40mm carbs, 48mm manifolds

[StewRat]

I find myself in the situation where I have a pair of 40mm DRLAs and 2 different sets of 48mm manifolds.

I got all excited when the studs fitted and only later when I went to look at how the ports matched did I realise the size difference. So ...

I assume this is a non-starter combo - slowing airflow, added turbulence, etc but just thought I would double check here before I add a 3rd set of manifolds to the shopping list!

Thanks

[pupjoint]

are you referring to the CB generic manifolds? if yes i believe they have been made in that size for a long long time. as a matter of fact, 9 out of 10 of the guys i know run those manifolds.

i was told years ago, CB did make some with 40mm throats, but were discontinued.

CSP makes them now in 3 sizes, 40,44 and 48mm.

last week i was trying to put a set of Dellorto 40 DRLAs on a stock 1600 using CSP 44mm throw manifolds, coz thats what i have sitting on the shelf. i was and am having some issues dialing them in, maybe need more time, but i am not sure if my issues have anything to do with the manifolds.

[StewRat]

pupjoint - thanks, your reply confirms there is maybe a bit more confusion around this and it wasnt *only* me being dumb.

At eurocarb, as you suggest, CSP are the only manifolds that come up as being 40mm specific - seeing other DRLA manifolds in their search results that weren't size-specific is one of the things that got me in the possible mess i am in now.

What you are experiencing in dialing that combo in does fit in with my fears - 40mm carb outlet flowing into 40mm tubes into the head you can kinda model how that might go.
40mm carb outlet flowing into a bigger void kinda feels like it will behave differently.

[alex d]

AFAIK most people run 48mm manifolds with anything and never look back

[StewRat]

Thanks Alex

I'm feeling better and better about this - sounds like folks are oftentimes using 48mm manifolds with carbs of varying throat sizes.

Still doesn't sit true with me though - given all the detail I've read about the importance of flow from top of velocity stack through the valves (and beyond). I understand that the exhaust manifold might be a little larger than the port because the gases are still expanding when they get there, but what is the positive logic in having air/fuel mixture being channeled through venturi and throat that are one size, into a larger manifold before they get to the head?

For clarity - I'd love it if the broader community confirms that 48mm tops on inlet manifolds are ok on 40mm carbs cos I have a set that could be well ported on the outlet end.

Not looking for problems, just checking.

[Joel Mohr]

I actually did a test about 12 years ago....2007cc with 44s... had the larger manifolds on, dynoed it, then got a pair of Berg manifolds, matched ported them to the carbs, and it only made 3 more HP...but lost 2lb of torque....my opinion? It doesn't hurt enough to worry about it....

[56BLITZ]

I actually did a test about 12 years ago....2007cc with 44s... had the larger manifolds on, dynoed it, then got a pair of Berg manifolds, matched ported them to the carbs, and it only made 3 more HP...but lost 2lb of torque....my opinion? It doesn't hurt enough to worry about it....

Soooooo . . . . save $150 and keep the 2lb/ft torque!

I was about to pop for a pair of Berg manifolds, but after reading, I'm holding off. Look here . . . http://www.thesamba.com/vw/forum/viewtopic.php?t=326526&highlight=40mm+idf+manifolds

[StewRat]

Great example of the power of the forum.

Totally turned round my assumptions, and maybe others.

@56BLITZ - great post. I had searched this topic and hadn't found that thread.

[dive!dive!]

I've run DRLA40's on the generic CB 48mm offsets and also on the CSP 40mm version, same engine, only difference was the manifold. I couldn't detect a difference in driveability or jetting requirements
If you have the 48s id use those and forget about it.
Steve

[Bruce]

Soooooo . . . . save $150 and keep the 2lb/ft torque!

The original question above was regarding a 40mm carb on a 48mm manifold.  The negative effects of this combo will be much more than with a 44mm carb.

Rant:
Anyone that thinks having a giant toilet bowl under the throttle is a good thing or won't have much effect, isn't a very good engine builder in my books.  Show me one automaker that does that or has done that.  Where are the 60mm manifolds to fit 48IDAs?
  
Sure air flow is important, but air speed is what makes driveability and throttle response.  Having a giant toilet bowl in the middle of the manifold kills air speed.  What does the whole hp and torque curve graph look like?  Let's see a comparison of the two manifolds that includes the BSFC.

I'd like to hear what Johannes would have to say about this.  Udo?  Steve Timms?  Clyde? Jeff?

[Joel Mohr]

Then explain the plenum under a 4 barrel...

[modnrod]

Hehehehe........

[Bruce]

Then explain the plenum under a 4 barrel...

Completely irrelevant.  We are dealing with an IR intake system.  Go back to Engines 101.

[richie]

Then explain the plenum under a 4 barrel...

