Thread: Need to pick your brains about headers

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Woke up at 3 AM this morning and it hit me:

Can using a 283 header on a 350 engine alter or affect in any way the vaccum of the engine?

Educate me por favor

No.


Don

>Can using a 283 header on a 350 engine alter or affect in any way the vaccum >of the engine?

Probably not to any noticeable degree. Most off the shelf small block Chevy "street" headers are generic with regard to primary tube diameter. When used on a 350 instead of 283 (all else equal), exhaust gas velocity may increase by virtue of the increased displacment, but that ain't a bad thing.

Bob

The reason I ask is because the common complaints Eric and Lonnie (2 previous owners) have about my current 350 engine is that the vacuum is not 'quite' right. Hector (friend of Lonnie) measured it and said it did not match what a 350 vacuum should be. Said maybe there is a different cam on it (but I don't think so). Vaccum being off, the 3 deuce set-up has not been working. I am currently running on 1 carb.

Now I also know that Eric swapped the previous 283 Chevy engine (which died on his way to the NSRA nationals in 2002) with a crated 350 chevy. He did the swap while at the show, in one day.

The same headers that were on the 283 still are on my current 350 engine.



I don't know enough about engines yet to know for sure, but by simple logic, it seemed to me that possibly if the headers were restrictive it could create some kind of "pressure"...??

Or maybe I am completely losing it - since i have NO CLUE what I am saying LOL

But just in case I may be right, I thought I'd ask. Is it AT ALL possible it might be affecting my vaccuum, throwing it off just enough to mess things up?

THose are whats called "rams horns" manifolds and they make a very distinct exaust note.

The manifolds you have are very good for that engine. If you have a different cam your vacuum will be effected. Could be a intake leak??? Have you had anybody look at it other than the people you bought it from???

My guess is that you have a vacuum leak. Check all of your vacuum fittings to see if the are tight, and check the hoses to make sure none are cracked. Vacuum leaks can cause havoc with the way an engine runs. That is a nice looking engine BTW.

Originally Posted by rumrumm

My guess is that you have a vacuum leak. Check all of your vacuum fittings to see if the are tight, and check the hoses to make sure none are cracked. Vacuum leaks can cause havoc with the way an engine runs. That is a nice looking engine BTW.

I agree ..sounds like a leak some place. On another note I dont like ram horns. Have you ever considered a header that goes over the frame?
Might not be the look or sound you are looking for but it is an idea

Faith,
If you have a stock 'mom and dad' car's engine, no matter what the make, the vacuum should be steady somewhere between 17 and 21 inches Hg (mercury) at idle speed (5 to 600 rpm). I prefer 19-21"Hg

A camshaft, different than stock can lower it - how much depends on the grind.

You have triple carbs - I almost can guarantee that one of them leaks vacuum somewhere, unless totally blocked off between the carb base and manifold. Possible leaks can be found at the throttle shafts, at the base and intermediate castings, cracked castings, missing vacuum takeoff point plugs or split hoses just to name a few areas. If triple carbs didn't look so great, my suggestion would be to put on a single 4bbl and manifold, but.....

The intake to head(s) gasket may also leak.

The timing could be set incorrectly.

The mixture screws in the middle carb may be set wrong.

The mixture screws in the end carbs may not be seated - they should not be open at all on a 3x2 setup.

You may have valve, lifter or rocker problems.

You may have a leaky piston ring (or several).

You could have a bad spark plug or wire

You can pick up a vacuum gauge cheap at Auto Zone (or whatever you have there on the Left Coast) - but these are the kind of things that can lower vacuum readings - and on a pre-computer engine are the cheapest and best diagnostic tools available.

But since I've said all this, how does it really run? Skip, idle too rough, poor gas mileage, shift lousy(if it's an auto - I don't recall). If none of those are a problem, you may be worrying over nits.

Originally Posted by shawnlee28

THose are whats called "rams horns" manifolds and they make a very distinct exaust note.

Can you explain what you mean by a "distinct exhaust note" ? How does this differ from the way a 350 SBC sounds with headers or other exhaust manifolds? Thanks, Topless...

