V6 manifold and exhaust design

[MattWanless]

Hi,

I'm in the process of transplanting a 3.0L V6 GM engine into an Opel Manta. I'm going to replace the cast headers with tubular manifolds, and build a full new exhaust system to suit. The main stumbling block is finding information specific to 6 cylinder engine manifold design. I've tried to adapt formula for 4 cylinder engines to formula for 3 and 6 cylinder engines, but I'm not that confident enough with it to actually start building the manifolds.

One idea I've been thinking about is pairing the primaries up with their opposite number so the pulses in that pair are 360 crank degrees apart, it'd be a plumbing nightmare, but unusually, I've loads of space in the engine bay, and would quote enjoy building it.

I'm not an automotive designer, just an enthusiastic amataur, so I don't have access to much information resource, so if anyone can point me in the right direction, I'd be very grateful. I've got A Graham Bells 4 stroke performance tuning, which is where I got the 4 cylinder equations from.

Thanks

Matt

 

[patprimmer]

I would try to have 2 collectors, one on each side with 3 pipes merging into each, with the pipes at 240 deg interval in fireing order. 240 deg lets the pressure pulses follow each other nicely down the tail pipe.

If you need to cross over primary pipes to the other side collector to achieve this, it might be prudent to try to keep the collectors merg point as close to the centre of the car as possible

If you want to keep it quiet with minimal restriction from a muffler, you should then merge the two tail pipes.

I have no idea of the appropriate primary or secondary pipe diameters and lengths

Regards
pat

 

[MattWanless]

Hi Pat,


6 - 2 - 1 sounds fine. I don't need to crossover (firing order is 1-2-3-4-5-6, banks are 1,3,5 and 2,4,6), so I can easily build manifolds in that configuration. I'm keen to keep the noise down to a level which'll let me run on most trackdays (98db ish).

I put together a spreadsheet with my guesstimated calculations on it, it's here

http://www.valvers.com/v6/exhaustdesignv1.1.xls

. I built it so you enter your numbers on the first worksheet, and then play with one value per sheet.

The cylinder volume/valve area/chamber design etc is identical to a 4 cylinder GM engine (2.0L ecotec), so would it be valid to use tube sizes based on that engine?

Cheers

Matt

 

[patprimmer]

Certainly yes for the primaries if you want to develope your power at the same rpm as the 4

I guess the 4 is a 4 into 1 system. I would expect the intermediate pipes, between the two mergers, could be smallish vs the 4 (maybe 25% less cross sectional area), but the tail pipe after the second merge, would need 50% higher cross sectional area.

Regards
pat

 

[MattWanless]

The sizes I'd earmarked were 1.75" primaries, 2.25" secondaries, and a 3" main system, as they felt (Highly scientific measurement) about right. Having just done some quick area calculations, they seem to fit the bill ok, the 3" main system is a little too big, being 85% bigger! Would that made a big difference, apart from noise?

Does anyone know if there are any published formula for 6 cylinder manifold design?

 

[patprimmer]

Sounds about right to me, but a 3" pipe will be real noisiy.

I once had (1970 something) a 3.3 litre inline 6 pushrod motor with about 250 HP for a daily driver. It had 3 Webbers with 42 mm chokes. I think the primaries were 1.75, I never measured the secondaries, but probably a bit over 2" and the tail pipe was 2.5"

Regards
pat

 

[GregLocock]

I can't believe that firing order!

Cheers

Greg Locock

 

[MattWanless]

Greg,
The firing order's great, it makes it hard to mess up when wiring the ECU in

Pat,
With regard to primary lengths, if I plug the numbers in for a 3 cylinder engine, it comes out with lengths of around 42". For a 6 cylinder, then it comes out as 24" (these are for maximum effect at 5500 rpm, 500cc/cyl, exhaust valve opening 65 deg after TDC (bit of a guess, that one, but probably not too far out).

The results make sense, but which one should I work to? I'm thinking that if I'm doing 6-2-1 then I should make the primaries 42" long as they'll effectively be working as a 3 cylinder engine, but then how long should the secondaries be, and how do I calculate the effect they'll have? I know some V6 exhaust designs use 2 systems and balancer tubes, so effectively the secondaries become the main system, and their length isn't too relevent. Should I just aim for a longish length (30"?), and see what happens on the dyno?

Thanks

Matt

 

[patprimmer]

My GUESS is 42" as the exhaust collector will only see a 3 cylinder engine.

I expect, that if the secondaries are long and the tailpipe is big, the engine will behave as two seperate 3 cylinder engines. This will be even more so if you run the secondaries into a two in one out muffler, as all pulsations and wave motions will tend to smooth out in the muffler. Is this a road car?

I must say, that I agree with Greg on the fireing order, but as I didn't know I took your word. I'm still astonished and a little doubtfull.

To test valve opening point, get a degree wheel, or if you don't want to buy one, get access to the ring gear.

