Moparts
Can compression hurt power?
Posted By: DaveDudek
Can compression hurt power? - 01/17/06 01:07 AM
Let`s assume a guy has the correct fuel, timing,ect. Can you over squeeze an engine and actually hurt power?
Let`s say a guy in a HEMI has a small cam and 9 to 1 compression is more than enough. Assuming the guy has the correct fuel will he *always* make more power with 12 to 1? What about 14 1/2 to 1. is there a point where it might hurt power?
Thanks!
Posted By: Anonymous
Post deleted by Defbob - 01/17/06 01:33 AM
Posted By: Leon441
Re: Can compression hurt power? - 01/17/06 01:45 AM
If you got enough heads cam you can run up to 18:1 with the right fuel and ignition.
Posted By: MR_P_BODY
Re: Can compression hurt power? - 01/17/06 02:24 AM
Quote:
Let`s assume a guy has the correct fuel, timing,ect. Can you over squeeze an engine and actually hurt power?
Let`s say a guy in a HEMI has a small cam and 9 to 1 compression is more than enough. Assuming the guy has the correct fuel will he *always* make more power with 12 to 1? What about 14 1/2 to 1. is there a point where it might hurt power?
Thanks!
There is a diminshing return when you get higher
and higher with the compression, its not a linaer
curve
Posted By: Cab_Burge
Re: Can compression hurt power? - 01/17/06 02:58 AM
I know of a Ford racer that thought he had 14 to 1 comp. ratio and the motor made peak power at 7100, he took it to a good machine shop that check all of the measurements and found that he had a actual 11.4 to 1 ratio. They order new pistons and made sure that they ended up with 14.7 to 1 and the peak hp (with no other changes, jetting, timing, LSA, valve springs, type of fuel or nothing else) moved up to 8400 rpm and gain a bunch of power. I can call him and get the exact differences and so on if you want. That Hemi is real hard to get to much compression in just like Hemifred said, I guarentee it. The tighter you squeeze good gas before you light the fire the better it will burn and the more power it will produce.
Posted By: Anonymous
Re: Can compression hurt power? - 01/17/06 11:15 AM
Changing squeeze requires you to change other things to "optimize" your return. Fred made a great point. Getting the most out of a fuel can sometimes be as efficient in getting some extra power as opposed to just changing the compression ratio.
This is one of Dvorak's pet arm chair engineering projects. He makes the piston top and valve relief to fit the chamber so he can get as much as possible. Here's a blurb and a few photos.
High Compression HEMI
Posted By: Mooosman
Re: Can compression hurt power? - 01/17/06 12:30 PM
It's a diminishing return after a certain point; the pumping losses from compressing the mixture more actually outweigh the extra power made from the compression.
Nick
Posted By: DaveDudek
Re: Can compression hurt power? - 01/17/06 01:13 PM
Quote:
It's a diminishing return after a certain point; the pumping losses from compressing the mixture more actually outweigh the extra power made from the compression.
Nick
That`s what I`m thinking? HemiFred also wrote that the prostocks only used 12to 1 because of the burn.
This year I`m increaseing the throw of the crank to 4.25. (497 cubes). I`m also gonna add more compression. I had 12.8 and now I`m guessing 14 to 1.
My cam baseline cam will be 256in/262ex 700ish lift, 112LC. 12.8 is More than enough compression for that cam. I don`t want to go backwards adding 14 to 1. But if it can help power I want it. What would you guy`s do. I need to make up some killer ET on the top half of the track.
Posted By: BobR
Re: Can compression hurt power? - 01/17/06 02:08 PM
You WILL make more power going with the higher compression. I have NEVER heard of anyone losing power by adding compression assuming the fuel octane was correct. Like Cab said the harder you squeeze it the more power it will make. -Bob
Posted By: Anonymous
Re: Can compression hurt power? - 01/17/06 03:20 PM
Quote:
What would you guy`s do. I need to make up some killer ET on the top half of the track.
If it will be a new combo all together, I'd opt for more CR since you're in the short block, but have someone pick a cam that uses all the available variables (gear ratio/shift recovery/etc.) to optimize/reduce your 330'-1320' times.
More squeeze without other changes in cam duration, fuel spec and timing may also result in more torque that can't be coupled to the track in the first 330', thereby slowing 1320' ETs even with more power. That balance is tough to achieve when you're traction limited. Been there, done that.
