1/4 Mile Issues

Discussion in '1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk-' started by sharp2377, Oct 27, 2013.


  1. John Dirks Jr

    John Dirks Jr Active Member

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    The OP is talking about a mildly modded engine. You are talking a different story. Put it in context.

    After all the OP's 600 cfm took him to 5800 rpm and 12.86 et in the 1320'. You think he would do better with a 750 cfm(considering everything else the same) ??

    Dominator? What? Gimme a break....
    #21
  2. NIKwoaC

    NIKwoaC 中國製造

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    I think you've mentioned this 358 once or twice before. What was the bottom end setup? Pistons? Rods? Just curious. I have 351 parts scattered around my garage right now, and I still have not 100% decided on what to do with them.
    #22
  3. clement

    clement Founding Member

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    you missed the point. I posted about how to choose a carb based on having the right size venturi for the application so you have the right amount of booster signal. and yes, a larger carb on an engine with lots of signal can help because it will slow down all that airspeed coming through the venturis that has to turn into the ports with fuel in the mixture. yes, too much airspeed can be as bad as too little. .

    the point was with that small cam, the small cross section over the 358" combo has enough signal that a dominator would probably have signal on it. I didn't tell him to buy a dominator. if you don't believe me call anyone that BUILDS carbs. none of them use that formula you mentioned.

    I have seen the correct carb for the application pick up .4 and 4mph in the 1/8th on a motor that made as little as 340rwhp, which is what that 10:1 358" made. many making that little power are going maybe high 11s at 3000lbs. but lets use the 'mild combo' excuse to give out 30 year old formulas that don't really have any relevance, at least according to those who build custom carbs. and I have seen one of holleys most expensive 650 HPs, that on paper should have been the perfect carb for the application (low power, low signal), not work as well as a my semi custom 750. and that particular 750 actually flows around 830. so much for the cfm theory.

    http://www.pro-system.com/pjames011900.html
    Last edited: Nov 16, 2013
    #23
  4. clement

    clement Founding Member

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    \

    stock block, crank, rods and a cheap set of windsor flattop hypereutectics. Cleveland 4v closed chamber heads (that we reused on the 410" motor) . 10:1 compression pump gas motor. an old restrictor single plane intake from the 80s that was easily the minimum cross section of the intake tract. the cam was a solid flat tappet, 255/261@.050, .610"/.617" lift @ 104* ICL. 110* LSA. the car ran high 1.4 60' times and 6.80s and .90s@ 98 mph on the motor with a set of 1 and 7/8" primaries to 3.5" collector headers. the motor didn't like collector extensions and liked mufflers even less. the car weighs 3000lbs. all in all it was a bunch of garbage. I would have liked to have seen a better intake an different valve events. but it was what was lying around or cheap. probably doesn't hurt that the car owner is a hell of builder and can take a bunch of unwanted crap and make it haul balls.
    #24
  5. John Dirks Jr

    John Dirks Jr Active Member

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    clement,

    Respectfully;
    I'm sure you know it's a mathematical equation. 600 CFM can move enough air to take a 351 to 6000 RPM. If its naturally aspirated, the engine is a basic air pump. A larger opening is not going to move more air. If anything, it will slow the velocity of whatever air is coming in. Lower velocity can have a negative affect on fuel atomization.

    You mentioned jet changes. I admit that changing jets can add power but it does not change basic air requirements relative to peak RPM on a naturally aspirated engine.

