Compression vs Airflow
Compression vs Airflow
(OP)
Hello All,
I need some help on one of those what is more important in the production of horsepower, compression or airflow? At what point does a reduction in air flow, due to piston valve relief, overcome gains in compression due to piston shape?
OK the picture is this, the piston is a near flat top with a significant valve relief cut into it for valve clearance. We work hard to get good air flow numbers through the heads and then in the motor the valve is shrouded by the piston in the area of highest flow (the long side of the port / valve) as the valve goes through .25 lift.
I understand the piston and valve are moving away from each other and the shrouding effect is not long term but it must affect air flow. So do we re-shape the piston top to reduce the shrouding at the cost of compression or not?
The current compression is 16 to 1, on a guess it would drop to 14 to 1 to re-shape the piston to help the air flow. The effect on air flow is at the moment unknown.
Thanks for any input to this delema.
PFM
I need some help on one of those what is more important in the production of horsepower, compression or airflow? At what point does a reduction in air flow, due to piston valve relief, overcome gains in compression due to piston shape?
OK the picture is this, the piston is a near flat top with a significant valve relief cut into it for valve clearance. We work hard to get good air flow numbers through the heads and then in the motor the valve is shrouded by the piston in the area of highest flow (the long side of the port / valve) as the valve goes through .25 lift.
I understand the piston and valve are moving away from each other and the shrouding effect is not long term but it must affect air flow. So do we re-shape the piston top to reduce the shrouding at the cost of compression or not?
The current compression is 16 to 1, on a guess it would drop to 14 to 1 to re-shape the piston to help the air flow. The effect on air flow is at the moment unknown.
Thanks for any input to this delema.
PFM





RE: Compression vs Airflow
increasing comp ratio puts a sooner harder hit on the Intake System/Chamber and sometimes raises RPM point of peak HP , but seldom moves the RPM point of Peak Torque .
lets say you increase flow 4.864 cfm @ 28 inches
should give you at least 1.25 hp per cylinder times 8 cyl = 10 hp
so if you loose 1 point in CR trying to gain 4.864 CFM it should cancel out, but there are times when Piston Dome might hinder combustion, so flow wins out
its always better to get more Flow and Comp Ratio if possible. Getting 16:1 CR with a flat-top piston is going to be better than getting 16:1 CR with a Pikes Peak Dome
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Compression vs Airflow
pickedup .05 tenths ET which calculates at least 15 HP gain
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Compression vs Airflow
I guess this will get down to two unknowns, first how much flow is being lost and second how much compression will I have to lose to "fix it". I could do a piston mock up to check the fix but a flow bench will not show me the lost air flow. So in the words of old Smokey, cut and try, and that is such a pain in the wallet.
You say the 15 to 16 CR jump got you at least 15 hp, how big is the motor and what is the % gain? I need to run a simple simulation for CR to HP gains on the PC to help the idea along.
Thanks,
PFM
RE: Compression vs Airflow
RE: Compression vs Airflow
As the intake valve opens faster than the piston moves down the bore (after overlap) the intake valve has a major shrouding as the edge of the valve is below the top of the piston (in the valve relief). On this motor the shrouding is along the edge of the valve with the highest flow (long wall).
I am confused by the comment that the increase in c/r should increase air flow. I have not seen this idea before nor do I understand how an increase in compression will increase the draw on the intake system. Cam changes, header changes, and of course changes to the valves, heads, ports, and in this case carbs would all have effects on the air flow into the motor. Please expand on this comment.
Regards,
PFM
RE: Compression vs Airflow
V1 = chamber volume.
V2 = intake port, manifold leg, plenum, carb to the venturi.
