Register now to get rid of these ads!

Technical Help w/compression ratio

Discussion in 'Traditional Hot Rods' started by Jenkins Competition, Dec 15, 2020.

  1. Budget36
    Joined: Nov 29, 2014
    Posts: 7,536


    Well, must ask the manufacturer in regards to CuIn, right? A big cam for a 396 would be much tamer in a 502. Just for comparison sakes:)

    A big cam for a 502, would require a lotta stuff for a 396 to make work.
    ekimneirbo likes this.
  2. ? Same 396 this thread is focused on
    bored to 402.
  3. Glad you worked it out. If you have a next time build, think strong about bigger bore (4.25, aka 427/454 block). That will give you better compression than the smaller bore 396/402. Excess dome height is not really the best for flame travel,which is what the smaller bore needs, at least with factory heads chambers.

    Or better yet even on next build, go with bigger bore and stroke. Like a 540 that can look like the 396 on first glance. I agree that lower compression is better for today's crappy gas and iron heads.

    I used to run a too big cam on low compression 454. Sounded and ran great, but was a little sluggish throttle response. Also needed quite a bit of initial advance on timing to work better. I still had lots of fun and no regret on the bigger than recommended cam.

    Sent from my SM-G950U using The H.A.M.B. mobile app
  4. 55blacktie
    Joined: Aug 21, 2020
    Posts: 506


    I had a 1969 SS 396 Chevelle L78/w L88 cam in 1973. It definitely had a rump, rump idle. With an M22 Rock Crusher behind it, leaving rubber behind wherever I went was easier done than said. I don't recall the octane rating of premium gas in 1973, but surely higher than today's gas. Being a senior in high school in 1973, I couldn't afford spending $10 a day for gas. I sold the Chevelle and bought a 1957 Chevy 3100/w 235 and three-on-the tree. I also bought a 1970 Kawasaki 500 triple, on which I received a ticket for going 120 in a 35, only because I was later found lying in a ditch. Oh, to be young and stupid.
    lippy likes this.
  5. Seems like once a Chevy motor
    exceeds 3.75 stroke, it morphs to
    a slow revving slug.

    IMO, no better way to de-value a SS 396 than to put a truck motor in it.
    (although this would have saved me some head scratching and a lot of $’s !)

    If this were a non-SS car, I’d install a 500 HP 383.

    As it is, original 396 will go back in. Whatever CR increase I get will be icing on the cake.
    Last edited: Dec 22, 2020
  6. 55blacktie
    Joined: Aug 21, 2020
    Posts: 506


    Earlier, today, I read some old posts, going back to 2003, regarding the L88 camshaft. I think many comments came from people who had no firsthand knowledge, but were only repeating what they might have heard. As I've already said, I owned it and drove it. The bad: poor gas mileage, and a garage that smelled of raw gas. The good: FUN! I will add the the OEM shifter discouraged speed-shifting from 1st to 2nd. I stopped doing it after fearing I might have broken something (I didn't). On the other hand, I do agree that after 51 years, there has to be a better cam available. I would consider a mechanical roller. I like the sound of solids when properly adjusted.
    Elcohaulic likes this.
  7. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,005


    3931063 Heads have a 109 cc chamber .
    They are a semi-closed chamber head which is a lot better than the "Bathtub" closed chamber heads.


    Your piston choice is wrong! .......... send them back.
    The piston you need is "Sealed Power" L2242NF30 which has a 38.3 cc dome.

    With those pistons and an off the shelf Felpro # 1027 head gaskets should get you 10.7: 1 compression [providing you still have 0.020" deck clearance]

    If your block has been Decked to zero [or less than 0.020"] you can lower the compression using Felpro # 1071053 which have 0.053" compressed thickness
    This will net you a 10.8:1 compression with Zero deck clearance.

    Cometic # C5816-060 is a 0.060" compressed thickness gasket that would net a 10.64:1 compression ratio with zero deck clearance.

    Buy the correct pistons, then measure the deck clearance [and buy the correct head gaskets to suit]
    10.5 :1 compression and a fat cam will really wake up a 396/402 with 3931063 heads.
    Baumi and ottoman like this.
  8. Mimilan,


    Are you sure about 063 head cc ?

    Not trying to be wise guy, maybe my chart is wrong ?

    Speed Pro piston calls for Bore (in):4.134 in. , correct ?
    Last edited: Dec 23, 2020
  9. 55blacktie
    Joined: Aug 21, 2020
    Posts: 506


  10. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,005


    You are not being a "wise-guy" .
    There is a lot of dis-information out there [or conflicting information]

    Check the 10th row!

    Most charts claim 107 cc
    But whenever I have CC'd factory Chevy cylinder heads, they have always been on the high side [if unmolested]
    Chevy's compression ratios were always overly optimistic.
    My advice is to CC the heads to get an accurate number.

