Compression for Gas / Alky blower motors
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Compression for Gas / Alky blower motors

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    Senior Member tkriley13's Avatar
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    Default Compression for Gas / Alky blower motors

    1.Instead of running a bigger blower why not run more compression & spin the blower less?





    2. Say this works & you have 8.1 compression in a 468 and you want to run alky now but you need / or want around 10 1/2.1 compression, couldn`t you just deck your heads to bump up the compression.


    I just ordered new pistions for next season but I was thinking How come I just couldn`t deck the heads & bolt on injection instead of spending more money on pistions & a blower.....

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    Senior Member wagspe208's Avatar
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    This will be a great thread.
    Wags

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    Senior Member Hass828's Avatar
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    Quote Originally Posted by tkriley13 View Post
    1.Instead of running a bigger blower why not run more compression & spin the blower less?





    2. Say this works & you have 8.1 compression in a 468 and you want to run alky now but you need / or want around 10 1/2.1 compression, couldn`t you just deck your heads to bump up the compression.


    I just ordered new pistions for next season but I was thinking How come I just couldn`t deck the heads & bolt on injection instead of spending more money on pistions & a blower.....
    1. Depends on if this is a max effort deal. If you are trying to run a class and can get there by doing that OK. But the more air you move through that engine the more hp you'll make. So more blower = more power.
    2. To many variables. How far are the pistons in the hole, head gasket thickness, PV clearance, which heads and how much can be removed before causing yourself other issues?
    Racing is who gets there first, not who gets there on schedule.


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    Quote Originally Posted by wagspe208:1531053
    This will be a great thread.
    Wags
    X2 lol, low compression will work
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    gn7
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    1. A low compression high boost gas engine will kick ass on a higher compression lower boost engine every single time. This isn't even dabatable. Any one that thinks it is, should surrender their blower. The ONLY reason for building a higher compression GAS blower engine is if your looking for part throttle, no or low boost power.

    2 You can never machine the heads enough to raise a 8.1 engine to anything near what a alky engine wants. You can raise the boost enough to make it work, but it will be lazy off boost. But 60psi with 8 to 1 has been done hundreds of times. It was a pretty standard set up at Indy in the Offy years.
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    Administrator Infomaniac's Avatar
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    Yes lower compression and more boost makes more power.

    You end up with a very narrow tune up window with the higher compression boosted engine. The higher the compression the narrower the window. Constant adjustments to changes in conditions.

    Plus there generally is not much selection between 8:1 and 12:1 for shelf pistons for a 468.
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    Distinguished Member David 519's Avatar
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    Quote Originally Posted by gn7 View Post
    1. A low compression high boost gas engine will kick ass on a higher compression lower boost engine every single time. This isn't even dabatable. Any one that thinks it is, should surrender their blower. The ONLY reason for building a higher compression GAS blower engine is if your looking for part throttle, no or low boost power.
    The OP said Gas/Alky and he probably should have seperated them. I do know he's gonna run alky. Never ran/tuned blown gas, but alky likes compression. Probably pretty obvious but higher compression will make more HP at the same boost. My suggestion to the OP was leave everything alone, pull the carbs and put an alky hat on it and hang on.

    Quote Originally Posted by gn7 View Post
    2 You can never machine the heads enough to raise a 8.1 engine to anything near what a alky engine wants. You can raise the boost enough to make it work, but it will be lazy off boost. But 60psi with 8 to 1 has been done hundreds of times. It was a pretty standard set up at Indy in the Offy years.
    Yup

    Quote Originally Posted by Infomaniac View Post
    Yes lower compression and more boost makes more power.

    You end up with a very narrow tune up window with the higher compression boosted engine. The higher the compression the narrower the window. Constant adjustments to changes in conditions.

    Plus there generally is not much selection between 8:1 and 12:1 for shelf pistons for a 468.
    Havent built a little motor in a while, but I know JE makes flat tops and that ends up around 10:1 on 4.25/4.5" strokes... (to lazy to punch in the numbers to check a 468). Also, 12:1 isn't so high alky would become a tuning nightmare... Plus you can make the HP you need without a bunch of boost (we're bracket racers, not class racers)
    Quote Originally Posted by gn7 View Post
    ....... David 519 is 100% correct........

