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Highest CR for 87 Oct.

Discussion in 'The Garage' started by 91GMCSuburban, Jun 17, 2005.

  1. 91GMCSuburban

    91GMCSuburban 1/2 ton status

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    What is the highest compression ratio you can run on 87 Oct gas? Iron vortec heads, 383 Cu. In.
     
  2. Triaged

    Triaged 1/2 ton status

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    Depends on how much cam you have and how much quench you have (among others). Do a search on google for Dynamic Compression Ratio (DCR) and quench. You will come up with some good info but in the end it will still be up to you to pick what you are comfortable with. If it ends up not running on 87 and you need to use 89 or 91 how upset will you be?

    Is this an EFI motor?
    Will you burn a custom chip for it?

    It isn't just as easy as saying 9.0:1!!!
     
  3. Mad-Dog

    Mad-Dog 1/2 ton status

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    To determine the minimal octane level for a given CR just move the decimal point one place to the right....example (8.7:1 CR= 87 octane fuel
    This is just a baseline guide and there are building techniques which allow a 10.9;1 CR motor survive with 93 octane when it should run on 109 octane race fuel..... it's all in the combination of parts, static vs dynamic CR's, air/fuel ratio, ignition system, type and size of combustion chamber, aluminum vs cast iron heads, the variables go on and on.....
     
  4. beater_k20

    beater_k20 Banned

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    you CANNOT accurately esimate octane like that. if that was the case, all 96+ Chevy trucks would run on 96 octane, and Camaros and Firebirds would only run on 104 octane.

    as was said before, you have to take into consideration static compression ratio, cam timing, quench distance, and ignition timing. with that said, i have put together combinations that would run fine on 93 octane despite a nearly 13:1 compression ratio.
     
  5. Mad-Dog

    Mad-Dog 1/2 ton status

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    And your point is.......?
    where did i say that was a accurate measurement....i said "baseline" guide and that is all.
    i also stated that was a minimal octane rating for a given CR.
    i could also run a 13:1 motor on 93 octane but the timing would be so retarded that it would not make max power,
    As you stated the quench area plays a very important role in this arena and tests have shown that quench areas larger than .060 were the major players in creating detonation.
    Have a nice day..
     
  6. moturbopar

    moturbopar 1/2 ton status

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    Hi, my blazer came with the 330hp 350 vortec crate motor, and is rated at 9.1-1 compression. it runs fine on 87 octane. We built a 400sbc that had 9.8 compression and vortec heads with a fairly big cam 230@.050 and it ran very well on 87 octane too. hope this helps some.
     
  7. 91GMCSuburban

    91GMCSuburban 1/2 ton status

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    Thanks for the opinions and examples. I'm undecided on the pistons to choose for my 383 build up. 28cc piston will give me 8.8 cr, and the 18cc will give me 9.7. I would rather go with 9.7 for more performance, but I want to be able to run on 87 oct.

    And it will be EFI (modified TBI, bigger injectors and chip reburn), headers, high flow exhaust.
     
  8. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    I'd personally run with the higher CR. Just get a thicker headgasket to bring down the static compression ratio a bit. You can probally get away with 9.4:1 or so on 87 (again, this depends hugely on the cam specs, dynamic CR, even elevation, etc)

    EFI typically likes a higher CR over a lower one, anyways :)
     
  9. k20

    k20 3/4 ton status

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    I got a 9.7:1 in mine, I always ran 93 cause I thought I had pingage when I would run 89. Well, that pingage turned out to be a loose t-case shifter knob. Boy I felt dumb, now its 89.
     
  10. beater_k20

    beater_k20 Banned

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    great idea, lets take something that you want to run on the lowest octane possible, and make it more detonation prone. you guys amaze me. :screwy:
     
  11. MTMike

    MTMike 1/2 ton status

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    My old '96 3.8L V6 Camaro had a 9.5:1 CR and ran beautifully on 87, but my LS1 powered 99 Z28 running 10.5:1 gets pissy on anything under 91.

    Like others said, there's too many variables to know for sure, but you'll get lots of examples and experience :)
     
  12. Russell

    Russell LB7 Tahoe Status Premium Member GMOTM Winner

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    Never claimed I was an engine builder by trade Jeremy :P

    However, it would seem to make sense as far as dropping down the static compression ratio. Why would it be more detonation prone? Rather than simply calling me an idiot, at least post a proper way of doing it, I wouldn't mind knowing either!

    EDIT:

    I suppose if you increased the quench too much, it could cause a tendancy to ping, but if you only increased the thickness a small amount, it should reduce the CR without causing too many issues.
     
  13. beater_k20

    beater_k20 Banned

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    .035"-.045" of quench distance is critical when you want to run on the ragged edge of detonation (which is obviously what the original poster wants to do here), especially so with an advanced design combustion chamber, such as the Vortecs. this tight quench distance serves two purposes, both at the top of the compression stroke. one is to better mix the air fuel mixture. as the piston approaches TDC it takes all of the air/fuel that was previously in the cylinder and forces it all into the combustion chamber. better mixture = more power. and then, if it werent for the proper quench distance, the head and the piston would develop hot spots, and could ignite the air/fuel before the piston reaches TDC. this would actually have the engine working against itself, trying to push the piston to TDC with the crank and rod, while the combustion process is trying to push it down.

    a couple good reads to pick up...

