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Dino compression?

Discussion in 'Technical Q&A' started by John Corbani, Jun 23, 2005.

  1. John Corbani

    John Corbani Formula 3
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    May 5, 2005
    1,153
    Santa Barbara, CA
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    John Corbani
    Just rebuilt a 1972 Dino 246GT(#0504, American specs) and am getting what seems to be very low compression readings. 115 psi average, 5 psi range from high to low. Leakdown is fine so rings and valves are doing their thing. Have 3500 miles since overhaul and it runs strong. Cams come on violently about 4000 and torque holds all the way to red line. Do not know if either pistons or cams are stock. Car now is at 184,000 miles and might have been raced.
    I have no history but engine had been apart and wheels/tires had been changed by the time I bought it at 51,000 miles in 1986 . I never did a compression test so do not know what is normal. Have the books but can not find any mention of compression readings.
    Would apreciate comments on what is stock normal compression and what might be the effects of either piston or cam changes. No complaints since performance is very very good; but curious.
    Thanks, John
     
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  3. Iain

    Iain F1 Rookie

    Jan 21, 2005
    3,222
    UK
    Bad compression tester? I'd try again with another tester. Also were these tests done hot or cold? If only at cold you might try doing one on a warm engine & see what you get.

    If that produces the same result dump a teaspoon of oil into each bore just before you do the test - if there is a ring problem that will show you because it will provide a one-off temporary seal & you should see a much better result.

    At 115 psi I wouldn't have thought the thing would run that strongly so I would err on the side of a duff tester & maybe a test done when the engine was cold.

    I.
     
  4. John Corbani

    John Corbani Formula 3
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    May 5, 2005
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    Have tried testing hot and cold, with and without oil, throttle open and closed, two testers. 115 to 120 psi is it.
    What is normal?
    Car runs strong. Needs platinum/iridium plugs to start easily and idle cleanly. Carbs have stock jets all around. Has been like this for 19+ years so no big deal.
    Had brake booster vacuum line fail and burned a valve on 3. Decided it would be prudent to tear apart everything after 181,000 miles. After complete rebuild with bearings, rings and new exhaust valves, I have it back to my original condition and it runs like it did when I bought it. 3,000 miles so far and still chasing oil leaks.
    There is a possibility that pistons and/or cams are not stock. I would like to find out what is normal.
    Thanks, John
     
  5. mikeyr

    mikeyr Formula 3

    Jun 17, 2004
    2,155
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    Mike Rambour
    my Dino's (also in Goleta) compression is cylinder's #1-5 155lbs. +/- 10, #6 has 90lbs, it runs good is not smoking but #6 plug is always wet with oil. Someday I will get to rebuilding the motor since it also has low oil pressure at idle indicating worn bearings. So just from my experience I would say your compression is low but I have seen your car hauling down the road and its not smoking either, keep driving it, I like seeing it on the road.

    Did Scott's do the rebuild ? I would think he would know what the proper compression should be since he has done other Dino's. I do my own work so I only have my car as the example. Others will chime in I am sure with more authoritative responses
     
  6. John Corbani

    John Corbani Formula 3
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    May 5, 2005
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    Thanks Mike. 150-175 psi was what I was expecting. The 115 put me off but the way the car goes sort of begs the question. I can go up San Marcos Pass in 5th and accelerate anywhere so what is normal? The car has never "pinged" so I have often wondered if the previous owner changed to a lower compression piston so the car would run on regular gas. I have tried regular a few times and no problems but always gone back to 91-92 for insurance.

    Scott did the rebuild based on my watching his work on a Fiat Dino. The detail diferences between the Fiat and the Ferrari are in the thousands. My Dino was his first. It was and is a learning experience.

    I have done all the work myself as long as I did not have to pull the engine. Heads were done about 10 years ago when car could not pass smog test. That I could do. Import Auto did the machine work. I got it all back together with no problems.

    Be happy to swap War Stories. I am in the phone book. Call and stop by.

