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Thread: Compression ratio reduction

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    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    Compression ratio reduction

    I am replacing my turbo right now and in the past I have had detonation problems. I have been using the #2006 head gasket which crushes down to .070. My compression ratio cc'ed out at 9.2:1 with this gasket my 782 head and Wiseco pistons. As long as I have the head off I am looking for solutions to the high compression ratio. I wonder if anyone has had any success by un-shrouding the intake valve on this head casting. I also wonder if there is a thicker head gasket available. I am using a stock T2 intercooler mounted between the front facia and the radiator. Could it be this is too inefficient at reducing heat in the air going to the intake (20+ psi). I'm not real sure which way to go here but I want to solve the detonation problem if possible.
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    Supporting Member Turbo Mopar Contributor zin's Avatar
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    Re: Compression ratio reduction

    Copper head gaskets can be had in thicker sizes, and de-shrouding the valves will gain quite a bit in terms of flow, and some in terms of compression. De-shrouding won't gain many CCs, so compression won't drop a whole lot, I'd do the thicker gasket as the main way of dropping compression.
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    Super Moderator Turbo Mopar Staff contraption22's Avatar
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    Re: Compression ratio reduction

    The stock intercoolers are decent for stock cars. At nearly 2x's factory boost levels, it is not enough to cool the charge well enough on pump gas.
    Mike Marra
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    Re: Compression ratio reduction

    Jeff,
    Why are you running so much CR on your car? Are you sure your compression ratio is really 9.2:1? What do you have, g-head pistons on swirl head? Maybe a heavily resurfaced cylinder head? What plugs are you running? Octane? What calibration? That is a LOT of compression for pump fuel and 20 PSI!
    Yes you can heavily unshroud the swirl head to lower compression (both intake and exhaust). Copper head shim can be used with sucess. You can also buy cometic gaskets in various thicknesses too.
    Todd

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    Re: Compression ratio reduction

    Same question as Todd, really 9.2:1???

    If you deshroud the valves, you'll also loose some quench area. I've done this on my current ported +1 head and I probably wouldn't do it again if I were to do it over.

    I've also done the copper shim head gasket and it worked to reduce detonation, but got all damaged when I had to remove it for another reason.

    I've also given up on custom cals because of the advanced ignition timing in favour of a ghetto stock 2 bar and cold starts.

    Cooler spark plugs worked for me as well. There were non-projected tips that JT suggested from Champion.

    I also picked up noticable power and detonation resistance with a larger fmic (srt-4) and a cold air intake (remote front mount filter)

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    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    Re: Compression ratio reduction

    Thanks to all,
    Todd, the engine machinist decked the block but I am unsure how far he took it down. I thought he may have done that to the head as well so I went out and checked. I couldn't see any of the normal evidence you would see, like cutting tool marks so he probably didn't. I have the WISECO piston that T/U sells for the 8 valve. It has a dish that includes about 80% of the piston top surface. When I assembled the engine I CC'd the head and piston at the deck and included the head gasket in my computations so I am sure of the compression ratio. I use RJ9Y plugs 91/93 octane and Cindy's stage 5 chip. I recently added 2 gallons of Sunoco 110 octane race fuel to about 4 gallons of unleaded 91 and I could still hear some clicks. What should my compression ratio be in the event that I wanted to run as much boost as my stock turbo can produce. (I don't know what that is, I suspect about 24 psi). I looked at Cometic gaskets at T/U, I could increase the thickness of the gasket only .015 over the #2006 which wouldn't be much but combined with un-shrouding might get me enough. Do you recommend a supplier for the copper head shim?
    Apparently I also need a larger intercooler, in you guys experience what is the largest you can fit easily into the space between my radiator and front facia? I have space for 22" long 12" high and 4" deep with out modifying anything. I could possibly modify the front crash absorbtion bumper to get more depth.
    Jeff
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    Re: Compression ratio reduction

