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Thread: Compression ratio for street truck and general build questions

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    xjsc16x's Avatar
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    Default Compression ratio for street truck and general build questions

    My truck is a daily and my #1 goal above anything else is streetability as I will continue to drive it daily in the summer.

    Is going anything over 9.5:1 a no-no on cheap pump gas? I would probably be buying 91 just for peace of mind but I won't go over what the 'limit' is for static compression for 89 octane, just so that I have that option. Some places around me skip 91 straight to 93. How high could I theoretically go?

    I am buying a set of stock eddy heads and already am planning on getting them shaved to the correct 58cc chamber size. How much shaving does that take usually? Is there any reason I'd want to get them shaved a little more since it's already getting done? I don't really have any plans right now to get them ported but I may or may not talk to a buddy of mine about cleaning them up some.

    I will contact Richard once I have a more defined plan to help me out with a cam.

    My stock engine right now is creeping up on 150K miles. I am planning on re-using my stock bottom end and just freshening it up with new bearings and rings. I don't really see any reason to replace my pistons or rods, I'm not shooting for the moon or anything. Never had a single oiling issue or noise come from the engine, but it is leaking a good amount through the RMS. I top it off just about every week.

    I've helped quite a few buddies plan and build their engines and have been working on small and big block mopars with my dad my whole life (we have a 76 440 power wagon that's been through a 383 and 2 440s), but this will be the first one that I build for myself.

    Sorry for the wall of text...

    -Justin

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    xjsc16x's Avatar
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    After playing with a calculator for a while, I came up with 9.44 static compression. That's with stock pistons (I found 11cc volume somewhere on here please correct me if that's wrong), heads shaved to 58ccs and stock block height .050 in the hole. And Fel-Pro 1008 .039 compressed thickness gaskets.

    I suppose I really shouldn't go any higher than that on pump gas?

    -Justin

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    99dart's Avatar
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    I run 10.8 to 1 on our 91 octane without any problems. It depends a lot on the cam timing. I am FRP tuned.
    99 DA CC R/T
    13.97@100 w/modded kegger & ported TB
    12.96 @103 new 365 C.I (guessatune
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    xjsc16x's Avatar
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    Quote Originally Posted by 99dart View Post
    I run 10.8 to 1 on our 91 octane without any problems. It depends a lot on the cam timing. I am FRP tuned.
    Hmm, well in that case sounds good to me. I wasn't sure if 10 would be too much or not. I haven't bother looking at dynamic compression calculators yet because I haven't looked too much into cam choices yet.

    I will be FRP tuned whenever it is that I decide to pull the trigger and put it all together, I'm in his Facebook group.

    -Justin

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    99dart's Avatar
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    Stock pistons aren't going to get you there, tho.
    99 DA CC R/T
    13.97@100 w/modded kegger & ported TB
    12.96 @103 new 365 C.I (guessatune
    http://imgur.com/a/lOVuZ

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    BIGROB's Avatar
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    I'm going for 10.5:1 on my 408 and Ryan said it was weak sauce with aluminum heads.
    Camshaft is a huge part of the equation but I'm sure he could get it dialed in no mater what cam you use.

    Project Maverick. I feel the need, the need for speed

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    99dart's Avatar
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    Have you looked at gasket thickness yet? My son's engine came out at 10.62 - 1 with Fel-pro 9898pt gaskets. FP 1008's would raise it to just a tick under 11.0!

    Here's a good calculator: https://www.uempistons.com/index.php...619fc9d4805c48
    99 DA CC R/T
    13.97@100 w/modded kegger & ported TB
    12.96 @103 new 365 C.I (guessatune
    http://imgur.com/a/lOVuZ

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    xjsc16x's Avatar
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    Quote Originally Posted by 99dart View Post
    Stock pistons aren't going to get you there, tho.
    According to Summit's compression calculator, milling .020 off the deck to give me .030 in the hole and shaving the heads to 56cc chambers would put me at 10:1. 11cc dish pistons (stock), fel pro 1008 .039 compressed height.

