The Stovebolt Site Forums The Shop Area The HiPo Shop home-made intake

I've been talking with my local engine building guru about building an intake for my in progress 261 build.After we get some time and cfm numbers on the flow bench, he says he has a formula for figuring out runner lengths and whatnot.We figure it shouldn't be too hard to improve on the stock manifold or even an offy-they're pretty primative. Any thoughts????

Yeah, I'd love to see pictures of the process and hear all about how well it works out!

offy, fenton etc - wouldn't they have already calculated flow and runner length etc and incorporated the specs into their existing designs?

I don't know about offy and fenton- they just seem to be one straight tube with 90 degree bends.They look to be kind of primative,like they just stuck 2 carbs where they fit, but they could be perfect. I'm thinking smoother bends with tubing, coming from a plenum sized to the appropriate cfm topped of with a single 2-barrel also with the appropriate cfm

I don't know about offy and fenton, either, but I would bet that back in the day it was less of a concern than fitment.

that's my opinion as well, they were just trying to get another carb or two on there.flow characteristics notsomuch.

I have seen curved tube runners on a home made intake,but couldn't vouch for its efficiency. Your design will likely be determined by the CFM of the carb(s) you plan on using.

Back around 1966,I was building a 235.A talented friend and mentor,decided a three single barrel manifold was what was needed.We couldn't locate one so he thought why not build one.It was a resonable success.Without computers it was his experience and some guess work.It always had an idle problem,but it had a really wild sound.This was on a 55Chev sedan so a hood scoop had to be added for clearance

I'd be interested to know what his formula(s) are for calculating that stuff. It's out of my league as of right now to build my own intake but it is definitely of interest at some point to try, not to mention I am currently taking calculus and we are just getting into real-life applications, so I'm curious if it relates...
Keep us updated on how it goes

I'm not sure about the formulas,but it has to do with the cfm he will get from the heads on a flow bench and cylinder volume.He builds alot of quick V8 drag motors and I get the impression that the 261 is a new challenge for him, you know, getting sick of small block chevys. Anyway, this is how we're gonna come up with a cam design as,well. I'm not to the point of starting the intake, but I'll take pictures once i get going.Please keep the
opinions and whatnot coming.
Dustman...

I modified one to accept a Rochester 2G but never finished it as it will never look as cool as multiple one barrels. It can be seen in my old album below in the intake project folder. If you are planning on putting the head on a flow bench I am in the middle of that now and can tell you this head is not exactly flow bench friendly. The adapter plate needs to be reversable for odd or even cylinders as well as slotted to allow for an inconsistent head bolt pattern. PM me if you want to compare flow values at some point as I can't find any other reference.

Curt B

As stated in another thread a ton of formulas can be found here:


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

curt, your absolutly right about the cool factor, my 235 has a 2g on it right now.Thanks for the heads (no pun intended) up on the flow bench issues.
Dustman

Just something for you and your engine man to check out is the intake on a slant six Chrysler. I'm sure they were in the same size range and had a very nice curved runner design. If I recall correctly one was offered with a four bbl from the factory. They had a runner for each of the six intake ports so the design is completely different but maybe some knowledge could be gained from profile and length of the runners. Here are some pics of manifolds already available with curved runners for 235/261 chevys. The 3rd shot has one in the foreground and the 4th shot has one with air cleaners in the background. http://s32.photobucket.com/albums/d41/coilover/

The slant 6 is an excellent example of runner shape, but note that there are 3 different lengths. I've read that the purpose was to stagger the torque a bit to provide a broader range?
You can't get a tuned length under the hood - it's 20" or more.

I've been disscussing the different length runner with my engine guy for a bit now. His theory is the same, different lengths spread the power over a larger range in rpm. We still haven't flowed the heads yet, I'm doing the bottom end right now. After the xmas shock on the bank subsides, I'll get started on the head and maybe figure out some cfm numbers and design runner size and length.Also plenum size is something to consider. Panic- you may be right about the 20" but I'm gonna see what I can do. I figure even if this idea isn't any better than stock, at least I can say I built it myself, and I don't think it can be worse than stock.

Have you ever played around with some of the engine analyzers that out there. Dad and I use the Performance Trends Pro quite a bit. They are easy to use and can tell you quite a bit about engines.

The 20" tuned length may not be that hard to get, since the runner length starts at the carb throttle plate and ends at the valve face. Stock intakes are not that far off.

I decided to convert my two carb intake I am making in to a Quadra-jet four barrel intake. The Q-jet has small primary's and should work very well on a inline. The secondary air valve can be tighten up so as to not bog it down when opened, but still operate with decent performance.

Joe

I once had a pontiac tempest sprint convertible ( 1966)..it had a stock inline 6, overhead cam, 4 bbl carb, and capable of a dual exhaust, 3spd on the floor. I belive it was a 230, 235, or maybe a 250.. the intake on that might work...( if you can find one )
oh ya, on a good nite, I could beat a 327 off the line..:>)

The Pontiac stuff won't interchange, they are longer or taller or something, just know they won't work. The OHC was a good engine as long as the cam followers stayed lubed.

Joe

About 15 years ago I made my own intake manifolds for a later 250 inline 6. I made 3 individual intake manifolds, for stock 1bbls those engines ran. Carbs were available at Kragen at the time for about $25. Each manifold consisted of a plate on the head that was close to the shape of the original manifold, with a pipe angled at 45 degrees, out and up, then a flange on top to match the carb. I made one to fit each intake port, 3 total since they are siamesed. The engine had a big cam from Clifford, a 194 head for higher compression, about 11:1, headers and dual exhust with glasspacks, with a Muncie 4 speed behind it.

I never could get it to idle well, I know now that my idle feed restrictors and idle air bleeds were too big. They were trying to feed 6 cylinders and I couldn't shut them down enough for 2 cylinders. The other problem I ran into was the carbs would ice up on a cold day. I had not provided heat to the manifold in any way. It does need SOME heat for atomization. If I were to do it again now I would run a water pipe across all the carb flanges from the water pump or something.

Oh, almost forgot, I sized the pipe the same diameter as close as I could to the port size. Not very scientific, but it worked.

Hope this helps,

Devin