DeLorean Posted June 29, 2009 Share Posted June 29, 2009 I have not found any specific info here on this, so I figured I would ask. I have a 73', currently stock l24 that I am about to add triple webers to (40s). I will also be putting in a cam (from Z specialities) and headers + exhaust. I am a simpleton when it comes to this sort of stuff, which is embarrassing (especially here, haha) so I apologize for my ignorance. What would you guys recommend for an exhaust on this setup? I want it to sound loud and aggressive but I would prefer more power to presence. Should the exhaust be 2.5" or 3"? Basically, my car is a daily driver and I just want a little more power than stock. I picked up a set of triple webers for a decent price and I have wanted to run them for a while now. Any advice or threads you can point me to would be much appreciated, thanks. Quote Link to comment Share on other sites More sharing options...
auxilary Posted June 29, 2009 Share Posted June 29, 2009 i think you should go with 2.5", should be more than enough. Your motor's not stroked, 3" would be losing too much backpressure so you'd lose a bit of low end. If I'm wrong, someone feel free to correct me. Quote Link to comment Share on other sites More sharing options...
smokeybear Posted June 29, 2009 Share Posted June 29, 2009 I have always heard that 3" tends to be too big on most NA applications. I would recommend 2.5" or even 2.25" mandrel. Quote Link to comment Share on other sites More sharing options...
m4xwellmurd3r Posted June 29, 2009 Share Posted June 29, 2009 i think you should go with 2.5", should be more than enough. Your motor's not stroked, 3" would be losing too much backpressure so you'd lose a bit of low end. If I'm wrong, someone feel free to correct me. a 2.5 would be good, but a 3 wouldn't be bad because you would lose backpressure, but is still bad what you lose is exhaust gas velocity, which reduces the exhausts extraction ability. High flowing High speed exhaust gases is better than High flow and slow speed exhaust gases. as far as I know from being on here for the past 2 years haha Quote Link to comment Share on other sites More sharing options...
johnc Posted June 29, 2009 Share Posted June 29, 2009 Which header? 40mm Webers are the lower limit on a stock L24 with a cam. A 2.5" exhaust will work fine but if you're planning on going to a L28 or bigger, head work, etc. then a 3" will be a better choice along with at least a set of 44s. Quote Link to comment Share on other sites More sharing options...
LowCarbZ Posted June 29, 2009 Share Posted June 29, 2009 I've got a set of Weber 40 DCOE's on my 260Z with a cam, header, and 2.5" exhaust. Works great and idles/runs perfectly. Quote Link to comment Share on other sites More sharing options...
DeLorean Posted June 30, 2009 Author Share Posted June 30, 2009 Thanks for all the suggestions, I have always heard that 3" is too big for NA motors too, but I wanted to confirm it. LowcarbZ: How do you find the performance of your car? I have heard estimates that these modifications should put the car around 170hp, which is not bad number (to me). Is the power more top end, or does it have decent low level response. Thanks again for the info, getting excited about these carbs... Quote Link to comment Share on other sites More sharing options...
johnc Posted June 30, 2009 Share Posted June 30, 2009 I have always heard that 3" is too big for NA motors too, but I wanted to confirm it. I will say that 3" is NOT too big for an NA engine and there's lots of information here to prove that. For you're particular situation its overkill. But you will not lose horsepower or torque anywhere in the rpm band with a properly designed 3" exhaust on your car. And if someone wants to continue this debate, let's take it to the threads specific to that topic. Quote Link to comment Share on other sites More sharing options...
