TheSamba.com :: View topic - 1 5/8 merged header on builds smaller than 2L

Fri Dec 06, 2013 2:45 pm

I was told by a well known shop that a 1 5/8 merged header would work great on my 1679 SP with Solex 40/44 EIS carbs and C25 cam.

This goes against my past experience but given that was over 2 decades ago and Im just getting back into VWs so things may have changed.

Anyone running this setup on such a small build? Should I anticipate any tuning difficulties, over/under scavenging?

Any info appreciated.

Thanks

Fri Dec 06, 2013 2:59 pm

1 1/2 " is as big as I would go. Plenty of merged headers are that size

Fri Dec 06, 2013 3:02 pm

What are the heads like (valve size and port work??). With that small cam I would imagine stock heads to I would lean towards the 1 3/8" merged header-CB carries one.

Fri Dec 06, 2013 3:17 pm

Best to stick with 1 3/8" with your singleport heads.

Fri Dec 06, 2013 3:22 pm

I agree the 1 3/8 headers would work best , I would just not exceed 1 1/2 if you had to use one.

Fri Dec 06, 2013 3:23 pm

Stock heads 8:1 compression.

I agree on the smaller primaries but the shop insists that SP engines LOVE the 1 5/8 header.

My mind is open so I would like to hear reasons WHY it would not work.

Fri Dec 06, 2013 3:26 pm

i have a LIGHT sand rail. 900 lb 2 seater kind of rail.

Has a 1641, went from a 1 5/8" merged to a home built 1.5" merged and i could feel the power gain in the lower rpms.

I would imagine that it would have been even more noticable in a full bodied bug.

Mine is a dual port heads though with kadrons and cam...

Fri Dec 06, 2013 4:22 pm

A large tube header on a small engine will provide a little more top end usually due to reduced backpressure, but the reduced velocity will reduce scavenging at usually low to mid rpms. With your cam you may not have as much of a hole to fill and the tuning of an engine is complicated. I wouldn't worry about the doom and gloom if you are able to fit it in.

Fri Dec 06, 2013 4:46 pm

Exactly which shop is telling you this? They are full of crap. A 1679 SP engine with dual Kadrons and a C25 cam only needs a cheap 1 3/8" header.

Fri Dec 06, 2013 7:17 pm

vwracerdave wrote: Exactly which shop is telling you this? They are full of crap. A 1679 SP engine with dual Kadrons and a C25 cam only needs a cheap 1 3/8" header. duh what???? yup I do agree with davy on this.

Fri Dec 06, 2013 8:56 pm

A local shop also recommends 1 5/8 for SP but said it wont work for DP. Could never figure out that one.

Fri Dec 06, 2013 11:02 pm

the shop is right. With such a mild combo the collector and muffler is fairly important, but primary size and length are not very important. If it was a normal car you could say that a cast iron header would perform just as well, but with the flat four there isn't such a thing.
Of course really if you have the RIGHT header it would run better with 1 3/8 tubes, but what length and type of header is the right one?? What kind of header is tuned to work well at 3000 rpm??? none sold. So you use one that is too big and it works better, because making it too big makes it act more like it isn't a header.....see?

IMO a "merged" header is not best, because the nice long merge gains you nothing except less ground clearance, but whatever, or is it a baha??
Use what fits.

Sat Dec 07, 2013 12:16 am

Mmmmnnaaaahhh I do not agree with you on that Modok. The merge headers change the power curve rather significantly. The last engine where I really tested the difference was when I built a 1914 sgl port for a german customer last winter. The difference in the best set up was + 4 hp @ 2000 rpm to a cheap Bugpack 1 3/8 and + 5 hp @ 5000 rpm to a CB 1 3/8" merge, both with a Dynomax muffler. The range up to 4000 was interestingly close. None of the combo´s were bad as such, but it is possible to move the power around to a certain extend.

WRT the 1 5/8" header. I have to admit that I have seen a few 1600´s run such a header when people have demolished a larger engine and run what they had on the shelf. A 1600 runs significantly better than one would expect with it. Add 1,25 rockers on the intake and it even does it without a dip in the power curve. It will never equal a correct size header on the torque, but it does´nt nessessarily fall on it´s nose either. As to why I do not know. It may be something with the total volume of the primaries that just happens to be something that the 1600 like. Whether if its a sgl or dual port doesnt matter. I dont know where they got that from. The sgl. port just doesnt pull as many rpm.
If the OP has the option I would of course recommend to buy the header that fits the combo.

T

Sat Dec 07, 2013 12:38 am

so consider the function of a merging header,
consider the rpm range you'll most like be in,
are the tubes equal length? and how many inches before the merge point?
what is going on it after the merge?
compression, displacement,valve size,cam,carb, even the intake length can play a role in the equation.(its all about flow) now add your own wisdom, dont second guess yourself and let some whippersnapper tell you what you KNOW has not changed in 20+ years.

Sat Dec 07, 2013 8:59 am

The SP is going into a commuter street car. Full weight, but lowered with spindles and drum brakes up front, 1500 trans gearing. Not sure what the jetting in the Solex are as the shop just says it "right" and grinds off the numbers. I ordered a jet gauge to find out exactly what they are because the plugs look fouled after only a few hundred miles. Before anyone asks, yes I have a holley regulator with gauge showing 2psi.

I was thinking with the C25 the rpm range would be all done at 4500 so the bigger header would be useless as the torque in the lower range would be compromised and when the header would start to work the cam would stop working so no real benefit in power. If it was a DP with a bit of port work and a big cam with a ton of compression I can see the 1 5/8 helping at 5K +, just not for this mild build.

How does this look instead

http://www.cbperformance.com/ProductDetails.asp?ProductCode=3674

These did not exists in this size back in the day, and is cheaper than the empi 1 5/8 with fat boy which I think would be too loud anyway.

I also see that decade offers a 1 1/2 sidewinder, is there any performance advantage to the sidewinder design over the traditional merge or is it just "cool"?

Sat Dec 07, 2013 11:56 am

For your build I think the CB system is right on, if you want a merge system. The muffler is very subtle in sound level. I do replace the muffler if I go above 90ish hp. I have not exploited the limits of the delivered muffler. Its just a feeling, but I´m pretty sure it will not supprt much more than 90 hp.

A sidewinder - should - bring more mid range torque due to typically 3" longer primaries. At the same time this also limits the peak hp above approx 6000 rpm.

But that´s no problem to you :wink:

T

Sat Dec 07, 2013 12:19 pm

Alstrup wrote: A sidewinder - should - bring more mid range torque due to typically 3" longer primaries. At the same time this also limits the peak hp above approx 6000 rpm.

T

Sorry to hijack but just wondering if this is true on all sidewinder headers. I am putting together a 2276 and was going to use a 1 5/8 sidewinder, cam and head choice should support 6500+. Will this limit above 6000 as well?

Sat Dec 07, 2013 1:46 pm

Alstrup wrote: Mmmmnnaaaahhh I do not agree with you on that Modok. The difference in the best set up was + 4 hp @ 2000 rpm to a cheap Bugpack 1 3/8 and + 5 hp @ 5000 rpm to a CB 1 3/8" merge, both with a Dynomax muffler. The range up to 4000 was interestingly close. None of the combo´s were bad as such, but it is possible to move the power around to a certain extent.

T
I don't find that unusual. I think you could change the collector length and muffler and make the CB header run better all around.

Being a singleport cylinders #1 and #3 scavenge themselves, that makes a difference IMO. The optimum header won't be symmetrical, because front and rear cylinders have different VE/torque curves. I am sure you would see if you made a way to test that.