Close



Page 1 of 3 123 LastLast
Results 1 to 25 of 73
  1. #1

    Default Compression Ratio, Octane, & Timing

    Okay, so my new engine has "increased" compression over a stock, let's say GT40P engine. Stock spec I could find was 9.1:1. How much more? I'm not sure, but both the pistons and head milling are altered vs. stock. Let's say it's 9.5:1 for a guess.

    At what point do I want to start running increased octane fuel? The only chart I could find that didn't relate to boost or have numbers that seemed to be way off said that you want 89 octane at 9.0:1. I don't know what you guys do, but I never ran 89 in my old stock engine (but admittedly it never ran very well--there was a number of reason for that I'm sure though). But that same chart said you want 93 octane at 10:1. So, if I were to run 91 in mine, that would sound about right? Or not? I really know NOTHING about this stuff.

    Then that brings us to setting timing. Do I do that differently given the different octane and compression?

    Thanks
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Transit Connect Titanium LWB
    '14 Ford Fusion SE Manual

  2. #2
    FEP Super Member
    Join Date
    May 2003
    Location
    Roseburg Oregon
    Posts
    3,277

    Default

    With that low compression, regular pump gas will most likely be fine .
    You dont mention if this is an efi or carburetor.
    Timing is another one that will be dependent on several factors .
    clowns to the left of me , Jokers to the right

  3. #3
    FEP Member
    Join Date
    May 2016
    Location
    Lake City Pa 16423
    Posts
    429

    Default

    10:1 with iron heads was normally the limit. The compression ratio and the octane in the fuel is going to determine how much timing you can run. The lower the octane, the lower the timing.

  4. #4

    Default

    I think you should also be looking at DYNAMIC compression ration numbers, not just STATIC.

    There's online calculators you can use to figure dynamic c/r, though I'm not sure how accurate they are. Put in the specs for my engine in different calculators, shows my static as being a touch over 11:1 and dynamic ranging from anywhere from 9.8 to 10.4:1. Not sure why the difference. Also, for some reason they seem to be reading .5:1 higher than when I used these calculators when I built my engine 8 years ago.

    Either way I'm running 91 octane in my 347 with what I think is a fairly stock timing curve (14 deg initial) with no issues.
    Thomas

    1985 Mustang GT - Build Thread
    347 (10.5CR, AFR 185's, PP Crosswind Intake, Custom-ground Comp Hyd Roller Cam, Scorpion 1.6 Roller Rockers, Holley 3310-4), T-5, 8.8 w/3.55's, MM SFC's, T/A, PHB, LCA's, STB, KMB, Bilsteins, MM/H&R Springs, SN95 5-Lug, Cobra Brakes, '04 Mach 1 Steering Rack

  5. #5

    Default

    Yeah, I guess I was vague about that. It's a SEFI 306 with iron GT40P heads. Since I have the two different things that are meant to alter the compression ratio, that led me to wonder if I need to make changes elsewhere. I have no exact numbers as to what those alterations are, but I'm fairly sure it's nothing too radical. The pistons are Racetec #1000336.

    Just so I'm clear, when you say "the lower the timing", that means the higher the number BTDC, correct?

    What will knowing dynamic compression do for me?

    I think I might just run premium anyway in this car so I can use non-ox. I don't want to have to deal with the problems of ethanol.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Transit Connect Titanium LWB
    '14 Ford Fusion SE Manual

  6. #6

    Default

    There just is no way to answer your questions accurately without real info. Even with it, each engine's ability to tolerate timing on a particular octane will vary. Really, just choose the octane level you want to use and keep advancing the timing (that means increasing the amount of advance on the spark before TDC) until it begins to ping under load or it stops making more power. These engine with iron stock-style heads will let you know when you've pushed it too far. They will loudly rattle under load. Wedge heads like these have little detonation forgiveness in them, unlike more modern three and four valve ohc style heads. My Coyote will happily make big power at 11.1 on 87 where my 88 engine says, nao, just nao at maybe 9:1. A part of that is modern engine management but the configuration of head and chamber matters too.

