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Circle track 305 & low end torque

General engine tech -- Drag Racing to Circle Track

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Fahlin Racing
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Circle track 305 & low end torque

Post by Fahlin Racing »

Ok, after reading threads on back-pressure in the exhuast. I see back pressure causes slower scavening which in turn affects exhaust gas velocity. Which is done by restriction of either muffler design and/or pipe diameter (including headers) thus slower response, am I correct in my thinking?

I remember when I first reworked my exhaust on my Ford pickup in highschool. I had no muffler for a couple days until I bought a cherry bomb for it. Now when I had it off, the low end torque production went down and my truck was slow at the start of acceleration (this is a 300-6 engine btw). When I put the muffler on the low end torque was brought back to where it was, well almost. I dont remember how long it was though. I know Cherry bombs are petty restrictive for a straight through design since the baffel in the center has high spots and low spots im sure you all know of. Making the "wave effect" as I call it in disrupting the flow.

As I thought about that, the question came up in my mind for our 305 muffler (since we have to run them). The exhaust can be dual but with a maximum pipe diameter of 2 1/2 OD pipe with factory manifolds. The torque converters have to be stock, heads can not be ported. Although we can use SS or Z28 heads (over the general 305s with 1.72s) with the 1.85 intakes & 1.50 exhausts with a .420 lift cam. Stock converter.

Since the Cherry Bomb mufflers have a perforated center tube done by most likely a press of osme sort, should I replace that with a pipe that has drilled center tube, OR should I just shorten the muffler to have a smaller length of area causing a restriction/disruption in the exhaust to keep the low end torque there for coming outta the corners?

What is the scoop?
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Post by W. Tripp »

Back pressure does not improve low end torque, changing collector lengths does.

I will bet you ran the truck with and without the muffler, but didn't change the collector length to compensate for the difference with the muffler.

When you ran the truck without the muffler, and then again with the muffler, you caused different tuning waves to reflect back at different times. This changed the scavenging of the cylinder at low rpm, not the back pressure.

If the engine has enough overlap, the exhaust pressure waves (positive and negative) can even be seen in the intake.
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Post by Fahlin Racing »

Yeah, I didn't know when I was 16 to change the collector length.

If back pressure doesn't cause anything besides reversion it must be a action that will happen no matter what then huh? This 305 is a completely stock engine (harmonic balancer to torque converter & air cleaner to oil pan) besides cam & compression ratio. Which I am sure is at 9.5:1 now from a proposed rule change.

What do you or anyone else suggest for me to look at to keep this thing strong coming out of the corners and down the straights?
AND
Do you think 2 1/2 OD pipe is too large for this engine with stock manifolds?

I am new to exhaust system tuning (obviously) and oval track engine tuning/building so I would like some help please.

Thank you for you response W. Tripp.
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Post by Fahlin Racing »

Here is some more information about our car.

1984 Monte Carlo (dont know how much it weighs when stripped)

The 305 will be getting the Z28 heads w/ 1.85 & 1.5 valve sizes
Iron intake only - 2bbl
Twin-jet 350cfm or 500cfm models can be used
Factory iron exh. manifolds (not log design)
TH350 trans OEM first gear - 2.52 ratio
The gear ratio I believe is 2.73 but I might go 2.65

Any recommendations on Camshaft purchasing? Max lift is .420 like previously stated for the racing class we are in.

Thanks in advance :D
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Post by 3V Performance »

Check your PM.
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Post by shawn »

As it has already been stated, you need to work on your tuned lengths. By Pipemax and model your engine. If your operating the engine at low rpm's then a properly tuned exhaust will make a HUGE difference in power under peek torque.Last week I showed one of my customers just how important it was. Using his headers I added a collector extension and gained 60, yes 60ft lbs of torque at 3000rpm. This was on an engine that made peek tq at around 4k.
Will they let you run a larger pipe after the 2 1/2" head pipe if you run single exhaust?
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Post by CamKing »

Fahlin Racing wrote: Any recommendations on Camshaft purchasing? Max lift is .420 like previously stated for the racing class we are in.
Call me, we have a line of limited lift cams, but I need to know a little more about the engine and rules.
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Post by skoda »

The torque converters have to be stock, heads can not be ported. Although we can use SS or Z28 heads (over the general 305s with 1.72s) with the 1.85 intakes & 1.50 exhausts with a .420 lift cam. Stock converter.