LowCarbZ Posted June 30, 2009 Share Posted June 30, 2009 Thanks for all the suggestions, I have always heard that 3" is too big for NA motors too, but I wanted to confirm it. LowcarbZ: How do you find the performance of your car? I have heard estimates that these modifications should put the car around 170hp, which is not bad number (to me). Is the power more top end, or does it have decent low level response. Thanks again for the info, getting excited about these carbs... The performance is adequate for me at the moment. I enjoy everything about the car. The lumpy sound of the cam at idle, the sound and response of the carbs. I've driven the car in the "snow-free" months quite a bit and it starts and runs great even in the cold. The car has a 5spd ZX transmission and R200 rear diff with 4.10 gears. So for a comparisson I can't say it'd be the same as yours. The car's a little underpowered and lacking torque to pin your shoulders out of the hole, but once it's wound out in 2nd gear into 3rd it really flies and holds its own for a 35 year old car. The "go fast feel" has a lot of variables in the equation, moreso than just fuel delivery. So unless it's got a V8 or it's boosted (to me) the Z doesn't feel fast out of the hole. It just lacks the torque to get moving. I've had it doing some pretty impressive speeds and it makes for a great road car setup. The power is always there in 3rd gear to make a pass or hit up some bends. But if you're like me it's never quite enough. I enjoy the car I have now but i'm looking for another to do a Turbo setup on. Quote Link to comment Share on other sites More sharing options...
josh817 Posted July 24, 2009 Share Posted July 24, 2009 40's flow 170 CFM or so with 36mm venturis last time I read and 45's with 36mm venturis flow 240 CFM. I have to check the numbers to be exact but I remember when I read it I was like well 40's you will strangle your motor if you have high flowing P90 head (whats that like 200+ CFM?) Right now I ported my own head so who knows its figures but the cam is .495" 290º and its a 3.1L which I run 1 5/8" primaries into 2 collectors. Now it gets weird here because I have two 2 1/4" collectors. I had a 3" pipe from before, now I just needed to mate the two. Paid a guy $100 to do it but he did it with smaller pipes so instead of a true 2 1/4" pipes merging together its some tiny thing like 1.5" or something. I figured, this is fine because even with 2 1.5" pipes, it should still be the same cross sectional area as a single 3" pipe. I must say the car feels much more torquey compared to the old 6-1 setup with a 3" pipe. Then again, the first intake gasket I had wasn't fit to my ports, since my ports were bigger. This time I used a drill (bad idea) and bored out the ports on the gasket. It was sloppy but it sure did fit a lot better. Next time I plan on using a die grinder to open them up... But... Next time... I'm looking into something bigger than 3.1L on an L engine. Yah know? Quote Link to comment Share on other sites More sharing options...
Leon Posted July 26, 2009 Share Posted July 26, 2009 Right now I ported my own head so who knows its figures but the cam is .495" 290º and its a 3.1L which I run 1 5/8" primaries into 2 collectors. Now it gets weird here because I have two 2 1/4" collectors. I had a 3" pipe from before, now I just needed to mate the two. Paid a guy $100 to do it but he did it with smaller pipes so instead of a true 2 1/4" pipes merging together its some tiny thing like 1.5" or something. I figured, this is fine because even with 2 1.5" pipes, it should still be the same cross sectional area as a single 3" pipe. I must say the car feels much more torquey compared to the old 6-1 setup with a 3" pipe. Then again, the first intake gasket I had wasn't fit to my ports, since my ports were bigger. This time I used a drill (bad idea) and bored out the ports on the gasket. It was sloppy but it sure did fit a lot better. Next time I plan on using a die grinder to open them up... Two 1.5 inch pipes do not have the same cross sectional area! I believe it was Tony who made a good example that I'll try to paraphrase: draw a 3" circle, and then draw two 1.5" circles inside of it. You can get a visual of the loss of cross sectional area, with the twin 1.5" losing 50% over a 3"! It may feel like to you like it has more torque, but only a dyno will truly show you the difference. Quote Link to comment Share on other sites More sharing options...
jasper Posted July 26, 2009 Share Posted July 26, 2009 "with the twin 1.5" losing 50% over a 3"!" I DID THE MATH....iT'S ALMOST EXACTLY 1/2 THE CROSS SECTIONAL AREA. It suprised me too. area of a circle= pi r squared Quote Link to comment Share on other sites More sharing options...
