Street 383 Compression

Discussion in 'Engine Topic' started by Cooldawg, Jul 25, 2017.

  1. Cooldawg

    Cooldawg Veteran Member

    Messages:
    196
    Likes Received:
    11
    Joined:
    Jun 3, 2014
    Location:
    Angola, IN
    Planning to run AFR 195's, air gap, and roller cam. Prefer to run 91 straight fuel. I was hoping for 9.7 or so, but builder says I need to choose:

    1)forged rotating assembly with flat tops/ 75cc heads = 10.1 or so
    2)the same with dished/ 65cc closer to 9.2 or so

    He's suggesting option 2 due to having seen too many get ruined by feeding marginal fuel. He can get me to 9.7 (decking block, I assume) or so but will cost $$. He will measure prior to getting cam recommendation. He's been building engines for a very long time.

    A 2nd opinion I also trust says go with option one: "will run fine on premium, better MPG, performance". He's a hall of fame sprint car driver... He's why I'm spending a premium on heads.

    Just looking for strong, reliable street motor to mate with TKO600 and factory rear end. 430-450hp territory. I intended to avoid blended 93 altogether unless I go EFI, which sort of busts the budget. But I'll tolerate the wait getting her back on the road as long as she's set up just right and reliable.

    I've learned a lot here at nasty. Based on what I've read, I'm surprised he's that concerned about running low 10's with the aluminum heads. But she will run cooler in the mid to low 9's.

    The flat-top assembly is $200 less. leaning flat top but would appreciate any additional opinions on the matter.
     
    Last edited: Jul 25, 2017
  2. 70lt1z28

    70lt1z28 Veteran Member Gold Member

    Messages:
    3,367
    Likes Received:
    662
    Joined:
    Oct 3, 1999
    Location:
    Beavercreek, Ohio, USA
    You haven't said anything about the cam. The secret, if it is a secret, to building a detonation avoiding engine is dynamic compression, not static compression which is what your numbers above are. A high overlap cam will effectively vent out thus reducing the effective compression ratio. That's why the 69Z "30-30" cam can many times run on pump gas where a 70Z "178" cam will knock like crazy running the same timing, heads, static compression etc.

    With the right cam, an aluminum headed street motor should run 10.1:1 no problem. 10.5:1 is do-able with a little attention to smoothing the sharp edges on the valve reliefs and combustion chambers.

    www.wallaceracing.com/dynamic-cr.php
     
  3. Cooldawg

    Cooldawg Veteran Member

    Messages:
    196
    Likes Received:
    11
    Joined:
    Jun 3, 2014
    Location:
    Angola, IN
    Ok, I see. The plan was to measure SC then he planned on getting a cam recommendation from his vender based on heads, gears, tranny, 91 octane fuel etc. So if I run 10:1 and want it to last drinking pump gas, the compromise is in drivability (lumpier, more overlap, less manifold vacuum).

    Been there done that. I'll go with lower compression before I build something with too much cam. Going EFI would help on that front, however.
     
  4. Rdobbs1977

    Rdobbs1977 Huge Guns N Roses Fan

    Messages:
    309
    Likes Received:
    15
    Joined:
    Dec 1, 2014
    Location:
    Nashville
    static compression plays its role in to detonate or not....imo,
     
  5. Lowend

    Lowend Administrator. .a car, a man, a maraca. Staff Member Lifetime Gold Member

    Messages:
    14,978
    Likes Received:
    446
    Joined:
    Mar 25, 1999
    Location:
    San Jose, CA, USA
    With aluminum heads heads, I would not be worried about 10-1 compression on 91 Octane fuel

    I ran 10.61 on my 383 (64cc heads, -5cc flattops) and it was borderline on California 91 octane, but 10-1 unless you are running a tiny cam should be just fine
     
    czizza likes this.
  6. biker

    biker Veteran Member

    Messages:
    846
    Likes Received:
    72
    Joined:
    Dec 7, 2014
    Location:
    New Liskeard Ontario Canada
    I just built a 10.1:1 383 with aluminum ATK heads, 64cc chambers and 195cc intake runner. Cam I used has enough overlap to bleed off compression and avoid detonation. Very happy with it, I think I'm in the hp range you are looking for. Cam is hydraulic roller by Howards, part number 180265-10. No temp, knock or other weird issues.
     
  7. Cooldawg

    Cooldawg Veteran Member

    Messages:
    196
    Likes Received:
    11
    Joined:
    Jun 3, 2014
    Location:
    Angola, IN
    Sounds like I'm in pretty good shape. That Howard's cam would work fine with EFI, but don't think I'd go there with a carb... Been there done that.

    I'm thinking something in .52/.53 220/230 110 should work. Not tiny, but not quite as rough either.

    Can't quite figured out why ABDC is the determining factor for DC. Seems like it would be all about duration...?

    Either way, the experts should provide a safe recommendation for camshaft given SC, gears, fuel requirement etc.
     
  8. G72Zed

    G72Zed Veteran Member

    Messages:
    1,121
    Likes Received:
    248
    Joined:
    Sep 8, 2015
    Location:
    Canada
    Cooldawg, I have this same 383 combo in my file, I pulled out my notes. I like your builder.......

    Given a zero deck, and a FP 1003 9.1 HG, the SCR comes in at 9.7 with the FT -5cc valve reliefs and 75cc head.

    Same setup with 65cc head, -5 cc valve reliefs and small reverse dome of 10cc, SCR is 9.7 also. Need to have a total of -20cc in the piston tops to get to 9.2 SCR.

    If I had a choice, I like the 65cc/reverse dome set up. I have done a few of those, last one was 400 SBC based, 65 cc/ reverse dome, what I found out in Dyno testing that it liked less timing than the FTops, BSFC was down, and I hit my best 91 pump gas/Flat tappet cam build record of 1.41 ft/lbs per Cubic Inch, also get 17 MPG Hwy!! lots of work!!!

    My current set up is a 10.6 SCR, 333 with Canfield Alum 195's (worked over), 57 cc with Flat tops -4cc. I am in the 530hp range on 91, no issues, runs 170*, and just nocked down 19mpg on my last trip to the road course 2 weeks ago.

    Late closing on the intake valve is a factor to managing the DCR (I crank 192/4 psi), but as VE % efficiency goes up, so does the risk of detonation. Those AFR's are very good heads, 9.7 SCR , 430hp is a cake walk, listen to your builder.

    Good luck in your build Cooldawg.
     
  9. NYH1

    NYH1 Veteran Member

    Messages:
    1,594
    Likes Received:
    12
    Joined:
    Feb 27, 2010
    Location:
    Central New Yorkistan
    You can't get 93 octane at the pump in your area??

    Also with that small of a cam in a 383, you might want to consider going with a 180cc runner heads. 195's work well on a higher RPM 383 or low/mid RPM 406 + engines. You might find it to be lazy on the low end kind of like it was over cammed. Larger runner heads don't produce the runner velocity that works well for lower RPM engines.

    Just my $0.02. NYH1.
     
  10. RS1979

    RS1979 Veteran Member Lifetime Gold Member

    Messages:
    3,726
    Likes Received:
    739
    Joined:
    Jun 18, 2013
    Location:
    Memphis, TN
    I agree with NYH1. With your set up right now, the 180's would give you more power in the low to midrange RPMs.
     

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.