Yellow Bullet Forums banner

1 - 18 of 18 Posts

·
Registered
Joined
·
1,193 Posts
Discussion Starter #1
For a naturally aspirated ls3, say around 700hp. This will be port efi with dual 4150 throttle bodies on top. Assuming runner length was equal (they're not), plenum volume was close and first intake had a flat top for throttle bodies, ignore forward facing tb... How would it affect performance with the runners protruding into the plenum, putting the mouth of the runner closer to the throttle body, as opposed to the second intake with flush runners and all of the plenum volume on top. The first intake actually has a one inch shorter runner. I'm thinking about this to try to get overall height of the engine down without running a really short runner. Thanks for any input.


20200212_135630.jpg

20200212_135846.jpg
300-113.jpg
 

·
Registered
Joined
·
478 Posts
Efi not near as crucial as with carbs.Its way easier to move air around corner than fuel mix.We have tried several things with efi and saw little on Tb placement or nozzle.So imo dont over think it or clusterfuck your hood area because it really won't matter how you install throttle bodies.
 

·
Registered
Joined
·
1,193 Posts
Discussion Starter #4
Efi not near as crucial as with carbs.Its way easier to move air around corner than fuel mix.We have tried several things with efi and saw little on Tb placement or nozzle.So imo dont over think it or clusterfuck your hood area because it really won't matter how you install throttle bodies.
I guess I didn't really describe it well enough. Both intakes would have 2 four barrel throttle bodies mounted on top. I would not be using a forward facing throttle body. I'm mostly concerned about how the plenum sits down over the runners and how that would affect the function of the plenum as I don't often see that on n/a stuff. The runner length might be a little shorter than I'd like but the overall height would be a good 2" lower allowing me to fit it all under a 4" cowl rather than running a lexan scoop. I'd rather my turd look like a 12 sec car than a 7 sec car haha. And yes, I overthink the fuck out of every detail.
 

·
Registered
Joined
·
478 Posts
Wow I reopened this and your first statement is way diff than first time I read it.Site has odd things sometimes.I really like to pay attention to runner length for cubes I have and intended rpm.I would get one closest to my needs and same with taper if building exact one was not a option.Idk if you looked at wallace site but their is good info there on helping get your combo designed together.
 

·
Registered
Joined
·
1,419 Posts
How long is the runner in each manifold? How long is the runner in your head? What is your intended rpm range?
 

·
A Senior Member
Joined
·
2,846 Posts
Isn't total runner length (valve to plenum) determined by the tuned harmonic length for the motor combination. Subtracting the head runner length from the total intake tract length will give the intake runner length.

So for packaging purposes extending the runner into the plenum with a bellmouth is fine as long as the volume of the runner inside the plenum is not included in the calculation of the volume of the actual plenum.

Desired runner entry area will determine the bell mouth shape
 

·
Registered
Joined
·
1,193 Posts
Discussion Starter #8 (Edited)
How long is the runner in each manifold? How long is the runner in your head? What is your intended rpm range?
Isn't total runner length (valve to plenum) determined by the tuned harmonic length for the motor combination. Subtracting the head runner length from the total intake tract length will give the intake runner length.

So for packaging purposes extending the runner into the plenum with a bellmouth is fine as long as the volume of the runner inside the plenum is not included in the calculation of the volume of the actual plenum.

Desired runner entry area will determine the bell mouth shape
That all makes sense. The engine is 377", target rpm range is 5000-8000, 2700lb rw, stick car. Runner length on the sheetmetal intake is 5.5", the cast hi ram is 6.5" and "As-cast Runner CSA - Tapered, 4.25 in² at the entry to 2.77 in² at the port flange." My heads are at the machine shop so I can't measure them and I can't find the length of the intake port online so I'll have to wait on that unless someone here is familiar enough with ls3 castings. *edit: Looks like they're around 5.6" according to another thread.
 

·
Registered
Joined
·
1,037 Posts
Runner lengths are posted Here on this site in this section....most to all heads. Used it to get a good avg for the CSA of my heads.
 

