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#1
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Hmmm, carb size! Any comments?
I was just reading in my new Car and Driver that a 3500 pound NASCAR Sprint Cup car has a 358 cu in engine that spins at 8500 rpm, makes 850 hp (and 550 ft-lbs torque at 7000 rpm) and uses one four barrel 830 cfm Holley carb. 830 cfm! You'd think there would be a 1050 Dominator on an engine that makes 850 hp!? Anyone got an explanation?
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#2
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Yes. They are trying to keep the speeds in check. For instance on the Craftsman trucks they do it a different way. They have a sealed aircleaner with 2 3" hoses fixed to two 3"openings in the grill. When they go to Daytona, they put a block off plate in the grill and neck it down to 2" to slow the trucks down. If the cup cars could run that 1050 they would make a lot closer to 950 to a 1000 hp with it.
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#3
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No they wouldn't, without huge other changes. PST truck engines were also 358 cu in, and only destined to run 1/4 mile at time, and even overall before rebuild some 1% of the miles the nascar engines run. They have completely different materials and masses inside the engine, and usually two 1050's and the best of them barely make that much. When the class was discontinued, the best were in the 900-950 hp range, with dual dominators, tunnel rams, extremely light internals, huge CR's etc. etc. We happen to have one R5/P7 engine on the garage floor, going in to a street Duster. A couple of months ago Michael Waltrips cup car was run at Concord Drag strip by several different drivers. The best speed was 140 mph, and best ET in the mid 10's. But the speed pretty much tells the power.
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#4
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Any chance it may have something to do with the air pressure at the base of the windshield?
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#5
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Air pressure at the windshield base has a lot to do with it. With no forward motion there's no pressure or ram air effect that the engine and carb was tuned for. They spend a lot of time working on those air ducts and air cleaners, even using wind tunnels to get it right.
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#6
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Well, I find it interesting that some folks say their 440's are choking on an 850Holley, while heres an engine making 850 hp with an 830 Holley. And I don't think the engine displacement is the factor here at all. To make 850 hp, you have to move a certain amount of air. If the engine displacement is around 350 cu in, then you just have to get the rpm up to around 8500 to make that power. If the engine displacement is 500 cu in, then you can make that power at 6800 rpm.
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#7
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Probably just comes down to efficiency versus mass!
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#8
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I agree with aar. I`ll bet the whole intake tract is a lot better in the nascar engine than the 440. The 850 is only making the restrictive[in relation to the state of the art nascar engine] tract easier to move air. Then, the choke factor moves to another location.
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#9
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That's true. When a street 440 head can't flow over 300 and a NASCAR head flows way over 300 and you run the big cams and turn them 9200 they can make that hp on the smaller cubes. And you have to remember those cubes are not on a bored 350 or 351 or 360 block. They have custom rods, pistons, and cranks. And use different set ups on the rotating assembly depending on the track they are running on. You want top end on a super speedway so you want to build the engine for hp and you want torque on a 5/8 short track, but they keep the same cu. in. with different rod and crank ratios to achieve it.
The carb really is the major restriction on those engines. Just take a look at the restictor plate engines. The plate makes that 830 less than a 750 and kills the hp down to like 650 and the rpms down to 7200. |
#10
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Well, I gotta chew on this for a while. Right now, regardless of restrictions or what else, seems to me the bottom line is that an 830 cfm carb is making the dyno machine indicate 850 HP! I suppose engine effeciency/design may have a lot to do with it. As far as air pressure at base of windshield, I've got a ram air box on my 496 Cuda, with the snorkles at the front of the car. I would guess the slightly positive pressure on my carb may be giving me 10 or 15 hp at high speeds. I know the cold air it provides is a biggee hp improver!
(1.5% to 2% for every 10 degree drop in inlet carb temperature!) On a 600 hp engine, a 50 degree drop in carb inlet air temp is supposed to be good for up to 60 hp !!) Amazing hp improvement for the money!! |
#11
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I have a question here.........
NASCAR use to limit the Datona and Talladaga races to 390 cfm carbs. Do they still or did they only restrict the cfm flow by the restrictor plate? Restictor plate engines only turn about 7000-7500 rpm as the unrestricted turn about 9000-9500. So the restrictor plate races should depend on torque more or just move everything in proportion? I know swirl head technology changed everthing and this allows smaller ci engines to pack in more punch, so to speak but it is really only and extention of the early mopar ram induction intakes into the heads.
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#12
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Here is flow numbers of one of our P7 heads:
intake 0.1" 73cfm 0.2" 151cfm 0.3" 234 cgm 0.4" 294 cfm 0.5" 343 cfm 0.6" 354 cfm 0.7" 368 cfm exhaust: 0.1" 53 cfm 0.2" 117 cfm 0.3" 169 cfm 0.4" 200 cfm 0.5" 257 cfm 0.6" 267 cfm 0.7" 271 cfm. The carb is not a major restriction, the engine is designed around that carb size. Just putting a bigger one would not dramatically change teh otuput. Same with the restrictor plate engines; you just don't install a restrictor plate, they are completely different engines, different port sizes, compression ratios, cams etc. The R5/P7 engine has got nothing to do with any mopar production engine, there is not a single interchanging part, not even close. Well, perhaps some bolts can be used at some points. |
#13
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Quote:
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#14
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Also, design a cam that makes power at 8000 RPM and you do have a peaky motor, the 390 CFM carbs are measured at street level inches of mercury. Pull more inches (more vacuum) and those 390's are looking more like 850's in flow number, and those 390's were built from the ground up as high RPM carbs as air bleeds and every other pore on those carbs were pulling in air. Put 2 1050's on them and it may not help much as the amount of inches the motor has to pull the air though increases and the motor starts to lose velocity through the carb. Hard to visualize but the Mopar 1050-1200 CFM six pack set up was closer to 780 CFM in total air passed as the motor couldnt pull any more than that. AS for the Ram effect, it must be something new as Im looking at a model of a Steve Kinser Quaker State Thunderbird #26 (1995?) and under the hood is nothing that resembles ducting, just an almost open air cleaner in a tubular chassis with a slot in the cowl above the air cleaner. You would think it would be almost sealed like a cowl induction GM or an air grabber setup to take advantge of any positive pressure at the base of the windscreen.