  7. #7

    Default

    Dynamic compression ratio is the amount of compression your engine actually makes when operating. It takes into account valve events and several other engine parameters, not just bore, stroke, piston and chamber volume.

    I have to agree with 4eyedblind though, at then end of the day pick a fuel to run and tune your engine to run on it. That's pretty much what I did. Given my increased compression I chose to run 91 octane to err on side of safety and have been tuning my car to run that fuel.
    Thomas

    1985 Mustang GT - Build Thread
    347 (10.5CR, AFR 185's, PP Crosswind Intake, Custom-ground Comp Hyd Roller Cam, Scorpion 1.6 Roller Rockers, Holley 3310-4), T-5, 8.8 w/3.55's, MM SFC's, T/A, PHB, LCA's, STB, KMB, Bilsteins, MM/H&R Springs, SN95 5-Lug, Cobra Brakes, '04 Mach 1 Steering Rack

  8. #8

    Default

    Quote Originally Posted by 4eyedblind View Post
    There just is no way to answer your questions accurately without real info. Even with it, each engine's ability to tolerate timing on a particular octane will vary. Really, just choose the octane level you want to use and keep advancing the timing (that means increasing the amount of advance on the spark before TDC) until it begins to ping under load or it stops making more power. These engine with iron stock-style heads will let you know when you've pushed it too far. They will loudly rattle under load. Wedge heads like these have little detonation forgiveness in them, unlike more modern three and four valve ohc style heads. My Coyote will happily make big power at 11.1 on 87 where my 88 engine says, nao, just nao at maybe 9:1. A part of that is modern engine management but the configuration of head and chamber matters too.
    Well, see these are things I didn't know. I assume by "real info" you mean actual combustion chamber size and stuff that it would be difficult for me to get?

    So, in the process of getting the timing right, what I want to do is keep advancing it until it pings and then dial it back? Is that how you get best performance out of it? The extent of most of my car knowledge is, here's your spec, set it to spec. I've never gone beyond that into the voodoo type stuff to get a few more ponies out of it.

    Thanks for all the replies so far, by the way. I really appreciate you guys tolerating my ignorance.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Transit Connect Titanium LWB
    '14 Ford Fusion SE Manual

  9. #9
    FEP Super Member
    Join Date
    May 2003
    Location
    Roseburg Oregon
    Posts
    3,277

    Default

    I generally start with 12* before tdc (top dead center) with the spout out at idle with engine fully warmed up .
    Drive the car , getting into 4th gear at roughly 45 mph.
    Mash the throttle and listen for any pinging .
    Wash , rinse , repeat , until i find where it starts to ping .
    Then retard 2* .
    On my carbed car , i use a vacuum gauge , at idle roughly 950 or so as i have a fairly aggressive cam .
    Adjust to get the highest vacuum i can , and retard 2" of vacuum .
    clowns to the left of me , Jokers to the right

  10. #10
    FEP Super Member xctasy's Avatar
    Join Date
    Jun 2012
    Location
    Dunedin 9011, New Zealand, South Pacific
    Posts
    3,350

    Default

    My timing story is about the H code Cleveland 351 2v heads used on the 1981 351M truck engine verses the 1981 G code 351 Windsor HO 2bbl in the Crown Vic Squad cars. 138 hp on 7.9:1 compression Cleveland with Explorer 5.0 210 to 215 hp cam verses a far inferior breathing 302 D8 heads but a well cammed up Marine cam spec knock sensor 5.8 with a feedback 2bbl Variable Venturi carb and 8.4:1 compression.

