4genaccordfreak

Hey all.

Getting ready to begin putting my new engine together and certain topics of discussion between friends and I have arisen about compression, making the most of it, taking advantage (without pushing the limits) and what things to consider when choosing pistons to gain an edge over others in similar conditions.

The questions of "the effect of elevation on dynamic compression" is a something I seem to preach about to my close HONDA friends, but the simple fact remains as I am NOT an engineer nor a mathematics expert.

For discussion purposes, I wanted to start this thread to see if others can provide their personal experiences on the matter and whether they have done the equations to properly calculate dynamic compression when an increase in elevation is apparent.

I understand there are other variables that directly effect dynamic compression, such as cam timing, rod length and boost pressure etc. This topic will be about choosing the correct STATIC compression ratio for pistons to yield the appropriate DYNAMIC compression ratio to be similar to those at sea-level.

Case in point:

Piston choice is currently: Arias flat-top - 10.8:1 (1.180 comp height, -3 cc dome)

Bore: 85mm (3.346")

Stroke: Stock GSR - 87.2mm (3.433")

Rod Length: Stock GSR - 137.9mm (5.429")

GSR head - 41.6cc

Currently where I live the average elevation 3500ft above sea-level.

Best pump gas to use in the area is 94oct (10% ethanol) blend.


I have tried calculations on several (automatic) websites that ask for bore, stroke, rod length, static compression ratio

I have calculated between 10.0-10.2:1 Dynamic

Can anyone confirm this calculation?

What experience have you had?

What calculations do you use?

Any further input to add or share...

I have built a number of engines prior to this one with the same idea (aiming 1.0 compression point HIGHER than normal, with good results) I believe I am right where I want/should be as far as dynamic compression ratio goes.

The reason for the discussion is that most people here do not take this elevation increase into consideration. For example: choosing 9.0:1 STATIC, which would yield 8.0-8.2:1 DYNAMIC at this elevation and so forth, at a cost of power across all rpms, especially out of boost, spool-up time and otherwise generally poor performance.

wantboost

iTrader: (2)

For every 1000 feet in elevation an NA motor will see a .2 drop in effective or dynamic compression. That's a constant value.

under boost dynamic compression changes as boost and elevation changes. at 5psi a motor will see a different value with elevation than it would at 10psi and the values aren't constant with each elevation change.

so instead of a constant .2 per 1000ft it could be a .34 change from 0-1000 and .23 from 1000-2000. And that is a rough number, every motor will be different because of things like turbo efficiency, engine volumetric efficiency, ring gaps, cam design, combustion chamber volume and shape, slight variations in piston dome size and shape as well as variations within each combustion chamber, ambient temp and humidity, amount of air each cylinder ingests, etc will all cause dynamic compression to be constantly changing.

4genaccordfreak

I am sure weather changes (air density) have effects on this aswell, which is near impossible to calculate unless it is done every day/every hour.

So, if that theory is correct, and the boost pressure value was made constant at 20psi (for example) would the correction factor also remain constant?
Reason I ask, I am not worried about the ratio in lower boost levels/spool up. More so, at full boost where the ratio is most volatile.

example: 0.23 change/ 1000ft = 0.25 x 3.5 (3500ft) = 0.805

10.8 - 0.805 = 9.995:1

wantboost

iTrader: (2)

to a point, yes. if you live at some extreme elevation over a certain point the oxygen density starts to drop rapidly the high you go compared to the drop from say sea level to 5000ft

For example a USDM b16a at 20psi at sea level has a static compression of 10.29:1

At 0ft its 24.29:1
At 1000ft it's 23.82:1
At 2000ft it's 23.35:1

so that's a .47 drop at 1000ft and .47 drop from 1000-2000ft

4genaccordfreak

Gotcha. Theoretically I should be aprx. 10:1 dynamic running these 10.8:1 static pistons, which is perfect.

Thanks

wantboost

iTrader: (2)

yea like in my case, the "ubermotor" I'm planning on building is an 84.5mm x 92mm stroked long rod motor (5.531" rod length or 140.487mm) which will get me awfully close to 2.1l (2063.72cc) I wish there was a way to run 85.5mm on boost safely, that would get me 2112.86cc, maybe I'll talk to endyn about that lol.

