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How much compression is really enough?

Posted By: frank

How much compression is really enough? - 09/30/10 02:48 AM

I'm building a 500 inch stroker from a 440 block.
Block 10.700 at .040 over
stroke 4.150
rod 6.760
heads 84 cc

Heads are eddy Performer RPM with port and valve work. Cam will be a hyd roller

I'm looking for mid to lower 12's on pump gas

factory 883 4 speed, 3:55 gear, 15 inch tire

Car and driver 3800-4000 pounds

Car is mostly for street

How much compression do I really need? My engine builder wants to stay below 10:1 and I have no problem with that but will say 9.5:1 get me where I want to be? Is it better to sacrafice some compression for timing?? I'm in Tucson and it's hot and elevation is about 2500' I want about 600 pounds of torque from the engine.

Your thoughts
Posted By: dOc !

Re: How much compression is really enough? - 09/30/10 02:54 AM

What kind of gas do you want to use?
Posted By: johnzgarage

Re: How much compression is really enough? - 09/30/10 03:01 AM

just depends on the spec of the cam ,It will affect your compression. Also buy using an aluminum heads, you can knock 1-1.5 compression off the top , due to dissapation of heat from the heads.
I would go with 11 to 1 and pick the right cam .
Posted By: BobR

Re: How much compression is really enough? - 09/30/10 03:02 AM

Quote:

just depends on the spec of the cam ,It will affect your compression. Also buy using an aluminum heads, you can knock 1-1.5 compression off the top , due to dissapation of heat from the heads.
I would go with 11 to 1 and pick the right cam .



I agree. Altitude also lessens octane requirements.
Posted By: Bob_Coomer

Re: How much compression is really enough? - 09/30/10 03:04 AM

I have personally built this very same engine, about 6 years ago. It ran strong, was able to push my 3800 lbs full interior 69 Coronet to 7.20's in the 8th mile, thats a easy high 11 second 1/4 mile pass.
I also ran pump 92-93 fuel also. Compression was right at 11:1
With a zero deck engine, your stroke and 84 cc heads your will also be 11:1+ my pistons were in the hole by .005-.008 with 4cc valve reliefs.
I set my timing at 32 deg total. This was plenty of timing for the engine, and help keep spark knock out of it also.
IMO having the compression up so high that it needs to be backed off a fair amount is a waste, and the little gain you tried to shoot for in the increased compression is now lost with the NONE optimal timing. You might even loose 2-3x as much pulling out timing. You would be surprised to know how much HP one can loose by pulling 8-10 deg out away from its optimal timing point. Maybe as much as 50HP in some combo's. Consider the gain you only get from 1 point in compression on a typical engine, what maybe 3%-5% overall.
So what on a 500hp engine you talking maybe 15HP??
Posted By: frank

Re: How much compression is really enough? - 09/30/10 03:15 AM

fuel will be pump 91 octaine. I don't want to use more compression than I need. I don't know if 9.5:1 will get me there or not
Posted By: fourgearsavoy

Re: How much compression is really enough? - 09/30/10 03:23 AM

I have just a tic under 11-1 compression in my 493 with flat Wiseco pistons and Indy EZ's.I run 93 pump gas with total timing set at 32 deg.and it puts my 3550# car at 11.13 @ 123 with a 1.8 60' time.
I just run a small street flat solid Comp XTQ series cam.
Gus

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Posted By: johnzgarage

Re: How much compression is really enough? - 09/30/10 03:29 AM

Dont get to hung up on base or static compression ratio. You have to really focus in on your cam grind .......that will give you your dynamic compression ratio, which will make or break your street combo.
Posted By: Crizila

Re: How much compression is really enough? - 09/30/10 03:56 AM

I'll just say this; Listen to your engine builder. If your motor gernades, no one on this thread will loose anything but you. As the CR numbers go up, the HP gains deminish - rapidly. Leave some room for error. You ain't always gonna be pumping 93 octane, even if that's what the pump sez. What happens when you are running on the ragged edge of detonation and you get stuck in staging with the motor running and gotta leave hot - or you launch the fan belt at half track? Trust me, it ain't worth the additional 15 or so HP.
Posted By: Darryls-Demon

