Either Low end torque or High end RPM flow/HP is the trade off made when selecting a runner length/width, Its one or the other,
The only good way to get around that is by using a "variable length runner intake". or Dual length/path runners intake with diverter valve. Other manufacturers have done it with success. (BMW, Audi, Ferrari, Dodge, Ford. Maybe on LS5 engine)

Has any one done it with a C4? fabricated their own? Used an adapter plate to mount another engine type's intake? or a third party product?

 

On my prior '88 Vette I siamesed the stock intake runners so that each cylinder pulled through two adjacent runners with some success. I was able to extend HP/torque to nearly 6k RPM (shown with comparative dyno pulls) w/o noticeable low end power loss.
Though she was building barely 300chp, it still beat bone stock C5s to the traps.

 

That's not variable, though. By siamesing the TPI, you're sacrificing the 3000rpm torque spike to probably eliminate any resonance tuning at all. The benefit is that there is no long a practical rpm where the resonance shuts down airflow, so you're basically making it a non-tuned intake like any short-runner intake (LT1, Miniram, etc.). That's often not a bad thing to do.

The OP's idea isn't off base, but I've never seen it done on a normal OHV chevy - not even on an LS. It would be expensive. You could basically create a midrange torque spike and then switch to short runners that don't limit the top end. The problem is that I don't think it would be worth the hassle and cost. You wouldn't gain peak power, just midrange power. It seems like in this decade, OEMs have gotten away from variable intakes and are preferring instead to use variable cam phasing. This is probably cheaper to do (for them), and is way better from an efficiency and emissions standpoint. There was a Chevy V6 with OHV based (I think) on LS architecture that used variable cam phasing. It could advance or retard the cam - of course it couldn't change the phasing of the intake and exhaust cams to each other like you can on DOHC designs. But if I were an aftermarket company looking to make a splash, I think I'd pursue that route before a complicated variable intake. It would allow you to run a wilder cam but retain good low-rpm drivability and emissions/efficiency.

 

The variable length design has been used, and as you can see is not common today. There has to be trade offs in port volume and peak flow. And these systems were not variable. They were normally 2 position systems.

Variable valve timing with shorter runners have proven to provide better overall output. It is also very common.

 

It's called a ZR1.

 

you are confusing dual/siamesed with variable length.

Even in a switched length manifold, there are two sweet spots... unless then second length is very short (like a mini ram) and has no "tuned" or sweet spot.

Tuned Port means just that, they were tuned to be most efficient at a certain RPM.

 

Right. The LT5 had separate intake runners for each of the two intake valves. The primary one was always operating, and the secondary one only opened above a certain airflow demand. The secondary runners may have been larger in diameter (I'm not sure), but they were the same length as the primaries. So this wasn't a resonant tuning setup - it was just closing off half of the intake capacity to keep airflow velocity high at lower rpm and load.

 

As far as I remember, same length, same diameter. They were shorter from plenum to intake valve though, allowing them to perform better at high RPM. Again, If I remember correctly, the LT5 runners did not cross under the plenum. The RH runner fed the RH side of the engine and same for LH side. Don't take this a gospel,. but I believe it to be the case.

 

All the runners on an LT5 are the same length, and they appear to be the same diameter:

 

Excellent point peoples. The ZR1 multi-valve double-runner design is actually quite excellent IMO, because its a simplistic way to adequately solve the problem. Narrow long Runner delivers a high velocity and torque for the low end, and then the second runner, when necessary to double the flow to support higher (double) RPMs, replicating the velocity curve of the first. Its sorta like the intake switching gears. Id argue it could be cheaper to buy a LT5 ZR1, than to custom manufacture a qty1 variable intake, without proper tools/machinery. As well, I agree a variable CAM methodology likely would yield more advantageous results, on numerous levels. Not only to widen the torque RPM curve, and improve drivability at all RPMs for dual purpose street and track. CAM timing likely would make more difference in improving power than intake mods. Great for new car manufacturers. But the point is, a C4 stock 350 block doesn't have a variable cam nor enough space in the compression chamber area to accommodate a head with dual intake valves, so those two options are out. A new Intake is something that is possible and often added to a stock C4 block. Thus a variable intake upgrade options, would be an exciting upgrade for a C4 owner, if it existed. The fact is there is as high as 40 ft lbs of torque difference at low RPMs (areas under 3000), comparing long runner intakes to race intakes like miniram and stealthrams. Solving that could drastically improve drivabilty for dual purpose cars. The question is what's the best way to address it.