More info? read my article:
http://victorylibrary.com/mopar/intake-tech-c.htm#vacuum
RE: Compression vs Airflow
Cheers
Greg Locock
RE: Compression vs Airflow
=========================================================
the Small Block Chevy engine is 393 cid
ran 7.166 ET w/15:1 CR in good air
ran 7.116 ET w/16:1 CR in bad air
.05 hundredths ET calculates 15 HP gain
engine is Methanol-Injected SBC , but i get same results
with Race Gas or Pump gas with increases in each point in Comp Ratio up to 16.8:1 CR tested so far on VP Race Gas
at 600 rpm/sec accel test rate on SF-901 Dyno
http://www.maxracesoftware.com/Dyno_console_4.jpg
BELLE ROSE, La. - Order after 2 rounds of qualifying in Competition Eliminator at the NHRA, Cajun SPORTSnationals:
Psn--Num--Class-Driver, Home Town, Machine-----------------ET---Index---(+/-)
1 4251 H/A Greg Porter, Edmond OK, '03 Chevy Cavalier 8.840 9.46 -0.620
2 214 H/SM Tim Freeman, Roan Mtn TN, '04 Chevy Caviler 9.183 9.78 -0.597
3 4 A/EA David Rampy, Piedmont AL, '32 Bantam 7.410 7.99 -0.580
4 5 PST Brian Self, Hempstead TX, '01 Chevy S-10 7.510 8.09 -0.580
5 56 G/SMA Cordis Johnson, Springdale AR, '03 Chevy C 9.041 9.62 -0.579
6 75 A/A Vincent Deceglie, Alta Loma CA, '63 Chevy 6.625 7.19 -0.565
7 23 H/A Mike Saye, Johnston SC, '92 Oldsmobile Cut 8.897 9.46 -0.563
8 429 K/AA Raymond Martin, Mt Belvieu TX, '32 Bantam 8.022 8.58 -0.558
9 2125 F/AA Tony Stephenson, Willow Sprg NC, '02 Chevy 8.230 8.78 -0.550
10 330 D/D Brian Browell, Lafayette IN, McKinney-Chevy 7.284 7.83 -0.546
11 423 PST Greg Porter, Edmond OK, '00 Chevy S-10 7.555 8.09 -0.535
12 456 C/SMA Jeff Miller, Pearland TX, '99 Pontiac Gran 8.445 8.98 -0.535
13 4337 A/ND Craig Bourgeois, Metarie LA, Hasse-Chevy 7.116 7.65 -0.534
http://www.dragracecentral.com/DRCPhoto.asp?ID=%7B621C4025-4FF1-41F0-921D-47C32AE460D0%7D
http://www.maxracesoftware.com/AND_Dyno3.jpg
http://www.maxracesoftware.com/AND_Dyno4.jpg
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Compression vs Airflow
RE: Compression vs Airflow
Thanks for expanding the idea, I do have a problem understanding the extreme example, it reminds me of calculus, great for the bulk of things and fails at the extreme ends. I took the idea to the PC to try a model, my motor at 9 to 1 at 7000 RPM shows 112.7 VE and 417 CFM, the same motor no cam change but 13 to 1 shows a VE of 111.4 and 412 CFM at 7000 RPM. The program is Engine Analizer PRO, and it is not the end all test but does not seem to agree with the theory when applied to the norm. The program cannot however model the valve shrouding I have a question with.
Running a change in just compression from 14 to 1 to 16 to 1 I see a gain a 3.8% hp (no cam change) If I reduce the air flow by 3.8% there is a 3.6% HP reduction. The air flow reduction is just a WAG to see where it might go.
I still see no clear cut solution to the question.
Regards,
PFM
RE: Compression vs Airflow
RE: Compression vs Airflow
RE: Compression vs Airflow
RE: Compression vs Airflow
Yes the computer modeling has come a long way. The program can change valve opening and closing numbers and show the results. What it cannot do is adjust the head air flow numbers for the short period of time the intake valve is shrouded by the piston, or the possible reversion in the intake track or at the minimum the reduced air velosity caused by the shrouding. The next detail is what if flow gains could be had with say a .2 or .4 point reduction in compression. You see it will get down to cut and try, no I am sure this will not show massive gains but as they say how do you make a 500 HP engine? with lots of little gains, or forced induction or more displacement or NOS but always with more money.
Regards,
PFM
RE: Compression vs Airflow
If I am making too many assumptions here please correct.
