Stihl MS 500i Custom Porting (Dyno Test)
A ported MS 500i should be judged by repeatable, documented tests, not a peak-horsepower claim. The saw is a 79.2 cm³ fuel-injected professional model, so its controls are not tuned like a carbureted saw’s. Ask for a stock baseline, controlled-load results, temperature and RPM records, and a safe field check before deciding whether the porting delivers useful gains.
Could a bigger dyno number still leave you with a slower, hotter saw in real wood? That is the question to ask before paying for custom work. I assess a porting report by looking past its headline figure: the test setup, the saw’s condition, and whether the change improves cutting under a repeatable load.
What custom porting changes on the MS 500i
Custom porting reshapes or alters engine passages and related parts to change how the engine breathes. The aim may be more power or a different power curve, but results depend on the work and test method. On the MS 500i, that work must also account for electronic fuel injection.
The MS 500i has a 79.2 cm³ engine and electronically controlled fuel injection. It is not a conventional carburetor saw with user-adjustable high- and low-speed needles. Porting can change cylinder filling, exhaust behavior, heat load, and the engine’s operating range. That makes a simple “turn the mixture screws” approach the wrong frame.
A custom build may involve changes to the cylinder, piston, intake, exhaust, or other parts. The exact work varies by shop. Ask for a written description of what changed, what stayed stock, and whether the shop checked that the injection system and related controls work as intended.
A ported saw is not automatically faster in every cut. A result depends on wood, bar and chain, cutter sharpness, operator technique, and load. A longer bar can raise the load and kickback exposure; it does not create usable engine power by itself.
What a dyno result can and cannot show
A dynamometer measures engine output under a defined test. A credible report states what it measured, how it was measured, and what was held constant. A peak number alone cannot prove that a saw cuts faster, runs safely, or will stay reliable in field work.
Power is the rate at which work is done. Torque is turning force. Engine speed is measured in revolutions per minute, or rpm. A dyno may report power across an rpm range, but the test must say whether the saw was tested under load and how the result was calculated.
Do not compare two headline numbers unless the test conditions match. Fuel, temperature, exhaust setup, sensor location, correction method, and the condition of the saw can affect results. A written baseline from the same saw, on the same dyno, is more useful than an unsupported comparison to another machine.
Set up a safe, repeatable test
A useful test begins with a sound saw and a written plan. The baseline and modified runs should use the same fixture, measurement method, and setup wherever possible. A qualified shop should use a guarded engine-dyno fixture, exhaust extraction, suitable RPM and temperature measurement, and a record of the saw’s configuration.
Before testing, inspect the clutch, sprocket, flywheel, mounts, fuel system, and chain-brake function. Do not run an exposed clutch or rotating assembly. The shop should stop if it finds a defect, fuel leak, abnormal noise, or unstable readings rather than pushing on to collect a number.
Stock reference figures for the MS 500i include a maximum engine speed of 13,500 rpm and an idle speed of 2,800 rpm. These are reference specifications, not tuning targets for an altered saw. Confirm figures and service procedures against the manual for the specific saw and its parts.
| Test item | Record or verify | Why it matters |
|---|---|---|
| Saw identity and build | Model, serial number, parts changed, hours if known | Keeps baseline and modified results tied to one saw |
| Test setup | Dyno type, fixture, sensors, correction method | Makes the result easier to assess and repeat |
| Conditions | Fuel, temperature, warm-up, exhaust setup | Helps explain run-to-run changes |
| Output | Power and RPM across runs, not one peak alone | Shows whether the change is repeatable |
| Safety | Guarding, extraction, clutch and mount inspection | Reduces risk during high-speed testing |
The shop should state how many runs it made and show whether readings repeat. Ask it to report abnormal heat or unstable behavior, not just the best run. Dyno data does not establish safe field operation by itself.
Read the results without overpaying for a claim
A useful report compares stock and modified results from the same saw under controlled conditions. It should show the measured curve or a clear summary of repeated runs, along with the test method. If the shop gives only a single peak figure, treat it as incomplete evidence.
A larger peak may not mean better cutting across the engine’s working range. Ask where the output changed, whether the result repeated, and whether the shop checked temperature and engine behavior under load. The test does not need to promise a particular gain; it needs to make its findings clear and traceable.
| Report claim | What to ask next | How to judge the answer |
|---|---|---|
| “More horsepower” | Was the stock saw tested on the same dyno? | A matched baseline is stronger evidence |
| “Higher RPM” | What was measured, and under what load? | Do not treat a stock limit as a modified target |
| “Cuts faster” | What wood, chain, bar, and method were used? | A controlled field test should name these conditions |
| “Runs safely” | Were temperature and abnormal behavior monitored? | A peak number alone cannot confirm safe operation |
| “EFI was adjusted” | What was changed, and who validated it? | Ask for a clear explanation; avoid bypasses or guesswork |
When I review a shop’s claims, I look for traceability more than impressive wording. A report that shows repeatable runs and a clear configuration is more useful than a large number with no test details. If the shop cannot explain how its work interacts with the MS 500i’s fuel-injection controls, pause before authorizing changes.
