Milling head on 1500cc engine

Discussion in 'Discussion Forum' started by beezee, Jan 24, 2019.

  1. beezee

    beezee True Classic

    Looking for some opinions...

    I want to increase the compression ratio on my 1500cc FI engine, and to do this the easiest way seems to be milling the head. I have a spare head so I wanted to determine how much to shave off to achieve a 10:1 static CR. Here are my calculations:

    Bore = 8.64 cm
    Stroke = 6.39 cm
    Cylinder Area = pi x rad squared = 58.63 square cm
    Displacement = Area x stroke = 374.64 cc

    Since the stock CR for this engine is 8.5:1, the combustion chamber volume is 374.64 cc / 8.5 = 44.08 cc

    To achieve 10:1 the combustion chamber volume must be 17.46 cc, a reduction of 6.62 cc. To do this I need to shave down the combustion chamber circular recess. The recess measures 8.7 cm. The area of the recess is pi x rad squared = 59.45 square cm. To achieve 6.62 cc I need a depth of 0.111 cm (or 1.11 mm). In imperial this is 0.044" .

    So to achieve 10:1 compression ratio I need to mill the head by 0.044". Does this seem reasonable? Most of the information I can find suggests milling the head to remove the recess, which is around 0.060" which would be too much by my calculations.

    Is 10:1 a realistic compression ratio?

  2. Mark


    Sounds very close to what I did on mine. Sorry, I don't have any measurements handy, but I shaved it just shy of removing the decompression ring. Runs fine on pump gas. You'll want to recalibrate the cam timing with an adjustable sprocket.
  3. JimD

    JimD Waiting for Godot... Moderator

    Missouri, USA
    Let me preface this by saying I know nothing. :) I got the 1500 FI performance head that MWB offers. I will let you read thru the full specs, but it says their milling results in "nearly 10:1 compression. You can read the rest of the description to get the specifics, since you will probably understand it much more than I do. I just wanted to post this picture of the milled surface, which is a bit better than the pic on MWB's site.
  4. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    Before I say anything, let me be clear that I am not questioning Beezee's info nor anyone/anything else. And I know less than JimD. :) So I'll try to qualify my questions as I go.

    I've wondered about the accuracy of the US spec 1500 stock compression ratio. Beezee, were you able to determine this yourself? Is the US spec 1500 head actually 8.5:1? Has anyone done the measurements and calculations to confirm it? Did it differ for various years (for example 10 bolt vs 14 bolt) or versions (eg. Calif smog spec vs other markets) or other reasons (eg. carb vs EFI)? Fiat seems to have been netorous for changing things regularly and without documentation. The factory manual has been found to be wrong in MANY places. The Euro spec engines are completely different, head gaskets varied, other design differences exist over the years, and likely lots of other changes happened. Is the stock 1500 compression ratio a known? I don't recall ever seeing much confirmation about it. In Steve C's manual he has confirmed such data for the Euro spec engine. But has anyone done that for the US spec models?

    I am honestly asking this. There is no intent to contradict anyone or to be offensive in any way, I really don't know. Hopefully someone can please enlighten me. I'd like to use accurate numbers to help determine my selection of (MLS) head gasket thickness for the turbo build, where the compression ratio makes a big difference in reliability. Thanks.
    Frank L. Di Gioia likes this.
  5. Mark


    Certainly, the correct method is to measure everything; not to work backwards from an assumed compression ratio. But going off reliable info such as provided by MWB is better than just winging it. I cc’d a bunch of heads years ago when I installed an 1100 Yugo head on my 1500, but can’t put my hands on it.
  6. fiatfactory

    fiatfactory Steve Cecchele

    Western Australia
    Your compression ratio calculation is too simplistic.

    A basic compression ratio calculation might look like this:

    Volume at BDC is (swept volume - pop up volume )+ flycut and ring land clearance volume + gasket volume + chamber volume.

