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Old 12-09-2002, 09:21 PM   #1
battsup
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Default How do i increase compression?

How do I do this? and is a compression ratio the same as boost on a turbo charger, like 10:1 would be the same as 10 pound boost? I don't want something sticking out of my hood and need something that is affordable, and under $1000.
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Old 12-09-2002, 09:30 PM   #2
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Default Re: How do i increase compression? (battsup)

wow! where to start? compression is similar to boost in a turbo, in fact a turbo just increases compression in the cylinders. for under a grand, the best way to increase your compression is to purchase a set of assembled heads with a smaller combustion chamber than what you currently have. i think the more important question though is what are you trying to do? increasing compression alone may not provide any performance gains, you may be better off changing the cam, intake and exhaust.
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Old 12-09-2002, 09:43 PM   #3
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Default Re: How do i increase compression? (clutchdust)

Ways to increase compression ratio:
thinner head gasket, smaller chamber (or shaved) heads, domed pistons, larger bore/stroke.

Compression is not the same as boost. Compression is the ratio at which the volume decreases from BDC to TDC.

10:1 -> when the piston is on TDC (top dead center, the upper most position) the volume of head/gasket/bore is 10 times smaller than when the piiston is at BDC (bottom dead center, lowermost position)

Boost is volumetric efficiency above 100%, the cylinders are filled from an outside pressure higher than athmospheric.

The compression ratio is a simple calculation, the dynamic compression ratio is similar but takes into account when valves open/close. It's a fixed number too, like the CR.

Cylinder pressure however is dependant on things like rpm, cam specs, intake efficiency,.. and so on. Turbochargers, superchargers and Nitrous Oxide increase the cylinder pressure (NO only on the expansion stroke, not on intake & compression)

Why do you want to lower the CR? Interested in running a longer duration cam?

Marck







[Modified by Twin_Turbo, 7:50 PM 12/9/2002]
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Old 12-09-2002, 09:45 PM   #4
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Default Re: How do i increase compression? (battsup)

Compression is key to a big cam motor. Keeps it civilized in the low RPM range.

In general, higher compression ratios give more power, better fuel economy and mandate higher octane fuel.

For your L-48, you could go with thinner headgaskets, a nice pair of heads with smaller combustion chambers(stock is 76cc, so just about EVERYTHING is smaller), and would reccommend a small cam. You could go with Vortec heads and a cam like the XE262, and get about 300 hp or so. Would be a great pick-me-up! Headers help too.

-Steve

PS Clutchdust- Do you want that book of yours back?
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Old 12-09-2002, 09:56 PM   #5
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Default Re: How do i increase compression? (battsup)

While the effects of increasing cr or adding boost are somewhat similar, they are two different things. To increase cr you reduce combustion chamber volume. Supercharging forces a larger amount of air/fuel into the cylinder but doesn't change the static cr. 10:1 cr refers to the cylinder volume with the piston at tdc being 1/10th the volume of cylinder with the piston at bdc. 10 psi boost refers to the amount of positive air pressure that the blower or turbo is adding to atmospheric pressure.

You can increase cr by using heads with smaller chambers, pistons that take up more space in the combustion chamber, or by using a thinner head gasket. If you can do the work yourself, changing pistons or heads can probably be done for under your $1000 budget if you shop wisely. A head gasket change would be much less but won't provide as much of an increase in cr as pistons of heads could.

While increasing your cr will increase power, the cost/benefit ratio would be high. A better cam combined with a low restriction exhaust could provide a much larger power increase but for the same or less money than a piston or head swap. Ideally, you could do the cam, exhausts, AND some good flowing heads but you'd have to rethink your budget.
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Old 12-09-2002, 10:21 PM   #6
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Default Re: How do i increase compression? (Pacin'California)

steve, sure-if your done with it.

marck, not disagreeing but i'm just saying the net effect is the same. positive boost accomplishes the same thing as a net increase in C/R, yielding higher cylinder pressure at time of combustion. i was in the "ask a simple question, get a simple answer" mode. that's why i asked what he wanted to do. without knowing more specifics about what he currently has or where he wants to go no one can give him any specific or effective information. i'd like to know what a '350' cam is and what exhaust he's got? how much power is he looking for and does he have to deal with smog?
the only bad thing about the forum is you 'meet' people for the first time and you have no idea their level of experience or the base of knowledge they have. i may be wrong, and if so i apologize, but the question sounds like something someone without a great deal of experience might ask. that's not a bad thing. we all started there and some of us are further along than others, i'll be the first to admit i am far from expert. marck, i'd wager your a more experienced engine builder than i am. still, if our posts overwhelm newer people with information they can't digest, maybe that would be a turn off. i still stand by my statment that boost is similar increased C/R
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Old 12-09-2002, 10:26 PM   #7
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Default Re: How do i increase compression? (battsup)