    John
     
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  8. lm2504me

    lm2504me Formula Junior
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    Aug 26, 2004
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    Richard
    John,
    I am still in the process of rebuilding my Dino 246GT motor and heads (waiting for parts from England and Germany to arrive), but I rebuilt a Fiat Dino 206 motor late last year. The compression readings on that motor ran ~180+ psi all cylinders after 300 miles. This was using stock components.

    115psi seems to be very low for a Dino motor. I would expect the 2.4 to be closer to 180psi ,also. Unfortunately, it will be several months before I run my motor since I am renewing the coolant, fuel, brakes, and electrical system. I just had my radiator recored to a 125 tube core, expansion tank completely resoldered, oil cooler ultrasonic cleaned internally, and alternator rebuilt. More work to come.... This is going to be a driver Dino, not show car. Mechanicals this year, driver level cosmetics next year.

    I live just north of Santa Maria and would like to visit to check your Dino out or if you head up north near Nipomo give me a call. I've helped Mike work on his Fiat Dino Spider. I have owned 2 Fiat Dino Coupes and 1 Fiat Dino Spider in the past, then I pulled the trigger on the Dino 246GT. PM me for phone numbers. You will also have a number to call if you need any help while driving the Dino in the Central Coast area.

    Mike, let me know if you need help putting the transmission on or anything else.

    Regards,
    Rich
    Dino 246GT#03510
     
  9. John Corbani

    John Corbani Formula 3
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    May 5, 2005
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    Took a good look at the top of the pistons. They are stamped as follows:

    502
    MONDIAL
    PISTON
    A 2

    What is the diference between Mondial and Dino pistons? Does anyone know how to get mechanical drawings of each (or either) piston so I can figure out what (if anything) is going on? These are the pistons that were in the car when I bought it in 1986, 130,000+ miles ago. Maybe some one has one of each and could get a few basic dimensions. Diameter is fine, pins were OK, new rings fit, valves don't crash. Only problem seems to be low compression readings. Curious.

    John
     
  10. lm2504me

    lm2504me Formula Junior
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    Aug 26, 2004
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    I think Mondial is the manufacturer of the pistons. I copied the below from Mahle's website on their Milestones:

    "1987 - Acquisition of Mondial Piston S.p.A., now MAHLE Mondial S.p.A., Italy"

    Are your pistons flat top or raised?

    Rich
     
  11. Gary48

    Gary48 Guest

    Dec 30, 2003
    940
    John, The stock com. ratio of the Dino is 9 to 1. I just recently overhauled my 72 246 VIN 03714 and the head gasket is .070 thick, pretty thick for a near zero deck motor, which would increase the com. ratio if it was thinner. You mentioned that you thought that your motor might have been modified(i.e.) bigger cams. If this is the case then the increased overlap of the bigger cams ( both intake and exhaust staying open longer at the same time) would then bleed off some of the compression. Is this a possibility with your motor?
     
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  13. John Corbani

    John Corbani Formula 3
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    May 5, 2005
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    Pistons tops are raised with valve cutouts. No easy way of telling about the cams. My original book says 9 to 1 ratio too. It is possible that everything is normal; that is why I asked for typical readings from US cars. I believe that some European Dinos were over 10 to 1 so there are many variables. I do not think that Fiat Dino engines were all that close in detail. Thanks for all the feedback.

    John
     
  14. miked

    miked Formula Junior

    Feb 7, 2001
    653
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    Mike Dawson
    Besides the usual problem areas, bad rings, bad valves/seats, leaking spark plug, head gasket etc., there is the posibility that the cams are not timed correctly or you may have a set of hot cams. If so they may not generate enough compression at cranking RPM. Your description of how the power comes on is typical of a racing spec cam on the street. If you do have long durations and overlaps the intake charge can go back out the still open valves instead of being compressed at low cranking speeds.
    Just another possibility to consider, and it would be much better than worn parts. :D
     
  15. mikeyr

    mikeyr Formula 3

    Jun 17, 2004
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    Actually they were the same engines, the reason you saw a difference is that the FIAT Scott's did was a 206 (as is mine) and not a 246, but they are the same engines. The FIAT Dino 2.4 is a Ferrari Dino 246 motor, and the 2.0L is the Ferrari Dino 206. Scott's did a FIAT 2.0 prior to your 246 and yes lots of difference there.

