Shoptalkforums.com

hey guys i building my efi intake using the 5in D tubing from ross machining and a 3in od cross pipe. right now i have a foot of d tubing on each side. right now my plan is to run it NA this season then add a turbo if it isn't fast enough. so the question is can i have to big of plenum? i think i could cut it down to 9 or 10 inches but don't think it looks bad now but i don't want to affect performance either.
jason

The best plenum for NA is the atmosphere which is pretty big

Are you running ITB's then or one single TB with the two plenums on top of each dual port intake manifold ? Maybe give Paul some more specs like engine size etc.

single 52 mm throttle body to sound like a real dumbass i don't remember which stroker crank i went with untill i dig out my receipt 92mm thick wall forged pistons w120 cam 1.25 rockers if remember right i got the compression. down to 9:1 still higher then i wanted but my heads are old cb big valves with small chambers. the end plan is to turbo it but for right now i just want it up and running as i've been building it for 2 years and have already sold the engine that was in my rail. the intake is getting welded next week. i'll be running 440cc injectors and ls coils. right now im just waiting on the temp sensor from mario then i can finish putting it all together. i ran it for about 10 mins last year with a set of kadrons my kid has but it was to lean. it looks like i could cut the plenums down to about 10 inches long and still have enough room for the top tube.

I'm guessing the motor has IDF manifolds with injectors but the main problem is too much volume between the throttle and the ports will produce very poor throttle response with no benefits
This was a mock up of a single throttle intake built with 2.5 inch tubing. Throttle response was bad and took a while to settle down to idle.
It was so bad I didn't even bother putting it on the dyno and switched to dual throttles.

A good rule is put whatever you want above the throttle but be careful what you do below the throttle

so i should just plan on putting the turbo on asap? with the cost of all the parts i think im past the point of wanting to change it up.

jpryorx2 wrote: ↑Fri Apr 21, 2023 10:53 am so i should just plan on putting the turbo on asap? with the cost of all the parts i think im past the point of wanting to change it up.

You'll have the same problems with a turbo, When you change gear and come off the throttle due to the large volume of air that will flip from boost to vac and then vac to boost you'll get turbo lag then due to the small volume between the turbo and the throttle combined with a slow signal to the dump valve the turbo will take a hit every time

I understand you are too far down the road with it
This won't effect peak power or steady state cruise but all the transients will be difficult to tune
My suggestion would be to reduce your 3 inch cross pipe down to 2 inch, cut those D sections as small as possible and increase the volume between the turbo and the throttle if you can

i'll have to see what tubing can come up with before it goes to the welder. would raising the compression help at all? with the small cc heads we had to shim under the barrels and used i believe .065 head gaskets to try and lower the compression so i could pull the head gaskets pretty easy. i was just always taught that turbo engines should be around 8 to 1 compression so was trying to get it as low as we could
jason .

i did have a small brain fart i was going by the od of the cross over tube its actually 2.5 id.after reading this i've been thinking about it alot. the way i had it laid out was the cross over tube sat directly in front of the shroud basically sitting on top of the alternator with a silicone 90 at each end dropping right into the d tube. now i'm picturing that i may be able to cut the D tube to only be as long as the idf intakes i'd just have to rotate the 90s towards the front of the rail. not sure how much less volume that would give me yet as it's been raining so i haven't been able it lay it out.

so i was able to shorten the plenums to 7 inches and drop the top tube down an inch so it's almost touching the alternator. do you guys think that's enough to make a difference? its a sandrail and gets the most use in the trails but is street legal so i want to be able to drive it around town. and i'm hoping to take it to the drag strip at the most twice a year when they have the bug run. i'm just confused on all the posts i've read on here and other boards people are saying that the plenum needed to be at least 1.5 times 1 cylinders volume and don't recall anyone saying they could be to big or cause response issues. i'm not trying to start a pissing match or anything like that i just want my engine to run good as i have alot of time and money into it now. im upgrading from a 1700 type 4 that had no throttle response so im sure anything will be an improvement but im starting to get worried and second guess myself here
thanks
jason

If you made one then you may as well try it & learn for it. Typically design theory suggests there should be individual port intakes with a length that has a resonance frequency in a useful Rev range. That is often longer than can be packaged so a compromise is necessary (perhaps using 2nd, 3rd harmonic). The stock carb intake is a poor example but does work. The plenum combines those runners so they can be controlled by a single throttle valve. It needs to be big enough to act as a transition and not a messy extension to the runners. It also smooths out the air pulses to achieve a smooth airflow through the throttle. Too big however creates a reservoir of air that dulls the reaction to a change in throttle valve setting. That has more impact at low revs when the airflow is less. Shutting the throttle at high rpm is going to kill the revs pretty quickly as the engine is sucking in vast amounts of air & the plenum will be under vacuum in a split second.

jpryorx2 wrote: ↑Thu Apr 27, 2023 9:09 am so i was able to shorten the plenums to 7 inches and drop the top tube down an inch so it's almost touching the alternator. do you guys think that's enough to make a difference? its a sandrail and gets the most use in the trails but is street legal so i want to be able to drive it around town. and i'm hoping to take it to the drag strip at the most twice a year when they have the bug run. i'm just confused on all the posts i've read on here and other boards people are saying that the plenum needed to be at least 1.5 times 1 cylinders volume and don't recall anyone saying they could be to big or cause response issues. i'm not trying to start a pissing match or anything like that i just want my engine to run good as i have alot of time and money into it now. im upgrading from a 1700 type 4 that had no throttle response so im sure anything will be an improvement but im starting to get worried and second guess myself here
thanks
jason

There's just a few simple guidelines to follow when deciding what intake to use
If using long duration cams eg 110 and up just use ITB's
If you must use a single throttle for whatever reason keep the volume between the throttle
and the intake low which is pretty difficut on a type 1 - see vanagon,type4 mexican for good examples of single throttle systems
and the applications
Keep areas of turbulance ie plenums splitters and tee's as far away from the intake runner as possible
For turbo applications the same applies but pay attention to the volume between the turbo and the throttle
if this is low you'll spike the turbo every time you shut the throttle and don't think the dump valve will save you as it'll react too slowly

i will post pics of what i ended up with once i have the time to bolt it on. i've been rushing to build a rail for my daughters graduation gift that she wants to be able to drive to her last day of school. between having to order parts and nothing seeming to go right i'm behind so haven't touched mine since i picked it up from the fab shop. for now i'm kinda stuck with what i have so i hope it runs decent. it is mainly a trail toy but it is street legal so i'd like to be able to use it to run into town and such and maybe take it to the drag strip twice a year when they do the bug run but if it falls on it's face i may end up waiting till next year and have a different intake made. right now between the 2 i have been just bleeding cash.

jason

Just finish it off it'll work OK . Throttle response maybe a bit soggy and you'll loose some top end HP no big deal

kind of a crappy pic but heres what i came up with now i'm on to the wiring