High Compression VS Low Compression Opinions

[BEATAV8]

I run E85. I decided to step up the static compression from 9.5 to 1 to something higher.

Neal

I was planning on 9.5:1 for my new build. Lol... Now you have me considering if I should target something a bit higher, especially after considering the dynamic compression.

Current combo with static 8.5:1 has a dynamic of 6.53:1 (18.97:1 @ 28psi boost)
New combo with a static of 9.5:1 would have a dynamic of 6.77:1 (19.67:1 @ 28psi boost)

 

[BEATAV8]

^and this is the one that you use to show how the biggest cam you can buy usually isn't the right one....

http://www.wallaceracing.com/dynamic-cr.php

Thanks for posting that Earl. I use that wallace site all the time but never messed with that particular calculator.

They say the V/P ratio is a good "barometer" for gauging the engine's low speed power. I find that useful when using my existing combo as a reference point and then trying to determine if the "new" combo will behave better or worse at low speed. I was pleased to see that even though it has substantially more intake duration, the new engine with more compression and more displacement should still outperform the old one at low RPM. Very helpful when trying to determine whether or not I'm screwing it up! Lol

My old combo generates a V/P of 65 (IVC @ 67.5ABDC).
The new combo would have a V/P of 69 if C.R. is 9.5:1 and a V/P of 74 if C.R. is 10:1 (IVC @ 75.5ABDC)

 

[750H.P.V6]

I was planning on 9.5:1 for my new build. Lol... Now you have me considering if I should target something a bit higher, especially after considering the dynamic compression.

Current combo with static 8.5:1 has a dynamic of 6.53:1 (18.97:1 @ 28psi boost)
New combo with a static of 9.5:1 would have a dynamic of 6.77:1 (19.67:1 @ 28psi boost)

If you have the fuel octane for it I'd say static compression is your friend, the responsiveness of the engine will be vastly improved. I work with a group of ex-road race guys from Honda, Nissan, Porsche etc. In some applications they used to run 16:1 static compression with 21 psi boost on methanol. Needless to say 11:1 is conservative by comparison.

Neal

 

[TTipe]

More compression will let the power happen somewhat easier and with some positive side effects as already stated....
The gap will widen some as you push the turbo harder and approach the limits of the turbo/ compressor and turbine side limitations and efficencies....think less charge air temp from a lesser boost number and lower IAT,s that come along with it for free.....and possibly less turbine drive pressure since you are at a lesser manifold pressure for the same or more wheel power numbers.......Also the ability to use a bigger exhaust housing without killing the spool.....and converter selection can be tighter as well.

Cylinder pressure and the length of the pressure wave in the cylinder make engine torque and the way
that the pressure is generated makes a difference.....
25# boost with 10-1 may make the same cylinder pressure as 35# with 7.5-1.. which one makes more power will revolve around the air temps,drive pressures cam timing and so on and if you take advantage of the beni's that the higher CR can give you....

Supposedly turbo engines benefit from their expansion characteristics in the cylinder by having a somewhat higher
cylinder pressure at the tail end of the Useful part of the downward stroke meaning they can produce useable cylinder pressure for a few more crank degrees.... The deal is pretty much done by 70 Degrees ATDC.
Nitrous and NA engines build the pressure a little bit different than the boosted engines......why do you think the blower cars are so damn loud at the track?? Cause when the exhaust valve cracks open their is way more cylinder pressure than a NA car but the pressure is down to a point it does not produce much usable work on the crank so they begin to empty the cylinder early......

The N2O engine we are building which is a serious but not all out race piece will have the exhaust valve Open
.085" at 95* ATDC on the power stroke.....When the piston is only half way down.....the reason I bring this up is a higher compression lower boost engine may benefit from the exhaust opening event begining a few degrees earlier
since their is less total air in the cylinder to expand and drive the piston down than a low CR Turbo engine......we are talking small changes here but if you are purpose building a high compression turbo engine why not go for all you can get?

