Compression ratio effect on horsepower

[RECHRGD]

Just curious-------As some of you may remember, a couple of years ago I redid the top end of my 440 with a ton of help from Ron, Runner and others here.  The build was based upon the engine already having sixpack pistons and assuming we would have around 10.0 or above cr with new aluminum MP452 heads.  When the old heads were pulled, we found some trw flattops that were down in the hole (not zero deck as thought).  As I recall it calculated out to be around 9.3 or 9.4 with the new heads.  I went ahead with the build using the existing pistons and hoped for the best.  We/I were/was originally hoping for close to a 500hp engine that would easily run in the 12's on street tires.  When completed it dyno'd a 325 RWHP which was a gain of 60+ rwhp over the previous set up.   Although it gained good power and runs great, it's still no where near that 500hp mark at the flywheel, maybe 410hp or so I'm guessing.  The one time I was able to get to the strip last year the car ran a best time of 13.8 at just under 102mph (2,200ft. elevation).  Even though traction was marginal, the top speed seems to calculate out to confirm around 325rwhp for a 4100lb race weight.  When reading about all the different combo's you guys have here and comparing them to mine, it seems that I'm significantly down on power considering my build.  I'm wondering just how much power you guys think I lost based upon the lower compression.  Here's a quick run down on the build.  Yes, it's been properly jetted and timing dialed in.
71 440= .030 over.
750 Holley DP with proform main body (no choke horn).
Performer RPM intake.
MP 452 Aluminum heads with stage 1 port job. (Dwayne Porter)
Crane Gold adjustable roller rockers.
Crane ProMagnum Hydraulic lifters.
Hooker 1 7/8" Competition headers.
2 1/2" exhaust all the way back with crossover.
Comp Cam custom grind per Dwayne Porter.  Very close to the 275EXHL, 112 lsa, 526/526 lift hydraulic.
New dual timing chain.
Carter high volume mechanical fuel pump.
6 quart street Hemi oil pan.
High volume oil pump.
Mallory Unilite electronic distributor.
Fresh 727 w/ TransGo-2 kit built by CRT.
Turbo action tight (street) 10" converter.  (flashes around 3,200rpm)
3.55 8 3/4" sure grip rear end. W/Gear Vendors O/D.

Thanks, Bob

[skip68]

I'm no expert but I think you need lower gears.      3.73 or 3.91       Ron will chime in I'm sure.   

[RECHRGD]

I'm no expert but I think you need lower gears.      3.73 or 3.91       Ron will chime in I'm sure.   

Thanks Chuck.  I'm no expert either, but in my experience your trap speed is a better indication of the power your making than ET.  I've heard of many people here running 12's with 3.23's without a ton of hp.  I remember my NEW '68 R/T back in the day would run a 14 flat at 101mph with 323's and F70/14 Polyglass tires (stock except for headers) at about 500' elevation.  Bob

[skip68]

    Didn't the cars back then come with higher compression ?   Like 10 to 1 or something.  I know that my car is around 10.25 to 1 and hope it runs good when I get it dialed in.   Maybe run some octane boost.      What gas are you running ?    I have run 89 in my car but it lacks power and pings with full throttle so I only run 91 in it.   I don't know what the line is for running low octane fuel verses compression but I know my car runs like crap on 89 and it's ok on 91.       I wonder if I should be running more octane then 91 ?        You may need more compression.   All this is still new to me so I'm not sure.  

[RECHRGD]

   Didn't the cars back then come with higher compression ?   Like 10 to 1 or something.  I know that my car is around 10.25 to 1 and hope it runs good when I get it dialed in.   Maybe run some octane boost.      What gas are you running ?    I have run 89 in my car but it lacks power and pings with full throttle so I only run 91 in it.   I don't know what the line is for running low octane fuel verses compression but I know my car runs like crap on 89 and it's ok on 91.       I wonder if I should be running more octane then 91 ?        You may need more compression.   All this is still new to me so I'm not sure.   

Yes, as I recall, the 440's in the R/T's came with 10.5 CR and would hate today's 91 octane/10% ethanol garbage.  I always ran Chevron Custom Supreme (white lightning) 101 octane  ($0.36 a gallon back in '68).  I should think with the aluminum heads that I could still get away with 10.0+ CR running on today's junk though.  Bob

[Runner]

hey bob, i dont know if there is and golden rule for compression to power,  but your cam shaft will come into play.     did you race the old combo with the gear venders unit on? i wonder how much power they rob?.  i cant stess enough the difference in timing systems from years previous to this year.  acording to my gps  the alitude at the track is right at 2350, and spokane almost always has a corrected altitude of over 3500 feet and most of the time its well over 4000-4500 feet.    im sure traction is a big issue for you as well. more stall speed with a heavy car= more tire spin on heavy cars.  what was your 60 foot numbers between the 2 different combos.   you have added 60 hp and 1000 rpm stall speed?   sticky tires would do wonders for you im sure.

    it woulda been fun hooking up over the summer. we should pick a nice winter day sometime and meet up in colfax for lunch or something

[RECHRGD]

hey bob, i dont know if there is and golden rule for compression to power,  but your cam shaft will come into play.     did you race the old combo with the gear venders unit on? i wonder how much power they rob?.  i cant stess enough the difference in timing systems from years previous to this year.  acording to my gps  the alitude at the track is right at 2350, and spokane almost always has a corrected altitude of over 3500 feet and most of the time its well over 4000-4500 feet.    im sure traction is a big issue for you as well. more stall speed with a heavy car= more tire spin on heavy cars.  what was your 60 foot numbers between the 2 different combos.   you have added 60 hp and 1000 rpm stall speed?   sticky tires would do wonders for you im sure.

    it woulda been fun hooking up over the summer. we should pick a nice winter day sometime and meet up in colfax for lunch or something

Hi mike! I never ran with the G/V unit before, but it is not supposed to rob hardly any power, less than one percent they say.  Well, if Spokane is like running at 4000' then there's probably 4 or 5 mph right there as compared to sea level. I know when Dwayne ordered the cam for me he had 10+ CR in mind, but when I talked to him after discovering the different pistons he said it would still run fine, which it does.  I'm just curious as to what that extra point may have given me.  My 60' times didn't really change between the two combos, but I wasn't using the converter properly.  I was just slowly getting into it to keep from spinning.  I should have dropped some tire pressure and staged at 3000 rpm and let it jump off the line, then nail it to see what would happen.
Have you heard anything about who may be taking over the track now than Bucky has been canned?  Let me know when you want to meet up.  I'm a retired old fart now, so time is not a problem.  Bob

[Cooter]

Bottom line......Compression equals HP and Torque. Unfortunately, none of us can afford to run our high comp. cars 440's on race gas @ $7.00/gal...
You should be able to get away with more compression with aluminum heads(Not as much pre-ignition)...

He prolly set it up to be "Street" freindly instead of a Strip stormer. If I was running a GV overdrive, I'd have something like 4.10 gears in the rear with ALot more Compression and camshaft, but that's just me....

Mine would look something like dis:
10.5-11.0:1 TRUE compression
.590-.650 Lift Solid Roller Cam with around 240-265 Duration @ .050
Same heads, ported
Zero deck block
4.10 gears

[RECHRGD]

Bottom line......Compression equals HP and Torque. Unfortunately, none of us can afford to run our high comp. cars 440's on race gas @ $7.00/gal...
You should be able to get away with more compression with aluminum heads(Not as much pre-ignition)...

He prolly set it up to be "Street" freindly instead of a Strip stormer. If I was running a GV overdrive, I'd have something like 4.10 gears in the rear with ALot more Compression and camshaft, but that's just me....

Mine would look something like dis:
10.5-11.0:1 TRUE compression
.590-.650 Lift Solid Roller Cam with around 240-265 Duration @ .050
Same heads, ported
Zero deck block
4.10 gears

Cooter--- the set up I have fits my needs well.  I take some pretty long highway trips to distant shows etc..  So if I want to run up 90 or 100+ from time to time I can do it very comfortably.  Yes, the build is for a strong street machine.  I'm just wondering approximately how much hp was lost due to the lower than anticipated CR.  Thanks, Bob

[Musicman]

The effect of compression changes with the output of the build... the greater the output, the greater the difference. The overall effect is not all that dramatic however... The difference between a CR of 9 and a CR of 10 on a stock 440 magnum build will amount to about 10 HP... you'll never feel it. The difference between a 9 and a 10 on a 500 HP build will be about 20 HP, but again you will probably never notice the difference. If your vehicle is designed for the track then maximum compression is something that you may desire. If it's meant to run on the street however, then a compression ratio in the 9 to 10 ballpark is a safer bet... pump gas isn't going to get any better in the near future.

[Cooter]

Well, I do know this, I've taken a 440 with 906 heads on it and swapped them for a set of 915 heads and steel shim gaskets and could definately "Feel" the difference in power...

[RECHRGD]

The effect of compression changes with the output of the build... the greater the output, the greater the difference. The overall effect is not all that dramatic however... The difference between a CR of 9 and a CR of 10 on a stock 440 magnum build will amount to about 10 HP... you'll never feel it. The difference between a 9 and a 10 on a 500 HP build will be about 20 HP, but again you will probably never notice the difference. If your vehicle is designed for the track then maximum compression is something that you may desire. If it's meant to run on the street however, then a compression ratio in the 9 to 10 ballpark is a safer bet... pump gas isn't going to get any better in the near future.

Thanks Musicman, that's what I was after and is about what I was thinking.  So lets say if I had the extra point of CR I might pick up 15hp at the wheels.  Then I pick up and move to a sea level location and maybe pick up another 25hp or so.  I would be around 370hp at the wheels and be closer to 450 at the flywheel.  Still a little short of what I was thinking, but in the ballpark anyway.  Thanks, Bob

[skip68]

    Bob, if you move to sea level can I have your water rights at the ranch.   

[RECHRGD]

   Bob, if you move to sea level can I have your water rights at the ranch.   

Chuck, I know you guys in Vegas need all the water you can get, so sure.  But building the pipeline down there won't be very cost effective.  Bob

[Challenger340]

Your mph would seem to backup your Dyno numbers, and yes, you are down on Power from what should be expected, HEAT being the Culprit.

Aluminim does NOT like low Pressure Burn Starts, especially with the better aftermarket Chambers.

The "problem" with running Aluminum Heads, at the lower Compression ratios of even 9.5:1, has more to do with the PEAK Combustion Temperatures reached, FAR MORE, than the mere 3/4 to 1 point of Compression your are down.
Without getting too long winded about Timing, Peak Burn Pressures, and Tunes to try and to punch up "Heat",

Just Mill your Heads until you have an Honest 10.3-4:1, and get that 112 lsa stick up around 106 if it ain't there already !
(Don't forget to correct the Intake Face Side of the Heads to retain Manifold "fit")
YES, it will run FINE on Pump Premium !

I suspect you'll probably see a dramatic improvement,(50 + hp)
YES,
even with the lowsy quench having the Pistons still down the Hole !


Bob out.

[firefighter3931]

I have to agree with Bob....aluminum heads run better with increased heat in the chamber. 

The gains going from 9.5:1 > 10.5:1 are greater than going from 10.5:1 >11.5:1  There are deminishing returns once you're past a certain point.


Ron

[RECHRGD]

Your mph would seem to backup your Dyno numbers, and yes, you are down on Power from what should be expected, HEAT being the Culprit.

Aluminim does NOT like low Pressure Burn Starts, especially with the better aftermarket Chambers.

The "problem" with running Aluminum Heads, at the lower Compression ratios of even 9.5:1, has more to do with the PEAK Combustion Temperatures reached, FAR MORE, than the mere 3/4 to 1 point of Compression your are down.
Without getting too long winded about Timing, Peak Burn Pressures, and Tunes to try and to punch up "Heat",

Just Mill your Heads until you have an Honest 10.3-4:1, and get that 112 lsa stick up around 106 if it ain't there already !
(Don't forget to correct the Intake Face Side of the Heads to retain Manifold "fit")
YES, it will run FINE on Pump Premium !

I suspect you'll probably see a dramatic improvement,(50 + hp)
YES,
even with the lowsy quench having the Pistons still down the Hole !


Bob out.

I have to agree with Bob....aluminum heads run better with increased heat in the chamber. 

The gains going from 9.5:1 > 10.5:1 are greater than going from 10.5:1 >11.5:1  There are deminishing returns once you're past a certain point.


Ron

50HP!!  Very interesting!  So how do I figure how much to shave off the heads.  The reason we went with the 112 lsa was because I do a lot more low rpm cruising than anything else.  If I left the cam as is and just got the cr up would I still see the same gain?  Would it run hotter in traffic?  For that kind of gain I would really like to go for it, as that would get me where I was thinking I'd be in the first place.  If there is only 10-20 hp to get from bumping it up then I don't think I'd bother.  Thanks,  Bob

[greenpigs]

What about desktop dyno? I played with a copy and that would give an idea by just changing C/R. I have a auto math book that has the formula you need.

Amount to mill = NEW DISP RATIO - OLD DISP RATIO
                      _______________________________   X STROKE
                     
                       NEW DISP RATIO X OLD DISP RATIO


                      9.4 - 8.4
                      ________ X 3.75 =  1      X 3.75 = .047
                     9.4 X 8.4              78.96

For both the new and old disp ratio you subtract 1 from value.

Plus this figures the gasket thickness remains the same and no material was removed from the deck.

How accurate was I?  Wouldn't hurt to double check.

[Musicman]

Your math is correct, but the formula assumes that the inside diameter (volume) of the piece being milled is a constant, which would not be the case in most instances.

[greenpigs]

If the heads were matched by cc'ing them would the formula be more accurate?

[RECHRGD]

What about desktop dyno? I played with a copy and that would give an idea by just changing C/R. I have a auto math book that has the formula you need.

Amount to mill = NEW DISP RATIO - OLD DISP RATIO
                      _______________________________   X STROKE
                     
                       NEW DISP RATIO X OLD DISP RATIO


                      9.4 - 8.4
                      ________ X 3.75 =  1      X 3.75 = .047
                     9.4 X 8.4              78.96

For both the new and old disp ratio you subtract 1 from value.

Plus this figures the gasket thickness remains the same and no material was removed from the deck.

How accurate was I?  Wouldn't hurt to double check.

So what exactly is the product of that formula telling me?  Certainly not 78.96 hp.

[greenpigs]

You need to mill the heads .047 of an inch to achieve your C\R of 10.4

[RECHRGD]

You need to mill the heads .047 of an inch to achieve your C\R of 10.4

Thanks Greenpigs!   Me not too bright sometimes.   Bob

[RECHRGD]

I'm taking the car to Shelly's Performance next week.  Brian is going to yank the heads and measure everything properly to get a true base line CR.  He will have the heads machined to get me to around 10.5.  He also will do a compression test before and after, then put it on the dyno to see what we gained.  He's guessing around 20-25 HP but said the torque gains will be more impressive.  We'll see.  Any guesses?  Bob

[RECHRGD]

Well, I had some issues with the valve guides on the heads, but finally got it back together and dyno'd yesterday.  Cranking pressure went from 150-155 to 165-170 and it should be right at 10.5 CR.  It only picked up about 10hp.  I think the torque increase was much more, but we were not able to get a good reading.  If we held it in 2nd gear the tires would loose traction on the rollers and in 3rd it would kickdown to 2nd at the lower rpms.  Anyway, without a lot of tweeking (it was getting late) we ended up with about 335hp and we're guessing 400+ max torque.  It does sound healthier and feels more responsive.  I could not get it on the road due to weather, so I'll have to wait until Spring to see if it feels any different.  Bob