    Both Summit and Diamond Racing's calculators seem to agree with this number

    -Justin

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    BIGROB's Avatar
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    Quote Originally Posted by 99dart View Post
    Have you looked at gasket thickness yet? My son's engine came out at 10.62 - 1 with Fel-pro 9898pt gaskets. FP 1008's would raise it to just a tick under 11.0!

    Here's a good calculator: https://www.uempistons.com/index.php...619fc9d4805c48
    With Felpro 1008's I'll be at 10.94:1 static compression ratio and 10.11:1 dynamic compression ratio.
    But my machinist said not to worry about it until the rotating assembly comes in. He hasn't decked the block yet and the Edlebrock heads are measuring to be 60.5cc chambers so we have room to compensate.

    Project Maverick. I feel the need, the need for speed

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    Alright, seems I was being way too conservative with how far I could go with compression. I can play with the head volume and deck height as I please so...what should I shoot for?

    -Justin

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    xjsc16x's Avatar
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    Quote Originally Posted by 99dart View Post
    Have you looked at gasket thickness yet? My son's engine came out at 10.62 - 1 with Fel-pro 9898pt gaskets. FP 1008's would raise it to just a tick under 11.0!

    Here's a good calculator: https://www.uempistons.com/index.php...619fc9d4805c48
    I swear I read on here somewhere that we couldn't use those gaskets because they cover up some water holes? I'm using the 1008s regardless

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    BIGROB's Avatar
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    I struggled with indecisiveness on my components for a long time also but in my opinion aiming for 10.5:1 would be safe and conservative. As long as you're serious about getting a good tune from Flying Ryan.

    Project Maverick. I feel the need, the need for speed

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    Duner's Avatar
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    Like everybody already said - camshaft is a very big part of the equation.

    As a young lad - I tried 12.5:1 with a stock cam. That thing pinged on premium at idle.
    Duner
    4.7 Turbocharged CC in white - 12's
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    Quote Originally Posted by BIGROB View Post
    I struggled with indecisiveness on my components for a long time also but in my opinion aiming for 10.5:1 would be safe and conservative. As long as you're serious about getting a good tune from Flying Ryan.
    That's a given. I was about to buy one from him before I got these heads - now, I'll just wait until everything is said and done with instead of paying twice.

    I still have a bit of research to do. Mostly, how far I can safely deck my stock block. I don't want to zero-deck it, I want to leave a little room so I know the valves aren't coming close. Target is 0.550" lift.

    -Justin

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    Quote Originally Posted by Duner View Post
    Like everybody already said - camshaft is a very big part of the equation.

    As a young lad - I tried 12.5:1 with a stock cam. That thing pinged on premium at idle.
    I'm not quite that adventurous lol

    Honestly considering a Hipotek HPT220XP2, on a 108LSA for the lopey lope lope. With a stall I haven't seen anyone complain about driveability of 108. I could go 110 if it's too poor (or if it's not enough vaccuum to power my A/C on full blast )

    -Justin

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    lots of 10.5:1 factory cars out there. As stated a few times already cam timing plays a large role. If staying NA, I would not keep the quench that large. 30 down hole plus 39 gasket = 69 quench. 40 is ideal safe.

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    Duner's Avatar
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    Quote Originally Posted by xjsc16x View Post
    I'm not quite that adventurous lol

    Honestly considering a Hipotek HPT220XP2, on a 108LSA for the lopey lope lope. With a stall I haven't seen anyone complain about driveability of 108. I could go 110 if it's too poor (or if it's not enough vaccuum to power my A/C on full blast )

    -Justin
    No problem!
    236/242 cam still gives me power brakes and A/C controls.