    Depending on Head CC and piston to deck clearance, there are plenty of graded head gaskets available.
    Cometic do .040 .051 .055 .060 .066 .080 compressed thickness.

    Here is the Summit listing , It shows a 4.124 bore

    After CCing the heads, If it comes out at 101.9cc as being speculated.
    Speed-pro do a piston with a 21cc dome #L2240NF30

    Deck the block so the piston is 5 thou [0.005"] above the deck, and use an "off the shelf" Felpro 0.039" head gasket.

    That would give you a 0.034" quench and a 10.21:1 compression ratio

    Start measuring!
    jimmy six likes this.
  11. Thanks Again,
    this is helpful.
  12. Budget36
    Joined: Nov 29, 2014
    Posts: 7,536


    @Mimilan is .034 a safe quench. I’ve always known .040-.045 to be what to shoot for?
  13. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,005


    "Safe" is purely subjective.
    We are running just that on our '57.
    The block was decked 0.050 to get the pistons 0.005" above.[using crap rebuilder pistons]

    I've been doing this on High revving "Kent" crossflow race engines for years.[9000 rpm +]
    Because the swept area is greater , you can mill less off the block than milling the head to get the same cc gains.
    Budget36 likes this.
  14. Ericnova72
    Joined: May 1, 2007
    Posts: 480


    It may be apples to oranges since it is SBC and not BBC.... but we have run a street/strip 383 stroker SBC, Scat cast crank, I-beam rods, and lightweight TRW forged pistons(L2491F) which are just 490 grams.....with .034" quench clearance (pistons .005" out and .039" gasket).
    Motor regularly sees 7200-7400 rpm and when torn down for a swap to a competition style oil pan and general examination you could see the carbon build up on the pistons, the quench areas of the flat top piston that matched the head chamber shape, just "buffed" nearly see appeared any tighter on quench or higher on rpm and we would be in danger of actually hitting the head.

    Every combination will have different minimal limits relating to component weight, rpm turned, piston-to-bore clearance, steelor aluminum rods, etc. which is why the general .035-.045" minimum number is typically safe.
    Truckdoctor Andy and Budget36 like this.
  15. Jmountainjr
    Joined: Dec 29, 2006
    Posts: 1,377


    It could just be my age showing, but I typically am involved in two distinct engine categories - race or street. And to me there isn't any gray area between them. They each have different expectation of performance, reliability, and maintenance requirements. So they each have a different set of specifications and tolerances. Lately I see a lot of gray area recommendations for street engine builds. You always give something to get something. Took me some time realize that one size doesn't fit all.
    kevinrevin and RodStRace like this.
  16. Mimilan
    Joined: Jun 13, 2019
    Posts: 1,005


    Here is my old Big Block race engine with open chamber heads. The pistons on this engine were 5 thou above deck. [You can clearly see the shadow from the chamber]

    Here is the chamber on the same cylinder

    This is a road racing engine that saw 8000 rpm for extended periods.
    So far I've never had issues with a BBC race engine, Ford Kent race engine, or my present SBC street engine [all were decked 5 thou above and used 0.039" off the shelf head gaskets]

    One caveat though. I wouldn't recommend decking the block if there is excessive wear in the bore.
    It is the piston to bore clearance that causes the problems.
    The pistons can rock in the bore causing the crown to make contact
    [This is more prevalent with short skirt pistons from stroker/long rod combos]
    Last edited: Dec 25, 2020
    Beanscoot and RodStRace like this.
  17. RodStRace
    Joined: Dec 7, 2007
    Posts: 2,752


    Good info mimilan!
    Hope this stuff is really sinking in.
    While many of us love the sound of cam and compression at idle, it is exactly that lope that makes off-idle transitions and light throttle low RPM cruise wasteful and weak.
    I did a SBM with the 3 Cs (Carb'ed, Compressioned, Cammed) a while back. It 'woke up' at around 2700 RPM. A beast above that, a bit of a slouch below. It sounded great at idle.
    A well built street engine will have enough torque and HP to never need to rev past 5000 RPM.
    A well built race engine will have a higher, narrower tuned RPM range where everything is optimized within rules and application.
    Those BBC pistons are big and heavy. Erring on the side of safe clearances and quench, and keeping the RPM reasonable is going to be safer and longer lasting.
    Boneyard51 and ekimneirbo like this.
  18. A shout out to Mimilan !

    Sent the pistons back, ordered some 21cc versions. Now, several variables plugged in compression formula and they all are close !

    Thanks man !
    Budget36 and David Gersic like this.
  19. GearheadsQCE
    Joined: Mar 23, 2011
    Posts: 2,843

    Alliance Vendor

    Better check Mimilan's profile.

    Probably should be, "Thank you, Mam."
    427 sleeper and David Gersic like this.
  20. That’s his wife
  21. GearheadsQCE
    Joined: Mar 23, 2011
    Posts: 2,843

    Alliance Vendor

Share This Page

Register now to get rid of these ads!


Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.