Subject: A different prospective on Compression Ratio
Camo
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posted on September 4th, 2013 at 09:48 AM
A different prospective on Compression Ratio
Hi all,
was just reading this over on the samba site and thought the author made some good points and will also allow you to tell me what you think. This is
also interest to me as I hope to turbo my engine soon.
This is quotes from other site:
[size=4]How do you guys build your engines? Do you base everything off static or account for the cam and use dynamic CR? Ive been doing a lot of
reading and have come across this a lot recently. People around here throw around static CR like thats all that exists. Either that or Im just not
realizing when people mean one or the other.For example, my new build has a static of 9.82:1 but because the FK44 duration my dynamic CR is 8.18:1 so
I base all my tuning, fuel and boost levels based on 8.18:1 correct? Of course not taking into account my boost levels but just plain old CR before
boost. [/size]
[size=4]I need to add this also. Some of you may remember my disgust with how my previous combo ran off boost right? Well after doing the calculations
based off the WAY TOO BIG cam that was in it before (320 advertised-287 @ .050, .400" lift at the cam (plus my 1.4 rockers put me at .560") on 110
LC. The static on that was about 7.5:1 and taking the cam into account I was running a WHOPPING...Get ready for it lol....4.4:1 Compression ratio
before boost UN....BELIEVABLE! How the hell did it even run? LMFAO! No wonder I could boost 20lbs on pump gas haha.[/size]
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posted on September 4th, 2013 at 11:12 AM
This is very interesting!
I suppose the camshaft manufacturer's take note of this and thats the reason why the static compression ratio recommendations increase with the
increase in cam profiles, to keep the dynamic comp up.
What is even more cool then, if if you adjust your cam timing, you could increase or decrease your dynamic comp ratio
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posted on September 4th, 2013 at 04:16 PM
Quote:
Originally
posted by bajachris88
This is very interesting!
I suppose the camshaft manufacturer's take note of this and thats the reason why the static compression ratio recommendations increase with the
increase in cam profiles, to keep the dynamic comp up.
What is even more cool then, if if you adjust your cam timing, you could increase or decrease your dynamic comp ratio
yep, I have know about dynamic CR for a while, plus exhaust design in conjunction with cam profile. There is a lot to it.
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Camo
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posted on September 4th, 2013 at 07:02 PM
Quote:
Originally
posted by bajachris88
What is even more cool then, if if you adjust your cam timing, you could increase or decrease your dynamic comp ratio
Exactly right.
People always ask, What compression ratio are you running". I know that it is only a starting point for reference, but I have had a couple of people
say I should pull the heads off my engine and check what the compression ratio before you turbo charge it.
Looking at this bigger picture now, does it really matter. Yes I might have to run lower boost but If you have a good tuner and he can tune to
maximum performance before detonation, correct mixtures etc that would be the main thing wouldn't it (OK, start the Kev bashing now)
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posted on September 4th, 2013 at 08:01 PM
Hi Rose,
I understand the lower the CR the more boost you can get in. But just as an example, if you had say 7.5:1 CR and threw in say 15 lbs boost and got
200HP is that not the same as say 9.5:1 CR and 10 lbs of boost (not exact figures, just made up) and still got 200HP.
Or another way of looking at it, your CR plus boost equates eg 15:1 compression and the higher CR plus less boost equates to 15:1 compression are they
not very similar.
Kev
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posted on September 4th, 2013 at 09:24 PM
It's all guess work but chances are the lower boost combo won't produce as much peak power. Although it may be more efficient with the higher comp
the motor just isnt processing as much air. You'll gain off boost response etc so it depends on what your after.
Or you could man up and run big boost and high compression
bajachris88
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posted on September 4th, 2013 at 09:28 PM
running low comp will result in less power prior to turbo spooling up... just to add more thought to the thread.
Can be an issue for a streetster... particularly when you got say a sizable turbo that boogies from 4k rpm onwards. It will feel like centuries pass
before zoom kicks in.
Now... if you could have variable cam timing through the rpm range, you could run high dynamic comp at low rpm, and tailor a lower dynamic comp at
higher rpm for boost . That'll be groovy!
(ô_!_/ô) (ô_!_/ô)
69' baja: kombi box, thing spindles, irs, disc front, type 3 rear drums, 2 inch lift kit, 31x10 rears.
New engine in process: 94mm p&bs, 74mm C/w chomol Crank, 35.5x39 SP heads, turbo. Wierd combo, hopeful torque monsta!
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posted on September 4th, 2013 at 11:27 PM
Quote:
Originally
posted by Camo
Quote:
Originally
posted by bajachris88
What is even more cool then, if if you adjust your cam timing, you could increase or decrease your dynamic comp ratio
Exactly right.
People always ask, What compression ratio are you running". I know that it is only a starting point for reference, but I have had a couple of people
say I should pull the heads off my engine and check what the compression ratio before you turbo charge it.
Looking at this bigger picture now, does it really matter. Yes I might have to run lower boost but If you have a good tuner and he can tune to
maximum performance before detonation, correct mixtures etc that would be the main thing wouldn't it (OK, start the Kev bashing now)
No, you are absolutely right. Some many people say you have to have turbo cam grind, low comp etc...
Stock engines with CR of 9:1+ tripling HP somewhat reliably says it all. In all these cases the turbo setup chosen was more or less matched to the
stock engine and tuned to the safe limits for the configuration. I was blow (pun indeed) away by the results.
71 notchback,
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posted on September 5th, 2013 at 07:48 AM
A higher static compression ratio and lower boost makes the car more drivable, no lag, that what i am after when i build my new engine, loose the
carbs/carb from the equation and fit fuel injection for the best resalt.
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posted on September 5th, 2013 at 11:16 AM
Quote:
Originally
posted by vwo60
A higher static compression ratio and lower boost makes the car more drivable, no lag, that what i am after when i build my new engine, loose the
carbs/carb from the equation and fit fuel injection for the best resalt.
yup +1
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