Re: How much compression is really enough? - 09/30/10 04:03 AM

I am running 11-1 with 170 pounds of cylinder pressure in my 440 with indy heads. Tight quench, top of the pistons are thermocoated so are the cambers and the valves.34deg all in by 3200RPMs low first gear, 3.55 gear in the rear a 28 inch tall tire and 3900lbs. I seem to be on the edge with 91 octane mule piss that we have here.
Now this was with a tight converter, I have since installed an A-1 8inch converter and it seems to work great.
If your engine guy is recomending 9.5 compression it is time to find some one else. Give Bud yancer a call, he is not cheep but he knows his stuff and is easy to work with.

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Posted By: Twostick

Re: How much compression is really enough? - 09/30/10 04:17 AM

Mine is at 8.9:1 (493) and made right at 500 hp and 600ft/lbs. If those numbers will do the deed then 9.5:1 should put you over the top.

You are 1000 lbs lighter than I am and I'm running 13.90's at 101 with 2.76's and no traction (2.23 60ft)

Kevin
Posted By: KOS

Re: How much compression is really enough? - 09/30/10 05:25 AM

dont go 9.5 10.5 minimum.....the motor will more efficient also.
Posted By: Cab_Burge

Re: How much compression is really enough? - 09/30/10 05:42 AM

I've built and dyno tested a bunch of Mopar wedges motors over the years, stock stroke N/A pump gas street motors, NHRA legal "Stock" motors, high compression bracket motors and so on. 10.0 to 1 up to 10.5 to 1 comp. ratio is very safe with todays pump gas using the heads you have as long as you use a big enough cam to feed the motor not a RV cam The 4 speed and tires will determine how good the car ET, the motor will determine the MPH I built a iron headed (ported 906 heads) low deck pump gas 512 C.I.(4.25 stroke, 4.375 bore) stroker motor that had a true 9.25 to 1 comp. ratio. It had (I have since hopped that rascal up some more ) the ported big valve iron heads(same size valves as the stock Eddy RPM do), stock type six pak intake and carbs. comp Cams custom solid roller cam and kit. I had the motor dyno tested on a DTS engine dyno in Ontario, CA on CA pump gas, it made 644 ft. lbs at 4500 RPM and 612 HP at 5600 RPM I put the motor in my 1971 Duster, 3450 LBs with me in it with full 3 inch exhaust system and a full tank(stock 14 gal.tank) of pump gas, 727 and 315x60x15 ET Street radials, 727 trans, ten inch Continental converter, 4.10 gears in a 8 3/4 with ladder bars. the car ran 10:69 at 124.5 mph the first time at a 1/4 mile track. My car does not spin the tires at the track, it flat hooks and moves. I lived in Hesperai, CA 3300 ft. altitude, when I built that motor and car so the compression ratio was determined by the CA pump Gas and where I lived at that time The clutch and 4 speed trans will be the real channel to hooking your car up, sorry The motor will make all the power you need, maybe to much power Good luck
Posted By: 383man

Re: How much compression is really enough? - 09/30/10 11:54 PM

I plan to start building my 500 inch stroker real soon and it will be close to yours. I plan to use CNC ported Eddy RPM heads and I will run 11.0 to 11.2 comp at least and I still plan to use 92 or 93 pump. I plan on a pretty stout solid roller as I want to make alot of power out of it. I also plan to have nice quench in it as it will be a zero deck closed chamber head eng using a .039 head gasket. I want to have aleast 600 to 650 hp as I want to put my 3700 lb 63 solidly in the 10's. Thats why I will run at least 11.0 comp and then use a stout cam to keep the cyl pressure where I want it. Ron
Posted By: MRMOPAR622

Re: How much compression is really enough? - 10/01/10 12:12 AM

If I was spending the $$$$ to go up to 500 cu. in. and aluminum heads. I think I would want all the CR I could get and be safe on pump gas. You can run 11-1 and be safe on pump gas. So why drop down to 9.5 CR and give up the extra HP you just spent all the extra $$$ to gain?