>Hector (friend of Lonnie) measured it and said it did not match what a 350 >vacuum should be. Said maybe there is a different cam on it (but I don't >think >so). Vaccum being off, the 3 deuce set-up has not been working. I >am >currently running on 1 carb.

Vacuum is affected primarily by cam timing and the induction/intake system. It can vary from what you'd see on a stock engine to less vacuum on engines with more radical cams. Those look like small base Rochester 2G's on your three duece set up. I'm assuming you're running some type of progressive mechanical linkage?

>I don't know enough about engines yet to know for sure, but by simple logic, it >seemed to me that possibly if the headers were restrictive it could create >some >kind of "pressure"...??

You're not completely off base with your reasoning regarding the exhaust/vacuum relationship. Extreme restriction in the exhaust system can prevent complete purging of exhaust gas from the cylinder on the exhaust stroke. Any residual exhaust gas left after the exhaust stroke will diminish the volume of intake charge on the intake stroke and thus affect vacuum.

Bob

As the Irish guy said..........vacuum leak. If you are not using the other 2 carbs, it would be best to seal off those carbs. A simple sheet metal plate under the carb between 2 gaskets would do it.

Originally Posted by poncho62

As the Irish guy said..........vacuum leak. If you are not using the other 2 carbs, it would be best to seal off those carbs. A simple sheet metal plate under the carb between 2 gaskets would do it.

Yes they have been plated )sealed off) already by last owner, who reverted to that to solve the problem. He tried to make the progressive linkage work, but the car would not idle right, it ran OK once going faster. He ran out of money and gave up LOL. He did not have money really and that is why he sold the car when he moved to Northern CA. he couldn't keep it financially. Anywhoo, this is WAY down on my list of "to do" things but I like to start investigating ahead of time so i am ready when the funds are.

Engine runs good, never stalls. It does seem to idel a bit rough (possibly - but not sure) and may be eating up too much gas (about 10 to 12 miles per gallon). Otherwise powerful and pretty fast.

Originally Posted by HiboyGal

Yes they have been plated )sealed off) already by last owner, who reverted to that to solve the problem. He tried to make the progressive linkage work, but the car would not idle right, it ran OK once going faster. He ran out of money and gave up LOL. He did not have money really and that is why he sold the car when he moved to Northern CA. he couldn't keep it financially. Anywhoo, this is WAY down on my list of "to do" things but I like to start investigating ahead of time so i am ready when the funds are.

Engine runs good, never stalls. It does seem to idel a bit rough (possibly - but not sure) and may be eating up too much gas (about 10 to 12 miles per gallon). Otherwise powerful and pretty fast.

I'm with Mike P, what is the vacuum reading, the idle speed and where are you sourcing the gauge from?


One experiment you could do would be to try each carb in the center position with the outer two carb mounts blocked off by a plate.
Sounds like you already have plates that would work.

If all the carbs function ok why not run them with straight linkage?
I've had many different multi-carb setups and ran them all with straight linkage.

Progressive linkage is ok . . . if you get a good one . . . there's a lot of junky stuff out there posing as progressive, but some of it is downright dangerous.
One setup I see now and then is nothing more than bent wire for the sliding part.

You can make a high quality straight linkage for about $30. - $35. for parts and a little time with a hacksaw, grinder and die.

Depending on what you're running for a throttle control (cable - ok, rod- best) you could make up a nice throttle rod setup to match.

HBG,
What you need to find is a greybeard that can tune a real motor.
Three two's should run fine on your 350 with straight or progressive linkage. If it has a stock cam (which I suspect) that would explain why it runs so well with end carbs. blocked off. Vacuum leaks and the way the pvc and breather are installed is something that should be checked. I have used a vacuum gauge to tune for years and it is how you adjust a carb. and I use it to set timing (although a race motor is another story). Some folks will probably not agee with this method but there's more than one to skin a cat.
I adjust carb. for higest vacuum reading (adjusting mixture and idle speed) and adjust dist. untill I get the highest reading. I usually have to back off timing a hair if I have trouble starting, but that is usually not a prob.
So much for my rambling. I hope you figure it out soon.
Keep on Truckin' (Roadsterin' )
Jim
P.S. 12 MPG sounds awfull low, 18-20 seems more in line with your combo.