    Knock resistance is Everything to modern performance. The Windsor GT40 heads might not have 2.05 and 1.64 Cleveland WV valve sizes and curtain area specs and port sizes and volume, but on an engine that might run 50 degrees total advance on over run like the 138 hp Bronco and F-150/250/350 351M option did, the engine post about 25 to 35 hp as the HO for 80 was rated at 173 hp, then derated to 165 hp. None Cleveland headed 351s made 145 hp for 80-81.

    Your base timing can be pumped ( or bumped!) up, and the spark advance in the TFi module adjusted back down. The Duraspark III cars had a crank position sensor, and the 351s that had that system had a knock sensor to "means test" delivery of extra spark advance.

    Down here, the iron GT40 headed Falcon GT's and XR8s with 5.0 and the last iron headed 5.6 had a new calibration for Shell 95 Optimax and a boost to 9.4:1 compression from the standard 9.1:1 the normal 225 hp MN12 and F150 hybrid based export Sequential and 96 on wards Explorer based engines used. The US industry gained permission to raise the octane options after Desert Storm. The control of optional fuel grads since Yom Kipper in 73 was due to the environmental cost of pool oil refinery octane blending. In Australia and New Zealand, we had 97 octane unleaded till 1990, with a stock higher grade unleaded option of 95 from 1996 onwards. The 5.0 loves advance ramp increases and a diet of higher octane. The heads, especially the aluminum variants of the GT40P's have great plug positioning, great knock resistance, and inserts which allow the aluminum to rocket the compression up to 10.5:1 on our 95 or 98 octane if you have any of the HO roller cam durations in the 266 to 270 at lash figures.

    Ford spent time with Honda and Cosworth working through options reducing incipient knock in engine families. The Clevelands. It and the Lima 2000/2300 were the two worst preignition/detonation candidate engines. After the spark plug repositioning with the GT40P and even before then, the wedge GT40 heads like a reasonable amouny of spark advance.

  11. #11

    Default

    I have been running at 14 degrees advanced on mine and always use the highest octane I can find, but my compression is only 8.8 at best. It's like 6 dollars more per tank, max, and I maybe put gas in it once a month, during the summer.
    1984.5 G.T.350 5.0 CFI AOD Convertible (TRX package, loaded)
    Hooker Super Comp Shorty Equal Length Headers
    GT40 heads, Edelbrock 3721 intake
    K&N filter in stock dual snorkel
    Comp cams XE254H, hypereutectic pistons
    catted BBK H-pipe, full custom duals
    Maximum Motorsports caster/camber plates and strut tower brace, 3.73 rear.
    Firehawk A/S 225/55/16 on LMR TRX r390 wheels (street)
    Federal 595 rs-rr 225/45/15 on 10 holes (race)
    Everything else stock and fully functional.

  12. #12

    Default

    Quote Originally Posted by xctasy View Post
    Your base timing can be pumped ( or bumped!) up, and the spark advance in the TFi module adjusted back down. The Duraspark III cars had a crank position sensor, and the 351s that had that system had a knock sensor to "means test" delivery of extra spark advance.
    That sounds complicated.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Transit Connect Titanium LWB
    '14 Ford Fusion SE Manual

  13. #13

    Default

    Quote Originally Posted by ashley roachclip View Post
    I generally start with 12* before tdc (top dead center) with the spout out at idle with engine fully warmed up .
    Drive the car , getting into 4th gear at roughly 45 mph.
    Mash the throttle and listen for any pinging .
    Wash , rinse , repeat , until i find where it starts to ping .
    Then retard 2* .
    On my carbed car , i use a vacuum gauge , at idle roughly 950 or so as i have a fairly aggressive cam .
    Adjust to get the highest vacuum i can , and retard 2" of vacuum .
    That, I can do.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Transit Connect Titanium LWB
    '14 Ford Fusion SE Manual

  14. #14
    FEP Super Member bwguardian's Avatar
    Join Date
    Jul 2010
    Location
    Houston (Clear Lake), Texas
    Posts
    4,430

    Default

    I have a stock bottom end 1985 roller motor. That year was the first year of forged flat top pistons with valve reliefs. I did install an E-303 cam and tweaked the stock 4180c carburetor. The heads are GT40p and the combustion chambers were cc'd at 59cc's. The stock compression ratio with the original 69cc heads was 8.4:1, so I figure the compression ratio with the new GT40p heads puts it at about 9.4:1. I run 87 octane with timing at 15* advance with no issues.