It is however wise to ask others who have run that just to be safe.
Posted By: 440Jim

Re: How much compression is really enough? - 10/01/10 12:49 AM

It depends on several things, including how much cam duration you will be running. I would build it with 10.5 to 11.0 CR and 260ish duration at 0.050". And that will be mild.
Posted By: BradH

Re: How much compression is really enough? - 10/01/10 01:02 AM

Quote:

I'll just say this; Listen to your engine builder. If your motor gernades, no one on this thread will loose anything but you.


, but true...
Posted By: 602heavy

Re: How much compression is really enough? - 10/01/10 01:10 AM

Quote:

dont go 9.5 10.5 minimum.....the motor will more efficient also.





Good advice............anything less than 10.5 CR will cause sooting when running alu heads.

Posted By: codfish

Re: How much compression is really enough? - 10/01/10 01:18 AM

Quote:

It depends on several things, including how much cam duration you will be running. I would build it with 10.5 to 11.0 CR and 260ish duration at 0.050". And that will be mild.



Yep.

I have 11.2-1 comp with an engine identical to the build you are describing. My cam has 263 duration at .050". Runs fine on 91 octane. 36* total timing.

codfish
Posted By: moparniac

Re: How much compression is really enough? - 10/01/10 01:18 AM

I ran 11.12 compression in one of my old combos with a puny 248/254 @ 50 solid roller and I raced on pump 93 and no signs of detonation whatsoever! quench on that was .052 (just sayin) good luck
Posted By: dOc !

Re: How much compression is really enough? - 10/01/10 01:43 AM

LOOK at what A BUNCH of the other guy's combos are ... over 11 to 1 ...

I say ... CHANGE yerr engine builder !!
Posted By: DRDart

Re: How much compression is really enough? - 10/01/10 02:06 AM

16-1 Might be too much
Posted By: dOc !

Re: How much compression is really enough? - 10/01/10 02:15 AM

Quote:

16-1 Might be too much



NOT running E85 and a big gear and open headers !!
Posted By: Crizila

Re: How much compression is really enough? - 10/01/10 02:27 AM

Quote:

Quote:

16-1 Might be too much



NOT running E85 and a big gear and open headers !!

Sure, diesel motors do it all the time. Coarse there bottom ends are a little stouter than our stuff
Posted By: dOc !

Re: How much compression is really enough? - 10/01/10 02:33 AM

I knew a guy in Detroit who was running a RED with an RB engine ...pure alky running something in the neighborhood of 16-18 to 1 ..IIRC. Stock block I think.
Posted By: Crizila

Re: How much compression is really enough? - 10/01/10 02:34 AM

Quote:

fuel will be pump 91 octaine. I don't want to use more compression than I need. I don't know if 9.5:1 will get me there or not

Personnally, I would not go over 10.1 with 91 octane fuel. You should have enough motor to get you where you want to go at 9.5.
Posted By: frank

