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compression and boost ?

This is a discussion on compression and boost ? within the Forced Induction forums, part of the LSx Technical Help Section category; I'm looking to up the boost on my 98 SS. I have a vortech G-trim supercharger, with a procharger inter-cooler. ...

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    Member SmokinSS's Avatar
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    compression and boost ?

    I'm looking to up the boost on my 98 SS. I have a vortech G-trim supercharger, with a procharger inter-cooler. I was running 6lbs and I'm thinking about throwing on the 13lb pulley. This was the next jump up that vortech offered to me. I was looking for something around 10 pounds but Vortech only had 6 pounds and then 13 pound pulleys. I realize that I won't truly make 13 pounds of boost with the inter-cooler. However, I'm worried about the compression being to high. Do you guys think that I should look into lowering compression? If so what would be the best way to lower the compression? Does any company make larger head gaskets to lower compression(can't find anything)? Anyone else running a similar setup, I'd be interested in knowing how it's working out. Thank you.

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    Member NoscamaroSS's Avatar
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    A set of 317 heads from a 6.0 will lower it to approximately 9.5:1 and will also flow alot better than your factory 98 heads.
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    Lower compression pistons would help too. As flow is'nt as important as detonation on a boosted engine..
    Last edited by Smittro; 03-15-2010 at 11:20 AM.

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    If I had to choose between either of those two options, I would choose to go with the heads for a number of reasons.

    For starters, lowing the compression with heads will be better for you then lowering the compression with dished pistons and keeping the currents heads on there. If the heads flow better, your whole engine will run better, regardless of what compression ratio you're running.

    Next, theres the bang for your buck aspect. I'd suspect you could find someone selling a set of lightly used 6.0 heads for a cple hundred, maybe drop another cple hundred and have someone port them for flow. Plus, they're easier to change. Granted, there is more room to work, but a few weeks ago, 3 of us (all extremely mechenacilly inclined though none of us are mechanics) swapped heads on my friends truck in about 8 hours start to finish. Even if you're paying a shop to do it, its the difference between a $500 labor bill and a $1500 one.

    There are at least 2 other options that might be worth considering.

    A water/alcohol injection kit. Pretty good on price at $500 or so, and easy to install yourself. Lets you keep stock compression and run that boost, and keep your timing advanced since by injecting alcohol you're increasing the overall octane rating of your fuel mixture.

    The other is a cam swap. You could keep the stock heads, and by specing out your cam properly get the effective compression lowered to a level that you want. You'll need a custom grind, as you'll be leaving your intake open a bit longer than probably any of the standard cams out there, but by going this route, (you'll need new springs at least too) you can pick up some lift and overall duration so you'll grab even more power that way.

    As for whether or not you even NEED to lower your compression, thats another question entirely. Hell if this is your weekend and beautiful fridays only car like my Camaro is(its seen about 5k miles in 2 years), then maybe the extra $3 a tank that putting in a bottle of octane booster in every tank would cost you isn't so bad. Compression isn't bad if you've got the fuel to handle it. The C5R vettes ran something like 19:1 compression on their race fuel I believe.

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    Quote Originally Posted by gashkerwin View Post
    If I had to choose between either of those two options, I would choose to go with the heads for a number of reasons.

    For starters, lowing the compression with heads will be better for you then lowering the compression with dished pistons and keeping the currents heads on there. If the heads flow better, your whole engine will run better, regardless of what compression ratio you're running.

    Next, theres the bang for your buck aspect. I'd suspect you could find someone selling a set of lightly used 6.0 heads for a cple hundred, maybe drop another cple hundred and have someone port them for flow. Plus, they're easier to change. Granted, there is more room to work, but a few weeks ago, 3 of us (all extremely mechenacilly inclined though none of us are mechanics) swapped heads on my friends truck in about 8 hours start to finish. Even if you're paying a shop to do it, its the difference between a $500 labor bill and a $1500 one.

    There are at least 2 other options that might be worth considering.