    Jeff,
    Attached is a bone stock 2.2 breakdown of compression ratio. Can you fill in the blanks for your application? Nothing should be really very different except how far pistons are down or out of the deck of block. The Wiseco pistons are probably close to this. Gasket is pretty much same. If cylinder head isn't highly modified, CC of head won't change much either. I don't get why you are coming in so high?
    BTW I was fighting a detonation issue with a vendors cal. All the the sudden my vehicle got REAL detonation happy. After checking and changing EVERYTHING LIKE THREE TIMES, I put in another cal and problem disappeared. In my case I was running a true 7.8:1 motor. It was rattling bad at 10 PSI! Just changing cals and nothing else it now can run about 21 PSI on pump fuel during the heat of the summer before any sign of detonation. At least something to consider...
    I am really not a believer in running turbos in "all of it mode". In most cases you are serously over-spinning them, and it will lead to failure of center section of turbo. Besides, they are not efficient at those speeds either. Ultimate PSI will depend on lots of things. Pressure drop across intercooler, how much CFM head flows, intake, exhaust manifold, etc. I know of one guy that was overspinning his turbo at 15 PSI. His top end of motor was very effcient, the stock TII turbo was about maxed out at 15 PSI. Went through a turbo a year until we bolted on a bigger turbo.
    Todd
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    Moderator Turbo Mopar Staff Vigo's Avatar
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    Re: Compression ratio reduction

    You will not need more depth than a 4" intercooler core, as that's the thickest core that's readily available anyway and i think you may run into problems restricting airflow to the radiator if you went any larger than that. I do think the intercooler should be the first thing you address (after you put the engine back together). Everything else you can change externally is relatively incremental (cool air intake, colder plugs) compared to good intercooling. If you're still getting detonation after doing this other stuff i would just retard the timing until it goes away, or add water/meth injection.

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    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    Re: Compression ratio reduction

    I went back over my numbers but couldn't find the information on my cylinder head volume. The head was a brand new stock 782 I got from TurboDave and as stated before I can see no evidence of milling. I used a 60 cc syringe to fill the chamber and I seem to remember that it was 58 cc's. What is the stock volume of the 782 head. I have looked all over and can't find the spec's. If 58/60 cc's is about right I am actually around 8.7;1/8.8:1. These numbers were created with a calculator apparently the same as 4l-bodies. I wont go back to a stock cal because I am set up for three bar and it cost way to much to get there, so I have to make this work. Perhaps the stock intercooler has been the problem all along as this motor has detonated with every three bar chip I have tried. I am definitely going to find a different one. 4l-bodies, do you have another chip like the one you solved your problem with?
    Jeff
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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    Re: Compression ratio reduction

    Jeff,
    Unfortunately no I don't have another chip. Currently the car is running a Paul Veccky calibration in it. Actually Jessie Buhr now owns the car. I would try another vendors calibration. As you may or may not know I have ported and flowbenched a lot of cylinder heads. I can say with quite certainty that a NOS stock swirl head will average out around 50cc. 50.4 cc to be exact for mine. I also have a mint (used but uncut) swirl that came in at 51cc average for the 4 chambers. The g-head is around 56-57cc.
    I highly doubt the stock intercooler is causing your detonation issues. Have you cleaned it out? Many times they can become saturated with oil. It obviously isn't the ultimate intercooler as it is a little small (flow impaired) compared to a larger FMIC, but many people including me have easily put cars well into the 12's running a stock intercooler or Indy cooler.
    Like previously mentioned you can try running a non-projected tip plug. I ran into one friends motor (3 bar cal) that had to run them or it would rattle no matter what I tried. I used a NGK BR6ES (4922) or you could even try colder BR7ES.
    Have you read your plugs or had DRB hooked up to see what cylinders are being retarded? Have you had your injectors cleaned or balanced? I've seen brand new +40's be as far off as +/-7% in same batch. Flowed between 520 to 560cc. That could do it right there. I've used Mr. Injector to clean, rebuild, flowbench many injectors for me over the years. At a maximum of $16 per injector, it's too cheap not to do this. He turns them around in 24 hours and charges like 5-6 to ship back. A no brainer for me.
    Todd
    Last edited by 4 l-bodies; 08-21-2013 at 04:07 PM.