Surely that's there to feed 8 cylinders off 2/4 holes depending on if its a vacuum carb or not? Same as single barrel carbs have a small plenum on dual port manifolds, I am far from an expert but would have thought it would be totally different to an individual runner set up?

cheers Richie

[Torben Alstrup]

Hello. Here´s my 02c.
The 48 mm universal manifolds can generally be used on all sizes of engines (in our range) I have used them from hopped up 1200´s to 2332´s, generally with good results. That said, some cam/head combo´s care less than others. Generally also, if the engine has some dynamic compression it works better due to the better tumble in the chamber. In a few cases I have noticed short strokers with some cam duration, like say a W120, C45, web 110 and large intake ports in the heads at the same time, have some stumble just off idle. I never pin pointed exactly what is going on there apart from that it must be some reverse pulse anomali. I was often able to cure the problem with a slight reduction of "plenum" size in the manifolds. I got 4 rings machined that I adapted to fit in the upper part of the manifolds so the port volume was reduced and throat corresponded with the butterfly size. Maybe 10 years ago I got a 1776 in that I was to jet and dyno. that engine absolutely did not want to idle decent and the throttle respond was awfull to non existant. It turned out that this particular engine had a copy cat W110 that was maybe a little uneven, Go3 041 heads, 7-5-1 CR and a regular 1½" Bugpack header. It was litterally thrown together. I did all the trick I know to try and make it work, and the engine did not care at all. In the end I had no other choice but to try and replace the manifolds with some that had smaller ports. So I did. After that swop the engine immediately idled much better, but the off idle stumble was still there, though not so evident as before, but still very annoying. After 1½ day of tuning and trouble shooting I threw in the towel and told the guy that he had to live with it or take it apart and fix whatever problem he got built into it. The guy was pretty pissed at me, because obviusly it was my lack of skills that was the problem. He then approached another guy that works with these cars and engines, and after a day on the rolls in that shop he got the same answer. I know he ran it for a couple of years after that. Then he sold the car. I guess what I´m trying to say is that yes, in some cases you can mask or even cure a problem in the intake with less volume manifolds, but it is rarely a problem with the manifold. 

So, from a univerdsal point of view, don´t worry.

T

[nicolas]

OK so i must say i am really starting to get intrigued.


on one hand you can just make it work with almost whatever and have very minor power losses. from the example given i could assume a 3HP difference on a 120 -150hp engine is marginal (same difference on a different day can be had due to temperature and air density,…) it's 2 to 2.5%
on the other hand there are reputable enginebuilders who all say power is in the heads, porting is so important, parts need to match, are cut and welded up again, and i have read in almost any performance book that even the gaskets between head and manifold need to be ported or cut the same size.

so where does this leave us? i have trouble getting my head around this especially if you take into account what a decent engine may cost in time, labour and in the end money.

as a sidenotewhat is a realistic output (HP/l) for a wellbuild, not too exotic, non turbo engine. for instance my E30 has a 1800 engine and has 108hp stock, no frills, no porting, very conservative, even a modest upgrade would be putting that engine in the 140 region. and we on our VW's almost all have better carbs, modified exhausts, bigger cams, bigger valves, etc in our 1776's and up engines.

[Joel Mohr]

4 valve VS 2 Valve...not fair!

[Fiatdude]

And the other side of the engine -- Just look at what can be done to the headers

https://www.youtube.com/watch?v=azPKIjxmmdU

[56BLITZ]

And the other side of the engine -- Just look at what can be done to the headers

https://www.youtube.com/watch?v=azPKIjxmmdU

That is AWESOME!!!
Take that theory into the pits . . . you could get guys to arm wrestle ya all day long!

[modnrod]

Tunnel rams, Mmmmm!!!!
No big ugly mismatch under the carb there......
 

PS: I especially like the part in the vid where the guy holding the Oxy torch falls off his stool. Safety first, that's what I like to see! 

[nicolas]

4 valve VS 2 Valve...not fair!

nope, it has 4 inlet and 4 outlet valves, i did have to say that it isn't the M40B18 european engine i was referring to, not the more potent M42B18 engine.
just for kicks i looked up the valvesize and it is 41.9 inlet and 36.9 outlet, so in a way very comparable to a VW 42x37 head… 


so now it is fair, but my question is still valid   

[Joel Mohr]

Ok...More modern motors almost always run in a higher RPM range than VWs, and that helps the numbers. What kind of torque does an E30 make? My 1654 in my Puma (72 x 85.5) makes 120 hp, with stock size valves and 44s. Hemi chambers, 9 to 1. But with a Web 122, it comes alive about 5,000...and it has manifolds with 48mm holes...

[nicolas]

just for kicks here it is. however i must say that 120hp for a sub 1700 engine is very decent. i also think puma's are quite heavy, so i suspect it to be torquey as well.

these are the specs of the E30

83 kW (111 hp) @ 5500   162 N·m (119 ft·lbf) @ 4250 redline is 6200

sorry for hyjacking this tread, but in my defense i hope there is more input on how to build better engines and not to mismatch parts that in the end are expensive and may result in a desapointing engine combo altogether

[Joel Mohr]

109 torque...but my Puma "feels" about 1600lbs.... it runs 14.60 in the quarter...and I have a BERG 5 speed...

[StewRat]

sorry for hyjacking this tread, but in my defense i hope there is more input on how to build better engines and not to mismatch parts that in the end are expensive and may result in a desapointing engine combo altogether

That's what it's all about.

But sometimes disappointing isn't a disaster, it just means less than perfect but given the parts/time/budget available, actually ok.

Given that vid about headers, and another thread here about wings, it seems you also need to be careful not just to follow the crowd and buy into the "obvious" route.

Personally as the OP I started out thinking that using 48 manifolds below a 40 throttle was patently a bad idea. Now I'm much more relaxed about it.  Prob won't make max power and will keep eyes open for 40 manifolds but won't lose any sleep over it now until the car is on the strip for the first time. 

[Fiatdude]

There have always been the stories of those great running "sweep-the-floor" engines -- -- Sometimes there are those magical combinations that work fantastic no matter what crap has been thrown into them