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#15
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Quote:
http://images.hotrod.com/techarticle...ar_engine+.jpg |
#16
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Ram effect is from the early 60's............
Quote:
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#17
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To go back to the o.p.'s question, NASCAR mandates the engine displacement and carb size so the players get to throw major $$ building a 358 c.i. 2-valve pushrod V-8 that will make the needed power within those constraints. 8500 rpm with 830 cfm. of air going through apparently is good for 850 horses. If I had the same budget and brains I could do the same thing. Of course I'd need a few big tracks to test and tune, but that would be the fun part. And I'll bet that finding any semblance of a NASCAR engine under the hood of any Dodge or Chevy is not possible. Ford and Toyota don't even have pushrod engines in production anymore, and in fact Dodge doesn't even have an inline-valve pushrod V-8 any more. NASCAR needs to take the "S" out of it's name.
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#18
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delete duplicate post
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#19
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Quote:
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#20
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Quote:
Here's a Daytona with the cowl air back in '69 before they stopped that in the '70's. http://www.conceptcarz.com/view/phot...CAR_Photo.aspx Toyo engine http://www.sfmreport.com/Nascar_Toyo_Motor2.jpg There's some Ford pics at the bottom with cowl air. http://www.randyayersmodeling.com/mo...ic.php?t=36222 |
#21
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This is where I got the setup for my Cuda.
They also sell an air cowl setup! One of the best bang for the buck mods you can make! I use the 4AL trumps like those pictured on the 70 Buick Skylark. Ram that cold air in there! (Good winter project, boys!) http://ramairbox.com/ |
#22
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I have them book marked. They have some good products.
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#23
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A couple of comments---Nascar mandates the carb size to be used on the Cup, Busch(I will always call them that) and Truck engines. The Cup engines must use a "830", annular discharge carb----the Busch engines use a "390" carb and the trucks use something like a "650" carb. As a point of interest, the Cup engines must use a flat tappet cam while the Busch and truck engines do use a roller cam. Don't ask why---that's just the rules. Why the different sizes? To regulate the speed/horsepower of the three classes.
Don't expect to ever buy one of these carbs. Why? They are not Holley---at least not the Cup carbs. I had the pleasure to hold one of these carbs---it had been on a dirt late model engine----been crushed by a wreck. It weighed a mere 20 ounces! It was billet titanium---cost is a mere $4800. Looks like a Holley---but Holley it ain't! Don't know about the Busch or truck carbs but the Cup carbs are like that. You never see a used Cup carb for sale! A little about the engines---in a word they are "design" specific----meaning all the engines used are designed and built to Nascar specs---PERIOD! Nothing interchanges with a standard engine---even the bellhousing pattern is "generic"! How "generic" are they? You can interchange the intake gaskets between a Mopar, new GM R-07 and a Toyota engine. The new Ford engine will be the same next year. And the new generation of Mopar R5/P8 will be the same as well. The primary difference in the new Mopar engine will be improved cooling passages and the distributor moves to the front---like everyone else. A little about the heads---Nascar specifies the port location and port size. And since the ports are all CNC machined and weight is a major consideration, very little aluminum remains for enlarging the ports. There is some excess there but not much---the same is true for the valve sizes. |
#24
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General comment.....
There is no question that the nascar engine is tuned by the best of the best, and is completely maxing the combo. That said.... I also think this ads to the theory that carb size is only a very small part of the equation. I saw an article in a mag a few years back. They built a big cammed up motor, that made big power. Then they started putting smaller carbs on it. The only difference that showed up on the tests was less and less peak power. The power in the curve stayed the same, and in some places even improved. In short, you wouldn't have noticed a change in drivability of the car, or in general performance. Personally, I feel a lot of motors are over carbed. I tow with a 440 winni. I use a 625 road demon, and it works awesome. I've used 750 mghty demon to go 11's with a 440 in a 3800 pound car too. Of course I had to try a 950 hp holley to see if it would make a difference. Nope. |
#25
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I agree. At 6000 rpm on a big block, going from an 850 to a 1050 Holley may only get you 5 to 10 hp, if anything, and proabably cause you to lose some power below 6000 rpm. One reason may be that there is another component in the engine and exhaust that may be more restrictive to air flow, such as the intake, or headers, or air filter, or exhaust! Also, the solution may be just to increase jet size, rather than carb size!
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#26
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You can't just change jets. That will only effect the fuel air ratio and wont effect the carb size.
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#27
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I agree. But, changing jets may net you 10, 20, or 30 hp on the top end. Just something to try before you spend money on a new larger carb. I guess the color of the plugs would be also be a clue as to what you should do.
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#28
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So very true.
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