Put a dial indicator on the exhaust valve or the spring retainer or rocker arm or pushrod, whatever is convenient, mark TDC on the compression stroke, then turn the engine by hand until the dial indicator starts to move. measure the # of degrees from TDC with the degree wheel, or count the teeth on the ring gear, vs the total # of teeth, then do the sums to work out the degrees per tooth.

Regards
pat

 

[MattWanless]

Really, the firing order is correct! Here's a link to the engine on GM's site!

http://media.gm.com/division/powertrain/products/engine/carengines/04_car_engine_specs.html#l81

Thanks for the methodology for working out exhaust valve opening degrees, I'll figure out a way of doing that tonight, it'll be slightly different as the engine isn't 100% built up yet, but I can easily figure it out from the cam teeth, and then translate that into crank degrees.

It's going to be used for sprints and hillclimbs, and I'm wanting it to stay road legal. I don't think it'll do that many road miles, but it'd be good for sorting out any reliability issues, and for testing/mapping.

Could a 2 in 1 out muffler be effectively the 2 pipes entering a chamber, which then tapers into a conventional 'straight through' silencer? If so, that's something I could make relatively easily. Alternatively, would it be enough to merge the secondaries immediately before they enter the first silencer?

Thanks very much for your help, Pat.

Cheers

Matt

 

[GregLocock]

I'm pretty sure (90%+) that is completely wrong. Can you chase it up with someone?

Cheers

Greg Locock

 

[MattWanless]

It's also in the Haynes manual for the Vauxhall Omega (the only one which covers that engine), and in a Vauxhall mechanics 'watch us build a V6' training video I've got.

Out of curiosity, why do you think it is wrong?

 

[GregLocock]

Firing each cylinder in order down the crank will create very strong half order torsional stresses in the crank, and will encourage charge cannibalism (! is that what it is called?) between consecutive cylinders.




Cheers

Greg Locock

 

[MattWanless]

I'm with you.

The concecutively fired cylinders are separated by a minimum 70 cm (roughly) of intake tract at low rpm, this changes at 3.5k rpm.

The cranks a weird thing, the journals for the opposite cylinders are slightly offset relative to each other. I'll see if I can find a photo of it, as the explanation of it leaves a lot to be desired. The big ends are quite big relative to the 4 cylinder varient (54mm vs 49mm)

 

[patprimmer]

OK
I believe you about the fireing order.

One thing I forgot, if it's a hydraulic cam, is that you need to consider how to avoid errors from the followers bleeding down as you test.

If you have the inlet manifold off, simply put the dial indicator on the edge of the follower where it does not move to adjust itself

Re mufflers

It depends on how quitet, or noisy you can accept, and I am guessing, but you could carry the 2.5" pipes right through as twins, with no merge of the 2 into 1. Then use a hotdog on each. This will be simple and effective, but noisy.

If you can blend all the pulses together, you can get a quieter result for less backpressure and weight, but it usually takes a few experiments and some dyno time to get it right.

My purely speculative idea of the two in one out muffler was more for a road car with the two in at either side and running pretty well full length internally with a series of holes and baffels allowing the gas a relativly free, but indirect route with various path lengths into a central chamber. The tail pipe would come from near the front of this central chamber. I expect this could confuse any waves travelling in the exhaustand kill of a lot of sound.

I'm sure it is not a new principal and is certainly how some oem mufflers are made, I just havn't seen many 2 in 1 out with a larger outlet

Regards
pat

 

[JayMaechtlen]

The same website has the Buick/Pontiac 3.8 motor using the following firing order:
3800 Series II and Series III 3.8L V-6 (L26/L36)
Firing order: 1 - 6 - 5 - 4 - 3

No, I didn't leave anything out- but methinks they did!


Jay Maechtlen

 

[MattWanless]

As I work for GM, I thought it may be a plan to let them know about the missing no 2. They changed it PDQ, I'm impressed (it normally takes an age to get anything done!).

Anyway, I've not had a chance to get near the exhaust cam, lifters and head to measure the degrees when the exhaust valve opens.

I did have a thought about modeling the secondaries though. I could treat the secondaries as a 2 cylinder engine running at 3 times the rpm of the engine. I put the numbers in, and the tube sizes look about right.....

What would be more effective, tuning the primaries to where I want peak torque to be, and the secondaries to peak power, or the other way round? Am I right in assuming that the primaries would provide more of a usable effect than the secondaries?

Thanks for the help with this, Pat. I know you class what you've told me as speculative, but it makes sense, and has helped me immensely.

 

[ginetta]

Years ago, if we did get on a dyno we fabricated an exhaust that had one tube inside another. Watch the numbers and slide the tube moveable tube for peak power. Computers sure have made life easy. That is unless you have a V6.

 

[GregLocock]

That's still the best way to tune an exhaust. The computer gets you in the ballpark, everything else is done with a welding torch.



Cheers

Greg Locock