    I really question your low compression reading since you say the car runs so well and I have seen your car on the road and it does not smoke, I know you mentioned 2 different compression testers but I still think with the readings you have there should be visual signs. Just by chance were all the plugs pulled when you did the tests ? or were the plugs pulled one at a time leaving the other 5 in ?

    Have you personally pulled the valve covers off and checked the valve timing ?
     
  16. Rifledriver

    Rifledriver Two Time F1 World Champ

    Apr 29, 2004
    27,086
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    Probably half of the 206/246 cams on earth have been reground, many more than once. That being the case I would not trust the original marks to be in the correct locations relative to the current lobe location. I would degree the cams to be sure of the correct timing.

    Also you say the car runs strong. Have you recently driven another good 246 or had another 246 owner drive yours as a basis of comparison? Better yet do a 4th or 5th gear roll on with another car and see how it stacks up.

    115 is simply not good compression if it is a true number.
     
  17. John Corbani

    John Corbani Formula 3
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    May 5, 2005
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    Thanks to all who have replied to this thread. The reason I questioned the compression readings is because they were very unusual in my experience. All seem to agree. I still think the car is as strong, or stronger, than when I bought it. Before rebuild, #3 had a burnt exhaust valve and the car was still no slouch running mostly on 5.

    I want to know what is normal and what happens when someone plays with cams and/or pistons. I have tried all standard methods of reading compression (for new and run-out engines) and all come up the same. I personally checked cam timing, etc. It is back together in reasonable shape. I have no means or intention of checking cam profile or exact profile timing with reference to marks on cams..

    Leak down is fine. Engine has 3500 miles since rebuild. I did compression test just to see if high oil use and some smoke had something to do with valve seals, valve seating or rings. Smoke may be from stuck float valve in one or more carburators (found and fixed one Sunday, less smoke!) Oil loss may be from 2-3 places we have found (One fixed, still working on others).
    This still leaves compression as a puzzle. If we can get a range of what is normal, other Dino owners may have an easier time (or at least, an easier mind.)

    Again thanks, keep the info coming.
    John
     
  18. mikeyr

    mikeyr Formula 3

    Jun 17, 2004
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    Well I went and saw this Dino today, it does smoke a little, its really minimal smoke and easily ignored it is so minor at startup or idling but I agree with John, it should not be there with a new motor. As far as the high oil use, I think its best to see what happens after John's mechanic works on some of the oil leaks this week.

    I must say the car ran great, when John got on the freeway and revved it up there was no odd sounds from the motor. When I drove the car, I did not rev it high because well, its not my car :) but the thing was quite impressive with its power. I was able to take off from 3,000RPM with no hesitation to just under 6,000RPM in any gear and accelerate quite smartly. Oil pressure was very good throughout RPM range.

    It short it runs just like a Dino should, I gently questioned the compression readings but one gauge is new and both gave the same readings. Its a little puzzle.

    I guess he needs to know how to easily measure up the cams to see if they are stock or modified cams, I know its easy to do acurately with them in the car but a rough check would work. But in my opinion the valves could account for low compression but not oil loss. Those could be leaks which there are a few.
     
  19. Verell

    Verell F1 Veteran
    Consultant Owner

    May 5, 2001
    7,000
    Groton, MA
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    Hmm,
    The results are confusing:

    -Good leakdown test - Usually indicates that both rings & valves are sealing properly.

    -low compression, doesn't change with oil in cylinder - usually indicates a valve or valve timing problem.

    Combining the 2 does seem to leave cam timing as the likely area.

    - compression consistency across all cylinders - indicates a common cause.

    -Signs of oil smoke on startup & idling - These are max. intake vacuum conditions, could be pulling oil past the valve seals.

    Is there noticable smoke while accelerating after coming to an idle? If so, again indicative of valve seal leakage.
     