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    .....I think people forget that racing is supposed to fun. Losing shouldn't be discouraging it should motivate you work on your pile to make it faster.....

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    Senior Member tkriley13's Avatar
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    [QUOTE=David 519;1531349] My suggestion to the OP was leave everything alone, pull the carbs and put an alky hat on it and hang on.

    David - how do I cheat on my tune up next season if I dont get compression close enough to my buddies new motor? I have not pulled the motor yet for the upgrade just getting parts lined up. I wanna do the Tom Papp deal in Jan, as well as the Billy deal in Feb. so I`m doing what you told me to do for now...Hopefully by mid season I can step on it & not go swimming..


    Thanks for the info guys,new question though.

    1. what style injector hats do you guys think would work the best for my 568bbc drag boat? I`m assuming a hat doesn`t really affect anything as long as the butterflies are big enough for the amount air/boost someone is wanting to run? Thanks

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    I wonder why the Blower Shop and BDS recommend higher compression? Maybe, it is because there is a limit to the boost or overdrive before the fuel air mixture starts heating too much with lower compressions.

    I know what they told me, but maybe they don't understand blowers.

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    gn7
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    When you say the blower shop and BDS recommend higher compresion, are you talking about alky? Thats pretty much a given. A low compression alky motor with low to no boost is a real loser. But if you run enough boost and do it all the time, like the old F1 and Indy stuff, the compression isn't nearly as high as a A/FC or a TAF or H

    On gas? They do recommend higher compression on street deals with less boost because a unboosted low compression engine on the street is gutless. There is no difference between a low compression blower motor and a unboosted blower motor except your dragging the blower along like a over grown A/C compressor, and the cam in the unboosted blower motor with low compression just makes things even worse.

    Its like was said earlier. A blower motor without the blower on it is a pig. Having the blower, but at part throttle/no boost is no better. If you put some compression to it, its aleast decent when its not under boost.

    Its not that they don't know what they are talking about, its more like the use.
    We have to run a limited blower speed so our compression is pretty high for a blower gas deal. But I would trade 3 full points of compression in a heart beat for twice the boost.
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    I am talking about gas. What I understood was the use of 9.5:1 on gas is more efficient than say 7:1 with higher boost. (using FCR for comparison)

    It seems like once you start turning more than about 20% overdrive (no intercooler) there isn't a great deal of power improvement due to heat build up. There are a lot of variable so little is set in stone, but generally, this is what I believe.

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    gn7
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    Yeah, there is definitely a point where boost over compression falls down. And I would say below 8 to 1 is getting into that territory. Lower compression engines lose there cylinder pressure at an accelerated rate compared to higher compression engine. The cylinder pressure degrades much quicker from its peak.
    But I would love to see a 7 to 1 max effort, max boost gas deal against a 9 to 1. My money says the 7 to 1 engine would take it. Below that I would be come more doubtfull.
    I looked up the compression ratios for alky Indy turbo deals in the late 60's and 70s. They were higher than I first thought. They were up around 9 to 1 which is pretty high considering the boost they ran for a race that long.
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    I may have been comparing apples to oranges. When I set mine up, it was not a max effort set up. I was running a 540 BBC on AV gas with 9.5 CR. The blower is a Blowershop 14-71 that is under-driven 5% producing about 9.5 lb. of boost. I have now switched to e85 and wish I had more compression, but what I have is OK. I just need to switch pulleys and make about 12 lb. of boost and be happy.

    If anyone is interested, other than converting the carburetors and timing, everything is exactly the same and the e85 does make more power. Everything is not exactly the same if you include the need for a bigger fuel pump, regulator and fuel lines.

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    gn7
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    Jim, on anything less than a max effort deal I will trade compression for boost. The closer to a max effort deal, the less compression and the more boost. Like I said, I will give you 3 points of compression is you can convince the powers that be to let me have 12-15 pounds more boost. I would much perfer to squeeze twice the air into a slightly larger area, than squeeze a half air into a slightly smaller area,
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