·
Registered
Joined
·
1,037 Posts
On your thoughts...did the same thing with a thought on the Cathedral low ram idea for my Ls7 heads...with help from my guys here...

 

·
Registered
Joined
·
1,193 Posts
Discussion Starter #11
On your thoughts...did the same thing with a thought on the Cathedral low ram idea for my Ls7 heads...with help from my guys here...

Great info in there, thanks!!
 

·
Registered
Joined
·
1,037 Posts
No problem 👍....I always Bookmark good information. Have to many and still growing.
 

·
Registered
Joined
·
262 Posts
Why would you go With 2 x carb TB's on top if Space is Limited (unless you have them on the shelf and won't sell w/ loss)?
Calculate:
Runner length
Runner CSA
Runner taper
Plenum volume

for engine CID and RPM.
Let the runners stick into the plenum if needed (as 1. pic) and make the entries With generous wide bellmouth radius, more than in the pic. Make floor area and heigt of plenum to suit needed volume and room under the bonnet.

What I would do, anyways.....
 

·
Registered
Joined
·
1,419 Posts
Found the intake port length in the LS3 is about 6.3".
I'd go with the Holley, race proven and falls closer into the OP's desired power-band to 8000
 

·
Registered
Joined
·
1,193 Posts
Discussion Starter #15
Why would you go With 2 x carb TB's on top if Space is Limited (unless you have them on the shelf and won't sell w/ loss)?
Calculate:
Runner length
Runner CSA
Runner taper
Plenum volume

for engine CID and RPM.
Let the runners stick into the plenum if needed (as 1. pic) and make the entries With generous wide bellmouth radius, more than in the pic. Make floor area and heigt of plenum to suit needed volume and room under the bonnet.

What I would do, anyways.....
I want to run the dual throttle bodies because I feel the power is more important than hood clearance, but if I could have both that'd be great. I do already have a majority of the parts.
Found the intake port length in the LS3 is about 6.3".
I'd go with the Holley, race proven and falls closer into the OP's desired power-band to 8000
The hi ram is definitely proven. A lot of fast shit and big power running them. Makes a really nice broad power band in the range I'm after. I already own the hi ram, was just considering other reasonably priced options that might eliminate the need for a scoop. Street car also. Oh, and I'm figuring the sheet metal intake to be about half the weight. I guess I could put together both setups and see what works better and sell the rest.
 

·
Registered
Joined
·
275 Posts
From what im seeing is the 1 inch less from the sheetmetal is the only part to effect you. If you'd rather a 12 second looking car, than a 7 on the street I'd give the sheet metal a try. You may lose a little power if it's under length, but also weigh less, less drag, and less stiff sticking out. May break even overall
 

·
Moderator
Joined
·
35,217 Posts
The runner length is going to be more fundamental in the decision. I think based on your rpm 'range' you want all the length you can get. For EFI I like all the plenum I can get as well and that will be lacking on the Holley..... but, runner length will take precedent.
 

·
Registered
Joined
·
1,193 Posts
Discussion Starter #18
From what im seeing is the 1 inch less from the sheetmetal is the only part to effect you. If you'd rather a 12 second looking car, than a 7 on the street I'd give the sheet metal a try. You may lose a little power if it's under length, but also weigh less, less drag, and less stiff sticking out. May break even overall
That's pretty much exactly my thoughts. I guess there's only one way to find out.
The runner length is going to be more fundamental in the decision. I think based on your rpm 'range' you want all the length you can get. For EFI I like all the plenum I can get as well and that will be lacking on the Holley..... but, runner length will take precedent.
Thank you for your input. The sheetmetal one is actually a Holley also (not that that matters), I'd just be modifying it. I guess I could make the plenum volume whatever I want if I'm making a top, or adjust it with whatever I can fit for spacers under the throttle bodies. It'd be a little more work to modify the runners. I feel like the hi ram appears to have more plenum volume with the top on it. I'm willing to turn it as high as it'll hang on for, but it is hydraulic roller. Shift drop shouldn't be more than 2500.
 
1 - 18 of 18 Posts
Top