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post #1 of (permalink) Old 02-13-2002, 07:57 PM
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4 brl manifold or not?

Ive asked some questions about this already but I need some honest opinions and/or experiences instead of info. Ive got this 4 barrel holley manifold that Im thinking of puting on my 225 V6 in the next few days. I also have an adapter so I can still use my 2 barrel carb(I dont have the funds to fund a nice 4 barrel). I was told that I would gain more torq by using this set up. Is that because the fuel is atomized better? The real question Im asking is whether or not I should put it all on. Am I going to actually gain some low end or am I going to lose any horspower. Is it worth my time? I also was told that the offenhauser manifold was better than the holley one Ive got. I got the holley for free but I was wondering what about the design of the offenhauser is better? Thanks for the help.


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post #2 of (permalink) Old 02-13-2002, 08:03 PM
 
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Re: 4 brl manifold or not?

the only gain will be in the high rpm range not where a jeep needs it in the low rpm range if the 225 is stock than a duece is all it should need if the carb itself is a problem than rebuild it or swap a different duece a fourbarrell on a stock v6 would be overcarbed hell i have a mc2100 two barrell on a warmed up 304 v8 in my cj5 and it is plenty strong with the 2barrell headers and a mildly rumpy cam

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post #3 of (permalink) Old 02-13-2002, 11:04 PM
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Re: 4 brl manifold or not?

will I lose any low end?


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post #4 of (permalink) Old 02-14-2002, 08:41 AM
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Re: 4 brl manifold or not?

btt


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post #5 of (permalink) Old 02-14-2002, 04:54 PM
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Re: 4 brl manifold or not?

There is a good chance you will. A lot of factors are involved....and even though you are keeping the 2b, the velocity on the 4b may actually be less, thus hurting low end.
The best comparision is to think of a water hose and that bucket you want to fill with water. Use a small diameter water hose and you have to run the water pressure and velocity up a lot more to get the same rate. Use a bigger water hose and you can slow the water down.
In simple terms, we want low restriction, high velocity air flow for best low end performance. For high end performance, guess what, we want low restriction with high velocity. See the problem? How do you get both with the same intake manifold...and you must admit, the designers have done a pretty good job. But in your case, I think you will be asking too much.

You may have heard the ol'e saying about racing.."How fast do you want to go? How much money do you have?" At the same time, don't confuse common sense with cheap. Some common sense things can be cheap....and bigger is not always better.

John...
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They say we learn by our mistakes.....I guess thats why we are all so smart.
post #6 of (permalink) Old 02-14-2002, 09:08 PM
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Re: 4 brl manifold or not?

Would I be better off buying a cam for a little power boost.

This is kind of unrelated but not really. Has anyone built there own turbo/supercharger? Anything that forced air into your engine? I have a friend that built his own supercharger (kind of anyways) out of an air pump from a cadillac back in the day when he was drag racing.


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post #7 of (permalink) Old 02-15-2002, 02:10 AM
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Re: 4 brl manifold or not?

A longer duration cam will give your more HP at a higher rpm at the sacrifice of power at the lower end. Don't believe the ads from the cam companies. Yes you will have X amount more HP, say at 1000 or more rpm of what you peaked out at with the stock cam, but you lose considerable power at the low-end, (below 3000 rpm). Idle quality will go down as well as vacuum, mileage and most important, low-end where those of us that don't race, operate in the majority of the time. If you want maximum low-end grunt, good idle, vacuum and mileage then stick with a stock cam. This especially holds true with pre-emission cammed engines like yours.

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post #8 of (permalink) Old 02-15-2002, 08:04 AM
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Re: 4 brl manifold or not?

My $.01327. Take a look at stock designs. The 1 bbl and 2 bbls on the 258 engine. I don't think they were any bigger than 250 cfm. Yet these would keep the engine feed through 4k rpm. Now an in-line engine won't rev like a V design. So your 225 should be able to go to a higher rpm, but it has less cubic inches. Now we just went to high rpm~ horsepower, not low rpm torque. A carb works by having a certain speed(velocity) of air through a restriction(venturi) to develop a low pressure(Bernouli) drop so fuel will be sucked into that moving air. The lower the speed of air moving the less fuel, to the point where the engine won't run. The faster the air moving the more fuel the more power made, but to a point. That restriction(venturi) will only pass so much air. Want more power, make the venturi bigger, i.e. a larger cfm carb. But then the slow speed mix might be too low. This is all based on how much your engine sucks. Hehe. Really, though. Back to the top. 225 cubic inches sucks less than 258 cubic inches, despite being able to do it for maybe a 1000 more rpm on the top end. Now the smallest 4 bbl that I know of is a 390 cfm Holley. You need to compare the size of the primary bores(the venturis that you run with for low speed use) to the size of the venturis of a 2 bbl that will work with your engine. If they are smaller that is probably OK cause it will keep low speed velocity up, thus keep fuel going to the engine. If they are larger then the possibility exists that the air velocity might drop in the low speed range causing a loss in power. Moving on to the secondaries, the carb should have vacuum operated secondaries. This will only allow the secondaries(the other 2 bbls) to open when the engine starts to suck enough(at higher rpm). If the engine isn't sucking enough when the barrels are opened it will bog. A sudden loss of power. That is why we have accelerator pumps to shoot the gas into the engine that the sucking won't pull in for its self. If the carb is too big this shot of juice just won't be enough. The purpose of putting a 4 bbl. manifold on is to use a 4 bbl. carb. Using the 2 bbl. carb with the 4 bbl. intake you probably won't see anything. If the 390's primarys are sized around your 2 bbls bores, put the free manifold on with the adapter and 2 bbl. for now to see that the manifold installation is correct and save your pennies for the 4bbl. As far as useing the air pump for a supercharger....have you seen this done more than once and do you think you're the first person to think of it since air pumps started to appear on cars? I think the reason is that an air pump will move a quanitity(volume) of air at a low pressure, not enough to force enough air into an engine to make a difference. An air compressor like the OBA air conditioner compressor conversions will make pressure but not a high volume. Most super/turbo chargers use a pull through carb set-up. A pretty much stock carb(s) is sucked with some additional means but requiring remounting on the suction side of a compressor. A blow through carb has the original mounting on the engine but has modifications to it to accept the high output side of the compressor. I think Avantis at one time used a McCoulagh(sp) belt driven super charger with a blow through carb. BTW mechanically driven(belt or gears or direct connection) superchargers can give power right off idle(low rpm torque) while turbo superchargers(exhaust driven) need a certain volume of exhaust to spin the impeller and compressor to get any power increase. This usually equates to a higher rpm when you begin to see more power.

Huh?
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