I have read that the '70 LT1 had a 2 1/2" exhaust system but that it is restricted to 2" in places, for example, straight out of the exhaust manifolds. My question is if it is restricted to 2" in places what good dose it do to open it back up to 2 1/2". In my thinking the maximum flow is based on the smallest restriction and can't flow any more than that restriction allows


[Modified by Raydon 3000, 12:50 AM 10/18/2002]

 

It was actually smaller than that. I measured the ID of the pipe and it was 1 7/8. Hows that for performance. That is why headers add so much power. It breathes much better. I have since sold the stock exhaust and put on a custom exhaust. Way better performance.

 

My question is why go to the 2 1/2" pipes between a 2" at the begining and a 2" at the end???? Dose the larger pipe in the middle do anything?

 

The pipes should be 2 1/2 straight out the back. They should not drop back to 2 inches.

 

It dose drop back to 2" at the tips after the mufflers.

 

GordonM, exhaust sizes are always quoted in outside diameter, rather than inside. The reason is that it's considered tubing. Pipe is usually always measured in ID.

I've never followed GM's thinking on exhaust pipe sizes. Why would the 67's have 2 1/2 while the 69's, even the L88's be stuck with 2"? I've heard the old theories about why L88's had restrictive components, but the L71's were definately intended as street warriors and they got the same choaked exhausts.

 

Answer: You can fool some of the people some of the time! :)

GORDON, You SOLD your original equipment???? :eek:

My theory on any of these questions about "Why did GM do this or do that" is usually they did (and do) these things to save money. Cheaper to use the same parts on all cars, and the cost savings of using 2" vs 2.5" may seem insignificant but saving just pennies per unit can translate into big bucks. MJ

 

Don,

The stock LT-1 exhaust system does indeed start out around 2" in diameter at the head pipes and continues at that size for about 6" before increasing in diameter to 2½". The exhaust continues at 2½" diameter to the muffler inlet; it decreases again at the muffler outlet to 2" diameter.

A couple of points of interest...

GM had Walker (the OEM exhaust supplier) swage the reduction in the front head pipes. Swaging is a mechanical process that reduces the diameter of the pipe by drawing the existing metal. This leaves a relatively smooth transition from the 2" to 2½" section... I'd be willing to bet that there is some sort of venturi effect induced by this process which would help improve exhaust flow from the smaller to the larger diameter section.

The front head pipes are mandrel bent rather than the standard process Walker used for bending exhaust systems for GM. It cost GM more to do it this way, but it certainly helps performance.

Exhaust flow is much more critical the closer you are to the exhaust valve. That's why there was considerable effort put into the head pipes. It also explains why GM had Walker bend the rear pipes in the standard manner (with compression "wrinkles") and actually flattened them out into an oval shape at various points for ground clearance. This is also the reason why the muffler outlet is only 2" in diameter... It just wasn't that big a restriction at that point in the exhaust.

So, to answer your question, while there were restrictions to 2" diameter at various points in the LT-1 system, it would certainly flow a lot better than a straight 2" diameter exhaust system. It's pretty obvious that GM spent some engineering time on this... They specified a couple of costly manufacturing processes that really isn't in their nature.

If you're interested in what an original (as opposed to the reproduction systems that are being sold) LT-1 exhaust looks like, let me know and I'll post a few photos.

Having said all that, Gordon is correct... For a race car, a set of correctly manufactured headers are the way to go and will provide more horsepower... Especially on the top end.

For the street though, I'll stick with my manifolds. :)

Regards,

 

Stan,

Now I will get to the reason I started this post. I just replaced the rear pipes and mufflers. The shop that did the job used 2 1/4" ss pipe and nonrestrictive mufflers. The head pipes are 2 1/2" and what they removed, the rear pipes, were 2 1/2". They also put the original 2" tips back on the rear of the mufflers. How much did this hurt my exhaust flow, or did it??? Stan, I would like to see some pictures of the LT1 system.


[Modified by Raydon 3000, 7:06 AM 10/18/2002]

 

It was an 2 1/2" aluminized OEM replacement exhaust system, and I bought it from him. There were very few miles on the pipes and mufflers and they looked brand new. It bolted up to the manifolds on my '69 perfectly and I've been happy ever since. At less than half the price of buying "new" I think we both made out OK. :cheers:

Shannon

 

Glad to see it worked out well. I'd rather see someone using it then sitting in the basement collecting dust.

 

The biggest restriction in the system would be the 2 inch connection to the manifold. As gases are heated they expand, and this is why headers work so well. I would not be worried about the 2.5 to 2 inch neck down in the rear. The exhaust gases are significantly cooled down and do not require the larger pipe as much as in the front of the car. My car had headers to 2.5 inch pipes that neck down to 2 inch for the exhaust tips. The neck down will cause some resistance, but the velocity of the air increases to a set volume of gas to escape.

Mark B.

 

Just a side note: 73 stock mufflers outlet and tips were larger than th 70-72 mufflers. Don't know why because the rest of the system was 2" for the L-48 but 21/2 for the L82.

 

Thanks Guys for all the input. I love my Vette!!!