Re: How much compression is really enough? - 10/01/10 03:59 AM

Thank you for your replies. The cam I was looking at is a Comp hyd roller

549/544 lift
242I 248E duration at .050

Again thanks
Posted By: Cab_Burge

Re: How much compression is really enough? - 10/01/10 04:43 AM

I forgot to mention, besides I had rambled on long enough on that post, that when I hopped my 512 up three years later I had switch cylinders heads from the big valve ported iron 906 heads (OEM productions ones) to a set of Eddy RPM CNC ported by Jeff at MCH and I swapped the crank out from the Ohoi Crankshaft CO. crank 4.25 stroke to a Chinese imported 6 bolt 4.150 440 crank that I had offset ground to 4.300 stroke(+.050 ) and modified to match the balance and clear in my motor by the same shop that had prepared the 4.25 origunally. It bolted right in with no hassles at all That moved the 22.0 cc dihed piston from -.025 in the cylinders to zero deck bumping the compression from 9.25 to to 10.29 to 1, it also increased the C.I. from 512 to 518. I reused the same rings, bearings, rods and pistons and the head gaskets No other changes other than the crankshaft and cylinder heads. I had swapped the cylinder heads onto the motor the previuos winter Those heads pick the cars ET and MPH up by about .3 ET and almost three MPH I dyno tested the motor here in Oregon during the winter of 2006- 2007 on the local Studka engine dyno using Oregon 92 octane pump gas bought at a local Chevron station, nothing special The motor made a best of 718 to 720 HP at 5800 RPM on two different days and 644 FT lbs at 4700 RPM several times This is the same Comp Cams solid roller as before, 260 degrees at .050 intake ground on 108 LSA with .710 net lift at the retainers using the Harland Sharp 1.6 ratio rockers( which measured out to be 1.65 net actual ratios ) Put the biggest street and strip shelf cam you can get or contact me if you want a really good custom ground hydraulic roller cam made
Posted By: BigDaddy440

Re: How much compression is really enough? - 10/01/10 07:01 AM

Quote:

Dont get to hung up on base or static compression ratio. You have to really focus in on your cam grind .......that will give you your dynamic compression ratio, which will make or break your street combo.



^ This.


I had a stock stroke 440 with 11:1 static, but absolutely no real compression. I could have ran 87 octane with 36* total advance in 100 degree weather without a ping or problem.
You need to calculate your Dynamic Compression accurately with different pistons and cam profiles. If you're planning on running pump gas, you'll want your D.E. compression to be in the low to mid 8s and no higher.

-dan
Posted By: 440Jim

Re: How much compression is really enough? - 10/01/10 04:26 PM

Quote:

Thank you for your replies. The cam I was looking at is a Comp hyd roller

549/544 lift
242I 248E duration at .050

Again thanks

That will be real mild in a 500 inch motor. They like to eat duration for breakfast. IMO, cam duration and compression ratio depend on each other. Also remember aluminum heads will tolerate CR (octane need) better than iron heads. And I think your altitude will tolerate a bit more CR than sea level also. Also, when comparing duration of a solid lifter cam to a hydraulic, the lash of the solid lifter will make the duration "feel" smaller, maybe 5-7 degrees? So a solid 260 might feel more like a hydraulic 253.

My street car 440 has iron heads at sea level (80 ft), 257 deg solid lifter flat tappet cam, IRON heads, and 10.75 CR. Pump gas all the way.

For your application, 260ish duration at 0.050" solid lifter cam, 11.0 CR (10.5 is OK with the smaller cam). You could likely get away with even more CR, so I think this is conservative and will work great.

Posted By: ademon

Re: How much compression is really enough? - 10/01/10 04:30 PM

very small cam, i would shoot for 11.1 and a cam in the mid 250's or better @.050. that should get you around 175 or so cranking psi, fine for 91-93.
Posted By: jbc426

Re: How much compression is really enough? - 10/01/10 05:52 PM

Why straight pump gas in your hotrod? How many tankfuls of gas are you planning on burning through this thing a month? Go high compression and add some good gas to that pump piss!
Posted By: BSB67

Re: How much compression is really enough? - 10/02/10 01:06 AM

My Eddy head 508 is 10.8:1, has an intake closing point of 76* ABDC (measured at 0.006" lobe lift), has a cranking cylinder pressure of 190 psi and it is pretty close to the edge on 93 octane pump gas.

With this you can get feel for what you might be able to get away with regarding cam around this CR.

If your goal is low 12s, there is no need to get too close to the edge on CR for your 91 octane.
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