Jim Elliott
Camper
Posts:64
Registered Users
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| 03/12/2008 12:18 AM |
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For those readers who like to crunch numbers, there is a commonly used formula for determining carburetor size, but it provides only a rough starting point as all the engine's variables are not taken into consideration. This equation assumes 100 percent volumetric efficiency (VE) and can be worked out using the following equation: VE = (displacement x rpm)/3,456. For example, if we were to run a 350ci engine at 6,000 rpm through this equation, the formula would tell us that a 350ci engine requires a 608-cfm carburetor. However, most 350ci engines do not produce 100 percent volumetric efficiency, even at peak torque. They more likely produce around 85 percent as a strong street/strip engine. The same 350ci engine with 85 percent VE would require a much smaller carburetor (516 cfm) using this equation. As you can see, compression ratio, bore and stroke ratio, combustion chamber design, and much, much more are not even considered. In this case, a good street/strip engine would probably work best with at least a 600-cfm carburetor, and the same engine dedicated to the dragstrip would use every bit of a 650-cfm unit and sometimes more....
Jim |
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Jim Elliott
Camper
Posts:64
Registered Users
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| 03/13/2008 6:39 PM |
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Lets try the formula on a 8.1 engine (496) and I'm NOT going to push it over 4500 RPM.
496 X 4500 = 2232000
/3456 = 645.83333
X 0.85 V/E =548.895833 CFM
So a 600 carb would work fine, In fact I have installed a Edelbrock (1400) smog certified carb 600 CFM
on a Chebby 454 with excellant results...Now if the person would quit messing with it everything
would be fine..................
Also the 0.85 V/E was a dab generous IMHO and the only heads that come close to the 100% are AFR heads
but I'm NOT a fan of aluminum heads on a motor home.......
Wanna play more? Since the head gaskets are rather thick (stock) zip over to the Chevy house and purchase
some stock steel shim head gaskets that are 20 thou thick, With the 20 thou gaskets and with the pistons
down below the top of the block about 22-25 thou that will give you almost a perfect quench and end the
ping and detonation problem mucho plus the compression ratio will increase a dab also.
Jim
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Gon Mad
Not a Newbie
Posts:18
Registered Users
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| 03/13/2008 10:36 PM |
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| Let me share some personal experience with the 600 carb. It's OK for driving & NOT towing anything BUT when you hook up a heavy trailer it wont pull a fat girl off a tricycle. I switched to the 650 Thunder series (it has an adjustable secondary) & it pulls better than the 750 carter carb & the 600 edelbrock. The mileage stayed the same for the 650 as the 600. Not to mention the factory uses 650 Quadrajet carbs stock . The 600 (I dont think) is as well engineered as the AVS or Thunder series whereas the 600 is the AFB.Both work & I own about 10 of them currently My experience is that the larger will pull harder & the 600 wimps out passing or steep hill climbing. Too bad Predator doesn't make a street conifugured variable venturi fuel metering device. I guess fuel injection would be better but more expensive. Gotta go now CYA! Steve |
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Jim Elliott
Camper
Posts:64
Registered Users
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| 03/14/2008 12:59 AM |
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Posted By Gon Mad on 03/13/2008 10:36 PM
Let me share some personal experience with the 600 carb. It's OK for driving & NOT towing anything BUT when you hook up a heavy trailer it wont pull a fat girl off a tricycle. I switched to the 650 Thunder series (it has an adjustable secondary) & it pulls better than the 750 carter carb & the 600 edelbrock. The mileage stayed the same for the 650 as the 600. Not to mention the factory uses 650 Quadrajet carbs stock . The 600 (I dont think) is as well engineered as the AVS or Thunder series whereas the 600 is the AFB.Both work & I own about 10 of them currently My experience is that the larger will pull harder & the 600 wimps out passing or steep hill climbing. Too bad Predator doesn't make a street conifugured variable venturi fuel metering device. I guess fuel injection would be better but more expensive. Gotta go now CYA! Steve
Steve, I'm toooo cheap and hate spending the money on the AVS or Thunder series carbs 
There is two different Rochester carbs stuck on those units.....750 is the average carb but the 800 CFM
was also on the books............
