Hey Barry, Doesn't the 70 gas tank have the pickup in front of the tank? If so, that sure would give you more room to work with than the later ones.
These are what mine look like . The only thing I can say is it is alot louder then if they go out the back . I have the mufflers tucked up into a recess on the outside of the car for better ground clearance but it was tough to get a tight enough bend after the header to make everthing fit . Mandrel bent tubes would be much easier I would think . The hottest thing for exhaust nowadays re systems that use an "X" pipe which would also be hard to do with your dumps out the side . I haven't got around to it yet but the pipe sticking out the side would look better cut at an angle to be flush with the side of the car .
so...from the orig.question,is it overkill to go 3"all the way back?(horsepower wise)my engine is basically stock and i was thinking of doing the same thing.
Yes it is overkill. Some do it to look tough. Some basic facts. The 302 at 6000 rpm will only intake about 450 cfm of air at 80% efficency. Considering heat, expansion, gas speed and acustics, any system that is able to pass much more is a waist. More power is (extracted) from smaller piping of the correct sizes and configuration using header configurations known to help gas extraction. There are many configurations that have an infuence on engine power. Take a look sometime when a Nascar car flips on TV and see the systems under the car. Boat racers even inject water into the exhaust to cause rapid cooling and providing an extra pulling action on the cylinders resulting in more torque. .......... For the gentlman who thinks the engine takes in more air than it expells. You have just rewritten the laws of physics. Tell me where the rest goes.
Im not into "tough" lookin when it comes to exhaust....I'm more into "Optimal". If a 3" exhaust is not as good as a 21/2" or better...I dont want it. I just want to know what size is optimal. Thx Rick.
Optimal is an individual condition on any motor setup. The only improved known setup is one that has been tested and applied to a standard setup for general use like a stock car otherwise you have to guess or test run yourself. The cam makes a difference as to were the exhaust will be an inprovement. For example a cam that peaks at 4800 won't make full use of an exhaust that is optimized at 6000 rpm so it becomes a waist of time and money trying for improvement over the last system, in that way. I am only talking the technicals of the subject since I don't know what you have and want to do with performance. This is only to get you to think deeper and that it isn't just a matter of an easy answer. Certinaly 3" on a small cube engine under 6000 is not prudent at all. Please don't take any offense. This is just discussion. There is alway some one who did a system that says it was better and that can be true because he had something to compare to. There are a lot of people that have various levels of knowledge and much of it misguided by hearing a buddy did this and that and another website said this and that without going into the science of it and trying to come closer to an assembly that is more apt to be of benifit for the time and money spent. Bottom line is I am not trying to tell anyone what is best but to say getting way out in some areas is known not to be of help. It's a very large subject so we could talk it into the ground and not everyone would ever agree.
It sounds like based on CFM intake of the carburetor (650) and the outlet size of the headers (3") the 2 combined would equal 4.5". there fore 2 1/4' would give me unrestricted flow not taking into account the static pressure and backpressure provided by the Flowmasters and the walls of the exhaust piping itself. My theory may be all off...Im basing it on the properties of air. Thx for all the input guys...keep it coming
Just one thing. The carb may be capable of lowing 650 cfm but only the size of the engine and rpm determines the max. air flow (not including the manifold and port losses). A 302 would have to be run at nearly 7000 rpm to use 650 cfm the carb is capable of flowing. One reason the tail pipes can be smaller is that the exhaust gas cools, slows down and no longer needs as large a pipe volume so the restrictions offered by the smaller pipe size comes into line with the gas conditions. These things are why there is no universal best system for every use.
i know i said this before but car craft did some testing with x-pipes on a dyno, i think that car was some where around 300 or so to the wheels i think (but don't quote me). now of course every car is differant however they had the best results with the set of of 2-1/2 after the collector to a 3 inch x-pipe then 3 inch to 3 inch muffers with cut downs. i decided to try the same... i like it but its up to you. bigger don't aways mean better but you can also restrict a engine as well. i've posted my exaust ten million times but if you need to see a pic of it again let me know.