    I tried to look up the Racetec pistons you listed in post #5 and was unable to find anything. Are these flat tops with valve reliefs?
    Last edited by bwguardian; 02-24-2020 at 01:11 PM.
    HAD
    '82 GT monochromatic (red)...black cloth

    HAVE
    '85 GT vert two tone (white on charcoal)...white leather
    '00 F350 two tone (white on silver)...gray cloth
    '00 Excursion Limited two tone (white on tan)...tan leather...wifes ride
    '08 Taurus Limited ice blue...tan leather...daughter ride
    '08 Edge Limited white sand tri-coat metallic...tan leather...other daughters ride

  15. #15

    Default

    Was able to find his pistons listed in this PDF on pg 38. Actually from their Autotech line. 5cc valve reliefs.

    http://racetecpistons.com/wp-content...ec_Catalog.pdf
    Thomas

    1985 Mustang GT - Build Thread
    347 (10.5CR, AFR 185's, PP Crosswind Intake, Custom-ground Comp Hyd Roller Cam, Scorpion 1.6 Roller Rockers, Holley 3310-4), T-5, 8.8 w/3.55's, MM SFC's, T/A, PHB, LCA's, STB, KMB, Bilsteins, MM/H&R Springs, SN95 5-Lug, Cobra Brakes, '04 Mach 1 Steering Rack

  16. #16
    FEP Super Member gr79's Avatar
    Join Date
    Jun 2009
    Location
    SE Michigan
    Posts
    4,093

    Default

    I always set timing by sound, drive feel, max power with fuel econ, smoothness all driving conditions.
    Finding ignition and fuel tune sweet spot for the particular engine. The last variables after all else is sorted out.
    Will test different settings just to see and learn what they do, then decide which one to go with.
    Need my dd to be reliable, economical, and fun to drive with no engine issues to live with.
    Will give up absolute max power tuning for this. So far so good for me.

    In 1969, dad bought a new sweet Chevy Kingswood Estate wagon. 350/300 4 bbl, 3 speed Turbo-Hydramatic.
    Same as in SS Camaro/Nova L48 4 bolt mains 750 Q-jet, 10.25 compression premium fuel. 300 @ 4800, 380 @ 3200.
    Single exhaust. 4400# 2.73
    Ran fine but idling, had a chug sound out the tailpipe with factory timing. Sounded and felt held back on accell.
    Bumped up timing with timing light. Now sounded normal, ran great, better fuel mileage too. Low teens to 15 mpg.
    Would not spin tires from dead dig, but had a healthy midrange-up from a roll. On our 1/4 mile test road, ran in the 16's.
    Dad commented how well it now ran to work and back and noticably better fuel mileage.
    Did not like the q-jet quicker secondary air valve mod i did. The easily reversed vac line trick. They sure kicked in (after the bog).
    Luved the short trips to wash it at the coin wash, mono FM radio rockin'.
    Car was slow enough to hear the q-jet wide open for a good 5-10 seconds.

    Same with my Mustangs, VW's, and any other car i tune. Finding the sweet spot. Factory specs only used for baseline tune.
    Left everything connected. Advanced the 67's 289, then backed off a bit. Regular fuel. Balanced fuel econ, drivability, idle vac, and power.
    Drove and tuned the 79's timing by sound (no pecking in 4th when boost starts) then back off 2. Regular fuel.
    Seat feel and overall sound, fuel econ, drivability, highest idle vac, factored in. When it felt right, timing was locked down.
    Seasonal weather have not changed the tune. Timing not touched since 2002.
    Figure there is more, but leaving it 'detuned' for longevity. The 'get it all, then back it down a bit' tune.

