Compression - Big Joe;Would you tell me more?

[oldhead]

bigjoe1 wrote:
The lower the compression is, the more important it is to get it higher if you want to make more HP. Below 11 to one, there is a pretty big gain to be had with each ratio increase=The biggest gain I have seen is from 8 to one to 9 to one, then 9 to one up to 10 to one is next biggest gain== It does help torque more than horsepower


JOE SHERMAN RACING
THATS what you said,Would you tell more,How much difference between 7 to 1 vs 8 to 1 and all between 15 to 1 vs 16 to 1...........Let me guess 7-8 is 15hp,8-9 is 14hp,9-10 13hp, losing 1 hp for every 1.0 to 15 -16 which would be 8 hp........THANKS,Oldhead

[bigjoe1]

There is no big message here== From very low compression as you can increase it, all the other things that are tied into building more HP will begin to come around,When the cam you wanted to use did not work out well with 8 to one, all the sudden it works very well with 9 to one= My experiance is that when dealing with a SH Chevy ( 350 ) going up by one ratio at a time will make about 15 to 17 HP, and a bigger amount of torque= When you get up to 14 to one, one ration still is only worth about 16 or 17 more HP= Depending on lots of things, at some point no mre increase in HP will showup==Matbe I should have said there is a big feeling of more power when you go from very low ( 8 to one )up to 9 or 10 to one.I dont know if this is what you wanted to hear


JOE SHERMAN RACING

[oldhead]

WOW,WOW WOW......I guess I said the wrong thing,Let me put it differently..................(Thinking)..........................Say I want you to build me a 250 hp SBC,and I would like 8.0 to 1,You build it,I want more power and you change it to 9.0. You build a max.power engine and I like the sound of 14 to 1,You build my friend a engine and he likes the number 15 to 1 how much difference between them Oldhead

[bigjoe1]

I have not done the apples to apples testing going from 14 to one compared to a 15 to one example== based on what I have seen, 15 to 20 HP differance is what you might see. Again, some designs do not keep getting better with higher and higher compression ratios=


JOE SHERMAN RACING

[MadBill]

Just from the the thermodynamics perspective, it's a matter of diminishing returns as CR is increased, but there are a number of other factors, e.g. charge loss at lower revs due to late IVC amplifying the effect of high CR, increasing bearing loads and piston side loading friction, flow obstruction from excessive piston domes...

FWIW, diesel CRs are determined as much by the need for high heat of compression to reduce white smoke on cold starts as by power and efficiency gains. Many older models were in the 22:1 range, but some newer examples are down to perhaps 18:1...

[Sean]

[quote="MadBill"]

FWIW, diesel CRs are determined as much by the need for high heat of compression to reduce white smoke on cold starts as by power and efficiency gains. Many older models were in the 22:1 range, but some newer examples are down to perhaps 18:1...[/quote]



Diesel motors are running lower compression to lower NOX emissions. It has nothing to do with cold starts, efficiency or performance.

[wjnielsen]

If you look at some old motors manuals, which show the horsepower ratings for engines, the old 218 MoPar sixes picked up a lot of zip going from the 6.XX to the 7.XX range. The 230 was the exact same engine, with a minor change in stroke adding twelve inches... and a higher compression ratio.

The 8:1 230 was a 130 HP engine; since this was <1/2 HP/CI, I'd be pretty surprised if the extra inches accounted for more than 8-10 of the horsepower gained over my 97 HP 218 at 6.75:1. Remember, same kind of exhaust, intake, carb, manifold, ignition... all the breathing stuff.

The gains are going to have to be bigger on lower CR engines, probably because the gain from 7:1 to 8:1 is about 14%, where the gain from 14:1 to 15:1 is only half that percentage.

Not that the power will go up as a straight function of the change in compression ratio.

For myself, I have a sneaking hunch that the gains in power might be more closely related to a change in dynamic CR.

Kinda' makes sense why an engine with a large cam (and resultant low effective CR due to late intake closing) will be such a dog at 7.5:1, but run pretty well at 9:1. But the same engine with a stock cam will run okay (albeit a little less peppy) with the lower CR. The dynamic CR isn't all the way down thru the floor due to the low CR and long cam events.

