| Re: Which carb should i use? |  | ||
| Re: Re: Which carb should i use? -- Ken Merring | Post Reply | Top of thread | Maverick Message Board |
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Posted by: Old Guy ® 2001/09/27, 13:24:41 Author Profile |
::First of all, here is my engine setup:
:: ::302, bored .40 over ::"step up from stock" cam ::high-rise, dual plane (1500-6500rpm) intake ::9.5:1 comp ::351W heads, 60.8cc with 3-angle valve job ::headers, most likely (getting there...) :: ::I have a Holley 750 which i'd LIKE to use, it has reasonable jets in there now (70--also have some 66's i could put in). I've been told that i can use it with no problems (i.e. gaps) by putting a stiffer spring on the vacuum secondaries (in the vacuum diaphragm). I know that NORMALLY a 750 would be too much on a 302, but is it possible to adjust it to where i would get reasonable gas mileage/performance? : :..I'm sure you will get more than one opinion on your question. Here is my take on it. Follow closly to what i'm trying to put across. The 750 will work at some loss of throttle response due to the larger venturis that only help at wide open throttle and higher RPM. :..The engine (302) is only capable of drawing about 500 cfm +/- of air at 6000 rpm for 100% volumetric efficency. Your engine's efficiency will only be near 70-80% at wide open throttle. :..That said,the 750 and the 600 both when setup with a springs to allow the secondary to tip in (at the correct point) would not fully open. :..The 750 would have a very small advantage at wide open throttle due to allowing air to pass with less restriction and contributing to the loss in volumetric efficiency along with the manifold,intake track, head port and valve losses. :..We don't run our engines like that just going to the store so the smaller unit would be the choice. Even better would be a spreadbore carb but it requires a different intake. Those carbs are quite expensive. They have small primary-large secondary so you have the best of both response and high RPM operation. :..Tried to give the scoop on this as the question is often asked. Again if the secondary is set up to come in at the right time,a vaccuum carb can't overcarb the engine but starting with a larger unit will tend to degrade throttle response. :..Good luck ,Ken This is one of the most common errors people make when trying for better performance. Too much carb for a particular engine. From my point of view, you don't have enough cam and heads to warrant the 750. However with some frustrating tinkering and adjusting it can be used. The only real test would be to go to a local drag strip and do the adjustments then make several passes to see the results. Ken has got it right, I run a 650 dbl pumper on my race car with a 302 bored .030. 11.5 cr and ported iron heads with 1.94 and 1.60 valves. .514 lift cam and RPM manifold. Took a long time to get it right, squirters, jets, etc. For street performance at a reasonable cost, right out of the box, the 600 Edelbrock or even a NEW 600 Holley would be my choice. As far as gas mileage goes, should get in the 15 range if you can keep the pedal off the metal. Just my two cents worth.
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