Posted By: MR_P_BODY
Re: Can compression hurt power? - 01/17/06 04:19 PM
Quote:
Quote:
What would you guy`s do. I need to make up some killer ET on the top half of the track.
If it will be a new combo all together, I'd opt for more CR since you're in the short block, but have someone pick a cam that uses all the available variables (gear ratio/shift recovery/etc.) to optimize/reduce your 330'-1320' times.
More squeeze without other changes in cam duration, fuel spec and timing may also result in more torque that can't be coupled to the track in the first 330', thereby slowing 1320' ETs even with more power. That balance is tough to achieve when you're traction limited. Been there, done that.
Another way is to run a lesser converter, soft out
of the hole and big charge on the top end, but its
a balance on the converter where its not a mutt
out of the hole
Posted By: Cab_Burge
Re: Can compression hurt power? - 01/17/06 05:53 PM
Don't you have a programmable MSD7? If so you can control traction out of the hole with the timing curve now. If so I would look to shorten up the lobe centers to 110 instead of the wider 112 you have now, especially with the stroker,
Posted By: mbogina
Re: Can compression hurt power? - 01/17/06 08:08 PM
Too much static compression can definitely hurt horsepower in most engines and especially in a Hemi. Horsepower potential is dependant on dynamic compression, not static compression. Too much static compression coupled with "short" cam timing will result in a engine that will not produce the desired HP. The cam timing must be matched to the static copmpression ratio to keep the dynamic compression within a workable range. You cannot make acceptable HP with a huge duration cam in a low compression motor, nor with a small duration cam in a high compression motor.
A 426 style Hemi also has potential trouble with adequate flame travel and flow through the cylinder during the overlap period. Too tall of a dome will inhibit the flame travel and/or detract from the "chamber cleaning" during the overlap period. The original prostock motors used 12.5 compression because anything greater caused these inherent problems. The 99 hemi uses a flatter chamber and dome to combat these problems, as well as offering a shorter and lighter piston compression height. Current Prostock racers start with 17 + compression, dyno the motor, work on flame travel until colorization/HP is optimized (usually around 15 to 15.5 Static compression). Large cube wedge style motors do not suffer from these flame travel problems because their dome is very short or non-existent. Flame travel will always take precedence over static compression ratio.
Posted By: DaveDudek
Re: Can compression hurt power? - 01/17/06 09:10 PM
Quote:
Too much static compression can definitely hurt horsepower in most engines and especially in a Hemi. Horsepower potential is dependant on dynamic compression, not static compression. Too much static compression coupled with "short" cam timing will result in a engine that will not produce the desired HP. The cam timing must be matched to the static copmpression ratio to keep the dynamic compression within a workable range. You cannot make acceptable HP with a huge duration cam in a low compression motor, nor with a small duration cam in a high compression motor.
A 426 style Hemi also has potential trouble with adequate flame travel and flow through the cylinder during the overlap period. Too tall of a dome will inhibit the flame travel and/or detract from the "chamber cleaning" during the overlap period. The original prostock motors used 12.5 compression because anything greater caused these inherent problems. The 99 hemi uses a flatter chamber and dome to combat these problems, as well as offering a shorter and lighter piston compression height. Current Prostock racers start with 17 + compression, dyno the motor, work on flame travel until colorization/HP is optimized (usually around 15 to 15.5 Static compression). Large cube wedge style motors do not suffer from these flame travel problems because their dome is very short or non-existent. Flame travel will always take precedence over static compression ratio.
Damn.... 
Posted By: BobR
Re: Can compression hurt power? - 01/17/06 09:19 PM
All of that stuff is true but using an increased stroke you will not need as much of a dome to get to 14-1. Flame travel is important but you should not have to give up optimum flame travel to get to 14-1 using increased stroke. -Bob
Posted By: moper
Re: Can compression hurt power? - 01/17/06 09:19 PM
It would take a lot, but perhaps the extra stroke means the pistons could be "worked" to better fit the chamger, and to better "herd" the mixture where you want it...Like Barton's hand fitting jobs. Are you simply l;ooking for more piston speed at a lower rpm Dave? Because I'd think that too much arm will hurt your first 330'. Maybe you should be looking at a higher rpm range instead of crank? I'm not into them, but I'd think a high rpm lower cube engine would breath better thru your manifolding, and loose a bit of torque which is what you want. With FAST rules, it would be very hard to get away with the MSD timing contorller ignitions..They neeed a #1 cylinder reference, and the probe will be visible, even if nothing else is.