    If you have any first hand experience testing the difference between different sized carbs on an otherwise identical setup, I'd be interested in hearing all the details including carb CFM sizes and performance results.
    #25
  6. clement

    clement Founding Member

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    There is a reason I have provided links. I know what has been explained to me by some very knowledgeable people, and everything they have explained to me has worked. I didn't even get into the differences in metering and emulsion. there is a reason some carbs run better than others. from a best of 7.28 to a 6.88 and from a best of 95mph to 98 with a carb swap in the 1/8 on an engine that made 340 to the tires on its best day. we tried carbs that worked on other stuff to no avail. going smaller is a band aid for a bad carb. back to topic, lower velocity wont hurt it when you have so much air speed trying to make the turn out of the carb through the plenum and into the port that the fuel falls out of the air.

    you are assuming that the engine will consume less air with a larger venturi because of a lower velocity. you are not taking into account that you are getting greater volume, albeit at lower air speeds. if too much air flow were a bad thing the argument for better heads and intakes would be invalid and every combination with a given cross section port, cam and displacement would run the same. it just isn't the case. you also have to remember that the carb feeds the plenum, which feeds the ports. the reason I know there is more to this deal than just cfm is because once you get to above 1.5 hp per ci static flow means very little in terms of the port, and shape is almost the sole determining factor in how much power the engine makes and how fast it runs at the track. an engine is a dynamic machine and flow benches and calculations don't even being to be able to explain the complex physics happening in the very high VE normally aspirated engines. not to mention that there is only so much air flow you can get through a given cross section before the port goes sonic.
    Last edited: Nov 16, 2013
    #26
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  7. f8tlfiveo

    f8tlfiveo My wife likes my spool and blow-off valve.

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    Wow.. Never knew there was such a science to carb selection. I grew up mostly in the fuel injection era though.
    #27
  8. 84Ttop

    84Ttop They make new pistons every day, so why worry? SN Certified Technician Mod Dude

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    You're better off in my opinion..... Fuel injection for the win!
    #28
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  9. clement

    clement Founding Member

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    the carb has its downsides. but ive gotten used to changing jets. if I had a driver with a carb it would be a pain in the ass. but in terms of getting down the track its hard to beat a good semi/custom carb.
    #29
  10. revhead347

    revhead347 I have face herpes.

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    I agree with the others above. Even though I have limitrd carb experience, I still.think you have enough carb.

    Kurt
    #30
  11. John Dirks Jr

    John Dirks Jr Active Member

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    clement,

    I'm certainly not assuming the engine will consume less air with a larger venturi. But you seem to be saying that it will consume more. That's where I disagree. Unless it has forced induction, the engine is a basic pump. It has no idea how large the venturi is and it could care less. As long as a given size venturi is big enough for a certain size cubic inch engine to get to a specfic RPM, a larger venturi will not make the engine pull more air. Its basic physics. You cant change that.

    Volume is volume, Period. No difference in volume when changing venturi sizes.

    Velocity will change with different sized venturi. A larger venturi will "slow" that given volume of air. It will slow it at the critical point where air and fuel are primarily mixed.

    All along I've been making my point about the air requirements of an engine and I stand by them.

    If an engine can get enough air with a smaller venturi, it will likely have better throttle response and better bottom end TQ and drivability characteristics. Furthermore, if it's just big enough, it will get the engine to its designed peak RPM. The best of both worlds.
    #31
  12. NIKwoaC

    NIKwoaC 中國製造

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    Are you familiar with the concept of volumetric efficiency? It is defined as the actual amount of air and fuel an engine digests at a given RPM, relative to what its displacement suggests it would. Most NA engines fall short of 100% VE at their peak- like maybe 80-90%. Some very well built engines will hit 100%. The new Coyote 5.0L supposedly achieves something like 110% VE- meaning it is actually so effective at moving air, it creates a supercharger-like effect.

    So, I bring this up to point out that engine airflow requirements are not strictly related to displacement and RPM. "Good enough" component sizing logic is a slippery slope, and for a lot of guys, leads to mediocre performance.
    #32
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  13. John Dirks Jr

    John Dirks Jr Active Member

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    I know about V.E. It most cases the naturally aspirated engines are less than 100% just like you say. That means they don't pull as much air as the simple equation would say. Therefore, smaller carbs can be sufficient.