Enough food for thought; now I'm off for pepto-bismol.

-Bill

[MadBill]

FWIW, diesel CRs are determined as much by the need for high heat of compression to reduce white smoke on cold starts as by power and efficiency gains. Many older models were in the 22:1 range, but some newer examples are down to perhaps 18:1...

Diesel motors are running lower compression to lower NOX emissions. It has nothing to do with cold starts, efficiency or performance.

Well Sean 4-poster, I haven't yet mastered the shrinking of images to suit ST's requirements so the image is too big to attach, but I'm looking at a copy of Richard Stone's Introduction to Internal Combustion Engines Second Edition On page 196 it reads: " Starting compression ignition engines from cold is a serious problem. For this reason a compression ratio is often used that is higher than desirable for either optimum economy or power output..." It goes on to talk about hydraulically variable CR pistons infrequently used to address the issue.

Perhaps you could cite your credentials and basis for stating otherwise?

[redcorvette]

MadBill; "Diesel motors are running lower compression to lower NOX emissions. It has nothing to do with cold starts, efficiency or performance." does not conflict with "" Starting compression ignition engines from cold is a serious problem. For this reason a compression ratio is often used that is higher than desirable for either optimum economy or power output..." , it is just saying that new diesels that need to be emission compliant are running lower compression than a previous generation diesel to lower NOX emissions.

So does the effect of CR change as rpm is increased, from the comment about torque increasing more than horsepower does this mean that a motor that peaks at 5500 rpm show a larger gain than a motor than a motor that peaks at 8500, specifically the shift to shift rpm range.

[MadBill]

MadBill; "Diesel motors are running lower compression to lower NOX emissions. It has nothing to do with cold starts, efficiency or performance." does not conflict with "" Starting compression ignition engines from cold is a serious problem. For this reason a compression ratio is often used that is higher than desirable for either optimum economy or power output..." , it is just saying that new diesels that need to be emission compliant are running lower compression than a previous generation diesel to lower NOX emissions...

I disagree. The thrust of my first note was that 20:1 plus CR is too high for optimum diesel performance and efficiency; it was necessary only for good cold starts. Single fuel rail technology with ultra-high injection pressures and sophisticated electronics permitting up to five or more injector events per cycle allow acceptable starts with the CR reduced to <20:1 for better performance and not just reduced NOx. In other words: Ignoring emissions, somewhere between the 15-16:1 of the more extreme gasoline engine CRs and the performance-optimized ~18:1 of modern diesels is the likely the best theoretical CR, all factors considered.

[Lazy JW]

Most of us would agree that Sir Harry Ricardo was a pretty smart fellow; he invented a swirl-type pre-combustion chamber type diesel called the "Comet"; I own a tractor equipped with this type engine which has 22:1 compression, these engines are notorious for their hard starting, particularly in cold weather. But it sure is stingy on fuel!

I have also wasted far too much of my misspent life starting old Caterpillar diesels with pony motors; those old Cats also had pre-combustion chambers with fairly low ratios and were also hard starters.

Just this week on a cold morning (25º F) I observed a brand-new diesel tractor start up which has a direct-injection engine; I believe the second piston up fired and it was running, no glow plugs, no ether, no compression release, just hit the starter and bang it was running. Mama had to grab my hand and get me out of that sales lot QUICK! I don't know what the compression ratio was but I have never seen any gasoline engine start quicker, much less a diesel.
Joe

[levisnteeshirt]

the old NTC Cummins only had 14-1 for a 400hp ,, i think the compression went up as the HP went down , i think a 350 cummins is 14.5 ,, a 444 cummins is 13.5 ,, if my memory is correct ,,,,, no either , no engine heater , no start when it gets cold for those bad boys ,,, I think a series 60 Detroit is 18-1

[62ChevyII]

New guy here (been lurking for a while though) but this topic reminded me of a chart that David Vizard made showing the dimenishing returns per point of CR in the higher CR ranges in gas engines. There is a copy of his chart in this article: http://www.popularhotrodding.com/tech/0 ... index.html

Not sure how accurate it is or anything but seems to back up what some of you are saying.