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    Re: Compression ratio reduction

    All great points and things to consider. I'd like to add the possibility of oil consumption for another possibility for detonation.

    As well, is the air fuel ratio rich enough? My own car would detonate if there wasn't enough fuel to help cool the chamber.

    I picked up my copper cylinder head shim from car quest. I forget the thickness, but 0.030" rings a bell.

  12. #12
    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    Re: Compression ratio reduction

    4 L-Bodies, I measured one of my chambers and they are indeed 50cc. This means my compression ratio is 9.7 :1 this may explain why the engine always began to detonate at around 7 to 10 psi. I talked with Bill Johnson today and am sending my injectors out. If I need to, after all is back together, I will go with the NGK plugs. In the past I connected OTC 4000 to the engine and it showed that as much as 9degree's were being pulled out of some cylinders.

    To get my compression back to 8.1:1 I need to increase chamber volume by 15 cc's. Since a .020 shim is only .9 cc's if my math is correct, I would need a very thick shim. I don't think I can even remove enough from the chamber to get that number. What do you think?

    I also considered oil as a source of detonation. The engine was rebuilt a year ago but has seen a lot of detonation. It was using oil but it remains to see if it was the turbo going bad.

    My air fuel has always been 10:1 at WOT but the AEM cant sort out an individual cylinder so some cylinders could be lean and you wouldn't know it.
    Jeff
    Last edited by jeff1234; 08-22-2013 at 02:54 PM.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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    Moderator Turbo Mopar Staff Vigo's Avatar
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    Re: Compression ratio reduction

    Just as a tidbit to throw in here, I've run a turbo on a tbi 2.5 bottom end with a crappy turbo and crappy intercooler and had no detonation on stock timing and pump gas up to 15-16psi. I believe those have low-9s compression. It's certainly not such a big issue that it detonates at 7psi!

    I honestly dont think low 9s compression is a big deal until you are pushing the setup, and even then i think you will uncover many weaker links (cold air intake, better intercooler, colder plugs, timing changes, uneven fuel delivery are all possible issues or improvements) before you can conclusively point to the compression ratio as being the MAIN culprit.


    As for the HIGH 9s number, im starting to feel really dubious about the measurements situation. I dont understand how you can run a piston that gives a low 8s compression ratio with stock parts (educated guess on my part that the piston would give same or lower CR compared to stock), have block decking as the only major unknown variable, and end up close to 10:1. If you're using low-compression pistons with a big dish in them i would think they'd have to be ABOVE deck to get that kind of CR and im pretty sure you would have noticed this.

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    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    Re: Compression ratio reduction

    Vigo, here is the answer. I was entering information on the online compression calculator incorrectly on head gasket thickness. When I changed the number so it was correct, I came up with 8.1 to 1. Arrgh, so it is not compression if I have it correctly calculated. This stuff can drive me goofy.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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    Moderator Turbo Mopar Staff Vigo's Avatar
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    Re: Compression ratio reduction

    Well, water under the bridge and one less thing to worry about. Still have to figure out your detonation issue!

    If you are still getting knock at 7-10 psi i would leave boost relatively low until you figure it out. Obviously you're not going to break it if you havent already but i think it will make diagnosis simpler because we more or less know for a fact that knock counts should be ZERO at 7-10 psi, so if you address the cause and knock counts go to zero it is conclusive, whereas if you run 22psi (for example) and make some improvement, you may only reduce but not eliminate knock counts, which is less conclusive and may leave you wondering if you've fixed the caused or just treated one of the symptoms.

    At SOME point knock counts are expected, but you want to know that there is no knock UNDER that so you can tell the difference between the limits of the setup, and an actual problem of something not working right.