    Volume at TDC is (volume in gasket - pop up) + flycut and ring land clearance volume + chamber volume.

    Volume of BDC over Volume at TDC is expressed as a ratio to one (xx:1)

    European 1500 X19 was 9.2:1 static compression, and shares the same basic piston crown shape and CH values as the USA specification engine. Unfortunately with the 1.5mm full circular decompression recess that the US spec engines have this drops to just 8.5:1. North American Spec engine with the 1.5mm full circular decompression recess in the head, we substitute the (**nominal) 33cc chamber volume for 40.5cc, so simply add 7.5 cc to the BDC/TDC volumes (1.5mm full cicular decompression recess has a volume of approx 7.5cc)

    For a standard Euro spec 1500 X19 (1498cc)
    (86.4mm bore and 63.9mm stroke)
    swept volume = 374.8 cc per cylinder
    Head gasket volume = 7.44cc (approx 1.5mm thick crushed) 1.65mm new ASTADUR type*
    Volume in head = 33.5cc (nominal)
    Volume in valve reliefs = 4.75cc (cast relief-nominal)
    Total at BDC = 420.49cc
    Total at TDC = 45.69cc
    Compression ratio = 9.20:1

    so for US spec we get
    Total at BDC = 428cc
    Total at TDC = 53.2cc
    Compression ratio = 8.04:1

    I don't think it was 8.5:1 to start with for US spec engines...

    **The combustion chamber volume needs to be measured by direct measurement. Approximately 33.5cc in the combustion chamber is a 'nominal' figure. The actual combustion chamber volume will change depending on variables like valve seat height and valve manufacturer so the only way to calculate it properly is to measure accurately for your particular head. Actual chamber volume for a Euro 1500 head is around 33cc + - about 0,5cc and the extra CC or so is to allow for the piston top ring land volume, which is something many standard calculations fail to mention. The piston is a smaller diameter above the top ring, and this gap has a volume which contributes to the chamber volume at TDC. When the piston is sitting at zero deck, I usually figure this in at around 0.75cc for an 87.0 bore SOHC. This also affects any volume figure you might get for a 'pop up' of the piston above the deck surface, most times it also has a chamfer or rounded edge which further alters the volume figure.

    maybe this clears it up.

    PaulD, dllubin and Dr.Jeff like this.
  7. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    Steve, thanks for the info. But please help me to understand correctly. For the US spec 1500, are you saying it was 8.04:1 originally and went to 8.5:1 later? Or are you saying the calculations indicate it is 8.04:1, despite the claimed 8.5:1? I'm not sure I completely followed your explanation for the US engines. I believe you never get our spec engines or even a sample head over there? But appreciate if you can add more clarity, because this is exactly the confusion I've always had on this. Seems the US specs are a bit vague in general and I don't trust the factory manual's data.
  8. MikeHynes

    MikeHynes True Classic

    Goodfield, IL.
    Did you miss the fact that the piston is not flush with the top of the block at TDC?
    Milling the block is an effective way to increase compression ratio on the SOHC engine. But just like milling the head there are things that you have to compensate for, such as the height of the head locating bushings.
    Be careful how much you mill the head, you can mill it so far that you end up with a deck surface that is too thin to seal properly in operation. I've seen this happen when the head deck surface was milled flat on a badly warped head.
    Last edited: Jan 24, 2019
  9. Eastep

