Carb Trumpet length

[Payco]

I know from research that the length of trumpet on the Weber side draft carbs is an important component to get right. Does anyone know or is willing to share what trumpet length you would fit or suggest on a twin set of 40 weber side drafts on the L16 engine ?.

 

Willing to listen and learn.

 

Martin

[banzai510(hainz)]

Really?

Longer is better I think but really what ever is for sale at the time that works.

In USA we go for the shorter ones cause out brake master cylinder will hit.

Since you guys use the other side  length of being to long is not a issue.

But really on a small motor like this I don't really think I would even worry about something like this. Just my opinion. Moving to a L20 would be more time well spent

[Payco]

10 minutes ago, banzai510(hainz) said:

Really?

Longer is better I think but really what ever is for sale at the time that works.

In USA we go for the shorter ones cause out brake master cylinder will hit.

Since you guys use the other side  length of being to long is not a issue.

But really on a small motor like this I don't really think I would even worry about something like this. Just my opinion. Moving to a L20 would be more time well spent

Thanks. I understand longer is better, just curious on what length to go for. I am buying new carbs so want to get the right length trumpet and wanted to know the impact of the different lengths so seeking some knowledge and advice.. I am keeping the original L16 engine as don't want to go down the L20 route.

[yenpit]

I doubt you would see a noticeable difference on a basically stock L16, with varying lengths of the trumpets (velocity stacks).........but I understand TRYING to get all the power you can, based on others experience.  Firstly, spend the mulla on a good set (regardless of what length you choose) of trumpets, that have a fully rounded OD of the "bell" (machined), as opposed to the ones that have a sharp edge on the OD of the "bell" (typically stamped).  The sharp edge trumpets tend to break up the air flow, whereas the fully rounded ones do not..........these are the potential details where you might find a HP or two!  Here in the States, British performance business owner David Anton at Advanced Performance Technology (APT) sells these rounded trumpets...........I would expect that you could find a similar product in the UK, if not his product!  British engine builder David Vizard wrote a number of books on these finite details, where you CAN find more horsepower in an engine, however you will likely find that a basically stock engine will NOT respond to a similar modification, as a "built" engine.  Vizard specialized in British cars & Chevy's (beleive it or not!).  Example, an L20B will likely produce more additional power than an L16, doing a similar mod...........simply because it is a bigger engine etc.  As Hainz says, we here in the States are greatly limited to what length intake runner, what length trumpets & what air filters we can use, because our BMC is on the left side, limiting our space for "the best parts". 

 

I would think that the intake that you use (various runner lengths) will be more important than the length of the trumpets (velocity stacks).  There are short runner intakes & long runner intakes........just like the British MG, if you have any experience there or other buddies/acquaintances etc that do.  One runner length produces more low end torque, the other produces a higher power band etc, with MANY variables in between..............sorry, I'm drawing a blank as to which runner length is best in any given application!  

 

Do you have the Frank Honzowitz (sp??) book and/or "How to Hot Rod Your Datsun"?  Those might be the same......??  One is considered the Datsun performance bible..........an absolute must have for reading & learning the tricks of the trade!!

 

Hope some of that helps guide! 

[datzenmike]

 Ideally you want a runner length around 10 feet. However you can tune for the 4th reflected wave which is under 2.5 feet. As the pressure wave has to go up the runner and back a length of around 1.25 feet will fit most cars. That's around 15" but this is just a very rough starting point and port area and cam timing and such will change this, up or down.

 

I have individual 1.5" diameter intake runners with R-1 carbs and small 'trumpets' on the ends. I used an on line calculator and got a runner length of 34cm. This is from the valve face to the trumpet opening. I just went out and did some measuring. It's 10" from head to trumpet and 4.5" from an intake valve face to the port opening for 14.5" or 37cm which is close to optimum.

 

 For a 1.5" diameter runner and 34cm length my peak power is at 5400. I should shorten by 3cm. Or make the runner diameter smaller which isn't happening. Everything affects everything else. I did have some much longer trumpets on it and (to me anyway) that mid range was enhanced which sort of makes sense. I shortened them so I could fit an air cleaner but it was accidentally in the right direction. I don't drive at 6,000 so for me longer runners would improve torque lower in the RPM range.

 

Here's the link but keep in mind this is a very simplified calculator. Some ask duration and lobe angles, bore and stroke, rod lengths and more. This is good enough for a stock engine where you don't have a lot of numbers to plug in.

 

http://www.exx.se/techinfo/runners/runners.html

[Payco]

Thanks so much for putting the time in to give me so much detail in your replies. As always I will take advice and use it.

 

[datzenmike]

 

Here I fitted simple and cheap muffler pipe. It took all of 3-5 minutes and a ball peen hammer to flare each end. These were the long ones and it felt more powerfil at lower RPMs where most people drive. Eventually I shortened to maybe 1/3 this length in order to fit a home made air filter box.

 

 

Generally shorter for higher RPM and longer for lower RPM torque. Smaller diameter pipe increases air speed and adds to higher RPM power but you risk having restriction. I like the R-1 carburetors and the angle they sit at. With the top off the air filter you can open the slide and look straight down at the intake valve. This gives side drafts an advantage over downdrafts that have an almost 90⁰ bend and a 'basket of snakes' intake with all the runners inter connected.

 

 

 

Here are some dyno testing of intake runner lengths. Starts at 2:20. Again you can tune to top RPMs but who drives in this range unless racing? I would be more inclined for 3-4,000 range power.

 

 

 

[Payco]

Thankyou..