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    boostaholic bfarroo's Avatar
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    Re: Compression ratio reduction

    I'd definitely try another cal, the stage 5 i'm sure is designed for a bigger build with a larger more efficient turbo, larger intercooler and all of the other supporting mods so it probably has a bit more timing added from a stock cal. are you running the 87 electronics? At what pressure are you starting to see the detonation at? Definitely make sure your PCV is working correctly and isn't sucking in to much oil vapor and that your intercooler is clean. Meth / Water injection could be another route to try although some think it is a bandaid and another possible point of failure.

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    Re: Compression ratio reduction

    Quote Originally Posted by bfarroo View Post
    I'd definitely try another cal, the stage 5 i'm sure is designed for a bigger build with a larger more efficient turbo, larger intercooler and all of the other supporting mods so it probably has a bit more timing added from a stock cal....
    Quote Originally Posted by 4 l-bodies View Post
    Jeff,
    ...BTW I was fighting a detonation issue with a vendors cal. All the the sudden my vehicle got REAL detonation happy. After checking and changing EVERYTHING LIKE THREE TIMES, I put in another cal and problem disappeared. In my case I was running a true 7.8:1 motor. It was rattling bad at 10 PSI! Just changing cals and nothing else it now can run about 21 PSI on pump fuel during the heat of the summer before any sign of detonation. At least something to consider...
    Todd
    Jeff,
    Try temporarily throwing your stock cal and 2 bar map back in and turn fuel pressure WAY back to around 26-28 PSI (static). If you have a Accufab AFPR it may only go down to around 30, that is close enough. At 28 PSI your +40 injector will be actually flowing around 37 lbs. and not 52 lbs. I bet you will find your problems are gone. Went through this already with my Stage 5 cal. See above post^^^. I even had lower compression (7.8) and it rattled really bad at 10 PSI. IIRC, I think Boostgeek had a cal do this as well 2-3 years ago. In my case cal worked great for a couple years then something changed. Of course I never suspected a cal or LM to go bad, so I checked and replaced everything. Turned out to be the cal and not the LM.
    Good luck,
    Todd

  18. #18
    Moderator Turbo Mopar Staff Vigo's Avatar
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    Re: Compression ratio reduction

    I diagnosed an issue down to the chip/cal file on your friend Steve's car down here. That was the first time i had ever been able to point to the chip itself as having failed.

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    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    Re: Compression ratio reduction

    Todd unfortunately I no longer have my stock calibration. Here is what I plan to do. I no longer have access to the OTC 4000 so I am going to re-install my cal from Shelgame. It has the detonation indicator still intact. At least I will see an indication which will tell me if I am getting anywhere. I will then follow Vigo's suggestion and turn my boost down and see if I can eliminate detonation below 7 psi. If not I will go to the NGK BR6Es plug. If that doesn't help I will get a different intercooler and see what that does. Additionally I will do the maintenance to my present intercooler that bfarroo (I have 87 electronics) and others have suggested. As long as the engine compartment is wide open I am going to find a way to get the air cleaner out between the nose and radiator support where the cooler air is. It will be three weeks before I can accomplish this because of waiting for parts and other obligations but I will update or create a new thread to let you guys know how I came out. Oh, one other thing, I have created a system of hoses connected to the valve cover, intake manifold and air cleaner that draws on the crankcase when the pcv is closed by boost.
    Thank you for taking time to share your knowledge, it is valuable!
    Jeff
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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    Supporting Member II Turbo Mopar Contributor jeff1234's Avatar
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    Re: Compression ratio reduction

    Update, I read an article on intercoolers on enginebasics.com that was very informative. I determined that a part of my det. problem may be related to heat. I ordered a new intercooler from CXRacing. The ic0005 will fit best in my Charger and still increase cooling. I am going to modify the nose and build a duct around it to allow unrestricted air flow. I still haven't figured out exactly how i am going to get my air cleaner out ahead of the radiator support without chopping the car up and running tubing all over the place.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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