Inliners International Forums General Discussion Hi Performance GMC/Old Chevy Single Headre Design Questions.

I have a 302 GMC powering a 40 Chevy. I have made two different single 4 into 1 headers over the 20 years I have owned the car. The engine has 9.5 cr pistons and a old ISKY 3/4 cam.

My first attempt was tho simply match the stock GMC head with 1 5/8 and 2 inch tubing into a 3 inch collector. These tubes give mismatched gas flow rates into the collector as the siamese ports carry 2x the gas of the single ports yet the siamese port area is only 1.55x the single port area.The car ran 84 mph in quarter mile. Next, I had the head ported but I ran the old mismatched and undersized header and the car did 89 mph in the quarter mile.

Next I built a bigger header to match the enlarged single ports and equalize the flow rate. This was 1 3/4 inch and 2 1/4 inch tubes into a 3 1/4 collector. These tubes have 2:1 areas to give equal the flow rates into the collector. The car turned 86 miles per hour. The car has less pickup!!

Questions.

Does anyone understand why the larger header with equalized flow rares into the collector slowed down the performance?

Why did GM undersize the siamise ports?

What is the best tube size for a a ported 302 GMC head with port areas equivalent to 1 3/4 and a 2 1/8 ID round tubes? What is the best collector size for this combination?

Does equalizing the flow rate into the collector by making the siamese port tube areas 2x the single areas make any sense?

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You pipe diameter is to big, you lost all the velocity of the exhaust. Once you loose the air speed, you loose the reversion and scavenging effects. Those tube sizes are big enough for a 500 inch engine. Use some of the calculators on line and be realistic with the rpm and air flow, and you should come up with some pretty good numbers.

I would think 1 5/8" for the single ports and 1 3/4" for the siamese ports. The siamese ports fire far enough apart that big diameter pipe is not needed. 2" would be maximum if you want to step it up.

How long did you make the primary pipes? 28" to 32" should be about right for under 5000 rpm, maybe a touch longer if the rpm is less. Shorter for higher rpm. Collectors should be 12" to 14" in length starting out. Watch for the bluing of the pipe or burning paint on the collector, thats the point at which you want to cut them or transition into exhaust pipes. 2 1/2" would be more then enough pipe diameter for collectors, 2 1/4" would also work. Three front ports into one, the other three into the second collector. Off setting collector length to even out the total header length shouldn't hurt it to bad if want then to end at the same spot.

Cross over tubes are not needed on 6 cylinder engines so connect the two pipes with a two in one out muffler or two separate pipes and mufflers. 2 1/4" is all you need for head pipe and tail pipe,
2 1/2" if running single pipe.

You main goal is to keep the exhaust hot so it keeps its speed. Large diameter pipe allows the gases to expand and cool, thus slowing them down. Its not just about getting the exhaust out of the engine, you also want it to work for you as it leaves. A really well designed header will create vacuum in the ports pulling fresh fuel and air into the cylinder at valve over lap increasing the efficiency of the engine which in turn makes more power.

Thanks for the inputs.

I made an error in my description of the larger header. The ported GMC head single ports have an area equilalent to that of a 1.75 OD tube which I used. But to get equal flow I transitioned the single tubes down to 1 5/8 OD. The ported head has siamese ports areas are equivalent to a 2 1/8 OD tube but I transitioned up to a 2 1/4 OD to get 2x the area of the 1 5/8 single tubes as they enter the collector.

The the tubes are all 36 inches in length. The collector is about 12 inches long.

As for the volume of gas moving thru the pipes, I figure the siamese tubes carry 2 cylinders worth of exhaust and the single pipes only one cylinder at any engine speed. I was trying to get the flow rates equal when the tubes merge together in the 4 into 1 collector. My thought was this would give a smooth merging of gas flow and less turbulance inside the collector.

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Keep the single pipes you have and replace the 2.5" with some 1.75" or 2" , then replace the collector with some 2.5" pipe.

The header you have now has no tuning effect to the engine, thats why you lost hp and torque. I bet the ET dropped as well as mph, 60 ft times are probably way off.

You keep talking about equalized air flow, I assume you had the head ported and the ports all flow about the same. This doesn't change the header design. If you didn't increase the rpm range of the engine or the intake system, the air flow quantity hasn't change all that much. Better heads only create more flow if you run the engine harder.

And talking about heads and porting, if the ports were opened up just to get maximum air flow, they you may never get your speed back where it was. You can trash a head pretty fast by just opening the ports. They have to be tuned and sized for the intended use and driving style. You can kill the exit speed of the exhaust with big ports just the same as with big pipe. You can also kill the intake side the same way.

