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Thread: IMCA MODIFIED MOTOR

  1. #1

    IMCA MODIFIED MOTOR

    Whats the best size motor for IMCA MODIFIEDS on 3/8 dirt? I ran a 355 last year, wasnt very impressed.

  2. #2
    Senior Member EXPERT BUILDER
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    Hi there Just10Time,
    I've been an IMCA modified builder and driver for several years in southern NM and west Texas. I have an 05 KraftKar 4 link on Chevelle chassis. Willie Kraft sold to Derrick Ramirez Motorsports a couple of years ago, and I believe Ramirez is currently holding top honors in the manufacturers challenge right up there with Larry Shaw and Jeff Taylor. Derrick is kicking their butts in points standings right in there with Jeremy Payne and Jason Hughes.

    Anyway... for several years a 383 stroker was the winning ticket... (3.75 stroke with 4.030 bore and 6" rods). Using a seasoned GM 350 block though left much to be desired in the durablilty and longevity area.
    But, now that IMCA has a rule change allowing aftermarket blocks due to cost efficiency, and Dart SHP has made buying an aftermarket block so much more affordable and no brainer over using a junkyard block, everyone is heading towards going as big as they can get for bottom end torque and taller gears.

    I just got my new Dart [Little M with 400 mains] back from my engine builder, punched out to 404 ci (4.125+.020 due to rebore/rebuild).
    My engine was originally a 388 stroker built with 4.125 bore, 3.625 stroke, 6" rods, and (illegal for IMCA) big roller cam. It was a screamer, but mistakes cost me several burnt pistons so my builder convinced me to bore and stroke to 404 and make is IMCA legal.

    If you follow other threads here on RJ, there is much opinion about staying away from 400 mains if possible, which is probably one good reason why Dart only makes the SHP block in 350 mains.

    With an SHP block, Dart only recommends a max of 4.165 bore and 3.75 stroke, to give a 409 ci. There is much debate about using 6" rods over 5.7" rods, so that is up to your choice of pistons you want to use.
    Some guys are experimenting going bigger to 414 and 420 cubes, but that is really pushing and streching it in machine work, and not leaving much room for a future rebuild without special order pistons. It appears the big bore and long stroke is heading the right direction for dirt tracks.

    I highly recommend a new SHP Dart block, finished at stock 4.125 bore, and 3.75 stroke, which is of course a 400 ci platform. This will give you two more standard overbore rebuilds in the future, knowing how hard dirt racing is on engine life. The big bore and long stroke will give you lots of bottom end pulling torque allowing you to use a taller gear to prevent blistering the tires coming out of the corners, yet a lot of speed at end of the straights with lower rpm range under 7800 revs.

    Good luck, and let us know what you come up with...
    Jim
    I'm not hatin... I'm just sayin...

  3. #3
    Senior Member RACING JUNKIE
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    dirt that also depends on if imca sport mod or a mod as here in iowa/sd sport mods here are limited to 360 ci and 9.1 cr ie. my short block the only thing right now that make my motor illegal is the top end (heads intake ect) instead of the race pan i put on a factory one. still have the cam for the track in it was to cheap to get a more streetable bump stick. i dont know if you guys have both sport and a mods there but thats what we have here. funny thing is same sport mod motor is also used in imca hobby stock. both classes mandate 2bl carb. and fact intake. sport mod can use crate if with in all but carb rule then you get 4bl and since fact sealed its dq if you open the crates
    CRIMEDOG
    Sam Wood

  4. #4
    Senior Member EXPERT BUILDER
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    Oh... I'm sorry... I read that just10time asked about IMCA MODIFIEDS, which usally means IMCA "A" Mods.

    Had he stated "B" Mod, Southern SportMod or Northern SportMod, I would have headed that direction.

    Since I am an IMCA engine and chassis builder I do know the rules for all the classes at the IMCA sanctioned tracks I race at, but if you say I'm wrong, I guess I am wrong...?

    I guess since we never heard back from just10time it really wasn't all that important to him anyways to even care enough, so it is all a moot point anyway...
    I'm not hatin... I'm just sayin...

  5. #5
    Senior Member RACING JUNKIE
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    not saying you wrong and yea mute point. just noticed rules very greatly in various parts of the country for imca it seems
    CRIMEDOG
    Sam Wood

  6. #6
    Senior Member RACING JUNKIE
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    BLOCK: Must use OEM steel passenger vehicle production block only. No GM Bowtie, Ford SVO or Chrysler W components allowed. GM approved block numbers are: 3892657, 3914660, 3914678, 3932388, 3932386, 3956618, 3970000, 3970006, 3970010, 3970014, 10066033, 10066036, 10243880, 14010207, 14010209, 14010287, 14016376, 14016379, 10054727, 14088528, 14088548, 14088552, 14093638, 14101148. Stroke must match block. No 400 or larger cubic inch parts allowed. Maximum 361 cubic inches (GM); 363 (Ford); 370 (Chrysler). Violation of cubic inch limit must be verified by removal of head and will result in disqualification, loss of all IMCA points for the season, $1,000 fine and a 30-day suspension. Maximum compression ratio is 9.0 to 1, no tolerance. Compression ratio checked using Whistler and cubic inches checked using pump, OR by visual inspection of part and/or casting numbers, pistons, etc (track option which method is used). Flat top or dished pistons only, no gas-ported pistons. OEM or OEM replacement steel crankshaft and rods only - cannot be lightened. Conventional flat tappet cam and lifters only, cannot alter lifter bores. OEM firing order cannot be changed. May use oil restrictors. ‘Wet’ sump oiling system only. Steel oil pans only. Racing oil pans allowed. One inch inspection hole recommended in all oil pans (mandatory effective Jan. 1 2010 – no obstructions to crank and rods. Accumulator allowed.
    CRIMEDOG
    Sam Wood

  7. #7
    Senior Member EXPERT BUILDER
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    HEY JUST10TIME... YOU OUT THERE...?

    If so, do you wanna clarify if you mean "A" Mod, "B" Mod, Northern Sport-Mod, Southern Sport-Mod, B.R.A. X-Mod, Wissota Mod, or whatever Mod you are interested in running...?

    just10time... in case you see this and don;t wanna reply, just get a rulebook for whatever track you are going to run at...!
    If it is an IMCA track you can find your answers at
    http://www.imca.com/rules.php?divisionid=1
    I'm not hatin... I'm just sayin...

  8. #8
    Junior Member SHOW GUEST
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    sam,
    I think those are the northern sport mod rules, southern sport mod rules require a hyd cam, and they check cranking psi(limited to 175 lbs after five rotations), no limit on c.r. per say, just on the cranking psi. I to assume he is speaking of "a" mods, here in texas people usally call mods mods and sportmods sportmods. I own, drive and build one of each of these money pits

  9. #9
    Senior Member MASTER BUILDER
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    Since IMCA Opened up allowing aftermarket blocks, 415 and 427 are all we sell..

    http://www.cnc-motorsports.com/category.asp?CtgID=23908
    Competition Engines & Hardcore Engine Parts
    www.cnc-motorsports.com
    info@cnc-motorsports.com

  10. #10
    Drtrcr22 l'm wondering could l put a set of 23 4.135 forged high end dirt track piston's ment for 6" rods on my stock gm400 5.56rods and be ok for a 010 overstreet car ww64cc chambers? These pistons have the typical1/16 1/16 3/16 4.135+.005cc ringset. Thanks ahead of time, l only ask bcuz of all your posts l know you would know thanks again


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