  17. #17
    FEP Super Member bwguardian's Avatar
    Join Date
    Jul 2010
    Location
    Houston (Clear Lake), Texas
    Posts
    4,430

    Default

    Quote Originally Posted by 85MUSTANGTGT View Post
    Was able to find his pistons listed in this PDF on pg 38. Actually from their Autotech line. 5cc valve reliefs.

    http://racetecpistons.com/wp-content...ec_Catalog.pdf
    That's basically a stock style 1985 piston. It shows 9.5:1 with a 60cc combustion chamber head...aka the GT40p head.
    HAD
    '82 GT monochromatic (red)...black cloth

    HAVE
    '85 GT vert two tone (white on charcoal)...white leather
    '00 F350 two tone (white on silver)...gray cloth
    '00 Excursion Limited two tone (white on tan)...tan leather...wifes ride
    '08 Taurus Limited ice blue...tan leather...daughter ride
    '08 Edge Limited white sand tri-coat metallic...tan leather...other daughters ride

  18. #18

    Default

    Quote Originally Posted by bwguardian View Post
    That's basically a stock style 1985 piston. It shows 9.5:1 with a 60cc combustion chamber head...aka the GT40p head.
    Yeah, they say isn't not too far from stock, but just a bit more. I guess maybe I don't know as little as I thought.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Transit Connect Titanium LWB
    '14 Ford Fusion SE Manual

  19. #19
    FEP Super Member bwguardian's Avatar
    Join Date
    Jul 2010
    Location
    Houston (Clear Lake), Texas
    Posts
    4,430

    Default

    I didn't see if that piston was a cast, forged, or a hyper slug. However, the earlier...1984 and back a couple years...5.0 Mustang engines had cast flat top slugs with four valve reliefs. The 1985 5.0 Mustang engine had forged flat top slugs with four valve reliefs...and moly rings. The 1986 5.0 Mustang engines had forged true flat top slugs with no valve reliefs. The 1987 and up a couple years 5.0 Mustang engines had forged and/or hyper slugs with a slight dish in the center...about 4cc-5cc...and four valve reliefs.
    HAD
    '82 GT monochromatic (red)...black cloth

    HAVE
    '85 GT vert two tone (white on charcoal)...white leather
    '00 F350 two tone (white on silver)...gray cloth
    '00 Excursion Limited two tone (white on tan)...tan leather...wifes ride
    '08 Taurus Limited ice blue...tan leather...daughter ride
    '08 Edge Limited white sand tri-coat metallic...tan leather...other daughters ride

  20. #20

    Default

    Brad,

    See the image at the link below.

    https://www.researchgate.net/figure/...fig5_325733354

    This is a graph of combustion chamber pressure versus crank angle for six different ignition timing options. TDC, 5, 10, 15, 20 and 25 degrees BTDC. This graph and timing combination is only for one fixed engine rpm. More on this later.

    Why have spark advance at all? It takes time for the cylinder pressure to increase once combustion has been started by the spark plug. When the plug fires, all of the air fuel mixture in the chamber doesn't instantly ignite. It takes time to burn, which means that the pressure rises slowly.

    When the spark plug fires before TDC, there is negative torque created, because the piston is still going up, but the gases are expanding. This is bad as it slows down the engine (creates negative torque). However, the upside to this is that when the combustion pressure hits its maximum value sooner after TDC, there is also more positive torque created. This is because the total combustion volume is smaller, so for a given gas expansion, you get more pressure. Combustion pressure times the area of the bore, times the lever arm of the crank, determines how much torque the engine creates.

    The areas on either side of TDC on the graph, cancel each other. The area on the left is pressure that creates negative torque. The area on the right side creates positive torque. These two cancel each other out. You get the most torque when the area on the right minus the area on the left is maximum. Ignoring the curve with 25 degrees BTDC timing, the 20 degree curve clearly has the most differential area, so this ignition timing would make the most torque and therefore power for this engine.

