I just spoke to my tuner earlier to get some more details of the latest data.
First of all, the throttle body was checked and is functioning as it should. There are no impairments to the operation.
From 6000rpm+ is where the airflow restrictions are showing.
The tuner is saying that the FAST with the long runners was pretty much maxed out with the previous camshaft and cannot provide enough air flow.
He also said, we have higher valve lift, a bigger camshaft and a new valve job which is providing extra air flow that the long runner cannot flow enough for.
The runners were checked and none were cracked or leaking and there were no leaks in the manifold either.
So I had no choice but to order the high hp runners today.
He guesses that with the high hp runners, the mid range torque will be stronger too as well as pull much better up top due to better breathing.
If this still proves to be a restriction, I will have to change the intake to the new FAST polymer intake when it comes out.
I'm a bit surprised the long runners could not flow enough for my combo.
I have seen other similar engines with the long runners not have issues.
Does any of this sound familiar to anyone or has anyone had a similar experience.

Also, with the shorter runner will the idle quality be a bit better too?

 

The shorter runner will weaken low end power, and how would that improve idle quality?

 

Not sure.
I was just asking.
I am thinking if the engine is breathing better with a shorter runner it may also idle cleaner.

 

Holley sniper low profile will help that top

 

Try this little experiment. Get a narrow straw like a coffee stirrer straw. Inhale and exhale through it. Now cut the straw in half cleanly (not crimped) and breathe through it. You will feel the amount of effort in your chest reduce.

I am still unclear where your data is showing airflow restrictions. I will make an educated guess that you are hitting peak power around 6000, and the curve is flatlining, possibly the tone of the engine is changing as well like it hit a wall or something?

What follows is what I have observed and deduced on my own stuff - May or may not be generally applicable, but it is what I saw directly.

My old 346 hit a wall at 510 hp on the fast 102. It was around 6500 rpm, flatlined, engine time changed, pretty much what I described above. The dyno operator told me I needed a larger plenum and shorter runners. So I told him - dude it a fast 102 ported by Mamo, there is nothing out there to go to. He laughed and said he was in uncharted territory himself given he had never seen a Ls1 over 470 hp before. But going off the power curve I needed a bigger plenum and shorter runners.

Enter Mamo ported msd. Here is what happened...

1. It peaked more naturally at 6900-ish and carried and faded. Revved clean to 7600. Not like a brick wall thing.
2. Peak power increased 9 hp to 519.
3. Peak tq dropped 6 lbs
4. Midrange torque in certain spots dropped 20 or so lbs. BUT......
5. The torque curve went from triple humped to smooth, broad, and predictable.

I theorized at the time that there was some interaction I did not fully understand between runner length and IVC. Somehow the later IVC just naturally worked better with a shorter runner. This was the reason the tq curve smoothed out.

Idle tuning was quite different on msd vs fast. It was not easy. Idle quality is good. Getting there was tricky. BUT... that was on a 346.

Enter larger motor. I sold my msd to a guy with a cathedral port stroker to replace his fast 102 after doing exactly what mine did. He reported back it pulled a good bit harder up top (as expected). He did not have anything near the issues I had with idle tuning, which kind of brings me to the point of this unintentionally long rant....

Ignore all the bullshit about x runner length does y result etc. combination, combination, combination. Those are the three keys. **** has to work together.

Larger motors, especially strokers can get away with shorter runners because piston speed and swept volume compensate for reduced airflow momentum when runners are shortened. Often this results in better cylinder fill and improved midrange torque with shorter runners. Look at msd vs fast on ls7 tests. The msd has more torque with shorter runners!

In your case, knowing your cam specs and your intake, I am not convinced you have maxed out your fast but I do think it is likely the longer runners are fighting your larger camshaft on your bigger engine. The shorter runners I could see going either way. I would not be surprised if you gained torque or lost torque. A 408 is kind of in the cusp. And basically square. If it had more bore I would guess more towards gaining torque with shorter runners.

You will gain power for sure just on rpm alone. But that long stroke is going to try to pull that peak rpm down some. Shorter runners will definitely help counter that by providing more total flow.

I do suspect you will like the way the engine FEELS with the shorter runners in that I think it will just pull cleaner and more predictably. If none of the above fits your scenario, feel free to print this post and use as toilet paper. Wanted to give you as much detail as I could in a post to maybe help you think through it.