im either going to be using an ls block or a benson sleeved gsr block with my b16 head (which endyn will port, not sure how the volume of the combustion chamber will change so i'll stick with stock values for now). the gsr block has a little over a mm more of deck height than the ls so that might give me some more flexibility on piston choice... again i'll be consulting endyn since I plan to use their custom rollerwave pistons

but for the ease of calculating accurate figures i'll use the ls block

so, 211.84mm deck height, assuming 0 is milled off the head and block (obviously after porting the head will be decked and when I send the block to benson for repair it might be decked as well so I'm going to say 0 until i know a precise value), 92mm stroke, 84.5mm bore, 5.531" rod length, pistons have a 4.5cc dome and a .993" (25.2222mm) compression height. with an OEM thickness gasket (.26") and at a rough elevation of 1100ft above sea level (between atlanta and where I live its 1100-1200 feet roughly but more on the 1100 side) I get the following

displacement - 2063.72cc
rod stroke ratio - 1.53:1 (with a 92mm stroke I shouldn't have to spin the motor much over 8500, 9000max to make the torque and power I want)
piston to deck height - 0.005in (value will change with gsr block)
static compression - 13.04:1
dynamic compression - 12.82:1 @ 0psi

with the 20-25psi of boost I plan to run, (20psi on 93 and methanol, 25+ on q16 and methanol) dynamic compression will range from 30.26:1 to 34.62:1

these numbers might sound high but with a good gasket sealing surface and Cosworths M11 headstuds I should have all of the clamping force in the world. Keep in mind Muckman runs a 13.5:1 static motor on e85 at 20psi on a gt3076r and makes stupid amounts of power (350whp at 5.5psi observed! he made almost 280hp NA with his motor as a baseline IIRC) but given that e85 isn't available here I have to take it easy, which means I may very well have to keep boost on 93 between 15-20 but just like muck the compression ratio and volumetric efficiency of the motor means absurd power all over the place.

It will take extra care and a dead nuts tune to accomplish this but I have faith in my engine building abilities and the abilities of my tuner. I've often been jealous of muckmans powerband and I was going to build a wild motor for my car and this is the path I've decided on. an 11:1 81x89mm stock sleeve motor will be going in my car till I get the unicorn uber motor done.

But I am glad you made this thread. Anytime I see [most] people talking about compression ratios they only pay attention to static and not dynamic, personally I think dynamic is a much more important number as it gives you the compression ratio of the motor under operating conditions not the compression ratio as based off of engine components

turbomaniac

the DCR fals when we use higher degree cam. thats why when we use higher SCR we must use higher degree cam. the DCR at a N/a engine must be not more than 11 and not less than 9
the BDC degrees can be calculated knowing the timing of our inlet camshaft.

ie a 260inlet duration cam with timing 20-60, will have 60 degrees ABDC

i used the wallace calculator http://www.wallaceracing.com/dynamic-cr.php


but now i am using one i ve made my own at ms excel that i think is better than this.

i believe that if using RON93 fuel the DCR at a N/A engine must not be more than 11 and not less than 9
and for turbo engines DCR must be more than 10 and less than 8.

i will be glad to hear your opinions

wantboost

iTrader: (2)

People are running 14:1 turbo motors without issue on e85 and race gas
people are also running as high as 15:1 na motors on race gas, you can run almost 14:1 on 93

It's all in the tune

Endyn has an 85x92 13.5:1 NA vtec motor that makes almost 300hp and 200lbft on 92 octane with no issues and has been running this way for almost 4 years

4genaccordfreak

Yes, basically, if I lived in an area close to sea-level, I would not be making this thread, due to the simple fact that STATIC will be close to DYNAMIC to a point that is not necessary for compensating, especially on a "street" engine.

I have always known Dynamic to be way more important which, in-turn, will help choose the correct Static ratio when choosing pistons, if elevation is something to contend with.

I have just come across too many builds in my current city that people are either not experienced enough, do not have any source of guidance or do not take the time to research these issues and have come up with mediocre results after spending thousands. I know 3500ft elevation isn't much in comparison to other places (like Colorado) where you can see 5000ft+. Granted knowing this information can give people an edge over those who forget how important it is to research before ordering anything or atleast ask a knowledgable person.

This is one I used also, it seemed to give me the most accurate result. Although I did not enter my camshaft (Skunk2 Pro1) spec into the equation, which I know will change it aswell.

turbomaniac

how much ignition advance is needed to run such high CR, especialy at high rpms?

wantboost

iTrader: (2)

not a lot

turbomaniac

mabe sg like 20-25?

above 25 after 4000 rpm (and CR14) i think there will be blood... lol

wantboost

iTrader: (2)

if anything you have to retard timing, depending on fuel quality and other factors.

also flame travel across pistons with large domes is slowed down, mostly resulting in an incomplete burn and other things.

you have to remember the optimum angle to have complete fuel air charge burn and full flame front travel is 20*ATDC so you have to base all of your timing events off of that golden number