    A water/alcohol injection kit. Pretty good on price at $500 or so, and easy to install yourself. Lets you keep stock compression and run that boost, and keep your timing advanced since by injecting alcohol you're increasing the overall octane rating of your fuel mixture.

    The other is a cam swap. You could keep the stock heads, and by specing out your cam properly get the effective compression lowered to a level that you want. You'll need a custom grind, as you'll be leaving your intake open a bit longer than probably any of the standard cams out there, but by going this route, (you'll need new springs at least too) you can pick up some lift and overall duration so you'll grab even more power that way.

    As for whether or not you even NEED to lower your compression, thats another question entirely. Hell if this is your weekend and beautiful fridays only car like my Camaro is(its seen about 5k miles in 2 years), then maybe the extra $3 a tank that putting in a bottle of octane booster in every tank would cost you isn't so bad. Compression isn't bad if you've got the fuel to handle it. The C5R vettes ran something like 19:1 compression on their race fuel I believe.
    I agree with just about everything you've said. But with more durration usually comes lope. How long do you think that blower will last with a bunch of lope? Cam lope is usually best left to the N/A guys. When it comes to boost lope tends to be harder on boost epuiptment especially on a DD..
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    TunedbyFrost.com Tuner Frost's Avatar
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    Quote Originally Posted by Smittro View Post
    I agree with just about everything you've said. But with more durration usually comes lope. How long do you think that blower will last with a bunch of lope? Cam lope is usually best left to the N/A guys. When it comes to boost lope tends to be harder on boost epuiptment especially on a DD..
    what??

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    Quote Originally Posted by Frost View Post
    what??
    Exactly what I said.. Pretty basic stuff dude. Long duration= lope, and pulsating turbo or blower.. Shortens their life and reliablity.. I highly doubt he's building a drag car.. Lopey cams on DD's tend to not work well..
    Last edited by Smittro; 03-14-2010 at 01:17 PM.

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    ughhh...

    I'm just gonna pass on this one

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    Quote Originally Posted by Frost View Post
    ughhh...

    I'm just gonna pass on this one
    Good idea.... Study up on cam profiles then we'll talk...

    "Since many hot rodders want their hot rods to sound the part, an idle with more lope to it might be desired. To get a stock-type engine to do this, a cam with more valve overlap—which can be achieved through either more duration or tighter lobe-separation anglewill be needed. Idle quality will decrease with the increase in duration or decrease in lobe-separation angle because the valve overlap causes weird vacuum signals at the carburetor, making the fueling inconsistent at idle. So if you are running a stock automatic transmission and converter and vacuum-assisted equipment (power brakes, for example), a lopey idle might not be what you want."

    http://www.hotrod.com/techarticles/p...ion/index.html
    Last edited by Smittro; 03-14-2010 at 07:32 PM.

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    ...
    Last edited by Wheels; 03-15-2010 at 02:44 AM. Reason: not worth getting into

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    Quote Originally Posted by Smittro View Post
    Good idea.... Study up on cam profiles then we'll talk...

    "Since many hot rodders want their hot rods to sound the part, an idle with more lope to it might be desired. To get a stock-type engine to do this, a cam with more valve overlap—which can be achieved through either more duration or tighter lobe-separation anglewill be needed. Idle quality will decrease with the increase in duration or decrease in lobe-separation angle because the valve overlap causes weird vacuum signals at the carburetor, making the fueling inconsistent at idle. So if you are running a stock automatic transmission and converter and vacuum-assisted equipment (power brakes, for example), a lopey idle might not be what you want."

    http://www.hotrod.com/techarticles/p...ion/index.html
    I am not talking about increasing overlap at all. I am talking about leaving the intake valve open during the compression stroke for a little longer. If my understanding of cams is correct, the exhaust valve had better be closed during the compression stroke, or you;re going to have real problems.

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    Quote Originally Posted by Frost View Post
    ughhh...

    I'm just gonna pass on this one
    LOL maybe i should take him for a ride in the TTA ?

