Max **SAFE** RPM for a stage 2 420

[LaithSaah]

I think the title says it all: What would be the safest theoretical redline for a stage 2 predator 420? (Bilet rod, cam, springs, intake, jet, flywheel). I read all over the place, and some people said they've gotten to 7k rpm with no problems, while others say anything above 6 will grenade the engine... I just wanna know what to set my little tacho to blink at. Thank you for all the anticipated help!

Thank you for your time!
Laith Saah

 

[Rat]

I think the title says it all: What would be the safest theoretical redline for a stage 2 predator 420? (Bilet rod, cam, springs, intake, jet, flywheel). I read all over the place, and some people said they've gotten to 7k rpm with no problems, while others say anything above 6 will grenade the engine... I just wanna know what to set my little tacho to blink at. Thank you for all the anticipated help!

Thank you for your time!
Laith Saah

AFAIK anything you see about RPM warnings is only relevant to primarily stock engines, most specifically stock flywheels which may or may not become high velocity shrapnel at as little as 4800 or as high as 6800.

With all the billet upgrades, I'm not entirely sure there's any such thing as a safe limit... let it rev until it can't rev higher type deal.

Just my 0.02, I'm sure Denny or someone else will have more reliable input

 

[Hellion]

Just how fast do you want to go? I think you should aim for your speed goals and work out the desired RPM for that.

 

[LaithSaah]

Just how fast do you want to go? I think you should aim for your speed goals and work out the desired RPM for that.

Wanna go 60, I will probably have a top speed gear for 60 and a typical play-around gear for around 45-50mph. I run split sprockets so it is easy to swap.

 

[panchothedog]

With the billet rod and flywheel you should be O K. If you installed stiffer valve springs to prevent valve float it should be able to rev until runs out of cam. The stock pot metal valves are a weak spot. They can break and ruin everything. For sustained high rpm use you should install stainless steel valves if you haven' t already. After that its a matter of gearing for top speed at maximum rpm.

 

[LaithSaah]

With the billet rod and flywheel you should be O K. If you installed stiffer valve springs to prevent valve float it should be able to rev until runs out of cam. The stock pot metal valves are a weak spot. They can break and ruin everything. For sustained high rpm use you should install stainless steel valves if you haven' t already. After that its a matter of gearing for top speed at maximum rpm.

I am a little tight on budget right now... Ill limit the RPM to like 5k while I save up for them. Thank you!

 

[vpd66]

The camshaft is the brain of the engine. Build the rest of the motor to hold up to the rpm range of the camshaft. I would be more worried about the engine failing from not being assembled right then part failure.

 

[BrownStainRacing]

The camshaft is the brain of the engine. Build the rest of the motor to hold up to the rpm range of the camshaft. I would be more worried about the engine failing from not being assembled right then part failure.

I agree 100% !!!!

IMHO, people are WAY too quick to give up on the stock cams.

They wanna spend that $ fast on useless crap they don't need, and not learn how to TUNE.

Alot of $ can be saved and MORE USEFUL power can be made...... in jus learning how to "TUNE" for the complete package.

This is the SMALLEST cam I have ever seen come STOCK in a small block.

I'm a ISKY RACING CAM fan.
This stock cam is not even close to being compared to any ISKY cam.

I TUNED the engine to what the cam WANTED,. .....not what the hottest selling crap on "the you tube told me I needed to do".

You might wanna gently remove your tampon, before you mash down the go fast pedal on this lil pi$$ed off STOCK camshaft.

It's all about the tune.

 

[Rat]

I agree 100% !!!!

IMHO, people are WAY too quick to give up on the stock cams.

They wanna spend that $ fast on useless crap they don't need, and not learn how to TUNE.

Alot of $ can be saved and MORE USEFUL power can be made...... in jus learning how to "TUNE" for the complete package.

This is the SMALLEST cam I have ever seen come STOCK in a small block.

I'm a ISKY RACING CAM fan.
This stock cam is not even close to being compared to any ISKY cam.

I TUNED the engine to what the cam WANTED,. .....not what the hottest selling crap on "the you tube told me I needed to do".

