ,this is sold nowadays as Motronic Piston !!

Quiet a bit of a difference.
I do have the Alfetta piston And I do have real 10:1 whereas the Series 4 normally only has 9.5:1

Salutations

Bernhard

PS My head is unshaved or better still in original thickness ( wish my english would be better)
 

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Discussion Starter #5 (Edited)
Ed, what dome volume and head volume did you measure? From my somewhat cryptic notes ( I need to keep better records) I recorded a motronic dome volume of 39.7cc and a head volume of 87cc which have me a CR of 9.8 with a 1.5 mm head gasket.
We measured the combustion volume between the piston and the cylinder head at TDC by sealing the top of the pistons and the valves with grease then added ATF through the plug hole until it just reached the bottom of the plug hole. We made a special dipstick to determine exactly when the fluid reached the bottom of the hole. The volumes were measured at 56.4cc & 57.0cc. I used the same technique to determine how much metal to shave from the top of my Venolia pistons to get 10.6 in my latest engine.
 

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CR Calculations

The method I use is to place the piston sufficiently low in a liner that a plastic plate with a hole will seal the top of the liner. I then make a measurement of the depth of the piston which is also sealed with grease, a little math will give the expected volume to fill with fluid. I then measure the amount of fluid to fill and subtract.... this gives the dome volume. Essentially the same method is used on the combustion chamber.

My measurement and calculations gives a combustion chamber volume of 55.6 cc so we are in very close agreement.

Regards,
Don
 

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I bought 79mm Spruell race pistons for my 1600 Super, alleged to be 10.5, and with a virtually uncut head and normal head gasket, measured at about 9.7. Norman Racing says this is pretty normal for such pistons, the makers err on the conservative side, and when "one size fits all" blanks are used for more than one specific application, the dimensions aren't held that exactly.

For race use I'd expect the piston makers could rely that the builder would check and measure, and that when you buy a particular CR, you want that CR. Unlike for street use, where they're often just dropped in without checking as long as they work. I'm surprised re the Motronics too.

Andrew
 

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It's worth including in our consideration the different sources for "Motronic" pistons. The pistons used in my new engine came from Turkey but there are several other sources of "Motronic" pistons. I bought these because Borgos were apparently no long being made. While I was happy with their build-quality, I also wondered about whether they were really 10.1 pistons. Since the head I used had been true-cut about 3 times, my guess is that I'm somewhere close to 10.1 at the moment.
 

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Discussion Starter #9
I bought these pistons that we measured from Highwood Alfa in the UK about 10 years ago. I am pretty sure that they came in Borgo or federal Mogul packaging.
 

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Norman Racing says this is pretty normal for such pistons, the makers err on the conservative side
To be honest, I don't find it surprising that a piston advertised at 10:1; in a stock, un-blueprinted motor is actually less than advertised. I have never come across a piston compression rating that is spot on without blueprinting the motor.
 

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Discussion Starter #11
To be honest, I don't find it surprising that a piston advertised at 10:1; in a stock, un-blueprinted motor is actually less than advertised. I have never come across a piston compression rating that is spot on without blueprinting the motor.
I think that some of it is hype. I raced a 500 Triumph motorcycle in the early 70's and their top performance pistons were advertised as 10.75 to 1. I cc'd them and they were about 9.8 to 1. This stuff has been going on for a long time.
 

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It's like the myth of E=MC2, The protons and neturons of the heavier elements weigh the same as the protons and neutrons in the lighter elements they are made from. So where is the mass that is converted to energy coming from? It's not from mass but rather from the binding energy holding the quarks together being liberated. A cute little trick the particle physics folks like to play on the rest of us.
 

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And that explains why the Venolia replica GTA 79 mm pistons (3 ring from 1972) are both heavier and softer than the identical pistons from JE in my other engine. Gotta be that binding energy!
(Neither of these pistons bind, but the old, 80 mm 2 ring short skirt GTAm forged pistons without expansion control inserts, would bind as they rocked in the bore, until they were heated up and became round! Never run those engines hard when cold! You liberated a lot of money to say nothing of quarks if you did!)
Just my opinion as usual.
 

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On the picture you see on the left Motronic Piston complete with rings 622 Gramm,ORGINAL MOTRONIC piston for ALFETTA !
right ,Spider Serie 4 piston kpl. mit Ringe u. Bolzen 628 Gramm ,this is sold nowadays as Motronic Piston !!

Quiet a bit of a difference.
I do have the Alfetta piston And I do have real 10:1 whereas the Series 4 normally only has 9.5:1

Salutations

Bernhard

PS My head is unshaved or better still in original thickness ( wish my english would be better)
Hi Bernhard,

Apologies for reviving an old thread. Can you adivse whether the oil control rings are the same between the 2 pistons?

Thanks,
Ryan,
 

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Hi Bernhard,

Apologies for reviving an old thread. Can you adivse whether the oil control rings are the same between the 2 pistons?

Thanks,
Ryan,
Sorry I do not know. But the alfetta motronic pistons are sold again by Bertelsbeck in Germany. Maybe you ask them.

Salutations from the Baltic Sea

Bernhard
 
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