I think if you swap heads with smaller combustion chamber, that would help. I woul dtry the aluminum "Vette" heads from the ZZ4 350 motor.
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Old 12-09-2002, 10:44 PM   #8
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Default Re: How do i increase compression? (clutchdust)

ya im 16 so i have very little or should i say no experience. all the stuff i know is from this forum, tv and books. i don't know what a 350hp cam is but thats what it says on the slip my dad got when the engine was rebuilt, i was hoping soomeone else would know because he doesnt. the reason i want higher cr is to make my car run faster et's. also how can i find out what type of compression ratio i have right now.

i dont think i have the original l48 but a truck block instead, because my dad said its a 4 bolt main whereas an l48 is a 2 bolt main(not sure exactly what this means though) the second thing is the original owner replaced the manifold and carb and put on true duals. i think the headers are stock but not sure.

this is the info i need:
1) how do i find out my compression ratio
2) how do i know if it a 4 bolt main
3) how can i tell if the headers are stock, (i have tools to measure the outside diameter if that helps)
40 im pretty sure that the mainfold is a performer because i have compared photos and that is the one that looks the closest
5) i have no idea how to figure out the type of carb i have and how much cfm it is rated at.

ahhhh, my fingers feel much better now, i will appreciate any input, i know these are newbie questions but im only a kid.

anyhow im off to play some hockey, were playing the americans tonight, ill let you know how we did.
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Old 12-09-2002, 10:48 PM   #9
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Default Re: How do i increase compression? (clutchdust)

I agree w/ ya, CD.

Not exactly the same thing, but have similar affects and intent!

For reference, here's the "350" cam, blueprint version by crane. http://www.cranecams.com/master/apps/chevy10.htm#2. Blueprint

When people say "300 hp" cam, or "350 hp" cam, they're usually referring to 327-300 or 327-350 cams. I do NOT think the 327-350 is the same as the L-82 cam, but could be mistake. For sure, the 327-300 is the same as the L-48 cam. (Junk!)

-Steve

PS CD- could you email me your addy?
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Old 12-09-2002, 11:56 PM   #10
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Default Re: How do i increase compression? (battsup)

Quote:
this is the info i need:
1) how do i find out my compression ratio
2) how do i know if it a 4 bolt main
3) how can i tell if the headers are stock, (i have tools to measure the outside diameter if that helps)
40 im pretty sure that the mainfold is a performer because i have compared photos and that is the one that looks the closest
5) i have no idea how to figure out the type of carb i have and how much cfm it is rated at.
1. Without knowing your head chamber size and piston type, you can't find out your compression ratio. Like the other post said, you have to know the volume of your cylinder and head chamber when piston is at BDC versus TDC.
2. To know if you have a 4 bolt main or two bolt, you have to have the pan off the engine. This refers to your main bearing caps. 2 bolt main is 2 bolts holding your caps on. 4 is 4
3. There are no stock headers. Your car came with iron manifolds. If you have headers and I think you mentioned true duals, then your exhaust is not stock.
4. Your manifold should say edlebrock performer on it if that is what you have. Edlebrock directly in front of carb and performer behind it.
5. You should be able to find something on your carb to identify it. A name and some numbers. Using the numbers, a part store should be able to tell you type and cfm.

Try to track down where your motor was built from the previous owner. If you had a cam slip then they should know. Find out from them what heads, pistons and everything else you can find out so you have a basis of where you need to go with your upgrades. :cheers:
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Old 12-10-2002, 12:06 AM   #11
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Default Re: How do i increase compression? (Pacin'California)

Quote:
I do NOT think the 327-350 is the same as the L-82 cam, but could be mistake. For sure, the 327-300 is the same as the L-48 cam. (Junk!)

-Steve

PS CD- could you email me your addy?
Steve, you are correct. The L-82 cam's profile is similar to the 350/350 with a .450/.460 I/E lift.
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Old 12-10-2002, 05:48 AM   #12
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Default Re: How do i increase compression? (clutchdust)

Quote:
steve, sure-if your done with it.

marck, not disagreeing but i'm just saying the net effect is the same. positive boost accomplishes the same thing as a net increase in C/R, yielding higher cylinder pressure at time of combustion. i was in the "ask a simple question, get a simple answer" mode. that's why i asked what he wanted to do. without knowing more specifics about what he currently has or where he wants to go no one can give him any specific or effective information. i'd like to know what a '350' cam is and what exhaust he's got? how much power is he looking for and does he have to deal with smog?
the only bad thing about the forum is you 'meet' people for the first time and you have no idea their level of experience or the base of knowledge they have. i may be wrong, and if so i apologize, but the question sounds like something someone without a great deal of experience might ask. that's not a bad thing. we all started there and some of us are further along than others, i'll be the first to admit i am far from expert. marck, i'd wager your a more experienced engine builder than i am. still, if our posts overwhelm newer people with information they can't digest, maybe that would be a turn off. i still stand by my statment that boost is similar increased C/R
I see what you mean now. :yesnod:
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Old 12-10-2002, 04:54 PM   #13
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Default Re: How do i increase compression? (battsup)