    My pistons are .005" out of the hole, with .052" thick Cometics. I couldn't get them at .045".
    Duner
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    I'm running 11.5:1 compression on my 410, but thanks to my nitrous cam it bleeds of so much cranking compression I could run my engine on 89 octane. I wouldn't cut the heads down to below a 58cc chamber just to try and get stock pistons to make more compression, not worth it in the future when you decide to build a real engine....
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    98Dak408's Avatar
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    He hasn't decked the block yet and the Edlebrock heads are measuring to be 60.5cc chambers so we have room to compensate.
    FWIW, I recently checked the cc of an Eddy head and it too was around 60.5cc +/-. That was with some unshrouding of the valves as sown in the pic. The pic is of an EQ head but you get the idea of the amount taken around the valve for unshrouding.

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    Quote Originally Posted by slammedR/T View Post
    I'm running 11.5:1 compression on my 410, but thanks to my nitrous cam it bleeds of so much cranking compression I could run my engine on 89 octane. I wouldn't cut the heads down to below a 58cc chamber just to try and get stock pistons to make more compression, not worth it in the future when you decide to build a real engine....
    I'm not trying to just throw money into it, but I'm also not trying to be ultra-mega-cheapo because I know that would end badly.

    If I buy new pistons, why not buy new rods. If I'm getting pistons and rods anyways, why not go the extra step and get forged so I can put a power adder on it later...I just want to buy what I need to throw this thing together and that's it.

    (and I have no plans for power adders, as tempting as they may be..)

    With that being said, I did look at KB107s. Their site says 10.9 compression at 62ccs and the description says that they may protrude on magnums. Can someone elaborate why that is? Is their compression height larger than stock?

    -Justin

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    nitroram33's Avatar
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    Deck height and machining tolerances between the Magnums and LA blocks.

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    99dart's Avatar
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    Lots of good input on this thread!
    99 DA CC R/T
    13.97@100 w/modded kegger & ported TB
    12.96 @103 new 365 C.I (guessatune
    http://imgur.com/a/lOVuZ

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    99dart's Avatar
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    Quote Originally Posted by xjsc16x View Post
    According to Summit's compression calculator, milling .020 off the deck to give me .030 in the hole and shaving the heads to 56cc chambers would put me at 10:1. 11cc dish pistons (stock), fel pro 1008 .039 compressed height.

    Both Summit and Diamond Racing's calculators seem to agree with this number

    -Justin
    Cool... glad you are playing with some different numbers. For my son's (and mine for that matter) build, we wanted the bottom end to be pretty much bullet proof from the start. As said, then we could throw whatever we wanted at it and be confident in the build.
    99 DA CC R/T
    13.97@100 w/modded kegger & ported TB
    12.96 @103 new 365 C.I (guessatune
    http://imgur.com/a/lOVuZ

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    I appreciate all the help you guys have given me. I talked to Ryan on FB and we came to the conclusion that I could get up to 10.5 with Mopar thin gaskets, and still be .015 in the hole.

    However, I may have jumped the gun a bit...I had a set of brand new assembled Eddy's pretty much locked in at $1000, was supposed to meet the guy on Monday and he didn't message me back until today. Apparently he's got someone that's supposed to come pick them up today now.

    If that guy falls through I'll be able to pick them up tomorrow. Otherwise, I'll be back to square 1 looking for heads

    That being said, there's a freshly machined .030 over magnum block , hyper pistons, stock rods and crank, stock rebuilt heads 360 for sale near me for $900. It needs assembling but the long block is all there. I'm not interested in the stock heads but I could probably resell them to someone local.

    -Justin

  25. #25

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    You stated you wanted a daily driver. The lope you want is just sound bullshit at your level. I'd stick yto your original premise of a nice running street truck. You can get a nice set of mildly worked heads from IMM in Cali for around 1200-1400 bucks. He makes sure there is no valve stem bind. You can ask for a 58cc chamber which I believe cuts about .025-.027 off the head and that's enough to want your intake to port match.. Put in the cam you wanted with a 112 lsa on a 108 center. You'll have some cam roll but you won't have the leakage inherent to a higher duration or low lsa cam. You want a balanced power out-put most of all based on your description of how you're going to use your truck and for you that is a reality engine.

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