Some argue, put in the mini and stealth rams, cause those high RPMs gains are more beneficial than the lost torque, considering tires can barely hold at the C4's high torque anyways.

This topic originally came up in my head, when I considering whether to do a 383 vs 350 in a future race engine build. Some experts suggest the 350 with a shorter stroke is more advantageous for a race engine, as it will rev higher than the long stroke 383. Boost can be a more efficient way to increase high RPM TQ/HP. Because high Boost is mostly at high RPM, it would mesh well with a high reving 350. Runner/Head flow is less important with Force injection Boost, as the flow is forced regardless, at a small expense of more heat and load. Sure higher flow RAM intakes would help a little to increase HP at some RPMs, and less HP loss due to load. But none of those systems help much with low RPM torque. With Force injection, maybe it would be more beneficial to keep the long runners to preserve the low end torque. Or maybe better to install the high flow intakes, and makeup the low end torque by going 383, and accepting the sacrifice to limit revs at the top most RPMs. Real world, the best choice really depends and very specific to the exact targeted driving RPMs or task.

I was leaning towards sticking with hardened 350 and just compensating by adding more boost. Once committing to a SuperCharger, adding more boost comes at a very small cost.

But my point was, if a Variable intake existed, it could favor both low RPM torque and high RPM HP, and fewer choices and tradeoffs would be necessary. It would compliments various displacements, cams, and boost situations. There would be absolutely no downside to adding a better variable intake, other than price.

I believe a (non-compliant emmissions version) bolt on Intake manifold could be designed of the dual runner style that used the pre-existing stock long runner or after market larger long runner as primary, and had second port to accommodate a second inner shorter runner, where the second runner would connect to bottom of stock size plenum. Many different way to manage the opening of second port.

Im guessing Manufacturers are considering C4s too old, for it to be financially viable to manufacturer new products of that nature.
To bad it doesn't exist.

 

I'd love to see someone make one. It would be different, it COULD be effective... Build one!

 

The Vortec v6 had a variable runner setup with a tuning valve that would open to essentially shorten runners. The purpose was to allow for a broad flat torque curve. The intent was to increase towing capabilities of the 90s S/T trucks and utilities. The L31 vortec doesn't use a tuning valve. The lack of under hood clearance may have prevented its development. The Superram on a LT1 broadened amd boosted torque curve. The Accel LT1 Superram base is extremely rare. The l98 base is more common.

 

The first part;






The second, the LT1 actually makes more tq from about 2500 on down, than the L98.

 

I didn't realize that LT1 had better torque that low down. Considering the L98 has almost 80ftlb more torque than a LT1 at peak torque.

Do you know what engine/heads that is, that has the 4 valves per cylinder? looks like an interesting read from Hotrod.,

 

It's been around a while. If it was worth it, but just costly... a lot of exotics would use it. A lot of vehicles have used it.

Read Here

 

Looks like a small block to me. If I were to guess, it could be the 350 GM was toying around with when they first reached out to lotus in the 80s.

 

Moser designed a dohc set up for sbc in 70-71. There were sohc heads in the 60's

 

I'm not sure where you're getting your numbers but the L98 doesn't make 80 lb more tq than an LT1 at any RPM. Both peak at 340, just do so at different RPM.





The 4v/pushrod heads are Dominion or Arao brand. They're out of business now, but I believe others are trying to pick up where Arao left off. HERE IS THE HOT ROD ARTICLE

And THIS is a place that currently sells them for BBC.

 

My sources say 330ft-lbs for the LT1 and 340ft-lbs. for the later L98. Not 80ft-lbs.

The LT1 got a good bump in compression over the L98. 9.5 to 10.4

 

330 was first year only for LT1 (shown in the "outgoing vs. incoming" graph above. All other 'Vette LT1 ('93^) were 340 tq too. I thought tdereggi was talking about max difference points from idle to redline....which there is not point anywhere in the curve where an L98 has anywhere close to an 80 lb advantage.