    If you look at the 25 degree curve, you can see all of the high frequency pressure spikes. These are from sound waves resonating inside the cylinder. This is the pinging sound you hear when the engine has too much ignition advance. The engine is beating itself to death in this case. The 20 degree curve does have some variations in the pressure curve, but probably not too much.

    From this type of data, you can determine what the optimum ignition timing is for a given engine at a given rpm. What happens when you change the engine rpm? Say increase it by a factor of two?

    Assume that the air fuel mixture in the engine burns at a constant rate (mass over time). If the engine is turning twice the rpm and the air fuel mixture burns at a constant rate, you now need to double the angle before TDC that the spark plug is fired, to get all of the air fuel mixture burned. This is because since the engine speed is doubled, for each degree of crank angle, it now rotates through that in 1/2 the time. If you then double the engine speed again, you will then need four times as much spark advance (degrees before TDC) for maximum power. This is why a typical engine will have 10 degrees of spark advance at idle, but 32 degrees of timing at 3krpm. A number of complex factors affect the optimum shape of the advance curve versus rpm versus engine load.

    Given that you have an EFI engine, when you turn the distributor you are adding or subtracting degrees of spark advance to every point of the advance curve stored in the EEC. Since you can't easily reprogram the advance curve in the EEC, you will be fairly limited on how close you can get the engine to its optimal ignition timing curve.

    Some important things to notice in the above graph. From 0 to 5 degrees of timing advance, there is a small increase in torque. From 5 to 10, there is a bigger increase. From 10 to 15, there is the biggest increase. From 15 to 20 there is a very small increase and there are standing waves forming on the pressure curve. The moral of this is that once you get close to the optimum amount of ignition advance for the engine, the amount of additional power you can get is very, very small, especially compared to the possibility of pinging. It is always better to err on the side of less advance so that the engine doesn't ping when it runs a little bit hotter or is under higher load or you have slightly lower fuel octane.

    If you use fuel with a higher octane rating (this means that it is harder to ignite), you can get more torque out of the engine by using more spark advance. If you don't advance the timing to get this, using higher octane fuel will actually result in slightly less torque, because the fuel burns slower.
    Last edited by Jack Hidley; 02-26-2020 at 07:13 PM.
    Jack Hidley
    Maximum Motorsports Tech Support

  21. #21

    Default

    I assume there must be some net gain between running lower octane fuel w/some advance vs. higher octane fuel w/moar gooder advance. Otherwise what would be the point?
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Transit Connect Titanium LWB
    '14 Ford Fusion SE Manual

  22. #22

    Default

    Brad,

    If you only have lower octane fuel available, you will probably need to use less ignition advance. The results will be less power, but at less fuel cost.

    If you have higher octane fuel available, you can usually use more ignition advance which will normally make more power, but at an increased fuel cost.

    Does that answer the question?
    Jack Hidley
    Maximum Motorsports Tech Support

  23. #23

    Default

    More advance = more power up to a point. I wasn't sure of that connection. Combined with what you said above, I think I get it now.
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Transit Connect Titanium LWB
    '14 Ford Fusion SE Manual

  24. #24

    Default

    Gasket thickness and piston’s distance down the hole at TDC make a huge difference on actual compression

  25. #25

    Default

    Doo dooo dooo, let's see, what does Eric The Car Guy have for me today.

    https://www.youtube.com/watch?v=RNzrdexY_Hw

    Ah, how timely. Ambiguous compression ratio? Pre-ignition DESTROYED your brand new engine?! Just what my paranoid brain needs right now!

    I do realize his compression is way higher than mine though...
    Brad

    '79 Mercury Zephyr ES 5.0L GT40 EFI, T-5
    '17 Ford Transit Connect Titanium LWB
    '14 Ford Fusion SE Manual

Page 1 of 3 123 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •