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Thread: Real world low compression vs. high compression

  1. #1

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    Real world low compression vs. high compression

    I am looking for some real world comparasions of high compression vs low compression. This is gonna be for a 4.0 stroker I am building for my YJ I figured since the 4.0 is a AMC engine I would come here for help.

    I have my crank and rods already, and just picked up a good core 4.0 at a junkyard so work can begin.

    What I am haveing trouble with is makeing my decision on weather I should build it with high compression or low compression. This is going to go into my daily driver YJ, so that needs to be kept in mind. If I build it with low (stock) compression I will still be able to run regular gas (87 octane) and with gas prices these days that is a definate plus! But I could build it with high compression in hope that mabey gas prices would go down (not likely) and make it hurt less when I fill up.

    If I just put it together with stock 4.0 pistons I will get high compression (about 9.5) If I want stock compression I will need to have the dish on the pistons increased to 30cc's I have no idea how mutch it costs to have that done, but I have to keep it in mind.
    I have been reading through this site. http://www.jeep4.0performance.4mg.com/stroker.html and they outline a few diffrent ways to build it, and the (I am gonna guess estimated) hp&torque.
    The first one is the low compression way.
    258 crank and rods
    Silvolite UEM-2229 +0.030" bore pistons
    Increase piston dish volume to 30cc
    8.8:1 CR
    Crane #750501 192/204 degree camshaft
    Ported HO 1.91"/1.50" cylinder head
    Mill block deck 0.035"
    Mopar/Victor 0.043" head gasket
    0.058" quench height
    24lb/hr injectors with stock 39psi FPR for '87-'95 engines
    235hp @ 4500rpm, 327lbft @ 2000rpm

    I really like those numbers. When driveing down the highway (every day going to work) I am turning just a hair over 2k rpm's at 70 (speed limit here) When doing normal driveing I usally dont go over 3k rpm's So it seems like the low compression build would give me number that match my driveing style pretty good.

    The high compression build is as follows
    258 crank and rods
    Speed Pro H825CP +0.030" bore pistons
    CompCams #68-231-4 206/214 degree camshaft
    9.6:1 CR
    Mopar/Victor 0.043" head gasket
    0.082" quench height
    24lb/hr injectors with adjustable FPR or MAP adjuster for '87-'95 engines,
    265hp @ 4900rpm, 325lbft @ 3500rpm

    Its makeing 30 more hp at 400rpms higer but 2ft/lbs less torque and makeing that at 1500rpms higher.
    That seems to put it out of my normal driveing rpm range. So I dont know if it is as good of a option as the lower compression one. With the high compression I would have to rev it higher to get the power and have to spend lot more money feeding it premimum (93 octane) fuel to keep it from pinging. Also it states that I will need a adjustable fuel pressure regulator or map adjuster if I go with the high compression. Neither one of those are cheap.

    So right now the low compression seems to be winning.

    I plan on haveing a 3 angle valve job, mild port and polish, balanced crank and weight matched rods/pistons. I already have a free flowing 2.5in exhaust and a free flowing K&N intake. Throttle bode will be either a 62mm bored stock one, or a moded 65mm ford 5.0 throttle body.

    any info or opinions would be great! Thanks.
    79 Cherokee, 401, T-18/D-20 hydroboost, otherwise stock.
    Soon to come: Caddy 500,NV4500, NP205, high pinion D-60 front, D-70 rear, SOA, 5.13 gears and 35X12.50's on re centered H1 beadlock wheels

  2. #2
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    Why not use the cam from the "low compression" recipe in the higher compression engine and get a little of both..The cam much more than compression ratio dictates power band and usable torque. and at the risk of getting in trouble on this thread for advocating "smaller is better" 9.6 to 1 doesnt seem that high on the compression, 10 to 1 is where I have always drawn the line for detonation problems and pump gas.
    "A man's got to know his limitation's"

    Dirty Harry.

  3. #3
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    Low or high compression??

    Here's something else to consider-
    LOW compression can cause serious pinging & force the use of race gas!!!

    I have a '69 390 that I pulled out of my BBO '69. It was FULLY modified & dynoed 518HP on the HotRod Magazine traveling chassis dyno. It pinged like crazy on anything but race fuel. Upon disassembly & measurement, I found that someone had turned a bunch of material off the piston heads (dished them), giving a .118" piston to head clearance. The math revealed a 9.1~1 CR.

    .045" is what that clearance should be for most steel rod engines (including motocrossers & go karts). The excessive clearance prevented the mixture from getting "excited" (heated) by the rising piston that was trying to squish it.

    In addition, there is much data available from many professional AM builders (Norm Brandes for 1) indicating that a cleaned up AM V8 chamber can go 11~1 without problems, given proper attention to the tune up. A good fuel system, ignition system, & cooling system obviously would be required as well.

    You can also go with over 11~1 and bleed off some of the compression with a more agressive camshaft. Take a look at all the cams CompCams has for AMC!!!

    As far as power numbers, 550HP & well over 500 ft/lbs of torque are easily attainable without pushing the 390/401 past the design's rpm limit, which is between 7500 & 8500, depending on specific demands & preparation.

    You did ask for REAL numbers, these are from the V8, your 4 litre design is not the same!!!!

    If you happen to get too much compression, a water injection system can be installed quite cheaply to fix, or you could go with thicker head gaskets. Putting extra fuel to it will cool it down & may stop the ping.

    Too much cam? That can be tamed somewhat with a lower rocker arm ratio. You could also have the cam advanced or retarded to change where the power is strongest.

    The best bet would be for a guy to go with forged 8~1 pistons & a turbo charger. An adjustable blow-off valve will allow you to set your compression ratio as needed. Actually, the SMALLER AM engines respond the best, with HUGE HP gains available!!!

    http://www.gatorsuperchargers.net/books/books.htm



    I just finished reading the next book, GREAT reading, on Ebay for $12





    .................................................. .................................................. .........
    '69 BigBadOrange AMX 390. Former NHRA 'Hot Rod", 9.80s @ mid 130s. Hurst/Airheart front discs brakes.
    Nostalgia ProStock.

    '71 AMX 401 w/5-speed Richmond RR trans. 4 wheel disc. 500++ HP. Nostalgia TransAm.
    .................................................. .................................................. .........
    mailto: dhoelcher@Comcast.net GPS: 41.78, -86.24

  4. #4

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    Quote Originally Posted by Goose
    Why not use the cam from the "low compression" recipe in the higher compression engine and get a little of both..The cam much more than compression ratio dictates power band and usable torque. and at the risk of getting in trouble on this thread for advocating "smaller is better" 9.6 to 1 doesnt seem that high on the compression, 10 to 1 is where I have always drawn the line for detonation problems and pump gas.
    I am not talking about pump gas vs. race gas, I am talking about regular gas (87 octane) vs premo (93 octane) There are 3 diffrent pumps at the station. Thats one of the things I like about my stock (but high mileage) 4.0, is that I can run regular 87 in it. Regular is usally around 20 cents cheaper per gallon than the premo (around here anyways) And although it might not sound like mutch, it really adds up after a while. Expecially when gas prices are close to $3 a gal, and you have about 45-50 miles one way trip to work every day. Iam not saying I wont build it high compression, I just dont know if the extra money I will have to spend on gas will be worth it. Lets say we build it like you say, high compression. But with the cam the low compression build uses. Just how mutch of a diffrence would there be? Would there really be that mutch?


    Holeshot, as for what you suggested. A turbo is out of the question, I simply dont have the money needed to CORRECTLY set up a system.
    But about what you said on your 390. The dish on yours being too mutch and giveing you problems. I think the shorter quench height on the low compresion build might be there to help prevent that. If you notice it states to use a .043 head gasket (stock is .051 I think) and to mill the block .035 to get a .058 quench height. While the high compression build simply uses just a .048 head gasket and gets a .082 quench height.
    Now I am still a newbie on engine math and other types of formulas and what not. But from what I read, quench height is how close the piston is to the head. The closer it is, the more it forces the air/fuel mixture into the combustion chamber. Too mutch and it hurts power, too little and the pistion can smack the head and you have problems. But I am guessing the low compression build uses the shorter quench height to try and help push the mixture back up into the combustion chamber because of the slightly larger dish, and get it running like it should.

    Now please correct me on anything I am wrong about. Like I said, I am still a newbie, so go easy on me. Just remember, this is not a race engine I building for the track. This is for my daily driver and I am want to have a engine that has good power, decent economy, and long life.

    Thanks
    79 Cherokee, 401, T-18/D-20 hydroboost, otherwise stock.
    Soon to come: Caddy 500,NV4500, NP205, high pinion D-60 front, D-70 rear, SOA, 5.13 gears and 35X12.50's on re centered H1 beadlock wheels

  5. #5
    Thank you from BT Ultra tech
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    Compression & turbos

    The only point I can see that I must stress FIRMLY is the .045" quench distance on any steel rod engine. I'm not talking about an increase in compression, just that .045" is what is suggested for optimum quench, or as I picture it, to get the molecules "excited" and ready to burn at the optimum speed & intensity to coordinate with the cam timing in getting complete burn & maximum work exerted upon the piston before the exhaust cycle begins.

    My opinion on my 390 is that the speed shop bowed to the former owner's wishes & gave him 9~1 because he insisted that was what he wanted. Most mechanics (employees) are trained to not argue with the customer. I'd bet the machinist was shaking his head as he turned the material out of those pistons.

    As a side note, I would like to suggest you consider TotalSeal gapless rings for increased performance of 5~7%. If you are aware of what a leakdown test is, std rings will leak 5~7% after breakin, gapless will be at or VERY near 0%, also offering additional piston cooling & less probability of ping.

    As far as turbos being expensive, they don't have to be. Ebay offers many fresh or new turbos in the $200 range & there are lots of websites that have formulas for determining sizing & ratios. All it is is a pump that you plumb to the exhaust on one side & feed the pressure side into the mouth of your throttle body, plus install larger injectors or computer adjustable pulse width. Just 5 lbs of boost will make your lil motor run way stronger with almost no risk, other than a stuck blow-off valve, so that should be premium quality. Even forged pistons are not mandatory at lower boost settings. Amazing how some creative Studebaker & Rambler guys have made screamers out of what some ppl may consider to be scrap!!!
    .................................................. .................................................. .........
    '69 BigBadOrange AMX 390. Former NHRA 'Hot Rod", 9.80s @ mid 130s. Hurst/Airheart front discs brakes.
    Nostalgia ProStock.

    '71 AMX 401 w/5-speed Richmond RR trans. 4 wheel disc. 500++ HP. Nostalgia TransAm.
    .................................................. .................................................. .........
    mailto: dhoelcher@Comcast.net GPS: 41.78, -86.24

  6. #6

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    So are you suggesting I sould go for a .045 quench height instead of the .058 that build suggests? I am guesing that to get that height you would mill the block a little more, so am I gonna run into pushrod length problems? I am planning on useing stock replacement stamped rockers and bridges (but they will be new) Not enough money for rollers and it would probably be better spent on something else since its a mostly a low rpm engine. But will the milling of the block and the thinner head gasket I worry about correct preload on the lifters useing the stock length pushrods. Should I look into custom length ones?

    I am planning on useing hypereutectic pistons unless there are any reasons I shouldnt. They are better than stock type cast, but not as good as forged, and although I like to use the best parts I can afford, forged are a little high for my budget. Summit has a set of speed pro hyper pistons for a pretty good price. I was actually looking at the total seal gapless rings and was wondering if they were better than molley rings.

    I bought this book a few weeks ago http://www.cartechbooks.com/vstore/s..._ID=30&CATID=3 and have been reading through it and I am learing a lot from it. With this being the 2nd engine I have built, I am wanting to do a better job with it and do a little more tweaking than I did with my last one. So any info will help me a lot, as I want to learn as mutch as I can.
    79 Cherokee, 401, T-18/D-20 hydroboost, otherwise stock.
    Soon to come: Caddy 500,NV4500, NP205, high pinion D-60 front, D-70 rear, SOA, 5.13 gears and 35X12.50's on re centered H1 beadlock wheels

  7. #7
    Thank you from BT ULTRA TECH MASTER!!!
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    Well as to my comment on the build..

    The Cam is what dictates the power band.. to simplify the more duration you have the higher rpm your
    power will be which is the difference between the two builds as I see it.. and as I said before 9.5 to 1 compression isn't "high" most of the stock engines pre 1980 were in the 9 to 9.5 to 1 range it should run just fine on 87 octane.. the thing to keep in mind is You are building an inline 6.. great for torque not so happy with high rpm.. use its strengths. low rpm grunt. you will "feel" torque in the seat of the pants.. every day.

    But It does get me to thinking.. If you are going to spend the money on all the machine work etc.. to build this motor.. have you considered a Good stock rebuild then using the money on a centrifugal supercharger or turbo?? I would bet that dollar for dollar they would come close to the same. I am a reliability kinda guy I guess.. and the simpler the internals the more reliable the package.
    "A man's got to know his limitation's"

    Dirty Harry.

  8. #8

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    Yea high rpm's are not what I am after. If I wanted a high reving engine I would probably swap in a caddy northstar V8 or something like that. But I want low end power, I hardly take it above 3k rpm's.

    But as for 9.5 compression running fine on 87 octane, that goes aganst everything I have read and have been told. I would just hate to build it like that expecting to be able run on 87, and then find out I have to run 93.


    But It does get me to thinking.. If you are going to spend the money on all the machine work etc.. to build this motor.. have you considered a Good stock rebuild then using the money on a centrifugal supercharger or turbo?
    The cost isnt going to be mutch higher than a stock rebuild. The only custom work would be milling the block and any work done to the pistons. everything else would be just like rebuilding the stock longblock. Nothing has to be done to put the 258 crank and rods in there, and you use 4.0 pistons. And theres the port work done on the head, but I think I am gonna do that myself. I have a few junk heads to practice on. So its not really mutch over a stock type rebuild.
    79 Cherokee, 401, T-18/D-20 hydroboost, otherwise stock.
    Soon to come: Caddy 500,NV4500, NP205, high pinion D-60 front, D-70 rear, SOA, 5.13 gears and 35X12.50's on re centered H1 beadlock wheels

  9. #9
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    Hmm, well In the end trust what you think will be right for you..
    I dont think I own a motor with less than 9 to one compression and I run 87 octane exclusively (I live in Podunk Nebraska it's either E-10 or 87 octane) I think I can get 90 octane in MCcook (about 40 miles north)
    But I havent had any detonation problems as a matter of fact I have a fairly warm 302 in a 38 Dodge street rod thats 10:5 to 1 and other than limiting total advance, I have no trouble.
    "A man's got to know his limitation's"

    Dirty Harry.

  10. #10

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    I guess on a older engine where you can adjust the timing you can run it alright. But the 4.0 has a non adjustable dist and you cant adjust timing. You have to go with the factory setting, whatever that may be. And AFAIK, it doesnt have a knock sensor to adjust itself if there is pinging. So I guess I am kinda screwed.
    79 Cherokee, 401, T-18/D-20 hydroboost, otherwise stock.
    Soon to come: Caddy 500,NV4500, NP205, high pinion D-60 front, D-70 rear, SOA, 5.13 gears and 35X12.50's on re centered H1 beadlock wheels

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