One of the big reasons I love a (properly setup) Rochester 750 is the primarys are smaller on that spread bore
and the square bore carb are 300 CFM VS the 275 or so on the spread bore......As long as my foot is out of the the back 2 toilets the milage is good.
Jim
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stacey honn
Not a Newbie
Posts:28
Registered Users
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| 03/14/2008 6:24 AM |
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EFI... Man if I only had the money tree in full bloom. Almost bought one on ebay called a "mega squirt" last fall but shyed away when it hit over 400.00
Here is what I'm up to. I have the 800 Edelbrock sitting on an adapter plate that funnels the 2 front barrels down to the little peanut 390 cfm barrels on my Edebrock Performer 2.0 intake. Old intake, I know. Probably not the best idea funneling down a Big 800 either. But hey, that's what I'm working with. I just welded a O2 senser in and I'm borrowing a wide band air/fuel ratio meter that will read idle, cruise, and wot. I'll probably never need to nail the trottle but I might get an itch at a street light if a mustang pulls up next to me.
If all fails I'll put the quadrajet back on and try and tweak that with the wide band. I bought the 800 cheap enough to horse around. I do notice I don't have my foot into the carb now as I did with the quad and my vacuum is a about 2 #'s higher at cruise. Does that mean anything? Probably not. In the end a 492 even with a Gear Vendor is going to have a very personal relationship with Exon.
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duct-tape
Newbie
Posts:1
Registered Users
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| 03/14/2008 8:29 AM |
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I have built 3 megasquirt cars
you can find it cheaper here: http://www.diyautotune.com/
I lOOOOOve my 454 with megasquirt and throttle bodies, it's responsive it's ridiculous.
(this isn't a spam, I was trolling when I saw your post) |
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Jim Elliott
Camper
Posts:64
Registered Users
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| 03/14/2008 2:31 PM |
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Posted By duct-tape on 03/14/2008 8:29 AM
I have built 3 megasquirt cars
you can find it cheaper here: http://www.diyautotune.com/
I lOOOOOve my 454 with megasquirt and throttle bodies, it's responsive it's ridiculous.
(this isn't a spam, I was trolling when I saw your post)
Duct tape, I looked at the web site.....No help cuz I'm a carb guy 
For the carb folk's I'll dig around for the modifications for carbs
on the Chebby high performance web site......
Jim |
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capt.dan
Classic Camper
Posts:126
Registered Users
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| 03/15/2008 12:49 AM |
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Hi, When i was thinking about getting rid of the Q-junk I went with the thunder series and 650 cfm and that really helped mine as it was a gutless wonder and now, I can race for the camping spot! With the vacumn gage I am using less pedel and getting more power and also a higher vacumn. That later means it is getting more power for less gas and that is Always a good thing!! I love that thunder series!!! The one I have has bigger secondarys as it was made for a thermoquad for Dodges like the 440! I needed a plate but there is not much on the primarys, only secondarys. I am getting around 8-10 mpg, somewhere in that range. Mine is small though at 25 feet.
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stacey honn
Not a Newbie
Posts:28
Registered Users
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| 03/15/2008 4:31 PM |
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Update on the wideband o2 censor hooked up to the engine!
My idle is easily adusted to get a 14.7 readout but my wot and my cruising air/fuel is a way too lean 16.
Now this is a 800 edelbrock dumping down into a 492 BB. I have a calibration kit for the carb and I'm spending the day tuning. I guess I'm shooting for a mid 14? |
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Jim Elliott
Camper
Posts:64
Registered Users
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| 03/15/2008 11:33 PM |
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Posted By stacey honn on 03/15/2008 4:31 PM
Update on the wideband o2 censor hooked up to the engine!
My idle is easily adusted to get a 14.7 readout but my wot and my cruising air/fuel is a way too lean 16.
Now this is a 800 edelbrock dumping down into a 492 BB. I have a calibration kit for the carb and I'm spending the day tuning. I guess I'm shooting for a mid 14?
14.9 on a light cruise would be nice, 16-17 inches of manifold vacuum would support the air/fuel ratio.
12.8 or a dab richer hill climbing with a vacuum of 5 or less inches would be safe also.....
I use a "Snap-on" smog test machine (old) to dial mine in but the (LM1) wideband is looking real good for the price.
Jim |
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