  20. frisbee54

    frisbee54 Rookie

    Apr 1, 2004
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    Napa CA
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    Douglas Morgan
    Did you make sure you had the throttle fully open while cranking engine over for the comp. test? I know that on my 79 308 that made a huge difference in test results. Doug
     
  21. Fiat Dino 206

    Fiat Dino 206 Karting

    Apr 19, 2004
    144
    Mississippi
    Full Name:
    David
    Crap fellows: I have been waiting for the engineers that are on this site to jump all over this post with all kinds of calculations of the cranking compression PSI that should be expected from a freshly rebuilt Dino 246GT engine. I guess that you guys were waiting for me to post an answer, and then jump all over me and tell me why you think I’m incorrect. Sooo … here goes:

    Please don’t be offended as I am in no way criticizing you or your testing methods … I am simply dumbfounded by your results as I cannot see a combination of “low compression” piston being the culprit nor can I see that a high lift, long duration cam would create such numbers.

    I calculated the cranking compression using the values in the workshop manual and other sources and came up with the following number: 191.839psi. This would be the high number based on a “perfect” engine with all stock parts and is probably higher than one might see in a real life situation if for no other reason than the calculation was made with “sea level” as the altitude factor … if you use the bottom of the Dead Sea as an altitude factor the psi will go up and if you use Mount Everest as the altitude factor, the psi will go down and still be “normal” for that environment. A Dino tested in Denver will have a lower “normal” cranking compression psi than the same Dino tested in Key West.

    The “normal” values that you might be looking for on a standard engine that has just been rebuilt and that has been properly and completely broken-in would be in a range from about 175psi up to approximately 190psi.

    There is an old “rule of thumb” that allows one to estimate a range of cranking compression in a stock gasoline engine if one only knows the piston compression ratio. The stock piston ratio on the 246GT is, I believe, 9:1. The “rule of thumb” is that the cranking compression should fall within a range of 15.5 times piston compression ratio and 21.5 times the piston compression ratio.

    15.5 x 9 = 139.5psi
    21.5 x 9 = 193.5psi

    The 21.5 times the piston compression ratio would be for a perfect engine
    The 15.5 times the piston compression ration would be for a well worn engine

    Regarding the cranking compression test:

    I would ask you about the cranking speed of the engine and the number of revolutions that you allowed each cylinder to make during the test.

    Regarding the power coming on “violently” at around 4000rpm:

    I might ask you about the condition of your intake cams and the condition of your distributor timing advance.

    I ask these questions as I see no explanation for the cranking compression to be well out of what one might call the “normal” range and experience no significant change with the throttle open or closed or when the engine is cold or warm or when the cylinders are tested dry or wet and still have a leak-down test with results that are, as you state: “okay.”

    I’m hoping that some of the qualified technicians and/or engineers will come in on this discussion and give you a hand.

    Best wishes
     
  22. Fiat Dino 206

    Fiat Dino 206 Karting

    Apr 19, 2004
    144
    Mississippi
    Full Name:
    David
  23. John Corbani

    John Corbani Formula 3
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    May 5, 2005
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    Thanks, this is interesting and maybe points to why I have what I have. The compression computer is neat!

    I plugged standard Dino numbers in (9-1 compression, 60 stroke, 118 rod lengh, 272 intake duration). Got 176 psi compression. Went to 300 intake duration and got 117 psi compression. Bingo?

    No other variable had such an effect.

    I may have a wild set of cams; sure does come on strong above 4000. I can live with it! Only problem is requirement for platinum/iridium plugs for easy starting and smooth idle. No sweat. Have been living with that for 19 years.

    I still would like for some Dino owners to let me (and us) know what is typical, normal, unusual, stock, modified, etc.

    Just to answer all of the other questions people have asked about tests:

    All plugs out, throttles blocked open, 8 compression cycles, two gauges, all cylinders. Just to be sure, put oil in only one, 4psi change with oil, forget it.

    Leak down done with Snap-On EEPV309A00 gauge, 100 psi at top dead center, all in the green, 16 to 28 % was range of readings. Gauge has 40% being the start of the yellow or doubtful range. Valves seal, rings seal!