But if you are going to be more of lesser boost say 17 to 22 pound kind of deal you can pretty much make up the power difference with some extra manifold pressure to make the power but you lose out on the other free benefits.

With that said my 8.2-1 CR 233 inch with an 18 blade ptc on a 94 degree day spooled my 71HPQ BB very fast And launching at 5# boost was at 30# boost in 5-7 tenths of a second. [.5 - .7 ]......I used a TA 3 1/2" pipe with a 1.00 inch hole in the housing a High pressure actutor and a Perrin adjustable controller and had great boost control...+ or - one-half pound.

My next engine will have more Squeeze and be much more purpose built But unless you want to scrape up an extra grand for new pistons and fitment you can get along with what you have except maybe that 17 blade. FBR

 

[TTipe]

Ultimately "chamber pressure" creates torque or is the ability to do work. The pressure builds under the torque limit curve. The curve can take different shapes depending on the power cylinder firing result. A high compression NA power cylinder( like the Hemi @ 12.5:1) is like an impact. A turbo charged engine is like an aggressive rounded curve to "push" max pressure. The majority of discussion points touched on in this thread are understood through the use of software such as "GT POWER & Star CD. The points made in this discussion are valid, but changes create the need for trade off elsewhere. The question is what can I give up / trade off without destruction or a large penalty.

 

[dank GN]

So I thought I would throw in my current build . I should have a compression ratio right around 9.6ish with a set of custom slugs that Earl help me with . My build should be done very shortly as my rotating assembly is at the machine shop getting balenced and will be completed this week . Attached are acouple pics of my beautiful custom slugs and a spec sheet .

 

[TTipe]

I was planning on 9.5:1 for my new build. Lol... Now you have me considering if I should target something a bit higher, especially after considering the dynamic compression.

Current combo with static 8.5:1 has a dynamic of 6.53:1 (18.97:1 @ 28psi boost)
New combo with a static of 9.5:1 would have a dynamic of 6.77:1 (19.67:1 @ 28psi boost)

I would be curious to see your method of "dynamic compression" calculation

 

[BEATAV8]

I would be curious to see your method of "dynamic compression" calculation

see the wallace racing calculator which Earl linked above:
http://www.wallaceracing.com/dynamic-cr.php
http://victorylibrary.com/mopar/cam-tech-c.htm

 

[ek02]

Be sure to use the intake closing degrees at .006, not .050, when using the Wallace racing dynamic compression calculator. To get that number, divide the advertised duration by 2, add the LSA, subtract any advance, and then subtract 180. That should be the intake closing point at .006.

 

[Wahoo]

I'm just waiting on my machine shop to finish my block so I can order pistons! Ugh I can't wait lol

I'm gonna do a stroked 109 9.5:1 compression ratio, ported heads and intake. Turbonetics 6668 turbo.

Also going to have a custom cam grind, not sure how big of a cam I should go with.

Also thinking of going Speed density, just not sure if I should do SD2 or after market ECM.

 

[bison]

Be sure to use the intake closing degrees at .006, not .050, when using the Wallace racing dynamic compression calculator. To get that number, divide the advertised duration by 2, add the LSA, subtract any advance, and then subtract 180. That should be the intake closing point at .006.

I've been watching this thread. So what is considered good and what bad when using the calculator? That calculator is only as good as what's entered into it and it doesn't take into consideration dilution on a turbocharged engine which has the same effect as chopping off duration. Using .006" valve timing events as you describe isn't accurate unless the correct master was used to grind the cam and even if so the lobe isn't symmetrical and there's still variance. Only way to get accurate into is to actually measure it. I've seen several degrees variance on the .006" numbers with different masters and the same lobe numbers.

 

[ek02]

The only reason I used .006 is that is where most cam company's measure their advertised duration. Duration at .050 will not work with the Wallace calculator. I agree that the only accurate way is to measure the cam being used to get the actual intake closing degrees. Then you have to figure out what fuel to use with the calculated results. Big question.

 

[BEATAV8]

So what is considered good and what bad when using the calculator?