    I agree that there are many with the wrong size carb. I'll bet that the majority of those with the wrong size are over sized.
    #33
  14. clement

    clement Founding Member

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    once again, its not about cfm. its about finding the best venturi size for the appication. does it run good with that carb? yes. would it pick up more mph with a nice carb? I am so sure it would that if he lived close to me and wanted to try it, I would be willing to put the unported 351w vic jr ihave on the shelf and once of my built 4150s on it. im wondering how many of those who think its only about cfm have actually done a comparison between the cheaper OOTB stuff vs a built 4150 at the track?
    #34
  15. clement

    clement Founding Member

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    and what do you define as oversized? bigger than what your calculator says it should have? what do you define as target airspeed? perhaps your builds that you are saying that don't pull as much air as they should is because you are putting a big fat cork in the induction system? maybe if you were to let it eat it might surprise you. run a vacuum gauge on the bottom port on the carb and lets how much vacuum its pulling at WOT. assumptions are what lead to lackluster performance due to not experiementing with the combination. I have seen gt40 irons go 11s on naturally aspirated 302s. 50 more ci better do something.
    Last edited: Nov 17, 2013
    #35
  16. John Dirks Jr

    John Dirks Jr Active Member

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    What was the weight of the car? What RPM's was the driver pushing it to? I ask because the weight of a car has a direct affect on how fast it can get down the track. Likewise, max target RPM has a direct affect on how much air the engine needs to ingest. If you are going to tell me that the engine was revving to 7k, I'd say slap a 700 on there all day long.

    You are tossing all kinds of variables into the discussion in an attempt to say my statements are wrong. I never said you were wrong and I'm sure your experiences are valid.

    My statements are very specific and yours are very broad. I think we're comparing apples to oranges here.

    Peace.....I'm done.
    #36
  17. clement

    clement Founding Member

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    3200lbs with driver and a 6100 rpm shift point with a fresh 91 5 liter shortblock. im simply trying to help you understand that the world is not flat. how much more specific can I get when I ask you what your target airspeed number is and what do you define as oversized? I know the airspeed window where air starts to go sonic. hows that for specific? I also know of a 289 that everything is wrong on, has a huge set of 225cc procomp heads, a 250*@.050 solid roller, a tunnel ram, nowhere near enough valvespring for over 5500 rpm, nowhere near enough compression for the cam size and cross section of the redneck home ported POS heads, a tunnel ram, two 600 cfm edelbrock POS carbs and it has gone 7.20s@91 mph in the 1/8th at 2900lbs on the motor! I couldn't even believe it because so much is wrong with the combo. I guarantee you the airspeed is low on that turd, and surprisingly enough it still runs; and runs good considering all that is wrong with it. I was at the shop when he assembled it and can assure you that there is something wrong with every part. so when you tell me a 750 is too much carb for a stock stroke Windsor with a set of 160cc GT40s and a little roller, im gonna have to disagree and have on track data to back it up. have a nice day.
    Last edited: Nov 18, 2013
    #37
  18. John Dirks Jr

    John Dirks Jr Active Member

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    Wouldnt a 3200lb car need to be putting near 400hp to the rear wheels to get into the 11's? That would be a sight to see on a naturally aspirated 302 limited to 6100 rpm. All motor huh? No nitros or boost? Wow! - is all I gotta say. You are way more enlightened than me in my flat little world.
    #38
  19. clement

    clement Founding Member

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    ive gone 6.80s@98 in the 1/8th (high 10s@122) with 340rwhp at 3000lbs. so no, you don't need 400 rwhp to go 11s at 3200lbs. this particular 302 made 310 and trapped 115 mph. there are guys who have gone 118 mph in the 1/4 with unported gt40 irons at 3200lbs. NMRA factory stock guys used to gohigh 10s with unported gt40 irons at 3050lbs, im pretty sure they aren't making 400rwhp. ive seen people with over 580rwhp at 3400lbs only go mid 11s if you want to talk about under achievers.
    Last edited: Nov 18, 2013
    #39
  20. John Dirks Jr

    John Dirks Jr Active Member

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    In any event, I'm glad we've kept the discourse civil.

    Thanks brother.
    #40

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