Exhausting Questions...

[AntParker]

Hi Guys,

As you may remember, I am in the process of putting together an M20 Stroker for my E30 Track Car, My Car is RHD, I have a reasonable stainless manifold on there currently, but I want to get something nicer made for the new engine (Current manifold has some ‘dinks’ in it to allow it to clear the column)

I was going to make something myself, I am capable, but do not have time currently due to work, I have a local place lined up to make a nice manifold for me, they make some lovely parts, however looking for some advice please.

I’m thinking 1.5” Primaries into either a 2.25 or 2.5” Collectors.. however I’m unsure on primary lengths, (and secondary lengths).

Does anyone have any information on the lengths on manifolds that are out there? Or what works well for an M20 Stroker.

Am I right in saying the ‘supersprint’ manifolds are pretty much the most highly regarded in the market currently ? Does anyone know what lengths we get here?

My Engine specs can be seen below for reference.

• M20B25 Block (85mm bore)
• M52 Crank
• M52 EU Rods
• 885 Head, 3 angle Valve Seats.
• Shrick 288 Cam & Vernier Pulley
• Shot Peened HD Rockers
• IE Pistons 11:1 Compression
• RHD ITB’s


I understand we can get some theoretical figures for a manifold using the calculations supplied by Bell in ‘4 stroke performance tuning’ but also appreciate there is often no substitute for experience.

Interested to hear your thoughts and ideas guys.

Could also look at the option of getting multiple made up if there was any interest…

Many thanks

[digger]

At first thought

1.5" x 26-30" primary
2" collectors about 1600mm for street 800mm for race

Collector length is more important than a few inches of primary length IMO on inline 6

[AntParker]

Thanks Digger, appreciate your input, Is your response based on your experiences, or measuring some systems, or both? Not questioning your knowledge, just wondering that’s all ��

I see there has not been too much of a response to this thread otherwise, is this because its not something which has been studied in much detail, I assume most people just use an off the shelf manifold ?

Many Thanks

[digger]

Here are my opinions based on my own dyno tests, observation of hundreds other dynos, calculations and simulations.

Collector


I have tested 4 collector (secondary pipe) configurations on my m20 on the dyno. This was with a BTB2/3 exhaust manifold (1-5/8” x 650 mm primaries). The collector configurations I tested are

A) 2 x 1-7/8” x 650 mm

B) 2 x 2” x 425 mm

C) 2 x 2” x 850 mm

D) 2 x 2” x 1650 mm

length is from end of primary to point of large change in volume e.g. X-pipe/Y-pipe / other large area change

If you want best torque well below 4000 rpm you really want really longer collectors i.e. on the order of 1650mm (this works for an engine that peaks about 6500rpm ).

A to B killed the engine torque and power.

B to C showed mid-range gains with no loss in topend though a slight loss at around 2000rpm.

C to D showed nothing but dramatic improvements in bottom end without out any loses in either midrange or top end.

On a race car with more cam overlap and rpm than your typical 6500 rpm street engine 850mm has been shown on other people’s dyno to work better than the short length 425mm often touted.

Really long length on race engine is not warranted as you don’t care about torque at low rpm and there is likely gains with shorter length than 1650 mm.

The longer collector length reduces low rpm reversion due to the timing of suction pulse arriving at overlap providing scavenging. when the pipe length is out of tune then the suction is actually a positive pressure at overlap which is reversion.

Shorter has potential for more midrange and top end but depends on engine specifics especially peak hp rpm, camshaft overlap and exhaust port primary length.
mine didn't like short length but other spec engines may behave a bit differently. e.g long primaries dont need as long collectors.

Keeping velocity up also reduces reversion this is why I recommend a 2” collector. On the S54 where you have variable valve timing you can minimise the low-end reversion somewhat thorough cam timing and with a good cylinder head make top end gains.

The separation of firing pulses in a 6-2 (2 x 3 into 1) manifold grouped properly is 240* which means in each collector really at any point in time there is only 1 pulse firing. This means the size can be very small almost as small as the primary.
For this reason with a 3-1 merge you can run a small venturi style choke and taper (diverge) to a slightly bigger collector pipe size.

