Compression vs Airflow 1

[PFM]

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

 

[MaxRaceSoftware]

Thursday May 13 , 2004
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

 

[mburgess]

PFM,
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

 

[richdubbya]

I think PFM is on the right track. I could be wrong but i dont think this 1930's design was made with swirl in mind, probably more that they didnt have efficent oil seals that could take the heat of the air cooled head, so the stems were pointed down, just a guess though.

 

[PFM]

Richdubbya,

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

 

[richdubbya]

PFM,


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.

 

[MaxRaceSoftware]

PFM, i finally got a chance to re-Dyno the 16:1 CR 393 cid
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

 

[NormPeterson]

If you're getting significant shrouding on the long side with the higher CR, doesn't that imply a somewhat greater pressure drop across the window between the valve and the seat? And even if it's of reasonably brief duration, doesn't that correlate to slightly "lazy" flow?

Norm

 

[PFM]

Larry,

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

 

[MaxRaceSoftware]

PFM..glad to hear CR is working for your combination .

=========================================================
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

 

[PFM]

Larry,

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

 

[dearbornblue]

A dome or dish style piston is desirable. Either of these two designs will enhance orientation the fuel charge (tumble and swirl). Most engine designs cannot acheive a relativly high compression ratio with a dish, so the dome will be the choice for most engines. One thing to consider on a high compression domed application is the overlap cycle. Take a look at your piston dwell time at TDC. Shape the dome as much as possible to enhance scavenging. Of course this is all relative to your head design.

 

[Tmoose]

Just wondering where the ignition timing ended up for best power with the high and low compression versions?

 

[PFM]

Tmoose,

I guess this is directed to Larry Meaux and his motor. For mine there is nothing to use as a comparison.

Regards,

PFM

 

[MaxRaceSoftware]

Just wondering where the ignition timing ended up for best power with the high and low compression versions?
=======================================================

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

 

[Tmoose]

Hi PFM, regarding my ignition timing question, I thought it was you who said -
"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

 

[PFM]

TMoose,

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

 

[MaxRaceSoftware]

Larry,

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

 

[PFM]

Larry,

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

 

[MaxRaceSoftware]

Larry,
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

 

[TTmotors]

I think one major reason power is down on lower compression engines is because the higher compression engine builds a higher vacuum in the intake tract at a lower piston degree after TDC(realistically a shorter distance from top dead center). The higher compression engine should generate comperable pressure in the intake port 14% sooner. Getting the cylinder filling faster and allowing higher turbulence and more homogenous mixture in the chamber for a more effective burn. More air may come in the cylinder at 14:1 but how well is the air mixed.

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.