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Just Some Dude in Jersey
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Just a little topic for discussion here, go wherever you want with it.

At which point does a "too small exhaust" actually affect horse power? All the time, from 500 rpm to 8000rpm, or primarily at high rpm?

I'm not personally very concerned about this on my car, I'm just curious from a general discussion and theoretical point of view.

What has piqued my interest on this subject is RallyBob's test results showing that a stock 2.4 engine should use a 2.25" exhaust diameter and that a modified 2.4 could benefit from a 2.5"+ exhaust. I think he said that using a 2" exhaust on a stock 2.4 would result in a 10-15 hp loss over using a 2.25" diameter. Now, I figure that this hp loss would primarily occur at high rpm's, rather than the low and mid range, because at lower rpm's there is less exhaust gas volume to get restricted by under-sized piping.

So, since RB is the source of my previously non-existent knowledge on this subject, I guess my question is a request that Bob elaborate a bit more on the physics involved.

So, here's the question Bob(and whoever wants to throw in their 2 cents):

Would my stock 2.4 liter engine experience this 10-15 hp loss at, say, 3500-4000 rpm, which is my legal cruising highway speed? Does an under-spec'd size exhaust cause hp loss at low rpms? At roughly what rpm would an under-sized exhaust start to reduce hp or would the smaller size affect hp throughout the rpm range?


:thinking:
 

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Opeler
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When I run my 2.0 motor, I had 4 in 1 exhaust, 2" pipe and OEM muffler and resonator. Last year I installed 2.5 engine but never changed the exhaust. The GT was going like crazy and I posted some video at that time. So I thought, what the heck, I will leave it as is.

Then, RB said that I was loosing a lot of power with 2" exhaust. Hm? Not that I needed more power but I decided to switch to 2.5" pipe (Opelitis...). As I was doing it, I went step further an replaced OEM muffler with 14" Magnaflow muffler. I also decided that the exhaust system should have no baffles, so the resonator was replaced with 11" Magnaflow muffler. That brought the noise to very acceptable level and changed the exhaust sound to deep, mellow roar. The performance improved dramatically. I demonstrated this to some of you at Springfield when the car was fishtailing during hard acceleration. Building up RPMs is super fast and I really have to watch the tachometer.

What can I say? RB was right again. Here are few pics of the new system. Some purists might not like one-pipe exhaust styling but my car is already heavily modified, so it fits overall appearance. I always thought that two-pipe GT's exhaust looks too similar to VW beetle.
 

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No....its not a Buick....
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I demonstrated this to some of you at Springfield when the car was fishtailing during hard acceleration. Building up RPMs is super fast and I really have to watch the tachometer.
I was one of the lucky passengers.:veryhappy I must say that it was by far the most fun I've ever had in an Opel.
 

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Now I'm not going to argue against RB's numbers but I will mention that as far as I can tell the Opel Omega 2.4i (CIH) used the same exhaust as the 2.0i (OHC) from the down pipe and back, looking at the web site of a FLAPS here in Norway.

Not only that but Simons, which is a very respected manufacturer of performance exhaust system lists a 2" system for the 2.4, as well as the 2.0

I don't doubt that a bigger exhaust might free up a few extra hp but nice 2" will probably work just fine on a stock 2.4 as that's probably more than they had from the factory. Meaning you'd probably have at least as much as stock with a nice free flowing 2"

I had a header on my 2.7 with a 2.5" pipe after the collector but from there back it was 2", never got around to swap in that 2.5" after the engine was converted.
 

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PJ, your GT looks really good, an example of how to build! :cool: Looking at the pics, the "14 inch" Magnaflow appears to be oval shaped while the "11 inch" model is round. What drives that decision and the sizes used in general? I do assume that the numbers are muffler length also? -- Doug

PS - I may be somewhat prejudiced as I actually covered over the exhaust cutouts when I made the revise rear panel! :yup:
 

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Opeler
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The size of mufflers was dictated by the available space, nothing else. I was initially hoping that just front muffler will be enough but the noise level was still high. Those Magnaflow mufflers do not have baffles, just straight through perforated pipe. Good for performance but not for cutting the noise. The addition of the second muffler did the job. Also, I was struggling to create multi-curved pipe over the axle, so I had to do some cutting and welding. It is not affecting performance but I am intending to eventually find some shop that can do one-piece mandrel curved pipe.

I would also like to close two original indents for the exhaust tips but cannot do it now without repainting the whole rear panel. I might do it in the Spring, will see.
 

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I have played with sports cars for many years and because i don't have funds to test different combos i have developed rules of thumb from experience for different things like intakes and exhausts.
efficient exhausts size:hp according to alex
1 1/2" :up to 70
1 3/4" :up to 100
2" :up to 130
2 1/4": up to 170
2 1/2 : don't know maybe 230 ish
I always run 4 into 1 headers around 40" from valve primary pipe diam between 1/8 and 1/4 larger than outer exh port No Bigger

hope this helps but at the moment i am trying a different header design long tri y with 36" stepped pri and 14" sec and seq pulsing 1 and 3 together and 4 and 2
been proven in NASCAR but not by my yet
 

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hope this helps but at the moment i am trying a different header design long tri y with 36" stepped pri and 14" sec and seq pulsing 1 and 3 together and 4 and 2
been proven in NASCAR but not by my yet
A straight 4 isn't "half a V8" if you're talking about a "classic" V8 because of the firing order and a header for a V8 won't be correct for your Opel.

American V8's do it that way for a reason, inline 4's pair up 1-4 and 2-3. You will occasionally see 4 cylinder engines with some more unusual headers but those are usually British engines with siamezed exhaust ports.
 

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Seems wrong at first, i thought i knew 4 cyl with 1342 firing order needed 1-4 and 2-3 together for even pulsing into each secondary. apparently this is a non sequential header with a delay in extraction pulse 1 then 3 before back to 4.
Whats new to me but not state of art racers is sequential headers ,long branch headers which joins 2 cyls next to each other in firing order creating an extra strong extraction pulse on the first cyl but there would be a delay for the 2nd if not for the short secondary getting scavenged by the secondary merg.

So its 4-1 header split with 2 collectors and short secondarys.
effect is top end =4-1 bottom end more torque like normal interference extractors.
Hope this makes sence.
Im still making my first set. we'll see what happens
 

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Seems wrong at first, i thought i knew 4 cyl with 1342 firing order needed 1-4 and 2-3 together for even pulsing into each secondary. apparently this is a non sequential header with a delay in extraction pulse 1 then 3 before back to 4.
Whats new to me but not state of art racers is sequential headers ,long branch headers which joins 2 cyls next to each other in firing order creating an extra strong extraction pulse on the first cyl but there would be a delay for the 2nd if not for the short secondary getting scavenged by the secondary merg.

So its 4-1 header split with 2 collectors and short secondarys.
effect is top end =4-1 bottom end more torque like normal interference extractors.
Hope this makes sence.
Im still making my first set. we'll see what happens
Yes it does.
I've been wanting to try a H pipe between the two banks for years.
 

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Back to the original topic i was trying to decide 2 1/4 or 2 1/2". my engine is 2lt it had 150hp with twin dellorto 45s and standard exhaust manifold.
neither will fit in the gt so i am making both the intake and headers (long tri y) the last merg has 2 1/4" ventury then expanding to 2 1/2. I am hoping for 170hp
but as my rule of thumb implies it is right on the cusp so I am thinking 2 1/2 up to and including the muffler and 2 1/4 after that. If i add a hotter cam i hope it will still be efficient.
 
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