Climbing a large dune on 2WD!
Been having a ton of fun the past few weeks testing out the 1FZ-FE Stroker. Especially in the sand dunes.
For my camshaft profile, I should be seeing the power peak around 6000-6500 RPM while holding the torque until 7500-8000 RPM. However, I realized that with my current setup the engine tapers off quickly after 5800 RPM (VE Drops), as also shown my the manifold pressure decreasing beyond 5800 RPM, with a significant drop when I try to rev it till 8000 RPM. Right now I feel a surge of power above 4000 RPM and that surge stops at around 5200 RPM.
Factors At Play:
1. Valve Spring Stiffness
2. Exhaust design
3. Intake design
Valve Spring Stiffness:
The higher the lift of the camshaft, the stiffer the springs need to be, or else you will get valve float. Now, I have not noticed any backfiring at very high RPM like 7500-8000 RPM but minor valve float can still cause weak airflow at very high RPM, and will not allow the camshaft to be used to its full potential, in terms of valve actuation and timing.
The solution? Dual valve springs instead of the current single valve springs.
Exhaust Design:
My current headers are 6-1 Long Tubes with 1 7/8 Primaries and a 3.5 inch collector with long primary tubes (32-34 inches). After some research I have discovered that for my target powerband of 5000-8000 RPM this is not optimal.
Larger Exhaust Header Primary Tube Diameter = Shift torque peak to a higher RPM + Hold torque better at higher RPM
Technical Notes:
- Reduces exhaust gas velocity at low RPM, but allows higher peak flow at high RPM. At high RPM, engines produce larger volumes of exhaust gas in shorter timeframes. A larger primary tube diameter reduces backpressure at these higher exhaust flow rates, allowing the engine to breathe more freely.
- Reduced backpressure means lower restriction. This enables improved scavenging, where the exhaust pulses help pull exhaust gases from adjacent cylinders, enhancing cylinder filling at high RPM.
- Reflection dynamics: Larger primaries slightly delay and soften the exhaust pressure wave reflections. The softer reflections are timed better at higher RPM, reducing interference that can restrict airflow at high speeds.
Shorter Exhaust Header Primary Tube Length = Shift torque peak to a higher RPM + Hold torque better at higher RPM
Technical Notes:
- Shifts optimal reflection timing higher: Exhaust headers use pulse reflections from the collector to improve cylinder scavenging. A shorter primary length means the exhaust pulses reach the collector sooner, reflecting the pressure wave earlier.
- Optimizes scavenging at high RPM: With shorter tubes, the negative pressure wave (vacuum pulse created at the collector) returns to the exhaust valve sooner, perfectly timed to help pull exhaust gases out of the cylinder during the exhaust stroke at high RPM.
- At lower RPM, these reflections return too early, losing the advantage: This is why short primary tubes are optimal specifically at higher engine speeds.
Larger Exhaust Header Collector Diameter = Shift torque peak to a higher RPM + Hold torque better at higher RPM
Technical Notes:
- Improves high-RPM scavenging effect by reducing collector backpressure: A larger collector allows exhaust gases from all cylinders to merge and exit more freely. This enhances the scavenging effect at higher RPM, making it easier for pulses to smoothly transition out of the header.
- Reduced interference among cylinders: A larger collector diameter reduces turbulent flow and gas congestion, providing a clearer path for each exhaust pulse, maintaining momentum at higher RPM.
- Enhances reflection strength at high RPM: At higher RPM, a larger collector ensures the reflection wave timing matches high-frequency exhaust pulses, enhancing scavenging and cylinder filling at high engine speeds.
You get the trend here—doing the opposite shifts the header tuning down to lower RPM.
The solution? Design a custom header with:
2.125 inch
Primary Tube Diameter
24-26 inch
Primary Tube Length
4.5 inch
Collector
Intake Design:
In short, the shorter the runners after the throttle body, the better the high RPM performance, you shift the powerband up. My intake manifold already has significant internal volume (in liters), but the runners are still quite long.
The ultimate solution here is to go with ITBs, individual throttle bodies per cylinder. However, I wont be doing that now, next year. But I suspect this will offer the best flow for my target RPM range.
