871 blower bbc 9.8 comp...under drive??
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871 blower bbc 9.8 comp...under drive??

  1. #1
    SUCK MY WAKE Sangster~bbc's Avatar
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    Default 871 blower bbc 9.8 comp...under drive??

    I have a mild bbc 427 60 over that is running 9.8 comp 4 bolt all gm forged bottom end rods crank pistons,edelbroke performer heads comp solid roller cam 570 lift 110 lobe .1.7 rockers gridle all arp hardware i would like change the cam and run a 871 on it, in a street car... ideal would be nice too make 600hp our more if can ...my question is could I get away with running it on pump gas and my current set-up and how much would I have too underdrive/ boost??? our would it be better too drop comp and run more boost my only deal is I just built this motor and thought would be enough...but would like more....thanks gents

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    gn7
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    Best case in a light boat, you're looking at 4 lb boost, about 20% underdrive



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    280 Profile Cat W/575sci JRider's Avatar
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    Leave the compression, change the cam and run E85 with the blower around 1:1...just a guess

    Just a suggestion that could save you some parts money
    Last edited by JRider; 04-20-2014 at 07:49 AM.

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    Blown, OHBA Member Wild Hair's Avatar
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    Quote Originally Posted by JRider View Post
    Leave the compression, change the cam and run E85 with the blower around 1:1...just a guess

    Just a suggestion that could save you some parts money
    9.8 static 871 striped @ 1to1 in 427 @ 12 lbs of boost comes out to 17.8 final compression on the BDS chart. still a bit much for E85 in my books. You will have to under drive the shit out of it to run pump gas.

    EDIT:

    Looks like 15% under will give you about 10 to 1. not worth the change

    10% under will give you 13.7 Too much for pump gas

    Note the cam you choose can affect that in either direction..
    Last edited by Wild Hair; 04-20-2014 at 08:13 AM.
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    280 Profile Cat W/575sci JRider's Avatar
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    That's why I said just a guess

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    Blown, OHBA Member Wild Hair's Avatar
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    Quote Originally Posted by JRider View Post
    That's why I said just a guess

    I'm going through the same thing, I run E85 safely on 8 lbs, but to flip my pulleys puts my final compression out of the safe zone for E85. Now I am trying to figure out what I have to do to run the E98. If it is too costly I'm just gonna start looking for a good deal on a MFI system and boost the shit out of it. LOL but I don't have to drive it on the street like the OP.
    Last edited by Wild Hair; 04-20-2014 at 08:29 AM.
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    280 Profile Cat W/575sci JRider's Avatar
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    What is your static?

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    SUCK MY WAKE Sangster~bbc's Avatar
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    thanks for the help guys looks like it would not be worth going that way with my current set up as I really want too keep running pump gas in it, as its my street car.. maybe ill look at doing it on my next rebuild what should I look too get for comp when I change the pistons out have about 8.0 static comp?? would it even be making power at 20% underdriven our would it be just robbing hp?? thanks for the info again...
    Last edited by Sangster~bbc; 04-20-2014 at 10:50 AM.

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    280 Profile Cat W/575sci JRider's Avatar
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    Quote Originally Posted by Sangster~bbc:3089090
    thanks for the help guys looks like it would not be worth going that way with my current set up as I really want too keep running pump gas in it, as its my street car.. maybe ill look at doing it on my next rebuild what should I look too get for comp when I change the pistons out have about 8.0 static comp?? would it even be making power at 20% underdriven our would it be just robbing hp?? thanks for the info again...
    E85 can be considered pump gas (fuel)

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    SUCK MY WAKE Sangster~bbc's Avatar
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    im not too sure were I can get it in Canada, alberta I might have too look more into it..

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    I have a friend who has 588CI, 420 B&M blower, 9.5:1, Dart 360 Alum heads. He runs about 3.5psi (850ish hp) on 93 gas. With a mix of race gas, he cranks it to 5-6psi (930HP). Engines are in a 33 Fountain, and have been together a long time. He doesnt baby it.

    Any chance you can thicken up the head gaskets, and knock the static down a little? Possibly play with cam specs to bleed off a little cylinder pressure?

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    Quote Originally Posted by Mild Thunder View Post
    I have a friend who has 588CI, 420 B&M blower, 9.5:1, Dart 360 Alum heads. He runs about 3.5psi (850ish hp) on 93 gas. With a mix of race gas, he cranks it to 5-6psi (930HP). Engines are in a 33 Fountain, and have been together a long time. He doesnt baby it.

    Any chance you can thicken up the head gaskets, and knock the static down a little? Possibly play with cam specs to bleed off a little cylinder pressure?
    Thicker gaskets are not the way to go. You will loose quench distance and that in itself could lead to detonation even though you're lowering the cylinder pressure.
    I'm not a blower expert but my take on it is, given the same quench total effect, raising the static compression (through the proper piston head shape) and running lower boost can net you the same cylinder pressures and not have all the heat from the boost. The higher the boost pressure you have the higher the intake charge temp will be, and that puts you closer to detonation. Cam overlap will bleed off cylinder pressure at lower RPM's. Most blower grinds have wider lobe seperations because the intake charge doesn't need the overlap (both valves open at the same time) to help it start moving into the cylinder because the intake charge is above atmospheric pressure already. So some blower grinds bleed off less cylinder pressure at the lower RPM's.
    Actual boost PRESSURE is in part a result of the restrictive nature of your particular intake tract. The more restrictive it is the more boost #'s it will show on a guage. A higher flowing intake tract will flow more volume making it less restrictive resulting to lower boost #'s, apples to apples.
    Lower boost #'s mean a cooler A/F temp and that much farther away from detonation. Bottom line is that you want higher cylinder pressures (as close to what your cylinder sealing capability can handle) with as cool of an A/F temp as you can achieve.
    Does that make sense? lol
    So, to try and answer the OP's question, if it were me, I'd put as high of a static compression as I could normally get away with for pump gas for your engine temp, altitude, chamber shape (burn rate) and quench characteristics and then put a small amount of boost to it.
    Rob

  15. #13
    gn7
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    Quote Originally Posted by JRider View Post
    E85 can be considered pump gas (fuel)
    Try running it in ANY gas class in ANY sanctioning body, even where race gas is allowed, and see how that is received.



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    Last edited by gn7; 05-03-2014 at 07:59 AM.

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