Intake runner length
 

I just wrote a bit on my blog about the LT5 intake runner length. In short, the factory manifold length is well optimized for torque at 4800 and 6000 rpm. With enough cam, there is potential to hit another tuning peak at 7500 rpm.

Getting a 1-d simulation to match actual performance was not easy. I've got it correlated well now, and can match within +/- 1% over the powerband. You have to account for the pipe end correction to get it to model the peaks correctly. The actual wave reflection occurs up to an inch above the actual runner entrance.

Anyway, I believe it's hard to beat the stock runner length for almost any build. Would be even better if we could get the same length but without all the bends in the intake path. But then the runners would have to cross over each other (LS-style), or stick out of the hood!

Re: Intake runner length
 

Thanks Todd.
Your technical knowledge & skills are so advanced that it leaves most of us with nothing to add except Wow & Thanks!

Re: Intake runner length
 

:handshak:

Re: Intake runner length
 

Thanks Jerry and Frank. I'll be posting more stuff on the blog from time to time--I have lots of ideas to talk about! Would be ok if anyone wants to ask questions or leave feedback too.

Re: Intake runner length
 

Ok, I'll bite.

It sounds like you got some time invested in this project.

What prompted you to begin it?

Was there doubt in your mind that the Lotus guys knew how to select port lengths and volumes?

Re: Intake runner length
 

Quite the opposite, actually Hib. Since there is but one intake option for LT5's, I was trying to determine how best to utilize it with the rest of the engine combination. In fact, the Lotus guys got it quite right for their intended purpose. I was initially surprised that my cammed 427 made peak torque so high in the rev range (~6000), when so many others peaked below 5000. Once I got the modelling done correctly, it became clear--the tuned length boosts 6000 rpm with the 3rd harmonic. Having enough valve time-area up there gave me the chance to access it. I've since found GM docs confirming that the design target was to optimize the runner length for 6000 rpm.

The other thing that intrigued me was why there was such a small difference observed between peak torque (6000), and peak hp rpm (7000) on my engine. That is not normal, you'd expect 1500 rpm minimum. Sure enough, there is a higher tuned length (7500), but I just didn't have the valve area to access it. And I have very large camshafts.

Conclusions (mine, yours may vary)--unless you are going to fit very large valves and camshafts (to fully access the 2nd harmonic @ 7500), the factory intake lends itself to designing the engine and camshafts for maximum breathing efficiency at 6000 rpm.

Second conclusion-- the shorter tuned lengths of a fabricated manifold will never match the specific output of the factory length at any speed below 8000. I think real-world testing backs that conclusion up.

As always, feel free to add or disagree with anything I've stated.

Re: Intake runner length
 

Phil and I were talking about a cross ram intake. the new OBX LS1 cross ram is only 1300 bucks. if we fab up a dual injector base adapter to lay over the ITB enough it might fit with a high rise hood

when you look at the C5R through the current C7R they all use a cross ram style intake and they make great torq and flow a ton for high end power

while the LT5 intake is a great compromise, I think there is some top end power to be had with a nice cross ram to eliminate the restrictive "s turns" in the factory manifold.

let me know if interested, it's only a $1300 experiment to get started and a suitable ITB "base" could be whittled out of billet or possibly even mod the stock base by cutting it up. not sure how well the injector housings would take to welding but it would be really fun to find out!

I'd love to try "printing" the base out of a high temp plastic media too as it would isolate the ITB from engine heat and you could better control the design.

calibration would be a bit of a bear with all those blades snapping open at once. hold on tight!

Re: Intake runner length
 

Not really feasible Ron. You're going to transition from two "round" 36mm holes to a single rectangle, maybe cathedral, port? That sounds like a disaster. To keep the area change manageable, you'd end up with a very long transition piece.

Calibration is the least concern!

Only way I see to make this work is to machine short transition pieces from the double ports into a single round port and try to lay over a Jenvey short or similar individual TB. Then you need curved trumpets. Fitting that under the C4 hood seems impossible. Not trying to be negative, but we are seriously constrained by hood height.

Re: Intake runner length
 

thanks Todd, yes it might be very hard to get that round hold to keep the flow decently laminar going into the head, but if done right it could work well

I agree the the hood clearance would be a nightmare.

not a trivial project! I also saw the "kirchoffer" intake which looked really good but I don't know that it really made that much power over stock.

the design looked great (exept the lid was to thin and flexed around)

I remember the old "calliope" stacks the rear engine Lotus race car had with the flat plane crank motor. those little beasties twisted up to 8k and made great power, but not many folks will have that kinda stuff sticking out of the hood :p

we also might look at the "msd" dual throttle body design. very low in profile, long runners, and great flow. only available for LS stuff but the "design concept" works well and meats the hold height constraints.

even the MY93 LT5 intake was more like the ford boss design with a tunnel ram rather than the crab style.

graham said that design worked really well!!! that would be much easier to make. as for "fixing" the stock one, love to see what it's capable of flowing. you got better head flow than I thought possible but it might be choked a bit by the intake if it can't keep up!

Re: Intake runner length
 

Could anything be done with the planned MY1996+ LT5 intake with its 8 intake runners vs. 16?

http://i1202.photobucket.com/albums/...ps9omrqfxs.jpg