2 1/4 is plenty big enough for a stock motor . I cant really tell you or anyone if 3" is better than 2 1/2 " exhaust . I can tell you that my car slows from mid 11 second ET to around a 12.00 with 3" exhaust using proflow mufflers . Maybe the flowmasters work better . I never tried 2 1/2 systems at the track so there is nothing to compare to . I can tell you that proflow mufflers are waaaaaaaaaaaaaaay too loud to use on the street !
Dennis, yes the pickup is in the front. The tank is offset to the left so that the muffler could fit on the right side. I measure 5 1/2" between the leaf spring and tank. Looks like where the real problem would be is where the pipe goes over the axle, avoids the shock, the axle tube and trunk. I guess the 9" rear doesn't help any there. A good bender could make 2 1/2" fit but I don't think it could be any larger. The "rule of thumb" for exhaust sizing going back to the late 60's - early '70's (and I've seen nothing lately to make me think it still doesn't apply): street driven small blocks use 2 1/4" pipes, street driven big blocks use 2 1/2" pipes, really big street driven big blocks use 3" pipes. Obviously if you race and your cam/ heads/ induction increase engine volumetric efficiency to 95 - 100% and you wind your small block 7000+ rpm you need to increase tubing size.
flowmaster provides a sizing chart on their website. Here is what they have to say about the size issue. "The size of pipe used in an exhaust systemis a critical item to consider. Pipe diameter will affect the sound level and performance characteristics of the muffler, but keep in mind that bigger is not always better. Too large of a pipe can actually hinder exhaust scavenging by allowing atmospheric pressure up the pipe. As a rule, most general street applications use 2.00" through 2.50" pipe, while modified street applications will generally use 2.50" or 3.00" pipe." http://www.flowmastermufflers.com/cgi-bin/flowmaster/muffsys.html?id=KsJgShDL
I am not gonna get into a peeing contest But VE is lessened BECAUse of exhaust limits..the exhaust has ALOT to over come to get out..there are alot of factors involved..an engine will achieve a higher VE IF the air charge is free of exhaust gasses...so instead of making larger exhaust due to limited space/configuration..engineers use thermal designs to try and extract MORE exhaust gasses from the power charge ( i dont communicate well so bear this in mind ). the reason the intake flows as much as the exhaust gets out is just for that reason alone..the exhaust resricts the amount of air that can flow..if that makes since..the perfect exhaust system would be larger exhaust valves, exhaust ports ( raised higher) AND pipe pointing in the same direction as the exhaust flow from the engine. the little exhaust valve ( no space in the head for same size as intake)will never flow as much as the intake is CAPABLE of flowing due to the configuration of the exhaust system..yes bigger CAN be a waste but thats only due to the restrictions of exhaust..this is just my theory based on what I have seen with IC engines..I could be wrong
3" pipes in most street applications, IMO, are a waste. Most engines cant use them. Head ports, displacements, etc rarely require it. BTW, the reason exhaust valves are smaller is because it is easier to move air when you are pushing it rather than pulling it. Unless you have some killer heads, a lot of cam and a lot more displacement than 302 cubic inches, 3 inch pipes will just be extra work.
Before I got heavy into hot rodding I had always heard that "some" engines like more backpressure..... I found out later that this is true in some street applications due to cam designs and other factors. I'd say for 95% of all "street car" applications 2 1/4" - 2 1/2" would do the job well and horsepower loss is minimal. If you want a deeper tone and just plain want it to be louder the 3" will do that. As far as backpressure goes, how can engines like backpressure when 99% of (performance) engines would gain power if you took the exhaust off and just ran it open headers!?!?!? Nobody can disagree with this because it's a proven fact that if you want to pick up a little power at the track you can just unhook your exhaust and run open headers.
