A different prospective on Compression Ratio

Camo Custom Title Time!

1,936 posts · joined Oct 2007 · 4 Sep 2013, 9:48 am

#968674

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:

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.

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.

He also put up this link http://johnmaherracing.com/tech-talk/what-effect-does-compression-ratio-have-on-bhp/

I just thought this was another way to look at CR and one that I haven't read on here before and others here may come up with some good answers.

The original link is HERE

Cheers, Kev

I thought rebuilding a VW would be cheap - Shit I was wrong, just don't let the wife know !!!!

Click here for my (slowy) modded 1961 Beetle build.

bajachris88 The international telephone dialing code for Antarctica is 6

6,661 posts · joined Apr 2005 · 4 Sep 2013, 11:12 am

#968681

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 👍

(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!

vlad01 Compulsive Aussie Vee Dubber

4,270 posts · joined Jun 2010 · 4 Sep 2013, 4:16 pm

#968703

bajachris88 wrote: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.

71 notchback,
Past owner of, 70 NB, 73 SB and 72 FB TLE

Camo Custom Title Time!

1,936 posts · joined Oct 2007 · 4 Sep 2013, 7:02 pm

#968721

bajachris88 wrote: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) 😂😂😂

I thought rebuilding a VW would be cheap - Shit I was wrong, just don't let the wife know !!!!

Click here for my (slowy) modded 1961 Beetle build.

rose doin it for d girls

2,036 posts · joined Dec 2004 · 4 Sep 2013, 7:19 pm

#968728

to get the best out of a street motor you need low comp.
the lower the comp means the more boost that you can get in & that's what a turbo is about, more boost with out rebuilding your motor regularly 😂
no silly lumpy cam and smooth ported parts.
or you end up with this high idle 😊
.

,

Camo Custom Title Time!

1,936 posts · joined Oct 2007 · 4 Sep 2013, 8:01 pm

#968737

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

I thought rebuilding a VW would be cheap - Shit I was wrong, just don't let the wife know !!!!

Click here for my (slowy) modded 1961 Beetle build.

SuperOwen Fahrvergnugen

868 posts · joined Jan 2009 · 4 Sep 2013, 9:24 pm

#968754

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 The international telephone dialing code for Antarctica is 6

6,661 posts · joined Apr 2005 · 4 Sep 2013, 9:28 pm

#968755

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!

vlad01 Compulsive Aussie Vee Dubber

4,270 posts · joined Jun 2010 · 4 Sep 2013, 11:27 pm

#968775

Camo wrote:
bajachris88 wrote: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,
Past owner of, 70 NB, 73 SB and 72 FB TLE

vwo60 Veteran Volks Folk

2,036 posts · joined Feb 2007 · 5 Sep 2013, 7:48 am

#968784

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.

vlad01 Compulsive Aussie Vee Dubber

4,270 posts · joined Jun 2010 · 5 Sep 2013, 11:16 am

#968807

vwo60 wrote: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

71 notchback,
Past owner of, 70 NB, 73 SB and 72 FB TLE