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Ships within 48 hours · Estimated delivery Sep 29 - Oct 4
For Your Every Summer RSVP, with Code: SUMMER15
Description
Holley HP EFI ECU and Harness Kits 550-604NHP ECU AND HARNESS, MPFI, NTK Looking for a plug and play alternative for your stock or built late model factory EFI engine? How about a replacement ECU for your older Holley C950? Holley has you covered. HP EFI ECU and harness kits are designed to be used with popular factory EFI engines as an alternative to the factory ECU and harness or as a replacement for your C950 ECU. The HP allows you to accurately control the fuel and ignition of your engine
HP ECU AND HARNESS, MPFI, NTK
Looking for a plug and play alternative for your stock or built late model factory EFI engine? How about a replacement ECU for your older Holley C950? Holley has you covered. HP EFI ECU and harness kits are designed to be used with popular factory EFI engines as an alternative to the factory ECU and harness or as a replacement for your C950 ECU. The HP allows you to accurately control the fuel and ignition of your engine along with additional controls for nitrous, boost or water meth.
Features:
- Optional 5.7” full-color touchscreen LCD allows easy on-the-fly tuning, data-logging or can be used as an easy to see graphical gauge panel
- Eight sequentially driven 8:2 peak and hold injector drivers, capable of driving up to 16 low or high impedance injectors as a standard feature. Multiple staged injector strategies
- Eight channel Distributorless Ignition (DIS)outputs capable of directly driving “Smart” ignition coils or Holley DIS coils. Will drive 2 wire coils with the addition of PN 554-112 high current coil driver module
- Integrated single channel wide band oxygen sensor control. Works with NTK or Bosch sensors
- Internal data-logging standard with a huge 2GB of memory
- 4 Stage progressive nitrous control
- Turbo boost control
- Integrated water/methanol injection control
- Self-tuning fuel table strategy greatly simplifies tuning process
- Individual cylinder fuel and spark control allows you to unlock your engine’s potential
- Works with 4, 6, 8 cylinder engines
- Unique lb/hr based fueling strategy greatly simplifies and commonizes tuning
- Advanced idle, closed loop, and enrichment strategies allow for very stable operation
- ECU is fully potted and can be mounted in the engine compartment or interior
- Sealed automotive and marine grade connectors
- Allows for use of common OEM sensors as well as customer sensor calibration input
- Ignition Plug and Play with GM HEI, Ford TFI, magnetic and hall effect trigger, and other ignition systems. New Plug and Play applications in process. Custom settings can be configured to allow many other applications
- 1-5 Bar MAP sensor capability
- Two channel knock control sensor Inputs for both one or two wire knock sensors
- Dedicated fuel and oil pressure inputs
- Controls both stepper and PWM Idle Air Control (IAC) motors
- Speed density, Alpha-N, or combination fueling strategies
- 4 Programmable high or low Pulse Width Modulated (PWM) outputs and 4 fully configurable inputs can be configured for use
- 4 Stage nitrous oxide control
- Fully featured nitrous control eliminates the need for a separate nitrous controller device
- Can be configured “Wet” or “Dry” with closed loop feedback
- Progressive control based on time, RPM, or boost, requires part # 554-111
- Lean or rich safety cutoff
- Full timing retard/control configurability per stage based on RPM or time
- Integrated Water/Methanol injection control
- Uses Holley Water/Meth solenoids specially designed and calibrated for use with this EFI system to allow the user to enter Water/Meth flow as a percentage of fuel injector flow for Water/Meth tuning precision unmatched in the industry
Application
- Universal
Specs
| Brand | Holley EFI |
|---|---|
| Emission Code | 3 |
| Ignition Type | Coil - (No Timing Control) |
| Ford TFI | |
| GM HEI Small Cap | |
| Holley Dual Sync Distributor | |
| Injector Connector | Jetronic / Minitimer (Bosch) |
| Product Type | HP EFI Kit |
| Part Number | 550-604N |
Shipping Notes
- Free Standard Shipping on $100+ Orders to the USA.
- Except Preorder products are shipped in 48 hours.
