Don Terrill’s Speed-Talk

Here's a question about weirdly restricted racing class.

The starting point is a 600cc four stroke four cylinder engine from GSXR 600 which makes about 106hp @ 13100 rpm. The rules in this restricted racing class are 9300 rpm redline, no changes whatsoever (other than factory manual repairs) allowed for the long block (including stock cams, no porting, stock compression, etc.), intake manifold can be modified but has to start from stock casting, free airbox and filter, free exhaust.

How to make the most power in the 7000-9300rpm range with an engine that is factory cammed for 13100 rpm peak power?!? The expectation is that one should get about 90hp out of the engine at 9300 rpm without violating the rules.

The ideas so far:
- Fabricate an exhaust with diameters sized for 90hp and lengths suited for 9300 rpm redline. (First version is up and running.)
- Fill in the intake manifold close to head to reduce the MCSA to be better sized for 90hp.
- Add longer trumpets inside the airbox to tune the intake to 9300 rpm redline.
- Tune the a rigid air box volume to create a four-cylinder Helmholtz resonator tuned to 9300 rpm redline.
- Add valve lash to reduce camshaft duration. It's ok if the cams wear out a little quicker...

Any expriences with projects like this? Anything else to consider?

What is the RPM range in use? You might consider tuning the int/ex for closer to the middle of the racing RPM range to get more power where it is needed on the track.

The rpm range is 7000-9300 rpm, with the limiter at 9300 rpm. The issue is that cams are from an engine specked for 13100 rpm peak power and that the cams can’t be changed or modified by the rules.

Another way to ask the question might be how to add mid-range torque to a way, way overcammed four-valve engine?

Can you change the cam timing?

SchmidtMotorWorks wrote: ↑Fri Mar 19, 2021 3:16 pm Can you change the cam timing?

The rule in general is that everything in the long block needs to be consistent with factory service manual repairs. It’s likely the case that both intake and exhaust cam sprockets can be “worn” a bit to be somewhat oval to adjust the overlap and the drive sprocket bolt jokes can wear as well. And chains do stretch. So a couple of degrees should be doable.

The maximum valve lash in the service manual is 0.20mm intake 0.30mm intake.

The valve seat angle has to be 45 degrees but I am guessing one could widen the valve seats and sink the valves a little within the factory spec.

Two things come to mind instantly, apart from those already mentioned.

1: Port filling plates/tongues. It's not porting if it comes off with the manifold. Same with the exhaust side.

2: An assembly error may be able to get cam timing a lot closer to where you want it.

Just a thought no experience with it seeing how everyone tries to lighten all there parts for more rpm ,can you run a heavier parts of the same type like pistons and rods valves etc,etc to work with the camshaft you have.Just a thought?

1972ho wrote: ↑Fri Mar 19, 2021 4:01 pm Just a thought no experience with it seeing how everyone tries to lighten all there parts for more rpm ,can you run a heavier parts of the same type like pistons and rods valves etc,etc to work with the camshaft you have.Just a thought?

I’m not sure that would work, and the rules explicitly state that rotating and reciprocating engine parts have to be stock weight.

Advance the intake cam.
Also can you run two exhaust cams?
I know on some of these engines the camshaft journals and such are the same and the dwarf dirt cars and 600 sprints and lightning sprint dudes liked to replace the exhaust cams with stock intake cams because they usually had similar duration but more lift.

Maybe
You want to use two exhaust cams?

englertracing wrote: ↑Fri Mar 19, 2021 4:11 pm Also can you run two exhaust cams?
I know on some of these engines the camshaft journals and such are the same and the dwarf dirt cars and 600 sprints and lightning sprint dudes liked to replace the exhaust cams with stock intake cams because they usually had similar duration but more lift.

Maybe you want to use two exhaust cams?

That would be cheating but I'm not ruling it out!

ptuomov wrote: ↑Fri Mar 19, 2021 4:13 pm

englertracing wrote: ↑Fri Mar 19, 2021 4:11 pm Also can you run two exhaust cams?
I know on some of these engines the camshaft journals and such are the same and the dwarf dirt cars and 600 sprints and lightning sprint dudes liked to replace the exhaust cams with stock intake cams because they usually had similar duration but more lift.

Maybe you want to use two exhaust cams?

That would be cheating but I'm not ruling it out!

But is it really cheating? It's stock....

englertracing wrote: ↑Fri Mar 19, 2021 4:24 pm

ptuomov wrote: ↑Fri Mar 19, 2021 4:13 pm

englertracing wrote: ↑Fri Mar 19, 2021 4:11 pm Also can you run two exhaust cams?
I know on some of these engines the camshaft journals and such are the same and the dwarf dirt cars and 600 sprints and lightning sprint dudes liked to replace the exhaust cams with stock intake cams because they usually had similar duration but more lift.

Maybe you want to use two exhaust cams?

That would be cheating but I'm not ruling it out!

But is it really cheating? It's stock....

Sorry Mr Scruitineer, I just got so confused with all these engine parts lying around

Different years came with different cams.
Will they be degreeing you? Or looking for Suzuki part numbers and checking lift?

englertracing wrote: ↑Fri Mar 19, 2021 4:59 pm Different years came with different cams.
Will they be degreeing you? Or looking for Suzuki part numbers and checking lift?

Only if you start winning. The driver is good so expect scrutiny.