I mostly agree with you, but I also must still disagree.
Having pushed my stock cam as hard as I did, more duration and lift was the direction my 208 had to go.
It's a torque monster out if the box considering it's meant to be on a snow thrower and eating a 24" wide path through 18" deep snow, and I've only recently got it tuned enough to keep the nose on the ground, but still run like a kicked dog

 

[LaithSaah]

I agree 100% !!!!

IMHO, people are WAY too quick to give up on the stock cams.

They wanna spend that $ fast on useless crap they don't need, and not learn how to TUNE.

Alot of $ can be saved and MORE USEFUL power can be made...... in jus learning how to "TUNE" for the complete package.

This is the SMALLEST cam I have ever seen come STOCK in a small block.

I'm a ISKY RACING CAM fan.
This stock cam is not even close to being compared to any ISKY cam.

I TUNED the engine to what the cam WANTED,. .....not what the hottest selling crap on "the you tube told me I needed to do".

You might wanna gently remove your tampon, before you mash down the go fast pedal on this lil pi$$ed off STOCK camshaft.

It's all about the tune.

Tuning helps, but you gotta consider how limited tuning can be done on a stock predator carb... I can really only change the idle and main jets. I would love to get a genuine Mikuni carb at some point, but I definitely do not have the money right now.

 

[Rat]

Tuning helps, but you gotta consider how limited tuning can be done on a stock predator carb... I can really only change the idle and main jets. I would love to get a genuine Mikuni carb at some point, but I definitely do not have the money right now.

Better than a stock carb is a Hauyi version (basically a stock clone with a dedicated fuel shut off cast into the body, and an A/F adjustment screw)

I prefer Keihin to Mikuni personally, so I've got a Koso 24mm PWK (the best Keihin copy out IMHO, way better than OKO) on a custom fabricated intake to hold it on

 

[LaithSaah]

Better than a stock carb is a Hauyi version (basically a stock clone with a dedicated fuel shut off cast into the body, and an A/F adjustment screw)

I prefer Keihin to Mikuni personally, so I've got a Koso 24mm PWK (the best Keihin copy out IMHO, way better than OKO) on a custom fabricated intake to hold it on

Im not that crafty, I will sell some parts and scrap some money together and buy it right the first time... may take some time but oh well.

 

[Rat]

Im not that crafty, I will sell some parts and scrap some money together and buy it right the first time... may take some time but oh well.

I make what I need, most often a bracket or some other crap that cannot be bought.
I follow the true Ratrod tradition of "built not bought" quite closely to the letter.

For where my engine sits on my "Mini Chopper" in orientation where I sit, the intake can't just sit in a shorty stub straight off the engine... I'd knock the damn thing off before I got the engine started. I needed an intake that would point the carb parallel to the head and can be leveled by a boot connector... if you can find such a thing being sold THAT ACTUALLY IS what I needed, you have far more free time on your hands than I do and I have a lot.

If I buy anything, it's materials or something I legitimately cannot fabricate.

I will cut up and welding together anything free or cheap, and made of steel to get what I need made done. Car parts, motorcycle parts, bicycle parts, or lawn mower parts, angle iron bed frame rails... I absolutely DO NOT care as long the functional purposes get served.

Hell I even carved up a beat up old cookie sheet to make blockoff panels for my shroud when I bailed on the stock carb to keep the airflow directed around the front of the cylinder properly.

 

[BrownStainRacing]

Tuning helps, but you gotta consider how limited tuning can be done on a stock predator carb... I can really only change the idle and main jets. I would love to get a genuine Mikuni carb at some point, but I definitely do not have the money right now.

Dont give up on the stock carb too quick.

There's more to tuning then jus the carb.

Ignition timing
Compression ratio
Coil air gap
Valve lash
Valve spring installed height
Exhaust pipe, i.d.and length
Muffler, i.d. and length
Cam and crank endplay
Piston to deck height
Piston to cylinder clearance
Ring end gaps
Cylinder head temp
Sparkplug heat range and type (projected tip or non-projected tip)

These are all part of the "complete tune".

Get em right, and you will be surprised how much better a stock engine will perform in the same rpm range.

Remember, these are utility engines designed to run for hrs and hrs at a constant rpm.

We have to TUNE them to run and be reliable for recreational fun vehicles

IMHO, there's no need to kill a big block with high rpm, they are low rpm thumpers and can easily handle 4200 rpm (gov'd) with the right tune.

Good luck with your build,