Regards,
MB
RE: Compression vs Airflow
Thanks for the input. I can check the crank degrees it is quite a few. The trouble is it is shrouded on the long wall side not the short side and in this small wedge chamber is very high flow to that side. In the hunt for compression the cut is also quite close to the valve circumfrence as well. This is what started the thread I would think there is a ballance here some where. As to how much compression would be lost due to piston trimming (In my first post I was back cutting the whole intake relief) in this small motor 1 cc makes a sizable differences (the dish in the bottom of an intake valve). I am now leaning towards a slightly larger diameter cut with a radius top to help the air flow, I hope to lose between .2 and .4 points of compression.
Regards,
PFM
RE: Compression vs Airflow
RE: Compression vs Airflow
Well this is a very popular engine, millions out there, try a flat 4 VW. The port layout and valve cant puts the piston notch on the long wall in the middle of the cylinder. I would not ask the question at all if this was the short side. I watched the edge of the valve vanish into the piston through intake port while doing a piston to valve clearance check. I have not built record holding car / engine combinations by following the crowd nor by not challenging the norm. I still think there is a ballance to be struck on this motor.
Regards,
PFM
RE: Compression vs Airflow
We did a speedway midget with 84 by 94 mm with Autocraft pistons and Scat crankcase and heads.
We found that we lost power when we went over 14:1.
We blamed it on cylinder distortion heat build up due to lack of a fan, and crankcase movement rather than valve shrouding.
It was over 20 years ago now, so I can't remember all details, but the valve reliefs were not all that deep, as the valves were only a few degrees from horizontal.
I would normally advise go for the compression, but on a VW, I think I would go for the airflow due to the other complications of very high compression in these engines
Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Compression vs Airflow
Order for Qualifying Round 1
COMMERCE, Ga. - SUMMIT FastNews - Order after one round of qualifying in Competition Eliminator at the NHRA POWERade Drag Racing Series, 24th annual Summit Racing Equipment NHRA Southern Nationals presented by Pontiac:
Psn Driver Class Index ET Ov/Un
1. Ian Landies Jr F/A 8.80 8.234 -0.566
2. Mike Saye H/A 9.46 8.912 -0.548
==>3. Craig Bourgeois A/ND 7.65 7.103 -0.547
Craig's dragster is definetly faster "after" going from
15:1 CR to 16:1 CR...solidly in 7.100's even in bad air !!
thru years of DragRacing testing, Hi Compression engines don't loose as much performance in bad air (weather conditions),..as Low Comp Ratio engines
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Compression vs Airflow
I am not familiar with that motor. With the long side radius near the port wall sounds like the chamber was originally designed with alot of swirl. Shrouding the intake valve as you say may no hurt airflow significantly but may change flow pattern within the chamber and directly effect combustion efficiency. Only way to tell would be to test or become very friendly with someone who has a CFD package.
Regards
MB
RE: Compression vs Airflow
RE: Compression vs Airflow
You would be correct, swirl was not part of the picture. To be fair this motor is a VW in name only, new design heads, big valves, healthy ports, good air flow, better rocker design, roller cam etc. etc.. This is not the 1930's plant power source, nor the 1966 40 HP bug of the era. I hope I can get my hands on a second set of pistons to touch up before we get it up on the dyno. I will not re work the only set for the motor to test my ideas but.... I have this feeling.
Regards,
PFM
RE: Compression vs Airflow
The technology of the latest VW air cooled aftermarket components is great, I didnt mean to sound like I thought you were dealing with 30s technology, just that back then i know they didnt have silicone or other high heat rubbers, I'm old enough to remember some of the first motors i worked on as a kid had hard exhaust valve seals(asbestos?), and motors with 30k would have the rubber valve umbrella seals as hard as glass from normal heat.
Not that it means much, but i think work unshrouding the piston notch will give a noticeable gain.
