Edelbrock LD340 Intake Manifold Usable Max RPM?

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Fish Bite

Fish Bite

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I'm hoping to get feedback from people who know firsthand (or can point me to others' results) about the RPM limits of the LD340 intake manifold with the plenum divider removed per Direct Connections' instructions and an exact deep-port gasket match.

The racing class rules I'm bound by restrict my intake manifold to either a factory cast-iron or LD340, and I know the intake will limit performance compared to a quality single plane. I'm trying to get an idea of the RPM range the engine can practically use with the LD340 to help me in considering cam profiles. The engine is a 360 gas race engine (no street use), 12.5:1 static compression, headers and unrestricted exhaust, four speed, and low rear gearing. I'm restricted to a Holley 4150 750 cfm.

The ported heads flow the following:
Lift Intake CFM Exhaust CFM
.400 256 171
.500 277 189
.600 285 195
.700 281 197

Can anyone give input on their experience of what RPM the power flattens out or starts dropping off with the LD340 on a race engine?
 
Your peak power will be 6200 -6400 just my estimate
The stock LD 340 will run efficiently to 7000.

With internal modifications by a very good cylinder head
man, you can run to 7300 to 7400 and bring up peak power
up a bit as well.

pittsburghracer could tell you a lot more accurate and detailed
information.
 
Last edited:
GTX JOHN said:
Your peak power will be 6200 -6400 just my estimate
The stock LD 340 will run efficiently to 7000.

With internal modifications by a very good cylinder head
man, you can run to 7300 to 7400 and bring up peak power
up a bit as well.

pittsburghracer could tell you a lot more accurate and detailed
information.
Click to expand...
Unfortunately no longer Sir,...
John Cadamore AKA PittsburgRacer Has Passed Away
 
had one on a 340 12.5 to 1 worked X heads and intake was port matched and had a notch machined outta the carb base, 833 built by maccandles, 3:91 and N50's turned it 7200 some times 7400 rpms, i mighta been turning it to much, idk, but it sure was fun and car seemed to enjoy it as much as i did!
 
I’ve owned a few. 340-4 speed cars.
6800 they stop making power.
Higher rear gears, lower that number.

Currently my 340 /4speed 3:91 gear will drop off around 6500. 10:1 compression and getting tired.
All of that factors in to the mix.
 
I had one on a hot 360 in a 65 Valiant years ago. I touched 7200 a few times and it was still pullin. That was with the .508 Mopar cam.
 
Thanks for all the feedback. I was assuming a running point of around 7,000 RPM, so it's good to hear that people are getting that and some a little more out of the intake.
 
Last edited:
Fish Bite said:
Thanks for all the feedback. I was assuming a ceiling of around 7,000 RPM, so it's good to hear that people are getting that and some a little more out of the intake.
Click to expand...
I am confident with the right solid flat or solid roller and proper port work to the intake and heads that 8K is not out of the question.
 
Syleng1 said:
I’ve owned a few. 340-4 speed cars.
6800 they stop making power.
Higher rear gears, lower that number.

Currently my 340 /4speed 3:91 gear will drop off around 6500. 10:1 compression and getting tired.
All of that factors in to the mix.
Click to expand...

Where the power drops off is going to be almost completely determined by how good the heads are, and the cam. To a somewhat lesser extent the intake.
So it’s impossible to throw a blanket statement over where a 340 is done at. Buddy Vic Bloomer runs his stock stroke 340 8 grand, runs mid 9’s with factory iron heads he tricked out. At 6800 it’s just beginning to eat.
 
B3422w5 said:
Where the power drops off is going to be almost completely determined by how good the heads are, and the cam. To a somewhat lesser extent the intake.
So it’s impossible to throw a blanket statement over where a 340 is done at. Buddy Vic Bloomer runs his stock stroke 340 8 grand, runs mid 9’s with factory iron heads he tricked out. At 6800 it’s just beginning to eat.
Click to expand...
Did you even read the OP’s question?
I was just answering with my experience not what an expert did on their engine and “yes” compression plays into that mix all day long not just cam and head flow. Does Buddy Vic Bloomer run an LD340 on that engine?
 
I’m gonna go against the grain here and say that on your specific combo, the LD340 intake is what will limit it. But you knew that. It’ll go 7000-7200 no problem if your valvetrain is up to snuff, and you have enough intake duration, but the real question is will it make power up there? I’m saying no it won’t. I think it’ll roll over about 5900-6000. Hard to say without cam specs though.
 
Syleng1 said:
Did you even read the OP’s question?
I was just answering with my experience not what an expert did on their engine and “yes” compression plays into that mix all day long not just cam and head flow. Does Buddy Vic Bloomer run an LD340 on that engine?
Click to expand...

He doesn’t, but if he did it would easily be capable of running well over the OP’s 7 k question.
You’re the one that says they stop making power at 6800. My point is there is no set “ limit” of where they stop at. Other factors come into play.
Compression isn’t a big factor in rpm. Vic’s motor is around 10 to 1, if not a hair less.
 