josh817 Posted July 27, 2009 Share Posted July 27, 2009 Interesting indeed. I should have known that because it was either Paul or John that I was PM'ing with and he gave me a chart... I was trying to find something that had the same area as my single 3" pipe (intent was to go with dual pipes). Short and sweet answer, two 2" pipes were a tiny bit less than a single 3" and two 2.25" were a tiny bit bigger. I made this for anyone who is curious now: http://www.filefactory.com/file/ahf84f5/n/Exhaust_xlsx Could have sworn we were able to attach Excel files but I guess not. Quote Link to comment Share on other sites More sharing options...
Leon Posted July 27, 2009 Share Posted July 27, 2009 IIRC, by working the equation backwards (starting with a given area and unknown radius) two 2.12" pipes would have the same cross sectional area as a single 3". So yes, the 2" and 2.25" are the closest standard pipe sizes. Quote Link to comment Share on other sites More sharing options...
josh817 Posted July 28, 2009 Share Posted July 28, 2009 Luckily I checked the pipe size and it is 2 inches. I was freaking out. Quote Link to comment Share on other sites More sharing options...
SHO-Z Posted July 28, 2009 Share Posted July 28, 2009 I found this when I was looking at installing a SHO in my Z. By his write up a 2.5 is as large as you want. John Holowczak on Exhaust Design January 18, 1998 -------------------------------------------------------------------------------- The following are some comments and observations on exhaust system design that I've absorbed over the years, as applied to the SHO. Hopefully they will provide some meaningful guidance to any of you that are pondering designing your own custom system. I welcome any input or debate anyone wants to make here, other than the "Doug sez they rattle" or "Ted sez they're better in his book so it must be so" dual vs single debate we've heard too much about. All of the below is IMHO, and FWIW, I do not design exhaust systems for a living. First off, the ability of one pipe vs another to flow gas can be roughly approximated by their cross section areas (a paper towel roll being easier to exhale through than a straw, for example). For a stock SHO, as well as the Contour SVT 2.5 liter, the intermediate pipe is 2.25 inch in diameter, for an area of 4.0 square inches. For a 5.0 H.O. in a Mustang, dual 2.25 inch pipes are used for an area of about 8 square inches. The interesting thing about these two comparisons is that the SVT has a high winding 2.5 liter DOHC engine, similar in some respects to the SHO engine, but with lower peak horsepower. The engineers at SVT felt a single 2.25 inch pipe was ideal for high peak power, without reversion hurting low end torque. If true, the larger 3.0 liter SHO engine should need a 2.5 inch intermediate pipe (area of 4.9 square inches, 22% increase) for its 20% higher displacement, and 13% higher peak HP. The Mustang engine is added here because its factory exhaust is often noted for having very low backpressure for an unmodified 5.0l V8 engine, and because its peak horsepower is equal to the SHOs, or very close. In order to effectively burn enough fuel/air to make 215 to 220 hp, the Mustang has double the rough exhaust flow capacity of the SHO engine, even though, at peak hp, the amount of fuel/air consumed is roughly the same. The reason for this difference in size has to do with reversion, which is back flow of the burned exhaust gases back into the engine. The bigger that the pipes get, the better the chance that flow can reverse and contaminate the incoming fresh air/fuel charge. On a Mustang, the large displacement keeps the total volume of gas going though the exhaust high (at low rpm), and a moderate amount of reversion is even desirable to help reduce peak cylinder temperatures and keep NOx down (the 5.0 having a basic layout and chamber design that is not ideal from an emissions standpoint). With the smaller displacement, bone stock SHO engine, dual 2.25 inch pipes would likely allow too much reversion at low RPMs, hurting low rpm torque. This latter effect can only be mitigated somewhat through additional fuel/spark a la an LPM, but it can not be eliminated completely, irregardless of what you may hear elsewhere Quote Link to comment Share on other sites More sharing options...
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