Posted By: moderncylinder
Re: Can compression hurt power? - 01/17/06 09:42 PM
dave, at 15.5/1 you wouldnt have anymore of a problem with flame travel, or crossflow at overlap than a 440 cu in race hemi engine with 13.5/1. the added stroke gives you more swept volume, therefore giving more compression.
now with what mike said,, with added static compression you would have more heat being put in the mixture, therefore having a greater chance of detonation or you will have to run a more stable(higher octane) fuel. with more compression you would make more torque all over unless detonation would come into play.
jeff
Posted By: mbogina
Re: Can compression hurt power? - 01/17/06 10:27 PM
Flame travel aside, by increasing the cubic inches and static compression, the dynamic compression will increase, possibly pushing the envelope on your current camshaft beyond its "sweet spot" and into a situation where longer timing events or smaller LCA may be required.
Posted By: moderncylinder
Re: Can compression hurt power? - 01/17/06 10:46 PM
yes,, with the added compression you can burn fuel faster due to the additional heat put into it by compression,, so you can open the exh valve earlier,, but in a sense youd have to close the intake valve earlier if the inertia is the same in the intake port/runner.
mike if you were building an unlimited engine where you needed the most power you could make,, youd put less piston dome in it than a ss race hemi,, 105 dome,, like 100,, or 95 or something? i was going to fit the same piston i used in a few s/s engines in daves engine which is 104ish dome,,, would yeild 15.5/1 because the added stroke.
mike,, i see youre getting addicted to moparts,, lol
jeff
Posted By: AndyF
Re: Can compression hurt power? - 01/17/06 11:23 PM
Dave, I think the Hemi is a special case when it comes to compression vs. power.
Vizard did a great engine project a few years ago where he dyno tested a SBC with old style dome pistons. Then he took the motor apart, machined some material off the tops of the domes and redid the dyno test. I believe he redid this test 3 times and went from something around 13:1 down to 10:1. If I recall, the power actually went up a small amount as the compression decreased.
Posted By: Leon441
Re: Can compression hurt power? - 01/17/06 11:37 PM
My answer would have been a lot different if I knew of the other variables. But the original question was general. Can Compression hurt power. That question by itself I stick with my original response.
But, if you are talking hemi which I know very little about other than you run a hollow dome to keep the piston light due to the huge dome to make compression. I would try and run someththing with the least dome I could and cram all the stroke I could in it making my compression that a way. This theory could be way off due to the hemi-culties but in theory this is the way I would lean.
Posted By: Outlaw10_5
Re: Can compression hurt power? - 01/17/06 11:46 PM
Quote:
With FAST rules, it would be very hard to get away with the MSD timing contorller ignitions..They neeed a #1 cylinder reference, and the probe will be visible, even if nothing else is.
Only if you want to use the ICT function.
Posted By: DaveDudek
Re: Can compression hurt power? - 01/18/06 12:14 AM
Jeff, for sure I`m just letting you do your thing on this engine. I`m just trying to learn/understand all this. As most people know I`m the 1st guy to ask questions.
Posted By: Streetwize
Re: Can compression hurt power? - 01/18/06 02:07 AM
Of course, the irony of a 4 stroke multi cylinder motor is that the compression cycle of the adjacent cylinder takes power generated from it's neighbor on it's power stroke. so increasing compression proportionally increases the internal load on the crank for it to overcome. Normally aspirated, as compression increases, power increases, likely to the motors very ability to maintain seal of the compressed mixture and the heads ability to flow gases in and out. but apples to apples compression alone doesn't tell the whole story, in simplest terms there is likely an optimum combustion chamber and cam timing for each ratio and for any given crank speed. And for example going from 10:1 to 13:1 in a hemi doesn't react like 10:1-13:1 in a wedge, too many variables in static vs dynamic especially with regard to timing events.
Posted By: dvw
Re: Can compression hurt power? - 04/28/13 01:38 AM
I can't see compression ever hurting power as long as these two variables are maximized. #1 No detonation. #2 Dome/dish/combustion chamber are optimised. A 426 style Hemi is a perfect example where using a large dome to increase compression may be worth less than the power created by an optimized chamber/piston.
Doug
Posted By: Rapid340
Re: Can compression hurt power? - 04/28/13 03:04 AM
I am convinced there is a point where piston dome shape ends up completely negating a given increase in compression but I think you will find that in this case (changing compression from 12.8:1 to 14:1 while at the same time increasing stroke to 4.25) the piston dome may be smaller. I would consider leaving your piston crown unchanged and changing just the compression height to match the crank and see what you come up with. If it is higher than 14:1 I would consider going with it.