There are somethings you can do to the heads to yield better results. I wouldn't call it portting, more like clean up. :wink: theres an article located here on speedtalk about the subject. I would reccomend you atleast look into it.
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Post by Fahlin Racing »

shawn wrote:. ...Will they let you run a larger pipe after the 2 1/2" head pipe if you run single exhaust?
shawn
No, it has to be 2 1/2 inches from exhaust manifold to the end.
AND
As for back-pressure, is there any need for it in any part of the racing world or vehicles in general? I am still trying to find a straight answer on why its there.
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Post by Fahlin Racing »

Correcting my post here, it has to be 2 1/2 in diameter AFTER crossover pipe. I will have to email the track to see if we can have a mandrel bent same diameter crossover pipe (of the factory size) made instead of the crappy "ribbed" factory crossover pipe.
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Post by Fahlin Racing »

W. Tripp wrote:Back pressure does not improve low end torque, changing collector lengths does.

I will bet you ran the truck with and without the muffler, but didn't change the collector length to compensate for the difference with the muffler.

When you ran the truck without the muffler, and then again with the muffler, you caused different tuning waves to reflect back at different times. This changed the scavenging of the cylinder at low rpm, not the back pressure.

If the engine has enough overlap, the exhaust pressure waves (positive and negative) can even be seen in the intake.
After finding and reading more on back-pressure. I understand WHY my truck engine happened to do what it in fact did. From what I read related to your post W.Tripp on changing the A/F ratio, allowing more air in to the combustion chamber from allowing better scavening of the exh. gases causing a leaner ratio. Since my engine was not able to tune itself after I took the muffler off, it did not produce the power it had made with the muffler/longer pipe on until I installed the other muffler. I also learned too much back-pressure on a stock factory engine will cause overheating more often than any performance racing mill.

Would this mean that back-pressure is something that just happens in the exhaust side?
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Post by RCJ »

It's been a few years since I raced under rules that restrictive.I do remember finding some power thru trial and error with the exhaust length.There is a big difference in weight of the chevy exhaust manifolds.I always took a grinder to the inside and outside.If the only requirement is to have a muffler cut it open and put a straight pipe in it.In a lot of restricted classes people want to get a motor rering it and go racing.This is the wrong approach.That little carb is only going to pull so much air.You have to make the best use of what air does get to the cylinder.Zero deck,angle milling the heads(if possible on 305 heads),deck plate honing will give you a more reliable and faster motor.If I was doing it today I would use 6.0''rods lightweight pistons with the little ring package and lateral gas ports
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Post by Fahlin Racing »

RCJ wrote:.......Zero deck,angle milling the heads.....
Can't angle the heads, if we have to resurface, it has to be level.
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Post by W. Tripp »

Fahlin Racing wrote: After finding and reading more on back-pressure. I understand WHY my truck engine happened to do what it in fact did. From what I read related to your post W.Tripp on changing the A/F ratio, allowing more air in to the combustion chamber from allowing better scavening of the exh. gases causing a leaner ratio. Since my engine was not able to tune itself after I took the muffler off, it did not produce the power it had made with the muffler/longer pipe on until I installed the other muffler. I also learned too much back-pressure on a stock factory engine will cause overheating more often than any performance racing mill.

Would this mean that back-pressure is something that just happens in the exhaust side?
Changing the pipe length can alter pulse tuning as well as air/fuel tuning. Sometimes it is difficult to separate the two.

There are many on this site that have far more experience and education on this than I ever will. But ever-evolving exhaust theory is complex enough than many experts do not fully understand or agree on it all. Read enough book and papers, and perform enough testing on different engines and you can see how confusing it all becomes.

How you define "back pressure" can make a big difference. Defining "back pressure" as a pressure increase from a flow restriction can mean that it can occur in the intake as well. Defining it as a wave-form can mean that it happens every cycle all throughout the engine.

--------------------------------
Pressure and temps alter tuning more than many realize.

I use a lot of coatings and wraps on exhausts on bikes. This is because motorcycles have their complete exhaust system exposed to large amounts of free flowing air that cools the SS and Ti tubing greatly, with both affected differently. Coatings and wraps can help to improve repeatability between the dyno and track.

I have had engineers and experts tell me that the boundary layers inside and outside the tubing, along with the short time cycles cannot cause enough thermal related difference to matter. But too many times I have seen enough difference between chassis dynos and tracks, that go away when coatings and wraps are used to tune the exhaust correctly, to not believe everything I am told, and trust repeatable results. Perhaps I am wrong in my interpretation of the cause of the results. Either way, the bikes go faster and quicker around the track.

Until we have proof, we have theories. Many times these theories conflict in different applications, but that doesn't prove them all wrong.
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Post by Fahlin Racing »

That being said, if I did not want to re-tune the carb I could just get out the chop saw and welder for doing exhuast work to bring it back to what it had before? well close anyways.

Have you ever heard of the "grease marker" trick? Make markings on the exhuast with a grease pencil/marker every so often inch wise (every 6 inches maybe) the length of the pipe. After the car has been running for a while, the first line that doesn't melt off or burn away from exhaust pipe is where you cut the exhaust pipe to end the system.

I dont know where to start the marking but have you heard anything like this before. Has anyone else?
Jim "Iron Giant" Fahlin ~ A high performance car is like a guitar, you have to tune it to achieve your best operation and pull ahead of the competition.

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