RE: Compression vs Airflow
SBC engine since going from 15:1 CR to 16:1 CR
w/15:1 CR = 853 peak Hp at 8700 to 8800 (best overall curve)..almost 186 mph 7.16 in good air
w/16:1 CR = 872 peak HP 8900-9000, ...almost 188 mph 7.103 ET bad air
--------------------------------------
just changed to new cam on Dyno
and the increase CR let us effectively use more camshaft
with new cam change = 892 peak HP at 9000-9200 (best curve)
with more "average HP" between 8000 to 9000 RPM than we were making "Peak HP" with 15:1 CR
(864 average HP from 8000 to 9000 w/new cam)
also almost 1/3rd of back-to-back Compression Ratio engine Dyno tests i've done thru years show that as CR is increased, the RPM point of Peak HP goes higher by a couple of hundred RPMs...same results in the A/ND SBC engine before the Cam change
i wish NHRA would let A/ND run some sort of HoodScoop over injectors because the MPH is low for the ET
on the 7.16 ET w/15.:1 CR
my Program shows at least 853 HP @ 60 FT mark
830+ HP at 660 Ft ...but only 800+ HP at 1320 Ft
the air at 185 to 188 mph blowing straight across injector tubes is not helping :)
PFM..suppose you had no clearance volume above the piston @ TDC ...then the very second the piston moved , you would be tugging on the Intake System air column
and there also would be no space left for exhaust gas residuals @ overlap , if you happened to get the Header pipes wrong..
thats an extreme example , but as you go to higher CR,
you are going to benefit most of the time unless you had some kind of burn-rate problem
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Compression vs Airflow
Norm
RE: Compression vs Airflow
Well I just got back from the dyno as well, and it looks like compression is king. I would love to try to give the valve notch a rounded corner but the power is up across the board, the fuel stand-off is down and we have not finished tuning as yet. And yes we put more cam in with the increased compression as well. We started moving the heads around a bit and lost a head gasket so we had to stop for now. I like the zero volume analogy, that one I can see. This is still not the end of the question for me but it looks like the compression increase wins over the momentary valve shrouding. Thanks to all that added there input to this post.
Regards,
PFM
RE: Compression vs Airflow
=========================================================
PFM..suppose you had no clearance volume above the piston @ TDC ...then the very second the piston moved , you would be tugging on the Intake System air column
==========================================================
increasing CR , is sort of like increasing cam duration...
CR lets you start moving the air column "sooner" ,
and CR lets you close the intake valve "later" !
and CR makes up for the fact that you are running out of air at the Peak HP RPM point and higher.
those reasons are probably why i sometimes see an increase in the RPM point of Peak HP with increases in CR
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Compression vs Airflow
2.27 Hp / CI is not a bad number for a pushrod motor, with no forced induction. That is on alki as I recall. Still a good number.
PFM
RE: Compression vs Airflow
RE: Compression vs Airflow
RE: Compression vs Airflow
I guess this is directed to Larry Meaux and his motor. For mine there is nothing to use as a comparison.
Regards,
PFM
RE: Compression vs Airflow
=======================================================
33 to 34 deg BTDC produced the best
overall combination of best average
between 8000 to 9000 rpms,
for both 15 or 16 CR in this engine.
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Compression vs Airflow
"Well I just got back from the dyno as well, and it looks like compression is king. ..snip... but the power is up across the board, the fuel stand-off is down and we have not finished tuning as yet. And yes we put more cam in with the increased compression as well. "
I'm a bit confused, as it sounded like there were dyno runs of a low compression and high compression engine (also with a cam change) .
Dan Timberlake/tmoose
RE: Compression vs Airflow
There were too many changes to have a comparison of ignition timing on this motor, bore, stroke, compression, rod length and cam were all changed from the previous build. For the hard number the first go around ran 32 degrees, the new build 30 degrees, we rushed the motor to the track so more dyno time is needed to get it all worked out, that is 30 degrees may not be final. My quote was more about not working on the piston top to aid airflow while reducing compression. In a perfect world I could build a mule motor and just do development work, I am sure there is a ballance to be struck, in this world I work with what I have.
PFM
RE: Compression vs Airflow
2.27 Hp / CI is not a bad number for a pushrod motor, with no forced induction. That is on alki as I recall. Still a good number.---PFM
======================================================
PFM, just ran the engine/dragster this weekend at Georgia Points Meet,..the increase in CR from 15:1 to 16:1 and new cam (892-895 Peak HP @9000-9100 / 864 avg 16:1 CR) ran 7.07 ET at 189.5 mph in bad air (cast-iron SBC heads)
came a long way from :
w/15:1 CR = 853 peak Hp at 8700 to 8800 (best overall curve)..almost 186 mph 7.16 in good air
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Compression vs Airflow
That is a solid improvement!!! I have a couple of questions, what does the dragster weigh in track trim with driver? In one of the posts you gave the timing that gave the best "average power", I will assume that there was a compromise in there, I know alki is forgiving but.... Is a programable ignition legal in your class? On a motor to motor basis I have seen some good gains with this setup. Give the motor exactly what it wants at all RPM, I am not talking cylinder to cylinder control here just a programable timing curve. Also do you have some form of data aq to let you know what the "used power band" is? By the by I have also seen near nill gains on some motors as well. Thanks for the info.