There is no good number or bad number, just a relative number. I found it helpful because I have an existing reference point in a combo which I already know what it does.

 

[bison]

I'm just waiting on my machine shop to finish my block so I can order pistons! Ugh I can't wait lol

I'm gonna do a stroked 109 9.5:1 compression ratio, ported heads and intake. Turbonetics 6668 turbo.

Also going to have a custom cam grind, not sure how big of a cam I should go with.

Also thinking of going Speed density, just not sure if I should do SD2 or after market ECM.

That turbo is a mismatch of wheels and the compressor will fall short big time especially if it's a turbonetics compressor wheel. The Garrett tv45 compressor will go a little further but still not a good match. I had the same turbo on a 9.3:1 engine and it fell off a cliff at 5400 on a 233ci. About 72lbs/min. As a test I put an old cast 72 diesel wheel on with a P trim and it picked up but still fell off at 75-78lbs/min where the turbine became the limiting factor. I have since used the MFS 6766 and hit 82-83lbs/min on a 8.3:1 231ci. You will need to take overlap out, tighten the converter and taller gearing if running that turbo all in especially with added displacement and the turbo being a dud at less than 75lbs/min. You will need plenty of octane


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[Wahoo]

That turbo is a mismatch of wheels and the compressor will fall short big time especially if it's a turbonetics compressor wheel. The Garrett tv45 compressor will go a little further but still not a good match. I had the same turbo on a 9.3:1 engine and it fell off a cliff at 5400 on a 233ci. About 72lbs/min. As a test I put an old cast 72 diesel wheel on with a P trim and it picked up but still fell off at 75-78lbs/min where the turbine became the limiting factor. I have since used the MFS 6766 and hit 82-83lbs/min on a 8.3:1 231ci. You will need to take overlap out, tighten the converter and taller gearing if running that turbo all in especially with added displacement and the turbo being a dud at less than 75lbs/min. You will need plenty of octane


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I'll be running e85. I think this turbo was a cottons performance turbo, it's used, I just got it for a good price, the guy ran mid 10s with it. But I'm sure it's not the best.

 

[TTipe]

I've been watching this thread. So what is considered good and what bad when using the calculator? That calculator is only as good as what's entered into it and it doesn't take into consideration dilution on a turbocharged engine which has the same effect as chopping off duration. Using .006" valve timing events as you describe isn't accurate unless the correct master was used to grind the cam and even if so the lobe isn't symmetrical and there's still variance. Only way to get accurate into is to actually measure it. I've seen several degrees variance on the .006" numbers with different masters and the same lobe numbers.

I have not seen a lot of consideration given to the fuel/s being used. This has a huge impact based on the burn rate, timing and where peak chamber pressure occurs. Obviously purpose driven configurations (strip/street etc.) have a lot to do with the cam, c/r, fuel etc. I did not want to burn race gas every day on the street so my static compression ratio is low (8.72:1) while not pushing my turbo too far out of it's efficiency island. I set my combo up to run 11's (street) on 93 octane if necessary.

 

[turbocamino1]

all i can add ...or have to say about all this...is, you guys are really smart.

 

[earlbrown]

Dynamic compression isn't a rock solid number, even when done right. Using the .006" still doesn't work as far as accuracy. Even at 6 thou, the valve is still open and cylinder pressure isn't always on the way up.

Also when the pressure starts to rise it can start at different points depending on if your supercharged or N/A.


With an N/A at the exact moment the intake seals, there's still a slight vacuum in the cylinder (unless you're over 100% due to ram effect) and the piston has to some up a little to equalize and then start building pressure.


Also, dynamic is waaaaaay inaccurate at less that WOT. But, then again, most people don't care about what's going on as much during part throttle.

 

[imjoesnuffy]

I will say this for a low compression motor, you can throw a mountain of boost and timing at it. I was running as much as 26* and 32PSI on 93 octane and alky.

Funny how we never see any timing info regarding the high compression motors in boosted applications.

 

[Wahoo]

I'm interested to learn what timing high compression builds get away with.