B3422w5 said:
He doesn’t, but if he did it would easily be capable of running well over the OP’s 7 k question.
You’re the one that says they stop making power at 6800. My point is there is no set “ limit” of where they stop at. Other factors come into play.
Compression isn’t a big factor in rpm. Vic’s motor is around 10 to 1, if not a hair less.
Click to expand...
I'm attempting to stay away from a theoretical debate about whether an LD340 and 750 cfm 4150 will physically make power at 8K RPM and, instead, am trying to get real-world feedback on the RPM ranges people have practically used with an LD340 on a race 360. To this goal, I'm not following your point but am open to hearing your input as it relates to my specific build. Where do you think a practical RPM ceiling is for this specific build considering it's a vintage class.

The way you have stated your position, it sounds like you are arguing that with the LD340 and 750 cfm 4150 I must use, I should build a race 360 that spends its time at 8K RPM (with heads/compression/cam/exhaust designed for that RPM) because you say the intake manifold is less of a limiting factor. I'm having a hard time believing that the 8K version of this engine would make much more power than one designed to run at 7,200 RPM, but it would cost far more in financial and maintenance investment to build and maintain an 8K RPM engine compared to a 7,200 RPM engine. Slap a Super Victor Jr. single-plane and larger carburetor on that 8K RPM engine, and the practicality goes way up, but I'm stuck with the LD340/750 cfm.
 
TT5.9mag said:
I’m gonna go against the grain here and say that on your specific combo, the LD340 intake is what will limit it. But you knew that. It’ll go 7000-7200 no problem if your valvetrain is up to snuff, and you have enough intake duration, but the real question is will it make power up there? I’m saying no it won’t. I think it’ll roll over about 5900-6000. Hard to say without cam specs though.
Click to expand...
Thanks. The main goal of me asking the question is to help determine the cam profile that will impact the rest of the build. A few cam companies I spoke to were thinking for solid flat-tappet in the .535"/255° @ .050" area running at 7,200 RPM. The largest that one suggested is .600"/260° to push up to 7,600. Your thoughts?
 
Fish Bite said:
I'm attempting to stay away from a theoretical debate about whether an LD340 and 750 cfm 4150 will physically make power at 8K RPM and, instead, am trying to get real-world feedback on the RPM ranges people have practically used with an LD340 on a race 360. To this goal, I'm not following your point but am open to hearing your input as it relates to my specific build. Where do you think a practical RPM ceiling is for this specific build considering it's a vintage class.

The way you have stated your position, it sounds like you are arguing that with the LD340 and 750 cfm 4150 I must use, I should build a race 360 that spends its time at 8K RPM (with heads/compression/cam/exhaust designed for that RPM) because you say the intake manifold is less of a limiting factor. I'm having a hard time believing that the 8K version of this engine would make much more power than one designed to run at 7,200 RPM, but it would cost far more in financial and maintenance investment to build and maintain an 8K RPM engine compared to a 7,200 RPM engine. Slap a Super Victor Jr. single-plane and larger carburetor on that 8K RPM engine, and the practicality goes way up, but I'm stuck with the LD340/750 cfm.
Click to expand...

Sorry, no, I don’t think that dual plane will carry 8000 rpm.
I think it capable of 7-7200 like you are thinking.
 
The runner length decides the rpm its tuned for but doesn't mean it won't rev/make power passed the tuned rpm range it just loses ram effect but there is higher rpm bands that get a lesser secondary ram effect. Darin Morgan was talking about tuning the headers above intake tuning to extend the power range. But cross section/plenum volume will limit max rpm if velocities get to high it will choke it out.

Richard Holdener did some test with runner length and found out above tune it could still rev but power just flatten out.
 
Fish Bite said:
Thanks. The main goal of me asking the question is to help determine the cam profile that will impact the rest of the build. A few cam companies I spoke to were thinking for solid flat-tappet in the .535"/255° @ .050" area running at 7,200 RPM. The largest that one suggested is .600"/260° to push up to 7,600. Your thoughts?
Click to expand...
It’s hard to judge really because that combo wants 260-270 @ .050 and a big single plane. And I’d want more lift, like .630-.650, but then throw the LD340 on there and I’d want to see 230 @ .050 and .500 lift and 9:1 comp. So I’m gonna tell you to not be set in stone on any of them. Have a cam ground and if it doesn’t do what you want tell the grinder and get a different one ground. And if you’re not rules limited to a SFT, I’d run a hard roller in that thing without thinking twice about it.
 
And if I was limited by rules to the LD340 it would have the biggest open spacer I could find on it.
 
If your choice is factory or LD340 it is what it is, obviously the LD340 is the way to go.
What mods are you allowed to do? people will cut the intake up in quarters and cut access holes everywhere to port them out it's up to you and the rules how far you take it.
 
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