Topic: 18HP Opposed Twin Performance Upgrade Path

[Scott P.]

So I work for an auto performance shop and for one of our company parties we all decided to build and race some lawnmowers. So I bought a MTD rider that is equipped with a Briggs and Stratton 18hp opposed twin motor, the model number is 422707. It is completely stock at the moment. What is the typical upgrade path to make this thing fast on a budget?

Here are some things I am for sure planning on doing:
-Remove governor, install throttle pedal
-Deck cylinder heads
-Build free flowing exhaust

Any info would be much appreciated.

Also, I already emailed Don about getting one of those DVDs.

[Jeff McKelroy]

So I work for an auto performance shop and for one of our company parties we all decided to build and race some lawnmowers. So I bought a MTD rider that is equipped with a Briggs and Stratton 18hp opposed twin motor, the model number is 422707. It is completely stock at the moment. What is the typical upgrade path to make this thing fast on a budget?

Here are some things I am for sure planning on doing:
-Remove governor, install throttle pedal
-Deck cylinder heads
-Build free flowing exhaust

Any info would be much appreciated.

Also, I already emailed Don about getting one of those DVDs.

What is your budget?
Here are some initial thoughts:

1. DO NOT deck the heads.  Flatheads need a large transfer port cross section.
2. The 3-screw carburetor will probably be fine for what you are doing, it has the bigger bore compared to the 4-screw.
3. Try to find a used aluminum flywheel, I'd hate to see you get hurt.
4. Lose the factory brake setup.  Spend the money and get a decent rear brake disc setup.
5. See if you can find a junk 46-cube, and rob the heads and camshaft.  The heads will say 'MOD 46', and the 46 cam will also have a code with '---46' stamped on it.
6. Make a header that will scavenge well, a 2 into 1 style.

There are more safety modifications you should do such as steering and front end modifications as well.

[Scott P.]

Ok, I wont deck the heads. That seemed to be a general theme on alot of sites as a way to gain HP..

I am not looking to spend more than a couple hundred bucks.

From what I have heard it seems the 1st thing that usually goes on these motors is the rods?

Can you point me in the direction of a good rear brake setup?

The racing I am going to be doing is nothing serious it will just be some oval track racing and a little drag racing. Its all in fun more or less.

What is your budget?
Here are some initial thoughts:

1. DO NOT deck the heads.  Flatheads need a large transfer port cross section.
2. The 3-screw carburetor will probably be fine for what you are doing, it has the bigger bore compared to the 4-screw.
3. Try to find a used aluminum flywheel, I'd hate to see you get hurt.
4. Lose the factory brake setup.  Spend the money and get a decent rear brake disc setup.
5. See if you can find a junk 46-cube, and rob the heads and camshaft.  The heads will say 'MOD 46', and the 46 cam will also have a code with '---46' stamped on it.
6. Make a header that will scavenge well, a 2 into 1 style.

There are more safety modifications you should do such as steering and front end modifications as well.

[Zippy 13N]

Listen to Jeff on the heads flat heads need flow for power not compression!!!  And if you could find the 46 cubic inch parts (crank, rods, heads and cam) Jeff was tlking about it would get you a good start.  (Find model number that starts with "46")

Send birdman_express a message on here and he will help you out with brakes. And I assume you are runnin a transaxle so getting the right set up will be a little more tricky but he will know what to get ya.

Now since you have brakes you can start on performance. First off you will hear it preached on here about the aluminum flywheels. Yes we all know they are "expensive" if you only look at price but expensive can be determined by how well the product works and if you get what you pay for.  Let me be the first to tell you that they are not "expensive" once you buy it you will not regret it! And a stock flywheel with no govenor is a safety consern.

Other than a flywheel and brakes for safety you could port the intake, intake and exhaust ports, and like you said a good flowing exhaust BUT yes like Jeff said you need a 2 into 1 collector set up.

O and search oiling mods on here.  You need those definatly with stock rods!

Goodluck!!!

[Rooster]

I am not looking to spend more than a couple hundred bucks.

You cannot safely modify that engine, for a couple hundred.
Unless you know how to delete the flywheel.

DO NOT TURN THAT FLYWHEEL OVER 4000RPMS

I personally have magnafluxed several of the opposed twin flywheels, and they are just simply dangerous!
I have also personally seen the aftermath of one blowing up - it is ugly!

you are taking your life and everyone's around you into your own hands!

[Scott P.]

Ok well knowing that I think I could possibly raise the budget up a bit more.

What else aside from the flywheel should I do to go real fast?

This the one I need? http://www.arcracing.com/servlet/the-121/Billet-Flywheel-opposed-twin/Detail
or is there another option out there?

You cannot safely modify that engine, for a couple hundred.
Unless you know how to delete the flywheel.

DO NOT TURN THAT FLYWHEEL OVER 4000RPMS

I personally have magnafluxed several of the opposed twin flywheels, and they are just simply dangerous!
I have also personally seen the aftermath of one blowing up - it is ugly!

you are taking your life and everyone's around you into your own hands!

[westcoast racer]

Do not remove the govenor.  Modify the spring so it will let the RPMs go to 4000. 

You can do a lot with gearing. Depends on how big of a track or how long of a drag strip you have.

Port and polish the intake manifold, intake and exhaust ports.  Get a velocity stack and make the exhaust a 2 into 1.

Must change the brakes and steering!

[Jeff McKelroy]

Believe it or not buddy, you will go faster with good brakes.  Spend your money on brakes and flywheel.

[Rooster]

Ok well knowing that I think I could possibly raise the budget up a bit more.

What else aside from the flywheel should I do to go real fast?

This the one I need? http://www.arcracing.com/servlet/the-121/Billet-Flywheel-opposed-twin/Detail
or is there another option out there?

Yup that is the one.
Then as everyone has said, upgrade brakes and steering!
After that, to go REAL fast, you need to get ahold of Tim Isky at www.smallenginecams.com 
Port, polish, etc etc

[Scott P.]

Can you guys give me  some leads on upgrading the brakes and steering? Does anyone make anything or is there a DIY out there??

This is what I have gathered so far,

< 4000 RPM, oiling mods are a good idea
> 4000 RPM, flywheel and oiling mods are a must
> 5000 RPM, flywheel, oiling mods, and pistons/rods are a must

Sound about right?

-Scott

Yup that is the one.
Then as everyone has said, upgrade brakes and steering!
After that, to go REAL fast, you need to get ahold of Tim Isky at www.smallenginecams.com  
Port, polish, etc etc

[Rooster]

Can you guys give me  some leads on upgrading the brakes and steering? Does anyone make anything or is there a DIY out there??

This is what I have gathered so far,

< 4000 RPM, oiling mods are a good idea
> 4000 RPM, flywheel and oiling mods are a must
> 5000 ROM, flywheel, oiling mods, and pistons/rods are a must

Sound about right?

-Scott

[Scott P.]

Just watched Don's video, Opposed 101. Good stuff!!

One question where should I relocate the pulse port to for the fuel pump to get a dry signal?

[Jeff McKelroy]

On the cast iron manifold, you will see some flat bosses that are perfect for adding an 1/8" NPT Fitting, between the carburetor and intake port.

[AZCOWBOY]

Yup that is the one.
Then as everyone has said, upgrade brakes and steering!
After that, to go REAL fast, you need to get ahold of Tim Isky at www.smallenginecams.com 
Port, polish, etc etc

Rooster,
Do you know if this link is still good to get a hold of Tim?
I called the # on the site and it just keep ringing...
Thanks

[redline]

Not Rooster, but yes, that is still a good contact for Tim. I know he is very busy, keep trying, and maybe send him an email also.