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    Quote Originally Posted by devil6rr View Post
    LOL maybe i should take him for a ride in the TTA ?
    Deal! Then I'll take you for a ride in an all one off TT with 4 custom cut cams one day...
    TT halted ugh but yay.....
    Last edited by Smittro; 03-15-2010 at 11:20 AM.

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    Quote Originally Posted by gashkerwin View Post
    I am not talking about increasing overlap at all. I am talking about leaving the intake valve open during the compression stroke for a little longer. If my understanding of cams is correct, the exhaust valve had better be closed during the compression stroke, or you;re going to have real problems.
    I was talking about lobe durration. Not so much overlap. But you're right.

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    Correct. You don't want a long-duration, big-overlap cam for a blown application unless you want all the blowers forced induction to go right out of the exhaust port.

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    Quote Originally Posted by Smittro View Post
    I agree with just about everything you've said. But with more durration usually comes lope. How long do you think that blower will last with a bunch of lope? Cam lope is usually best left to the N/A guys. When it comes to boost lope tends to be harder on boost epuiptment especially on a DD..
    Quote Originally Posted by Smittro View Post
    Exactly what I said.. Pretty basic stuff dude. Long duration= lope, and pulsating turbo or blower.. Shortens their life and reliablity.. I highly doubt he's building a drag car.. Lopey cams on DD's tend to not work well..
    Quote Originally Posted by Smittro View Post
    Good idea.... Study up on cam profiles then we'll talk...

    "Since many hot rodders want their hot rods to sound the part, an idle with more lope to it might be desired. To get a stock-type engine to do this, a cam with more valve overlap—which can be achieved through either more duration or tighter lobe-separation anglewill be needed. Idle quality will decrease with the increase in duration or decrease in lobe-separation angle because the valve overlap causes weird vacuum signals at the carburetor, making the fueling inconsistent at idle. So if you are running a stock automatic transmission and converter and vacuum-assisted equipment (power brakes, for example), a lopey idle might not be what you want."

    http://www.hotrod.com/techarticles/p...ion/index.html
    Intake duration + Exhaust duration /4 - LSA x 2 = overlap

    If duration somehow hurt a procharger or turbo, I highly doubt the EPP blower cam would sell as well as it does. Its used in everything from 370ci up to and past 408ci. With specs of 232/240 @ .050", its got a fair amount of duration and has no problems at all. I cant understand how duration would be harmful to any turbo/supercharger.

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    Quote Originally Posted by 35th-ANV-SS View Post
    Correct. You don't want a long-duration, big-overlap cam for a blown application unless you want all the blowers forced induction to go right out of the exhaust port.
    All I'm trying to say too bro. But seems to be getting unfriendly here over it..

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    Quote Originally Posted by Hi-Po View Post
    Intake duration + Exhaust duration /4 - LSA x 2 = overlap

    If duration somehow hurt a procharger or turbo, I highly doubt the EPP blower cam would sell as well as it does. Its used in everything from 370ci up to and past 408ci. With specs of 232/240 @ .050", its got a fair amount of duration and has no problems at all. I cant understand how duration would be harmful to any turbo/supercharger.
    Well finally the cam guy shows up. I'm thinking that a pulsating turbine or blower would serve to cause excessive wear on the their internals.. After all we are talking about a DD that will see (most likely) a lot of traffic congestion and lots of idle time. Lopey cams rarely do well as in dd drivablilty. Least non that I've seen over the years. For the track only is one thing but for a dd is another as you well know.. I'll be running a -6 +6 on the 4 cammer. Canceling out over lap.. Yes it will have a "mild" lope but won't effect it's drivablity..
    Last edited by Smittro; 03-15-2010 at 11:39 AM.

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    are we talking up the boost on a stock motor? if so i think you can only go about 8lbs. at most.

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    Part of the original poster's concern was compression. For the record, I'm running 10lbs boost with 10to1 compression. Cam spec is ADV DUR, int 281, exh 289, DUR @ .050 INT 232, EXH 240, LIFT .595 INT .608 EXH, LSA 115.0 works just fine on 93 octane, Thank You. Lope has no effect at all & you can darn sure tell it aint no sleeper. Just the facts, not hear say.

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