Field validation and practical case studies
Field checks answer a different question from dyno tests: does the modified saw behave well in a controlled cut? A fair comparison uses the same wood type and size, bar and chain, sharpness, fuel, and operator method. Record the result and stop if the saw shows abnormal heat, loss of power, fuel leaks, or unusual noise.
These examples are test scenarios, not claims about measured gains from a specific saw. They show how to assess two common porting goals without inventing results. In each case, compare like with like and keep the test within the saw maker’s instructions and safe work practices.
Case 1: A shop claims faster cutting
Start with a sharp chain of the same specification on both runs. Use comparable wood and cut dimensions, then record cut time and engine behavior. Keep footing stable, avoid forcing the saw, and keep the bar nose clear. A single fast cut is not enough to establish a repeatable gain.
Case 2: A shop claims stronger performance under load
Ask the shop to document its loaded test conditions and temperature readings. In the field, use a controlled task suited to the operator and equipment. Compare how the saw responds without forcing the cut. If the result depends on an unusually long bar or an altered setup, ask whether that configuration is approved and supported.
For field felling, follow ANSI Z133 and applicable OSHA rules. OSHA logging rules generally set a danger zone of at least two tree lengths from trees being felled, subject to the applicable standard and site conditions. Use task-appropriate PPE, including helmet, eye and hearing protection, chainsaw leg protection, gloves, and protective footwear.
Setup, chain care, and service limits
The right bar and chain depend on the cut and operator control. The MS 500i reference pitch is 3/8 inch; gauge depends on the bar and chain, and .050 inch is common. Check the markings on the actual bar and chain. They must match each other and the sprocket.
Use the specified 50:1 gasoline-to-approved two-stroke-oil mixture. Do not use straight gasoline or stale or incorrectly mixed fuel. Check chain tension, sharpness, brake function, handles, bar condition, fuel, and chain oil before use. Set depth gauges to the chain maker’s specification with the matching tool; there is no one clearance that fits every chain.
Start on stable ground using the manual’s procedure. Warm the saw under light load, not by revving it unloaded. Use suitable bar-and-chain lubricant and verify oil delivery using a safe, manufacturer-approved check. Do not assume a particular oil flow rate or viscosity from the model name.
The MS 500i’s electronic fuel-injection system is not adjusted like a standard high- and low-needle carburetor. Do not defeat or bypass its controls. Use qualified STIHL service or a specialist familiar with this model. For fasteners and assemblies, follow the applicable STIHL service documentation; do not substitute generic torque values.
At shutdown, follow the manual, let the saw cool in a clear area, and inspect the bar, chain, and powerhead. Clean the filter, cooling-air inlets, bar groove, and oil passages as specified. Follow STIHL’s storage guidance for fuel and preservation, and store the saw dry and secured with the chain covered.
Rotational kickback can occur when the upper quadrant of the bar nose contacts wood or another object. Keep the nose clear, use a firm two-handed grip, avoid cutting above shoulder height, and maintain a working chain brake. The brake does not replace kickback avoidance.
Conclusion: decide from evidence, not peak numbers
Custom porting may change how an MS 500i performs, but a headline dyno figure cannot establish the size of the gain or whether the saw is suitable for your work. Ask for a same-saw baseline, repeatable measurements, a clear parts list, and a cautious field check. If the shop cannot document its method or explain how it handles fuel injection, do not rely on its claim.
FAQ
Does the MS 500i have a carburetor adjustment screw?
No. It uses electronic fuel injection, not a conventional high- and low-speed needle setup. Do not apply carburetor-tuning instructions or bypass its control system.
What stock RPM figures are useful for comparison?
Reference figures are 13,500 rpm maximum engine speed and 2,800 rpm idle speed. Confirm them in the manual for the specific saw. They are stock references, not targets for an altered saw.
How much horsepower does custom porting add?
There is no single verified gain that applies to every ported MS 500i. The result depends on the work and test conditions. Ask for repeatable before-and-after data from the same saw.
What should a porting dyno report include?
It should identify the saw and changes, describe the dyno and measurement method, state test conditions, and show repeated results. Temperature and RPM records help put the output claim in context.
Does a higher peak dyno number prove faster cutting?
No. Peak output alone does not show how the saw performs across its working range or in wood. A controlled field comparison should use the same chain, bar, wood, and method.
Can I tune the fuel injection myself after porting?
Do not assume it can be adjusted like a carburetor. Have a qualified STIHL service provider or experienced MS 500i specialist assess the system. Do not defeat or bypass safeguards.
What fuel mix should I use?
The stock reference is a 50:1 mixture of gasoline and approved two-stroke oil. Follow the manual for your saw, use fresh fuel, and never run straight gasoline.
Does a longer bar improve power?
No. A longer bar increases load and kickback exposure; it does not create engine power. Choose a compatible bar and chain suited to the cut, and verify pitch and gauge markings.
Is a dyno test enough to prove the saw is safe?
No. A dyno result does not establish safe field operation. Inspect the saw, use correct PPE, and stop if you notice abnormal heat, unusual noise, power loss, or fuel leakage.
What if the shop provides only a peak number?
Ask for the baseline, test conditions, repeated runs, and measurement method. If the shop cannot provide them, the claim is not enough to judge the porting’s value.
(This article was written by one of our staff writers, Elliott Fowler. Visit our Meet the Team page.)