The intake mixture needs to cram itself into the port. If the speed is high enough, it will force it self into the cylinder as the valve is shutting even though the piston has reached the bottom of the stroke. This is why small runner intakes work better then large ones, they keep the velocity high.

A head that flows 300cfm will not work as good as a 200cfm head on a street driven 4000 max rpm engine, no matter what the cubic inch is. Turn the rpm up to 7000 rpm, now the 300cfm head will work and the 200 will choke it down.

Joe

I don't think the 2 trains on one track analogy works with gas flow. A single cylinder port exhausts every two revs. At 3000 rpm, 25 exhaust pulses go screaming down the tube every second. That is one exhaust pulse every 4/100ths of a second. With the siamese tubes 50 pulses fire each second or one pulse per 2/100 ths of a second. The tubes carry twice the volume of gas per unit of time. Trains don't run that close together and they don't expand like a hot gas.

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I agree in part with RADAR at least in the tract that he is on. If we start with the 3K Rpm he suggests (you can pick what ever Rpm you want your scavenging to go to work) that means there will occur 1500 exhaust events divided by 6 cylinder for 250 exhausts per minute or 4.17 exhausts per second. This means each cylinder will generate an exhaust pulse every 1/4 second approximately. Because of the firing order (1 5 3 6 2 4)and the 720 degrees that is required to complete a full cycle of any individual cylinder we now have 240 degrees or 1/2 second between exhaust pulses in siamesed ports. Stepping back to the firing order we see we still have an exhaust pulse into the header every 1/4 sec so we become aware that optimum merging of the sequential pulses becomes more important than the size of the siamesed port. Because this post has gotten a bit extended I'll wrap it up by saying that making all primary tubes the same size will optimize the scavenging characteristics. The size and length will control thr Rpm range that the scavenging takes effect. Best Regards.

Panic
Love the train track analogy.

Someone correct me if I'm wrong (on any of this). The goal of any header is to flow the exhaust better; to achieve more laminar flow and less turbulent flow.


Assuming your running the engine at the maximum operating RPM:
The volume and rate of exhaust at the siamese port for every exhaust valve opening event is always the same as at a non siamese port - correct? Therefore the same size pipes should be used at all exhaust ports. I believe the determination of optimal collector size is based on elimination of turbulent flow (3 trains on the same track running end to end - 'bump drafting' with no gaps in between)

Radar, there should never be any two cylinders at maximum exhaust flow rate at the same time (other than minorly from cam overlap).

240 degrees - that's the magic number. A cam with over this amount has both valves open at the same time in the siamese ports. Less is as Panic described. On a full race engine, 1 3/4's will do, some drag cars go up a bit from there but not much. Headers work best if smooth but we use them to provide exhaust and intake emptying/filling. Laminar flow is not in the picture. From a physical standpoint it's impossible. The siamese exhaust ports are that size because they are transitioning down to a single port size. Your pipes are simply too big. Length is OK for low RPM

I looked all over Youtube last night for a older video of a four cylinder engine on a dyno. The exhaust was glowing red hot and as they speed up and slowed down you could see the exhaust pulses in each pipe. They would stay the same distance from each other and move up and down the pipe with rpm changes, they never bunched up.

So Radar, back to the question, What went wrong?
To big of pipe!

Step all the pipes down to 1 5/8" diameter, length 32"-35", collectors 2 1/2".

Joe

radar, please share your camshaft specs. I didn't see it written above. Thanks.

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I don't think so.

Say you have two identical pipes with 1 sq inch area. One pipe carries away one 50 cu inch cylinder worth of gas every second. The gas velocity would be the gas volume per second / pipe area = 50/1 = 50 inches/sec. Say the second pipe carries 2 cylinders wort of gas away in one second or 100 cu inches per second. If these two pipes were merged into a collector, there would be great gas flow turbulance as on pipe is flowing 100% faster than the other.

That is why I tried to make the saimese pipes 2x area of the single pipes - so flow rates would be the same in the collector.

Not sure of your point.

You have now said both:
"My method is correct" (doubling the area is necessary), and
"Why didn't my method work?" (the car is slower)

This causes me to believe that you're looking for approval (i.e., "please tell me that what I've already decided to do is a good idea"), as opposed to asking for analysis ("here's what I did - am I missing something?").

