Low Backpressure Side Affects?

[SBC Maro]

Hey folks.
I just ordered a pair of Flowtech Race Ready 3" cutouts for my Nova (Gotta have fun with the new 355). I don't plan on blasting around the central coast on open headers consistently, but I would like to know if there is a significant risk posed to the motor with running with consistently low backpressure (read: Dumps open). I'd sure hate to destroy my brandy-new motor.

Thanks in advance.
-Rob

[R Pope]

There's nothing inherently wrong with low backpressure. A lean condition can occur, depending on your setup. Some carbs are worse for this than others. Usually nothing to worry about. All those diggers with zoomies can't be wrong!

[SBC Maro]

Heh, duly noted!

Just didn't wanna go blasting down the highway with coutouts open, and notice a piston hanging out in the passengers seat...

drinking iced tea...

Thanks,
over.

[Dave Severson]

I have seen a few instances on cars with cheapo headers (ie non equal length tubes and improper collector length) actually produce more horsepower through the exhaust then with open headers!!!! This was on a sbc in a Camaro, had a really nice exhaust system all mandrel bent tubes with an X pipe. Made 11 more horse on the dyno with the exhaust hooked up vs. a set of open Blackjack headers....... Interesting, huh????

[76GMC1500]

I think the back pressure is actually higher with a larger exhaust system than a smaller, under some operating conditions like low engine speed and exhaust flow rates. Something about Bernoulli.

[Dave Severson]

Friend of mine swears by the same thought. His idea is that the max diameter for a small block exhaust is 2 !/4".... He's going to come over when we build the tubes and pipes for the 'Maro, then go along when we dyno the car.... We have a huge side bet (20 oz. Diet Coke) on the results...... If he wins, I'm gonna have one fast old chebbie!!!!!!

[SBC Maro]

Hah, good luck, high stakes.

[firebird77clone]

I have heard that running 'open' headers can burn exhaust valves, but personally I feel you have to have another problem going on. IE, bad grind, ( valve edge too thin ), or engine running way lean.

[techinspector1]

I have heard that running 'open' headers can burn exhaust valves, but personally I feel you have to have another problem going on. IE, bad grind, ( valve edge too thin ), or engine running way lean.

I believe that the thinking on this is that after you shut the motor off, cold air has an easy entry into the open pipe and can warp a hot exhaust valve.

[NTFDAY]

Friend of mine swears by the same thought. His idea is that the max diameter for a small block exhaust is 2 !/4".... He's going to come over when we build the tubes and pipes for the 'Maro, then go along when we dyno the car.... We have a huge side bet (20 oz. Diet Coke) on the results...... If he wins, I'm gonna have one fast old chebbie!!!!!!

Dave, I believe that he is wrong. Hedman supposedly proved that misconception back in the '60's.

[Dave Severson]

Dave, I believe that he is wrong. Hedman supposedly proved that misconception back in the '60's.

I should clarify..... This will be a set of equal length tubes (size to be determined by some of the 383 chebbie guru's on CHR), done in a "step" design, coming into a set of 4 into 2 into 1 merge collectors.... then an X pipe aft of the collectors.... Theory being that all of the scavenging will be done before the exhaust pipes connect, all they have to do is supply a bit of back pressure and get the exhaust out from under the car.... Probably be another ton of labor hours to gain minimal horsepower, but it's all being done in the name of learning anyhow....

[pat mccarthy]

i get alot of back pressure when i eat at taco bell i have never see big pipe hurt on small blocks 3 inch works good but for a very mild small 2 1/2 is fine i have built some EX with the x pipe they seam to work if not they give the car a good sound. and have built headers to .it all boils down to dyno time and how many headers and EX you want to try?that makes or breaks what may or may not work with your engine. equal tubes is alot of BS have talk to guys that have dyno there headers they sell and you may see 10hp or less . now when i build a set i try like hell to keep it no more than 4inch from long to short . now some times it ant going to happpen . did a set for a S10 drag truck with 400 small block for a customer that had a set of hookers headers . he did not like them so i built him a set that fit the truck .they were not equal and no way in hell they would for this job .when done the truck went faster. i built bigger set of tube headers .the engine needed it more than having them equal plus the fact the collector comes in to play that helps

[gassersrule_196]

i get alot of back pressure when i eat at taco bell

i concur!

[Dave Severson]

Pat, have you played with the step headers much? I know the results of the equal length, played the dyno game myself.... I'm not talking street use only, and where I intend on going that 10 horse could be crucial. The only thing we don't have dyno data on is the distance from the header flange to the step... The merge collectors when paired up properly do one heck of a job on scavenging cylinders and ensuring the incoming charge doesn't have to contend with any leftovers from the previous burn. It's just more then anything going to be a dyno game for bragging rights at the coffee shop!!!! Then, I'm going to sell the car and get back on the Ranchero.... Don't ask why, I'm at a total loss for a good reason, just something I've been wanting to play with....

[iceburgh]

It will not hurt anything as long as cold air does not rush up when you shut it off....just close to the exhaust before shutting it off. You will see super comp dragsters have flappers tp stop this.
It will be louder and sound good but on some engines (L98,LT1,LS1,2and7) you need back pressure to produce ultimate horsepower