    Eastep True Classic

    Saying that the math indicates Fiat was either overly optimistic or they liked about the numbers. Perhaps the factory used another method of calculation or omitted something in their calculation (like head gasket volume, for instance) ?? Advertising basically 8:1 compression on a sports car isn't good business ;)
  10. fastx19

    fastx19 Administrator Moderator

    Santa Clara, CA
    Here is my suggestion on this. Don't get too caught up in the number. Mill the head to remove the piston recess, get yourself a nice cam, put it together and enjoy how this motor should have come from the factory. Sure, you can get into a lot more specs on this, but in the end the difference will not be noticeable for you. Remember, you will also need a 1300 belt tensioner pulley as well to make up the for loss of the head height. Anything more than this and you are going to start getting into the area of more race engine that hot street engine. Plus, I assume you would like to put this together and drive it for a number of years before having to go back in, correct?
    Ulix and Jepp78bertone like this.
  11. ghostdancing

    ghostdancing True Classic

    you can try to source an euro head, they are rather easy available here..maebe not cheap to get it posted to US though
  12. beezee

    beezee True Classic

    The compression ratio of 8.5:1 is what is stated in the factory service manual. If this number is incorrect, and is actually closer to 8:1, then my calculations are incorrect.

    MWB claims that machining away the recess in the head (0.075) will realize a CR of almost 10:1. Assuming that their claim of 10:1 is correct, and I for one do not doubt this, it means that the factory claimed stock CR of 8.5:1 is incorrect.

    So my plan going forward is to mill the head just enough to remove the circular recess, massage the ports a little bit, install new valves with a 3 angle cut on the seats, and install a slightly hotter cam. The goal is to retain the stock fuel injection and to keep things reliable. I have always used high octane fuel, not so much for the octane rating, but to avoid ethanol (all of the regular pump gas in my area contains ethanol).

    I have looked into the Euro head with it's higher CR, but it is not a "bolt on" option for a FI motor as it is missing a sensor hole and some other mounting boss that I can't recall. I already have a spare head so milling it seems like the easiest and most cost effective option for me.

    Out of curiosity, what sort of HP gain could be expected?

    Thanks for all the input folks, it is much appreciated.

  13. Pete Whitstone

    Pete Whitstone True Classic

    McKinney, TX
    If I recall correctly, the old school formula for estimating this is a 4% hp bump for each point of CR. So for a stockish 75hp engine, you are looking at a 6hp bump. But that's just from the CR. With the cam and some good port work, you might see up to 20hp depending on many many factors. What's your exhaust situation?

  14. ghostdancing

    ghostdancing True Classic

    this makes sense (forgot that yankee cars was injected since so early) anyway the euro1500 (cr 9,2:1 if i recall correctly) are rated for 85 hp at the crank so you should achieve an higher figure
  15. Pete Whitstone

    Pete Whitstone True Classic

    McKinney, TX
    I believe the Euro cams were different as well, I don't think all 10hp of that increase comes from compression ratio alone.

  16. Jepp78bertone

    Jepp78bertone True Classic

    I'm doing this next week!
  17. Rupunzell

    Rupunzell Bernice Loui

    Reduction in distance between the cylinder head to block via material removal from the block or cylinder head can and will affect the cam timing. As more material is removed between the block-head, the tensioner bearing will become the wrong diameter rendering the tensioner impaired to non-functional and will need to be addressed to correct this.

    More than 0.050" removed from the base of the cylinder head increases the risk of head gasket failure.

    Avoid pop-up pistons to increase compression as the pop up negatively affects gas flow and burn pattern within the cylinder.

  18. Right: it is the compression ratio + the higher lift camshaft + the 4:2 exhaust manifold.

  19. ghostdancing

    ghostdancing True Classic

    actually there were different state of tune of this motor for different models, ritmo 75 was a 1500 with single choke carb rated for 75hp, while ritmo 85S, regata 85 and lancia delta 1.5 had the same motor with twin choke carb, different cam (dont know about CR) and these were 85hp; all these cars are from 78 to 86 circa; note that FI in italy was widely introduced together with catalytic converters only in 94 due new pollution regulations
  20. Dr.Jeff

    Dr.Jeff True Classic

    Sin City
    It's also missing the scallops in the intake ports for the injector tips/spray. If you already have a spare head, then definitely the way to go. There is a lot of good advice here that should help confirm your plan. And over the years many others have done the same with good results.

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