marck, :cheers:

battsup, now we're getting somewhere. you want to improve your ET, and you want to do it for under $1000. first thing i would do is check the health of the engine you currently have. do you have good oil pressure (above 20# at idle while hot)? do you blow black or blue smoke? black would be an indication of too much fuel, blue (burning oil) could be worn or damaged engine components. can you do a compression test? this is relatively simple to do and the tools aren't very expensive. get a compression tester from a parts store, some may even rent them. pull all eight plugs and disconnect the hot wire to the distributor. then crank the engine over five or six times, just make sure you do each cylinder the same number of times. you can count the number of compression strokes by the number of times the needle jumps. your not really worried so much what the ultimate pressure is so much as that all eight are relatively even. if everything looks good there, then you should worry about everything that mp77 said. if you have a couple cylinders with low compression or your burning lots of blue smoke, then you have to figure out why and that is where your grand is going to go.
having said that, if the engine checks out o.k. and your worried about your ET, then the cheapest thing you can do is change your stall converter and new ring & pinion gear in the rear end. you'll have to figure out what gear you have but that's not that hard either. that's most likely to give you the best improvement per dollar short of NOX.
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Old 12-10-2002, 09:40 PM   #14
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Default Re: How do i increase compression? (clutchdust)

What's a stall converter?
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Old 12-10-2002, 09:50 PM   #15
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Default Re: How do i increase compression? (battsup)

The visco "clutch" between the engine & tranny. Also known as torque converter.
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Old 12-10-2002, 10:57 PM   #16
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Default Re: How do i increase compression? (mp77)

[QUOTE\]
3. There are no stock headers. Your car came with iron manifolds. If you have headers and I think you mentioned true duals, then your exhaust is not stock.
4. Your manifold should say edlebrock performer on it if that is what you have. Edlebrock directly in front of carb and performer behind it.
5. You should be able to find something on your carb to identify it. A name and some numbers. Using the numbers, a part store should be able to tell you type and cfm.

Try to track down where your motor was built from the previous owner. If you had a cam slip then they should know. Find out from them what heads, pistons and everything else you can find out so you have a basis of where you need to go with your upgrades. :cheers: [/QUOTE]

thanks for the info,

1)yup the manifold says performer in the back(cant believe i missed that)
2)i do have headers and not manifolds
3)i found some numbers on the carb, on the front left side by where it is bolted down, "L1 9625SA" but i didnt find any Edelbrock symbols, just writing that said "MADE IN USA" and "CARTER AFB" which were both on the back.

any ideas? ill phone around tomorow, o and almost forgot my team won 5-2
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Old 12-11-2002, 12:15 AM   #17
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Default Re: How do i increase compression? (battsup)

Quote:
[QUOTE\]3)i found some numbers on the carb, on the front left side by where it is bolted down, "L1 9625SA" but i didnt find any Edelbrock symbols, just writing that said "MADE IN USA" and "CARTER AFB" which were both on the back.

any ideas? ill phone around tomorow, o and almost forgot my team won 5-2
Carter is the brand of carb you have. Take the numbers to a good parts store who deals in performance, Oreilly usually does. They should be able to tell you your cfm. :cheers:
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Old 12-11-2002, 12:22 AM   #18
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Default Re: How do i increase compression? (mp77)

Quote:
Carter is the brand of carb you have. Take the numbers to a good parts store who deals in performance, Oreilly usually does. They should be able to tell you your cfm. :cheers:
o man your on a role, many thanks from me thats like 4 times you helped me in one post, thanks man you really know your stuff, or i dont know enough :lol: , either way :thumbs: :cheers:
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Old 12-11-2002, 06:56 PM   #19
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Default Re: How do i increase compression? (battsup)

Just got off the phone phone with the local parts store they told me that the the "625" in "L1 9625SA" tells you that it has 625 cfm, so there i have it i have a 625 cfm carb.
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Old 12-11-2002, 11:08 PM   #20
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Default Re: How do i increase compression? (battsup)

Ok. The original question was compression ratio. If you have a stock set of heads, there should be casting numbers visible under the valve covers. Get these numbers next time you have the covers off. With these, you should be able to find out what size chambers your heads have. You know you have a .30 overbore so that increases the size of your cylinders and decreases compression. You also know from the overbore that you do not have stock pistons. They would have to have been ordered to match your new cylinder size. Hard to imagine someone doing what sounds like at least a mild performance build and sticking in low compression pistons. But to know for sure you would need to find out what pistons you have. You also said you wanted to have a better 1/4 mile time. Do you know what you run now?
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