 

Thanks for that Darth.
Good information and thoughts. And yes, the tuner is saying that the engine is not very happy above 6000rpm. Especially from 6500rpm+
From what I remember, with my previous 236/244 114LSA camshaft the tuner was telling me that the intake was pretty much maxed out and that the long runner was probably at the point of no returns.
I also agree and have a feeling that the new camshaft is still peaking at 6000rpm like the previous camshaft and then flat lining. Likely fighting the runner and the cam wanting to peak later.
As stated earlier this camshaft wants to be revved out to 7500rpm which surprised me.
I also mentioned to my tuner to maybe try and mix and match the long and high hp runners to see how the engine responds.
I still think it will only need the shorter runners only.
Old cam had an IVC of 50 new cam has IVC of 51 so hardly any different,
EVO is 58 versus EVO 65 with new cam.
The high HP runners have already been ordered. I found them very cheap due to an ebay special discount.
I'm just hoping the engine is not a turd in the low end torque curve.
But we will see.
I have a feeling the peak power will occur at around 7000rpm with the newer runners and definitely pull to 7500rpm. This is the max rpm my lifters are rated to and certainly do not want to go over this.
It's one worry after another.

 

Mid runners or the shortest ones ordered?

 

The mid/high hp runners have been ordered.
The short runners are overkill.

 

Finally!!

 

I think it was you who was telling me to use the shorter runners.
Got no choice this time.

 

I had better idle (slightly) with the mid-length runners that I do with the full length runners.

Airflow issues seem odd....505hp to the wheels ain't that hard for the intake to feed, IMO.

KW

 

Why no graph from the dyno yet?

What lifters, rockers and valve springs are you running again?

Are you running out of injector?

Is it maybe just a case of the converter eating the power and not showing it on the dyno?

I agree I have seen plenty of Fast 102 dynod above 500whp, although the mid length runner is likely a better move for the total combo. I wouldn’t expect more than about a 20hp gain, but average power above 5500 should be way up if you take it to 7500.

 

I can tell you that my old 421 (4.125 stroke) with a ported rod mod stock intake my power peaked at 5800 and my truck ran 10.80's. NAVYBLUE210 advised me to try the fast intake with mid runners. I switched it out a few weeks later and the rpm now peaked at 6900 ran it ran 10.40's, and eventually got into the low 10.30's by the end of the season.

 

The lifters I am running are Isky hpx high rpm hydraulic roller lifters rated to 7500rpm
Rockers are yella terra pro street 1:81 ratio
Pac 1209x valve spring kit
And no my injectors are fine. I have 80lb injectors.
The converter is not the issue.
As stated the tuner was saying that his data is showing an air restriction in the manifold above 6000rpm.
Engine must be peaking around there

 

And I will try and get a graph asap.
Will post it as soon as I get it

 

Same springs as the last cam? That’s a hell of a lot of spring for a hydraulic roller.

 

No.
The last camshaft had a PAC1905 spring I think from memory.
Worked ok with the last combo.
However I have a feeling because of the spring and lifters etc it's really stretching out the powerband to 7500rpm.
Why would this be?
I have heard that short travel lifters can give you a few hundred more rpm but going all the way to 7500rpm for me is just something I did not expect the camshaft wanting to rev that high.
Also, I spoke to my tuner 30 minutes ago.
He pulled apart the FAST intake to check for cracked/leaking runners or a leak on the seals and all was good.
None anywhere so we know it's the runners that are the restriction for sure.
He told me engine was peaking at 6000rpm which is too early and then it's bottlenecked after that.
Too much cam and cubic inches for the intake to cope with.

 

I wouldn’t rule out valvetrain instability. If the engine suddenly isn’t reving like it should after 6000 it may be because the valve timing is getting all messed up by bounce and pumping the lifters down.

Do you know what your spring installed height is? Were they shimmed to 1.800, or even tighter? At 1.80” you have at least .140” to coil bind which is quite a lot. If you shimmed them closer, like .060-.080 to bind you have way way too much seat pressure.

Ls7’s run fast 102’s to ~550whp. I was a supporter of swapping to the shorter runners long before this, but I’m not ready to assign the intake the blame.

Is this a dyno that typically reads low like a mustang dyno or something?

 

All I can say is I know exactly how you feel

 

Valvetrain stability is not the issue.
Everything was all new installed by a professional.
He knows these engines inside out.
It revs out easy till 7500rpm but his tuning log is showing an airflow restriction after 6000rpm and the engine is not making any more power from that rpm because its being bottlenecked by the intake.
The power is maxing out to the same at 505rwhp as I had before with the smaller camshaft except this time it's peaking at the same place.
I certainly trust his advice.
Darth also had an issue with a ported fast maxing out around the 500rwhp mark on a 346CID.
And yes the dyno usually reads lower than most.