Thread: carb and heads to make better power

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I've always wondered about a carburetor versus heads for a better air flow. Now we all know that a goal for a carburetor is to mix the right amount of gas with air so the engine runs properly.Not enough fuel mixed with the air, the engine runs lean. and to much fuel mixed with the air and the engine runs rich. Now for and example I will tke my 305 engine. It's been bored .060 over with a mild cam, dual plane intake witha a 600 cfm carb. Compression is 9:2. This engine runs with slightly ported heads done by myself and I know that the engine is running out of air at around 4500 RPM's. My question is I know I would gain alot of both torque and HP with newer heads, but what would happen if I increased the carb from 600 cfm to 700 or 800 cfm? The carb would draw more air and that's what this enigne needs. On the other side, it will draw to much fuel and run rich. Now if I cut back on the jets wouldn't this solve the air problem(although it probably might not be enough to make a difference)? Just some thought on the matter.

Not trying to get off subject or take over your thread here but how do you know your engine is starving for air at around 4500 RPM's? I'm not questioning you like your wrong or anything, I am curious because I thought I might have the same problem but just dont know how to tell and you probably know a lot more than I know on this kind of thing. I am curious to know what people have to say on this subject as well.

My thought is that increasing the carb would leave you where you are still at, maybe even worse off. Its like the big open funnel at the top, but you get to the restricted part where it all needs to come out, like the heads or even exhaust. Its only going to flow or pass the amount of air that is allowed through the heads and exhaust. If you go with a bigger carb you will probably lose the responsiveness that you have with the 600.

I find that my engine has alot of pull for the high gears I'm running(2:76) and I can get up to around 90 very quickly, but in lower gear if I'm trying to turn the engine to 5,000 rpm"s for better time at the track, at 4500 RPM"S which is around 95, it just barely climbs above the figure. In better perspective I don't have a shift kit in my 350 tranny and it shifts at 4,000 RPM'S. If i keep it in a lower gear I 'm trying to increase my RPM'S just like a shift kit would do. My engine should turn 5500 with no problem but with my head issue, I'm just NOT getting the air I need.

Just talking out my rear here, but, To me it sounds like you are starving for fuel. A shift kit does not control when the tranny shifts, it cotrols the firmness or speed of the shift. The shift control module sets the rpm shift point. My TCI th350 had an adjustable modulator, you pulled the vacume hose off and adjusted the allen headed adjuster inside where the line went in.

I would have some kind of fuel pressure gauge in the truck that you can keep an eye on throughout the RPM range and be sure it is not dropping at high speed/rpms...

Its sounds more like a float level too low kinda thing to me, again just speculating here. Check out the big word, SPECULATING. I have been waiting all month to pull that word out.

[quote=mooneye777;343591]Just talking out my rear here, but, To me it sounds like you are starving for fuel. A shift kit does not control when the tranny shifts. The shift control module does that. My TCI th350 had an adjustable modulator, you pulled the vacume hose off and adjusted the al

len headed adjuster inside where the line went in.[/quote


I understand what your saying about the kit, but when I say kit I meant the module for the 350 which is part of the kit I bought for my tranny. As far as gas starving I don't htink thats the problem as the engine is running real good at higher RPM's with out any hesitation. It just doesn't want to go higher than it should. I can eventually get it up to 5,000, but just takes awhile.

, again just speculating here. Check out the big word, SPECULATING. I have been waiting all month to pull that word out.[/quote]


SPECULATING.to meditate on or ponder a subject. I don't know about you , but that seems like all I do now a days.

could be a lot of things... maybe even valve float but I wouldn't expect that till 5500 or so... I think most likely you're running out gas.... you get up in the revs and it just noses over... it won't necessarily miss, just stop pulling.

As for the carb, your 600cfm should be good for feeding a 305 well over 7000rpm. A bigger carb will not help you and might hurt low end throttle response. A bigger carb on the other hand will not necessarily make you run richer... this is a common misconception. Fuel mixture has virtually nothing to do with cfm of a carburetor.

What are the specs of your cam? It may be it just ain't meant to rev that high. Have you checked your valve lash? Springs? What intake do you run?

So if the cam is made to operate at higher revs (say up to 6000), the valvetrain is properly adjusted, your heads can flow enough to keep up, and your intake can keep up.... then it could be fuel delivery.... but those are a lot of "ifs"...

pretty helpful huh?

-Chris

The cam is good to 6,000, corect springs, valve lashOk. I know the heads from the 305 are worthless and I did a minor porting job, didn't really take much off. I guess the best solution is to spring for better heads. I know when I ran a tri carb set up with 3, 250 cfm carbs set up with progressive linkeage, when the two outer carbs open, the 305 really took off and rev up with no problem. Theonly reason I changed to a 600 edellbrock was I had a warped intake and it sucked the oil from the valley so I was blowing enough smoke to kill off all the bugs for a mile around me.

Chevy that thing should have no problem getting past 4500. Our 305 sprint car motor is 60 over zero decked with flat tops and stock heads as we can't touch em but they are shaved to the max,we did put smaller light wieght valves to help unshroud them,550 lift cam and roller rockers. In 2007 we ran it with a 650 alky carb,although the carb calculator called for under 600cfm don't remember exactley but 570cfm comes to mind. That thing pulled very hard all the way to 7500 rpm never pushed it past that, but I know it will. So I don't think your carb or heads are the problem...

Same thing happens with my 327 Gets up to adout 4500 an just flattens out. My problem is the secondary float is set too low. I should fix it -but it helps control both my 20 year old and his brother inlaw

Found out the problem . My secondaries weren't opening more than half way and since I never liked the edelbrock carb, I replaced it with a carb I had off a old truck motor a (to big) 750 Rochester Quadrajet. Except for the bog when I step on it from being to big, I can now rev it to 5500 rpms with no problem. Will drop down to a 650 carb.

With a 305 engine you should be able to drop down to a 490 cfm carb and still have enough carb for the job. The 600 was a tad to big, and I know some 350's will run ok with a 750, but it's oversized for the engine. But if it works for you then that's all that matterss.