Exhaust Manifold Primary Lengths.

[bren_1.3 1]

hello,

 

im hoping some-one can point me in the right direction here! im having a stab at making my own 4-2-1.

 

what do we know about exhaust manifold primary lengths for a small cc TU engine or an 8 valve in general? and what's the general theory?

 

i cant seem to find anything on the web that gives an in-depth approach!

[Cameron 16]

The general theory is very complex to work out, you need to know a lot of variables such as valve size, lift, duration, displacement etc etc and put them all into a formula which can give you a good approximation. That or you need years and years of experience in building engines so you'll know what lengths work best with what type of engine.

I think very basically you have shorter primaries for more peak power and longer for a bit more bottom end grunt. The smallest engine we cater for (other than bike engines) is a 1600 8v but they're a 4-1 so have long primaries. A 1600 16v 4-2-1 has primaries that are about 400mm and secondaries that are about 800mm, primaries are 1 3/4" diameter and secondaries are 2".

 

Isn't there already a manifold available for this engine? You could always "reference" your dimensions from that.

[rally ax 0]

for what its worth , I tried 3 different exhaust manifolds on our 1360 8v Tu , Pumaracing cylinder head , 280 cat cam , bike bodies , DTA ecu ,,,producing 118@ wheels. I did back to back testing on a Dastek rolling road comparing - S/S Raceland 4-2-1 (38mm primarys) a m/s GMC 4-1 (38mm primarys) and a m/s Janspeed 4-2-1 (34mm primarys) . Overlaying all the results they were all fairly similar,, crossing over here and there but not much in it but the Janspeed manifold gave me 2 bhp more at the wheels between 7k and 8k. So I stuck with the Janspeed because it worked best for my application. It seems that the 8v TU engine is less fussy with its manifold configuration compared to the 16v TU , A friend had his 106 mapped on a 4-1 on his Tb'd ,cammed 16v then changed to the 4-2-1 Raceland and picked up loads of power/torque everywhere (without remapping the ECU )- bottom end all the way to the limiter at 7800 ! In his application the 4-2-1 worked far better.

Imho - just buy a manifold "off the shelf"

[bren_1.3 1]

hmmmm! lots to think about! thanks for the comments!

[JeffR 51]

Try & get a copy of A. Graham Bell's 'Four Stroke Performance Tuning'. (ISBN 1 85960 435 8)

 

It explains exhaust dynamics quite comprehensively.

 

If you know the the number of degrees the exhaust cam opens BBDC & at what rpm max power is being developed, you can work out the primary pipe length. It also allows you to work our primary (& secondary in the case of a 4-2-1 system), pipe diameters.

 

Well worth a read if you intend to do-it-yourself.

 

Good luck!

[DamirGTI 342]

Agreed , i have two books written by Mr. A. Graham Bell and they're must have if you're at least a little bit interested in this engine tuning stuff - excellent books

 

Id go for DIY option only if you have proper tube bender , "fish-mouth" table drill attachment and if you're good with MIG welding (although TIG will be better , but by all means MIG will do the job just fine..) , cos if done bad it can rob you a good bit of torque/hp ..

 

Apart from pipe length/diameter you'll need to fabricate collector (the lower part where all 4 pipes joints in one segment , and this part is very important ..) , anyway if you're already decided to go this route i can recommend that you contact Mr. Guy Croft : http://www.guy-croft.com/ , can ask him directly via e-mail providing that you write him all your engine specs and he'll advise you about precise pipe length diameter and collector type based on your car type and engine specs ..

 

Damir

[JeffR 51]

Here's a page full of various online calculators-

 

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

 

Cheers

[Cameron 16]

You don't necessarily need your own tube bender as you can quite easily buy mandrel bends and collectors and then put it all together. They're quite expensive though, between £20 and £40 each depending on material and diameter / angle, so not a good idea if you don't really know what you're doing.

 

The online calculator is pretty handy, but a bit irritating that you have to convert to imperial.