It seems that if the pipe is able to carry the hot gases from a single cylinder's pulses it could carry the pulses of any number of cylinders as long as no two pulses come at the same time and it doesn't slow down. If it is put into a larger pipe and allowed to expand it will slow down and the train behind it will rear end it. Then the trains start piling up all the way back to the cylinders. This is a great thread. I am getting ready to build headers for the 4 cylinder. Beater

Thanks again for all the help.

I think I need to make header #3 smaller. I wonder if there is any info available on the optimum size header for a single cylinder of 50 cubic inches running at say 3500 rpm?

I have noticed the 454 chevy v8 headers I see seem to be about 1 3/4 OD. They service 57 cubic inches and off course turn much higher rpm than my old GMC. Maybe a 1 5/8 or 1 1/2 OD header pipe on the single ports would be ok for the GMC?

http://www.maxracesoftware.com/

re: optimum size header. PipeMax is available for sale there.

Here's one of my favorites and it's free. You will need to know your cam timing, no good formula excludes it. You have been given some very good recommendations and explanations. Good luck.


http://www.wallaceracing.com/Calculators.htm

AFAIK they all assume individual exhaust ports, and have no selection for different port window size.

Thanks for the Wallace Racing reference. I used their header equation. I am not sure about peak power rpm or volumetric efficiency numbers but I used these inputs for my 302 GMC: stroke =4.0, Bore = 4.03, Ex. Valve dia. = 1.6, Ex. Valve opening pt. = 61 BBDC., ex. valve duration = 264, ex. Valve centerline = 109 BTDC, peak power rpm = 4,000, Vol. Eff. = 80%. Cam is an old Isky E-2.

Here are the equation outputs: header length = 55 in., tube I.D. = 1.38 in., collector I.D. 2.74 in. This equation is for dual header design with 2 collectors.

As I was just saying...

As you can see, it leans toward the small size. Personally, I'd stick with the 1 5/8th OD. Internal pumping loses will take their toll on a 1 1/2 X 55 tube. Panics right, the calculators do not take into account the siamese port but with this short of a duration his analogy of trains on a track is applicable. Sometimes you have to work with what you have and make the best of it. You are doing exactly what you need to do, ask questions, make changes, note the results, repeat as necessary! Maybe Panic can help with the "What do we do with a siamese port" question. The racing community has been waiting for 70+ years for a good answer and he is absolutely tenacious about ferreting them out.

I agree. Maybe I,ll try to modify my smaller 4 into 1 header. It has 1 5/8 & 2 inch header tube 36 inches long and a 3 inch collector.

I will start by enlarging the flange ports and flair the 90 degree bends to match the larger head port sizes.

This still doesen't cure the difference in gas velocity entering the collector. The siamese tubes carry 2x the volume of gas but are only 1.6 times area of the single tubes so velocity has to be 1.3 faster.

Look at your firing order and the port arrangement. Note that the siamese (2/3, 4/5) exhaust ports are separated by one other firing (1 & 6). This means that 3 & 4 would have to be open over 240 degrees before overlapping 5 & 2. You are thinking in terms of them flowing at the same time, they are not, it is one and then the other. When 1, 2 and 3 enter the collector, they are equally spaced 120 degrees apart same with 4, 5 and 6. Add them all together and they all flow, one after the other, into the exhaust. 4 into one makes more power because of this.

Radar, 80% eff. is pretty high for this type of engine, 70 would be as high I would go. The calculator assume the information is 100% accurate and you never plan to vary from it. Change some of the parameters around, one group higher and one group lower then average out the sizes. Vary the eff and rpm and even the timing a degree or two. Did you degree in the cam when it was installed? if not, then just going off the cam card will give false information, again vary the inputs and average.

When going from port size to pipe size, be sure to blend them in gradually. If welding pipes to pipe, always have the inserted pipe going with the flow. Avoid any edges inside the pipe that will catch the flow of exhaust. When you get to the collector, pull the two pipes together forming a flat (think of two letter D's laid flat to flat) with a oval collector over them.

Good luck and let us know what you do. Joe

One more thing about collector and why they work. As the pulse inters the collector it quickly expands creating a vacuum right behind it. This vacuum then pulls the next pulse out which pulls the next one etc....... The 180 degree headers do this very well since the firing order in each collector is one after another. They crisscross the engine with pipes so each side has one after another firing order.

Using you calculations of 55" x 1 3/8" pipes, the pulses would be really shooting out at a high rate of speed if you could keep the pipes hot for the whole length. Ever put a long straight pipe on a lawn mower? That will create a lot of vacuum in the adjoining pipe.

Joe