PFM
RE: Compression vs Airflow
That is a solid improvement!!! --PFM
======================================================
yeah..a pretty good confirmation on how important CR is in your original question you posted !!
just like
klonowski (Automotive) on May 10, 2004 Posted ==>
"If you drop cr from 16 to 14 you won't compensate it with better piston overflow. Belive me. I prepare racing engines "
1375 Lbs. with driver
and Mag in this engine and 16 volt system
no programmable ignition allowed !
you have to have a Mag with this much CR and Methanol injection..even this Mag looks like its borderline ??
http://www.maxracesoftware.com/AND_Dyno3.jpg
RacePak onboard data recorder w/EGTs
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
RE: Compression vs Airflow
The air flows into the chamber later giving it less time to swirl and tumble for an even mixture at 14:1. In my research through countless books iv found that the cylinder is layered from top to bottem of the chamber in fuel ratios. The top being as high as 22:1 and the bottem being as low as 7:1 air to fuel ratio. This doesnt come from my research but from others as I cant personnely attest to this.
If this data is true. You will effectivly get more "time" out of a mixture on a high compression engine to mix more effectivly. Giving higher efficiency on a spark engine and allowing a higher tolerence to detonation over one of lower compression. Saying pound for the pound the compression and homogenous mixture allows for more spark advanced per psi/heat in the chamber before the Pre-detonation occus. Which allows for higher pressure when the piston is at the top of the cycle where massive pressure can be produced on the piston face. Effectivly starting the "power stroke" earlier and having a flame in the chamber over a longer period of time. If the flame is in the chamber for a longer period of time it would seem the mixture would burn more thorouly as well and the lashing flames would stir the mixture and allow the heat to ignite otherwise unusable mixture ratio`s that at that paticular psi and heat level would not ignite in a lower compression engine.
The down side is the compression ratio is usually approxiamatly equal to the expansion ratio. Assuming a non adiabatic compression of the air volumn upon compression you will not reclaim all the energy on the expansion stroke and this is expecially the case on engines with early opening Exhaust valves for high rpm use. But with so much emphasis on early flowing intake ports it seems that the effect is cancelled by the additional fuel flowed into the chamber by use of wave tuning the exhaust header.
This atleast in my mind makes since. I appreciate any critic.
RE: Compression vs Airflow
RE: Compression vs Airflow
Quote from Darin Morgan =>
The Pro stock engines are only 15 to 1 static, but the dynamic compression ratio is about 17.5 to 18 to 1. The small blocks you are talking about probably have 16.5 to 1 but there dynamic is still 17.5 to 1. Why? because you fill the cylinder less so you have to compress it more. We fill the cylinder A LOT more so we need to compress it less in order to extract the same amount of energy. Some of our engines only have 14.8 to 1 because they are so efficient at trapping the air ( high VE ). When you say competitive comp engines what engines are you talking about? Single carb or restricted induction system engines of course need a couple points more compression to make up for the decreased VE.. The 358cid Pro Stock truck engines where only 14.5 to 1 and they made 968 HP . With the cam technology we have now there is no doubt in my mind that they could produce 985 to 990 HP now at 14 to 1. Its been a couple of years since I did one of those so I am speculating.
Darin Morgan
R&D-Cylinder Head Dept.
Reher-Morrison Racing Engines
1120 Enterprise Place
Arlington Texas 76001
817-467-7171
FAX-468-3147
Visit our web site at http://www.rehermorrison.com
Larry Meaux (maxracesoftware@yahoo.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers
Support Israel - Genesis 12:3
http://www.maxracesoftware.com/