    Smokey Yunick's Power Secrets by Smokey Yunick
    How To Build Max Performance Chevy Small Blocks On A Buddget by David Vizard
    How To Build And Modify Chevrolet Small Block Camshafts And Valvetrains by David Vizard
    and John Lingenfelter On Modifying Small Block Chevy Engines

    i own all four of these titles, and have read them cover to cover several times. every time i pick up on something new. however, for a first time engine builder, these are probably a little over your head, since you need a good general knowledge of how and why things work to understand some of the theories involved.
     
  14. beater_k20

    beater_k20 Banned

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    keep in mind that most off the shelf parts store head gaskets are .040"+/-. if you want to zero deck the block (shave off the full .025") to leave your pistons level with the deck, that would be great. however, doing so does a couple of things... it weakens your deck surface, as well as the upper portion of the cylinders. also, cutting off the full .025" eliminates the possibility of squaring up the cylinders again later.
     
  15. basketcase

    basketcase Registered Member

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    I'm assuming this isn't a drag truck and max performance isn't the goal here if you want to run on 87 octane to begin with, so I would error on the side of safety and keep the compression lower, unless you'll be happy on 89. I guess it depends how much you run the truck, the extra 10-12cents @ the pump really add up if you commute with it. If it was me i'd be staying under 9
     
  16. beater_k20

    beater_k20 Banned

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    if he didnt want to make the most power, he'd be asking about a stock cammed 8:1-8.5:1 engine with a stock stroke. he's not. he's asking about a Vortec headed 383, and the most compression he can run on 87. not only drag trucks can benefit from max performance. with max performance can also come max fuel economy, since an engine that has to work less to move a vehicle (because it makes more power) also uses less fuel.
     
  17. wy4tt

    wy4tt 1/2 ton status

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    Dynamic Compression Ratio calculation

    For a very helpful read on the topic of, "Will my engine run on 87 octane," check out this writeup by Pat Kelly (he's from the Team Chevelles forum, among others).
     
  18. beater_k20

    beater_k20 Banned

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    i use Pay Kelly's dynamic compression ratio calculator, and have talked with him many times at www.chevytalk.org great guy, and he really knows his stuff!
     
  19. Mad-Dog

    Mad-Dog 1/2 ton status

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    I agree with ya, kinda....the idea is to reduce the quench area by bringing the piston as close to the head as possible.

    Negative .015" on the deck and a .041" head gasket would give .059" piston to head clearance (ideal), for most people....
    I prefer zero deck with a .041" head gasket.

    The use of small chamber heads with flat top pistons are the best way to improve compression while promoting a efficient flame front,
    dished pistons can be used to adjust the CR if flat top pistons will create a undesired (too damn high) CR with a particular combustion chamber size.

    More camshaft duration will also lower the static compression ratio at low rpm
    but once engine speed reaches a certain point the cylinder pressures will increase because of the added overlap.

    Thicker gaskets can also be substituted to lower the CR (within reason)....
    The thicker the gasket the more prone it is to fail and cause loss of compression.
    10:1 CR motors will survive just fine on the street if the motor is correctly tuned,
    My next engine is a de-stroked (+.030")400 (4.155"bore) with a 350 crank (3.48"stroke) creating (377cid) incorporating 6.0" rods for a 1.75:1 (perfect) rod ratio which will let the 180 cc intake runners feed the hungry small block with a healthy dose of air, flat top pistons with a zero deck and .041" head gaskets will build 10.9:1 compression and is borderline of being streetable but it is built by design to acomodate the high cylinder pressures.

    Horepower is the goal here man and every trick in the book must be used to remain in the game...
     
  20. dyeager535

    dyeager535 1 ton status Premium Member

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    When I rebuilt my engine, calculated compression was/is 9.2:1. (engine in sig) All "off the shelf" stuff, flat top pistons, no zero decking, etc. All the surfaces trued, thats about it. I run it on 87 and a stock 305 calibration, and it's ok. Needs timing played with for sure.

    Vortec heads (combustion chamber design) and fueling have a big impact on what you can run, along with all the other variables mentioned. TBI vs. TPI is also going to have an impact on what you can run compression-wise and get away with. It will probably be a bit better than a carb, since fueling is better controlled, but it still isn't getting the "better" fuel distribution/atomization of an injector right at the intake valve that you get with MPI. Just mentioning that because what works with TPI isn't going to be the same with TBI.

    One of the main things on the Vortecs vs. the older (junk IMO lol) heads is how much timing they want/tolerate vs. the older heads. The older heads NEED more timing, (big difference in chamber size and design) which can tend to put you in detonation territory in Vortecs with a chip in stock tune. Lots of idle timing is great on Vortecs, but up top *typical* results see people removing timing from stock calibrations. The ECM will do that for you via the knock sensor, but that's the wrong way to go about running lower octane gas, bouncing the engine off the knock sensor/timing retard.

    To quote Grumpy from thirdgen.org in relation to timing, give the engine what it wants, not what you or anyone else thinks it wants. I suspect that in a lot of cases, instead of fine tuning the application, additional octane is used to combat/hide pinging.
     

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