Posted By: LSP
Re: Can compression hurt power? - 04/28/13 03:06 AM
Quote:
You cannot make acceptable HP with a huge duration cam in a low compression motor
What would you consider huge duration for a 9-1 470 CI motor?
Posted By: Rapid340
Re: Can compression hurt power? - 04/28/13 03:54 AM
Quote:
i was going to fit the same piston i used in a few s/s engines in daves engine which is 104ish dome,,, would yeild 15.5/1 because the added stroke.
I should have carefully read all the replies, I see Jeff at modern already said same thing and you end up with 15.5:1.
Posted By: dOc …
Re: Can compression hurt power? - 04/28/13 01:17 PM
ANYONE remember the 366 short-stroke Hemi ?
Posted By: jim sciortino
Re: Can compression hurt power? - 04/28/13 01:44 PM
With the ability of a HEMI to overscavenge, I'd pay close attention to the camshaft. Especially in a class that doesn't allow crutching the effect with a properly designed header......or shrinking the ridiculously large exhaust valve diameter.
Has anyone seen a HEMI engine where the entire combination was "worked', actually pick up with cams in the 106/108/110 range vs 114/116/118???
Posted By: LSP
Re: Can compression hurt power? - 04/28/13 04:04 PM
Quote:
ANYONE remember the 366 short-stroke Hemi ?
Those never ran well, a 10.7" deck and 3.218" stroke has ugly written all over it. The combo that did work was the 383 block/hemi head combo, compression was 11.25, cam had 15* shorter exhaust lobe, set the PS speed record before they went to 500" motors.
Posted By: polyspheric
Re: Can compression hurt power? - 04/28/13 04:05 PM
"Dynamic compression" is this year's cliche, and greatly over-valued as an effect.
Posted By: MR_P_BODY
Re: Can compression hurt power? - 04/28/13 04:13 PM
Quote:
"Dynamic compression" is this year's cliche, and greatly over-valued as an effect.
Why do you say that... to me when the intake closes
is when you start the compression... so whats so over
rated about it... to me its more important than just
a math number for the cyl volume
Posted By: dwayne welder
Re: Can compression hurt power? - 04/28/13 04:21 PM
RIGHT ON MIKE!!!!!!
Posted By: Coldart
Re: Can compression hurt power? - 04/28/13 04:48 PM
More compression helps but narrows the tuning window, meaning the slightest weather change is going to affect everything - most importantly damage happens real quick if you miss!
Listen to Jeff
Posted By: cudabin
Re: Can compression hurt power? - 04/28/13 08:37 PM
Great post guys...
Any comments on the effects of the exhaust restriction from factory manifolds and 2.5" pipe/mufflers on this combo?
My understanding has been that this exhaust restriction is more of a problem at higher RPM's, so perhaps the added stroke will lower peak power range and have less exhaust pumping losses?
More compression should not hurt this.
At almost 500" would this motor not benefit from a wider LSA ??
Good luck with it Dave! IMO your in good hands with Jeff...
Cheers,
Arnie
Posted By: polyspheric
Re: Can compression hurt power? - 04/29/13 04:24 AM
It is - until engine speed goes above 1,000 RPM. After this point, and depending on manifold and port volume, inertia, gas speed etc. it approaches rhwn reaches full stroke - at some point before peak torque.
If you still have capture based on IVC anywhere near your power peak your cam is wayyy, wayyy too big.
Posted By: BIGSPEED
Re: Can compression hurt power? - 04/29/13 06:22 AM
When the effort to compress takes more power away than any gain by more compression it is a negative gain.Current NHRA ProStocks are around 15.5 CR,Comp Elim Single four engines around 16.2.Bill C.
Posted By: RAMM
Re: Can compression hurt power? - 04/29/13 01:03 PM
It has already been mentioned by guys smarter than me-but I would say that compression will help until the dome gets in the way of flame travel. Also you will need a different camshaft--again pay close attention to this with a big change in compression. J.Rob
Posted By: polyspheric
Re: Can compression hurt power? - 04/29/13 07:34 PM
True - the negative effect of dome intrusion on VE can be somewhat reduced by running a narrower LSA.
I don't think the more successful work done with complex chamber shapes has been made public. Hint: there's squish happening during overlap too...