- Delivery to the USA:
- Standard Shipping : 3-10 business days
- If time is of the essence, please consider selecting expedited delivery for faster service.
Exchange/Return Notes
- We offer a 30-day return/exchange service after receiving.
- Final sale items are not eligible for returns or exchanges.
- To process your return/exchange, please contact us at [email protected]
- Please click here for more details>>> Return & Exchange Policy
4.2 ★★★★★
Based on 24 reviews
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Product Reviews
★★★★★ 5
My favorite underwear
Size: Medium, Color: Multicoloured C (5-pack)
Super comfortable!! They are soft and breathable. No pinching, no shifting, and relatively invisible in skirts, pants, and loose dresses. True to size. Excellent quality, holds up well. Only thing is that the wording on the front does come off after a few washes, so you have to differentiate the front from the back of the underwear by how high the seam comes up. I almost exclusively wear this underwear now, I love it!
WAS THIS REVIEW HELPFUL?YesReportShare
Reviewed in the United States on April 30, 2026
★★★★★ 5
Incredible Comfort
Size: Large, Color: Multicoloured F (5-pack)
Amazing; I’ll be ordering more. Super soft, lightweight, smooth under clothes, incredible comfort! Best underwear! I ordered these because I’m tired of underwear lines when wearing biker shorts/leggings. Other boy shorts were too long or the bottom hem too thick and both of those issues left lines around my upper leg. THESE are *just right* - perfect length and fit. No panty lines on close fitting pants, comfortable under other pants and dresses, great with jumpsuits, and fit well under slip shorts with dresses.
NOTE: These take longer to dry, than similar clothes, in the dryer. No shrinking though.
SIZE: I ordered these in size L and they fit perfectly. A little snug when first on but they loosened up. For reference I’m 5’4” tall, about 175#, carry my weight in my hips and backside,wear size L sweats/pajamas/loose pants, size 12 jeans.
WAS THIS REVIEW HELPFUL?YesReportShare
Reviewed in the United States on September 23, 2025
★★★★★ 5
Full coverage and full comfort
Size: XX-Large, Color: Multicoloured C (5-pack)
These boyshort underwear are a dream. They’re very soft, very stretchy, and very comfortable. They do not shrink when they’re washed. They’re heavy enough to make you feel like you have something covering you for discretion, but light enough to wear under any type of fabric. I have wide hips and I’m older so I don’t have a tight perky butt anymore. Most underwear tend to either ride up on me or become very uncomfortable. These stay where you put them. And they come all the way up to your waist so that there is no panty line whatsoever. The waist line is soft and comfortably. It does not dig in. They do not ride up and drive you crazy they stay where you put them. I would recommend them to anyone young or old. I have tried so many different styles and materials to find underwear that’s actually comfortable. These are the absolute best I have tried in five years. I just threw all the rest of them out.
WAS THIS REVIEW HELPFUL?YesReportShare
Reviewed in the United States on April 8, 2026
★★★★★ 4
Bamboo is an amazing fabric
Size: Large, Color: Multicoloured I (5-pack)
Comfy and soft . I’ve never worn boy shorts before so takes adjusting to. Legs roll sliding pants up but easy to adjust and smooth before zipping up then all day comfort. When I wear skirts or shorts they are perfect for that! I use to wear bike pants under skirts that don’t have to anymore. I like that they don’t creep up like regular underwear. I’m a new fan of bamboo fabric and bought briefs in bamboo as well.
WAS THIS REVIEW HELPFUL?YesReportShare
Reviewed in the United States on May 9, 2026
★★★★★ 5
The softest, stretchiest, perfect underwear!
Size: Large, Color: Multicoloured F (5-pack)
I have struggled for years to find underwear that didn't ride up. I like bamboo, but had the same problem with them. These Bamboo Cool Boyshorts not only solved my problem, but they are so soft and stretchy that it almost feels sensual pulling them on. They never ride up, roll down, they fit perfectly, and are always super comfortable. I even persuaded my husband to buy the men's version and he loves them too. And I should add, I never write reviews!
WAS THIS REVIEW HELPFUL?YesReportShare
Reviewed in the United States on April 25, 2026
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