90 truck with 86-87 Corvette engine

[Schurkey]

I'm wondering if I could use the factory Corvette distributor that came with the motor instead of switching out with my 305 distributor...
...90 TBI truck w/ 305 factory
Now 86-87 L98 from Corvette.

What's the difference between the two distributors? Photos?

 

[L31MaxExpress]

What's the difference between the two distributors? Photos?

Small cap HEI with an external coil vs large cap HEI with an internal coil. Corvettes kept the large cap HEI until the end of the TPI.

 

[Schurkey]

Large-cap HEI using the 7-pin module?

IF (big IF) it physically fits around the TBI stuff; and if he stuffs a low-resistance bushing under the coil, touching the rotor, and if the truck wire harness is appropriately connected to the module (which is likely to be the tough part)...I think that would work.

ASIDE from the wider spacing of the big cap, there's no actual benefit to the 'Vette distributor. It's a lateral swap if anything.

 

[Scooterwrench]

I completely agree the aftermarket EFI manufacturers are not being honest. Dyno tests have shown repeatedly the longer runners will make more torque lower in the rpm range when tested back to back on an otherwise identical small block v8. I'd like to see an rpm air gap converted to use port injection compared with the single plane proflo type manifold. I expect the result would be similar to what your mercruiser intake is doing. I don't understand why one of the big manufacturers doesn't make a dual plane port intake for these motors.
Where did you find the mercruiser intake? I have been looking for months and haven't been able to locate one for a fair price.

It's all about runner lenght and size. It's basic physics. A body in motion tends to stay in motion. The dual plane has longer runners which helps get the air moving in the lower RPM range. Because air has mass once it gets moving it wants to keep moving down through the port and into the cyl. Most dual planes also have smaller runners which increases air velocity and increasing the ram air effect through the ports. I don't claim to know much about these Vortec motors but I have noticed the intakes have long runners that are narrow to make power in the lower and mid RPM range. Most single plane intakes tend to have shorter,larger runners and will eventually get the air moving but not until you get up into the upper RPM's. They have very little air velocity in the lower ranges. I think an optimum intake design would have two sets of throttle plates along the length of the runner. The primary plates would be up at the top of the runner giving you that long length for low end velocity then at about 3500 RPM the secondary plate would start opening. The secondary plate could actually be quite small so it would create a jet of air that would pull more air through the primary side taking advantage of the venturi effect.
Years ago a buddy of mine was playing with a 85cc three wheeler and shot air into the carb while is was running and man that motor tached out. That jet of air down through the center of the carb pulled more air along with it and really boosted the power. Another example is the flat head 5hp Briggs. Those carbs have a welch plug in the end of them. One night I was out on the track and all of a sudden my kart started pulling harder off the turns and screaming down the straights. When I got back in the pits I discovered that welch plug had fallen out and the air going in was helping to pull more air down through the bowl where the air filter sits. I was tempted to run it like that but the tech inspector would have picked right up on that and the dirt would have killed the motor.

 

[Erik the Awful]

I think an optimum intake design would have two sets of throttle plates along the length of the runner. The primary plates would be up at the top of the runner giving you that long length for low end velocity then at about 3500 RPM the secondary plate would start opening.

The Honda K24 has this setup. I had the intake off my daughter's 2002 Honda CR-V last week, cleaning the Intake Manifold Runner Control.

 

[L31MaxExpress]

The Honda K24 has this setup. I had the intake off my daughter's 2002 Honda CR-V last week, cleaning the Intake Manifold Runner Control.

My 4.0L Nissan engine has one as well. The variable intake cam timing actuators operate about the same time too. It feels like the secondaries coming in on a carb. My 1.8L Sentra had a similar setup. The mid 90s CPI 4.3L had variable runner length as well. Wish GM had carried that manifold over to the 350s along with the F/Y-car LT1 like cam specs those earlier 4.3Ls had. Those older CPI 4.3s had more power everywhere than the Vortec 4.3L that replaced them.

 

[L31MaxExpress]

In the Nissan VK V8 world, they are adapting over a M45 manifold with dual runner length off the 4.5L on to the 5.6L V8 and using a 90mm LS throttle body, called the PD90 Intake Manifold. The engine has the same architecture but spacers have to be machined to adapt it over and the M45 manifold has to be ported. Longer runners at lower rpm similar in length to the stock 5.6L manifold and switching to short runners at higher rpm. Stock 5.6L Pathfinder with the PD90 manifold. They inevitably lose a little torque when the runners switch, but it really wakes up the top end using the shorter length runners and keeps the low-speed torque with the longer runners. Stock VK56 manifold is in blue, PD90 switching at 4,800 rpm in red.

You must be registered for see images attach

You must be registered for see images attach

 

[L31MaxExpress]

Large-cap HEI using the 7-pin module?

IF (big IF) it physically fits around the TBI stuff; and if he stuffs a low-resistance bushing under the coil, touching the rotor, and if the truck wire harness is appropriately connected to the module (which is likely to be the tough part)...I think that would work.

ASIDE from the wider spacing of the big cap, there's no actual benefit to the 'Vette distributor. It's a lateral swap if anything.

Module latency curves from the Corvette calibration need to be put into the TBI calibration or the timing advance commanded will not be accurate. The EST connector for those is a bit hard to find unless you have the matching harness end already to graft on the harness. I prefer the large cap, less spark scatter and the cap and rotor last about 2x as long. With a DUI Module and Coil, 0.060" plug gap is not an issue even on a 10.5:1 engine. I ended up using a MSD6a box with mine eventually as well. I ran one on both my TBI and TPI setup on the old G20. I am sure the stock metal fuel lines would have probably needed some re-bending to clear. I had an external regulator setup and AN lines feeding the TBI unit.