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compression ratio vs boost

This is a discussion on compression ratio vs boost within the Forced Induction forums, part of the LSx Technical Help Section category; What's the highest safe compression ratio for 6 lbs boost and 226 cam?...

  1. #1
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    compression ratio vs boost

    What's the highest safe compression ratio for 6 lbs boost and 226 cam?

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    Moderator 35th-ANV-SS's Avatar
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    6 lbs of boost is pretty mild.

    There are a lot of people running 8-9psi on a stock 346ci which has compression of 10.5:1 without issues.

    It is when you run higher boost levels that it really benefits to have lower compression.

    Example: 15 lbs+ would be something like 8.5 to 9:1.

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    Senior Member Schmalgar's Avatar
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    04 Cobra 'vert - M6 mysti
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    Is that why a car like the Cobra that comes with a factory blower has, what, 8.5:1 compression? Just to easily allow for higher boost without issue? Not that it really needs it at the stock 8psi...?

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    I guess I needed more info. The good tuners that I have spoken with usually pull quite a bit of timing on stock shortblocks and 92 octane, when they go much over 6 lbs. I have a set of 61cc trickflows and wondered if a 226 cam would bleed off enough of the 11.0 compression to live without backing off the timing a bunch. Most of the tuners I have talked to basically see stock shortblock with 6-8 lbs or a really wild set up, not very much in between.

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    I would say twin turbo but thats just me. Worked for a guy about 2 years 8-9 lbs on stock is about all you can push. You will still need to do some work on the fuel system,tuning,so forth.Pro chargers are great but turbos are more efficient.

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    Moderator 35th-ANV-SS's Avatar
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    Turbo set-ups are also a lot more expensive and difficult to install.

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    OK, let me put it this way then. Will a 226 cam reduce dynamic compression ratio enough to run 6 lbs of boost without pulling lots of timing (to keep it safe enough to keep away from detonation). I realize we need adequate fuel, tuning, and turbos work, etc. Not what I ask. Will a 226 cam have enough overlap to make 11.0 to 1 compression ratio usable w/6 lbs of boost without have to run excessively rich and backing way off the timing to keep it from detonation? Almost anything will live with 5 degrees of timing and afr of 10:1. I don't want to spend 20 grand building a motor to hold 30 lbs of boost and use e85. I'm contemplating using the 61cc 215 Trickflows and 226 cam I have on the shelf for a mild street setup.

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    Moderator 35th-ANV-SS's Avatar
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    I understand your question, but I am not well versed enough to answer it...

    Are you going to be running an intercooler? This would definitely help with the detonation, as detonation is caused from unnecessary, excessive heat.

    I don't really see why a 226 cam would be an issue, but I can't say for certain. The duration on the EPP cam that they recommend for the Procharger has a larger duration at 232/240.

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    Quote Originally Posted by bsomm View Post
    OK, let me put it this way then. Will a 226 cam reduce dynamic compression ratio enough to run 6 lbs of boost without pulling lots of timing (to keep it safe enough to keep away from detonation). I realize we need adequate fuel, tuning, and turbos work, etc. Not what I ask. Will a 226 cam have enough overlap to make 11.0 to 1 compression ratio usable w/6 lbs of boost without have to run excessively rich and backing way off the timing to keep it from detonation? Almost anything will live with 5 degrees of timing and afr of 10:1. I don't want to spend 20 grand building a motor to hold 30 lbs of boost and use e85. I'm contemplating using the 61cc 215 Trickflows and 226 cam I have on the shelf for a mild street setup.
    I think you could get away with it,but you will always be riding the edge of boom.You will have to have race gas Im sure to keep from pulling alot of timing.Why not get a set of 70cc heads?

    IMO you would be better off running the 70 cc head and 10-12 lbs of boost(or more) versus a 61cc and risking 6 lbs every time you nail it,the cam isnt going to be perfect but it will work

    just so you know people are putting some nasty budget build 370 cubic inch motors together for boost

    me included

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    Your cam should be ok but I think you will just end up breaking parts if you try to run 11-1 and 6lbs on a stock bottom end.If you really want to learn something check out Bellengineering.net this man has been building turbo cars longer than most of us have been around. He also has a very good book out.Hope this helps.

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    TunedbyFrost.com Tuner Frost's Avatar
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    Quote Originally Posted by redneckcritter View Post
    Your cam should be ok but I think you will just end up breaking parts if you try to run 11-1 and 6lbs on a stock bottom end.If you really want to learn something check out Bellengineering.net this man has been building turbo cars longer than most of us have been around. He also has a very good book out.Hope this helps.
    You can run compression and boost with a stock bottom end; the tuning just has to be good. If you even think about trying to push the timing or mixture though, you can flush it in a heartbeat.

    I had a local with a set of AFRs milled to 59-60cc (SCR over 11:1), g5x3 on a 114LSA, big ole F1c Procharger and with a the biggest blower pulley available, it still made 9psi... on a stock bottom end. The car made 643rwhp at 67-6800 RPMs where I lifted, but it was still headed straight up. It had a six-speed trans then. It was beat on like this for a year + before moving to a built 408 ETP 245s... and 10.6:1 SCR. The blower now makes 13.5psi and over 700rwhp through a hefty dyno-unfriendly 4l80. All of these numbers are pump gas and Alkycontrol methanol injection.

    I have another local customer with a stock cube forged LS1 (well 347"), AFR 205s and a mid 230s duration cam and D1sc Procharger make 763rwhp at about 16psi, roughly 10:1 SCR. It is also pump gas and Alkycontrol meth kit.

    The Vette on the front of my website has a 100% factory stock LS6; never had valve covers off of it, 10.5:1 SCR, long tube headers, D1sc Procharger, about 13psi... 675rwhp. It also runs Alkycontrol meth. Combined with the driver, this is a record car. A 9-second totally stock engine, six-speed Vette with no weight reduction. It trapped 140.9 MPH over the winter. Many internet racers have said this is a ticking time bomb. It's been ticking for 4-5 years now and literally hundreds of dragstrip passes. The day he ran the pass below, I think he got in right at 20 runs at that track rental alone. That was a LOT of 10.0 passes!



    This link goes to a 44 MB video (right click and save-as) of a twin turbo C5 with a stock bottom end doing his first shakedown runs (no boost launches until the end) but was actually done during a race day. On the last pass where he launches in boost, he sees he's way out on the 10.0 race car in the other lane so he brakes but still runs under. You can see the lower MPH. This is heads (maybe 0.2 under stock SCR), cam, 9psi from TTi X twin turbo setup and again, Alkycontrol methanol... 140 MPH on his stock bottom end. http://www.tunedbyfrost.com/vid/randy.wmv

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