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]

i see 10 to 25 hp gains for each point in compression ratio

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

 

[MaxRaceSoftware]

just 2 weeks ago, we went from 15:1 CR to 16:1 CR with more dome , and ran 7.11 ET in 1/4 mile in worse air conditions than we ran 7.16 ET in better air with 15:1 CR

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

 

[PFM]

Larry,

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

 

[SMOKEY44211]

I think there may be a flaw in your logic. Increase in c/r. should increase air flow. I can't see where piston shape would have any influence on intake flow except during I/E overlap period. There should be additional gains by changing valve opening and closing events to compliment a higher c/r.-------Phil

 

[PFM]

Smokey,

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

 

[ejit]

Increase in CR will momentarily increase vacuum since the ratio of the volumes being compared has changed. Effective vacuum after TDC ~ V1/V2.
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

 

[GregLocock]

That's not what your article says. It says V1 is the full swept volume of the cylinder.

Cheers

Greg Locock

 

[MaxRaceSoftware]

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

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={621C4025-4FF1-41F0-921D-47C32AE460D0}

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

 

[SMOKEY44211]

Perhaps if we review the dynamics involved my statement will make more sense. The primary force available to fill a cylinder is atmospheric pressure. The only way we can introduce fresh air into it on a normally aspirated engine is to form a depression via piston downward movement. Let's take an extreme example of an engine with a 1:1 cr. The best we can do in this situation is achieve only about 1/2 cylinder filling. Go to the other extreme with a combustion chamber that is zero volume. Any downward piston movement is going to form a depression approaching a perfect vacume. In a situation where combustion chamber volume is very small a shorter intake valve event should offer better entrapment then what seems optimum on a relatively lower cr. engine. I can appreciate your concerns about intake valve shrouding via piston crown during the overlap period. If the engine in question is relying on exhaust gas inertia and intake sizing to achieve greater the 100% cylinder filling at a certain rpm,that may enter into the equation. However I believe that once you adjust the valve timing events to take advantage of the smaller combustion chamber, torque #'s will be up both at torque peak and throughout the entire rpm range.---------------Phil

 

[PFM]

Phil,

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

 

[ejit]

At TDC the swept volume of the cylinder is the chamber size.

 

[klonowski]

If you drop cr from 16 to 14 you won't compensate it with better piston overflow. Belive me. I prepare racing engines

 

[SMOKEY44211]

PFM. I am not familier with the software you are using. Is it sophisticated enough to adjust the valve timing events? If so I think you will find the lost flow and very likely some gain if you cut back the intake opening and closing events. Exhaust timing should have some effect also. As you probably well know in the real world its a very time consuming exercise to optimise the valve timing events for a particular engine system. Pretty amazing how with a few mouse clicks you can guage performance characteristics that used to take hours of cut and try.-----------Phil

 

[PFM]

Phil,

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

 

[mburgess]

Do you know how long (crank deg) that the valve is shrouded by the piston & can you measure from the top edge of the piston cut to the margin of the inlet valve. 2 points of compression is alot of material removal from a piston. Also the piston is cut on the short turn side of the port & may not affect the flow as much as you would think. How much of your airflow is in the section of valve coming off the short turn radius. Additionally the airflow into the chamber there would have high amount of tumble as such the maintain airflow you may no need to relieve out the piston that much possibly just chamfered back to the top of the margin point at maximum interference.
If I am making too many assumptions here please correct.
Regards,
MB

 

[PFM]

MB,

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

 

[richdubbya]

I would be interested to hear what engine this is, i have never seen an engine that the valve relief was not on the short side radius of the port and close to the cylinder wall. In a normal engine that the relief is on the short side, you will lose power by unshrouding on the intake at all. What you want is the absolute minimum that still allows the valve to not hit the piston on the valves outside edge. Draw a picture on a note pad, there is no place for the flow to go below the deck unless you make a place! lol. Seriously there is no power to be gained there, only a loss in compression and an increase in combustion space on the far intake side resulting in loss of combustion efficency.

 

[PFM]

Richdubbya,

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

 

[patprimmer]

Even though I have built and ported a few Dac Dacs in my day, it has not been for some time, so I forgot that although the inlet ports are on top, the valve stem points down, to help control oil from getting down the valve stems.


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
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