    Again thanks for the feedback,
    John
     
  24. Gary48

    Gary48 Guest

    Dec 30, 2003
    940
    John, other than using a degree wheel and dial indicators, how is the bottom and low end driveability? I have a 246 which all indications say it has stock cams as it does not behave like yours at 4,000 rpm, but rather has very good bottom and mid range driveability but has good pull all the way too 8,000 rpm. Another thing I get is a very nice exhaust resonance at 3,000 rpm, so much so that around town I seek this rpm by dropping gears whenever necessary to maintain this melody. Does your Dino do some of these things? Certainly the 4,000 rpm "kick" would be a cammed up indication and the lower compression numbers if all else being sound, would mean greater duration and more overlap than stock.
    The smoke thing has me puzzled. When I rebuilt my Dino it too was smoking and after checking stock clearances, the piston to cylinder clearance was .007-.008, way too much for a modern engine with modern pistons. We replaced with JE pistons and set the starting clearances at .0035 and have experianced no smoke. This clearance keeps the piston more stable in the bore which does the same for the rings. When you rebuilt did you re-ring only? Just a few thoughts
     
  25. Fiat Dino 206

    Fiat Dino 206 Karting

    Apr 19, 2004
    144
    Mississippi
    Full Name:
    David
    "Leak down done with Snap-On EEPV309A00 gauge, 100 psi at top dead center, all in the green, 16 to 28 % was range of readings. Gauge has 40% being the start of the yellow or doubtful range. Valves seal, rings seal!"

    The gauge must read in KioPascal (KPa) rather than percent

    1psi = 6.8948 KPa

    therefore, 28KPa would convert to 4.06psi or 96% which is good for a cylinder in a roadcar.

    If you have the parts manual, can't find mine ... check the cams out. I think one has a FIAT number and two have Ferrari numbers? Can't remember, but I believe that there where three cams offered through the production. The two Ferrari numbers have a different lift and different lash adjustment than the Fiat numbered cam.

    Wish the engineers weren't hiding!

    Best wishes
     
  26. mikeyr

    mikeyr Formula 3

    Jun 17, 2004
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    yeah, where are they hiding ? I can answer some recent questions that I can until John answers them.

    I am pretty sure they are new pistons, I think John said they were new. And when I drove the car last week it had very good low end, even accelerating from low RPM and lugging it still accelerated sharply with no problems. It sounded very good.
     
  27. Fiat Dino 206

    Fiat Dino 206 Karting

    Apr 19, 2004
    144
    Mississippi
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    David
    That should read KiloPascal (KPa) ... can't read my own typing.

    Best wishes
     
  28. John Corbani

    John Corbani Formula 3
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    May 5, 2005
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    Thanks for the input. Pistons are new, old pistons were masters, only diference is suposed to be in OD. I have old pistons and they have the "MONDIAL" marks I referenced in the earlier posts. Those pistons have none of the marks referenced in the Dino book. As I said, car has had a checkered life. When I bought it, it had a 10 1/2" OD MOMO steering wheel. That was fast, all right, but I replaced it with a 14 1/2" OD MOMO duplicate of the Dino wheel. You could then see speedo and tach and your arms did not get sore so easily. The car almost surely had been raced. The engine had been apart. Red silicone was on pan/gearbox gaskets.

    It was STRONG! Has been for the last 19 years. Burned exhaust valves have happened. When I could not get the car to pass smog 10 years ago, I re-did heads and replaced 3 exhaust valves. Cured the problem. One was cracked, couple burned. Noticed that rear bank exhaust cam showed wear on #3 but put it back in. On this rebuild I tried to pick up as many of the old problems as I could.

    Cams were selectively hard coated and reground to what we think was original contour. Each cam had at least one lobe that had .010" to .030" worn off the peak. You could see where hardening stopped.

    Engine now runs about like it did when I bought it. Distributor has been kept repaired every 3-4 years. Only "real" problem is that I checked compression when trying to find where the smoke was coming from and where oil was going.

    Numbers shocked everybody but seem to have nothing to do with current problems..

    So far we have found most of the oil leaks and found a stuck float valve in #1 carb. We are getting there but it is a learning experience! Will keep you all informed.

    John
     

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