For example Burns Stainless recommended me a 2-1/4” collector size with a venturi style 3-1 merge, my guess is that the choke would be 1-5/8” (they dont tell you until you buy) for something circa 320 bhp



Primary

I haven’t done any testing on primaries testing as its too expensive to do properly on a vehicle if you want it to be passable as scientific. So most of this is what I’ve learnt from others or through 1D simulation and calculation

The primary pipe size is predominently hp/cyl driven.

The general trend is the higher the compression ratio the smaller the primary pipe needed (for the same power) as the exhaust temps are lower and slower (opposite of what most think). from 8.8:1 to 11.5:1 CR this is worth about 1/16" size smaller

Also the more duration and lift on the exhaust lobe the smaller the pipe needed as there is more time to evacuate the gas so a slightly smaller pipe will retain the same exhaust gas speed. for example from stock cam to schrick 288 this means 1/8" change so 1 pipe size smaller.

The more generous the bends (centreline radius) the better (seamlessly) the transition from the port to primary and the shorter the primary length the smaller tube that is permissible as the flow losses are inherently lower. If the header is build crap then it needs to be a little larger than would otherwise be the case

There are thoughts that having primary bigger than the port is good for anti reversion but there are differing opinions as some believe a smaller perfect matching flange and pipe is better as higher velocity reduces reversion. i am a believer of the latter.

The consensus seems to be that primary length is not as important as collector length. However you could run really long primaries like 34” or longer and somewhat achieve a similar result as a long collector but they would merge under the car and possibly present ground clearance issues.
you dont want really short primary unless its a a high rpm engine otherwise the pipe is never in tune.

With really long primaries like over about 32" the collector length seems to be much less critical so going shorter collector is less likely to be detrimental unlike with short length primaries

I’m designing a new header for my new engine which should make around 320 bhp it will be 1-1/2” stepped to 1-5/8” about 26-28" primary as I’m expecting more hp than the average stroker. planning to use a Burns style merge collectors with small choke (maybe 1-5/8")to either 2” or 2-1/4” collector size which is based on their software. comparing my notes with theirs the primary setup i am planning on using is on the money

the reality is its very complicated and each spec engine is different so the only real way to be certain is a physical test(s). some of the dynos ive posted are more applicable to exhausts with primaries of similar length and if the length is different then you will get slightly different trends

[AntParker]

Hi Digger,

Firstly, my apologies for the slowest reply ever!

Your information and experience is very valuable when tackling something like this.

Very informative and detailed response, very interesting and will have to read through it a couple of times to take it all in, those burns collectors are really nice though.

As you say with a race engine i don't think there is any point going for a silly long collector,I'm happy to sacrifice a little low end driveability in the name of mid to top end power, something like 6/800 would hopefully work well at 2" Diameter. 1 1/2" Primaries at around 28" Long sounds like a sensible starting point if we can package it around the stupid RHD Steering column!

Will try and post some pics once its done if anyone is interested.

Your build sounds V Interesting Digger, going to browse your thread now!

Thanks again

[digger]

my engine didn't respond to collectors around the 400 mm mark at all, in fact it was a loss everywhere over the BTB2 that i first started with way back which is approx 650 mm to the x-pipe . the 650 mm BTB with 1-7/8" collectors made good mid range but felt very cammy due to the nature of it.

on my new exhaust i'm having a switchable length to allow testing with new spec engine just in case it was something unique to my current engine.
i'm going to have an additional cross over closer to the engine but have it blocked off but easily removable to act like a H-pipe. i even thought about a electronic flap that you could open and close based on rpm if it proves positive but i think that's too complex and me getting carried away

[AntParker]

Thanks Digger,

I guess in reality its not actually too difficult to have collectors which can easily be adjusted in length, but only of value if the Dyno time can be afforded to running back to back tests with each different length tested.
Switchable with an electronic flap does sound a little bit too technical for an old E30 mate ;)

Thanks again