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4 stroke gas motor straight pipe questions


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#1 Greasy6020 OFFLINE  

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Posted May 03, 2016 - 10:02 PM

I have a Sears tractor that I cannot find any info on and I was told it was only worth scrap price in current condition.

It has the 12 hp Briggs vertical shaft motor typical of Sears lawn tractors. I want to put duals on it and cut off the muffler to add on dual stacks.

I've heard straight pipe on gas motors will burn out the valves, or cause other damage...

Yet I see all kinds of straight pipe gasser pickup trucks.

Any experience with this idea? Any caution? Any advice?

I will be parting out this tractor once something on it breaks, for now it's gonna be for having some fun...

Edited by Greasy6020, May 03, 2016 - 10:02 PM.


#2 toppop52 OFFLINE  

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Posted May 03, 2016 - 10:07 PM

Straight pipes aren't the problem with burnt valves, might cause exhaust gas velocity issues, but won't hurt the valves if long enough. The problem occurs when straight pipes are too short and the natural reversion caused by overlap, draws cold air back across the hot valve.
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#3 Greasy6020 OFFLINE  

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Posted May 03, 2016 - 10:16 PM

Straight pipes aren't the problem with burnt valves, might cause exhaust gas velocity issues, but won't hurt the valves if long enough. The problem occurs when straight pipes are too short and the natural reversion caused by overlap, draws cold air back across the hot valve.


Ok, I wanted stacks that extend something like 24" above hood so that shouldn't be an issue...
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#4 toppop52 OFFLINE  

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Posted May 03, 2016 - 10:30 PM

As long as they don't leak near the port and you keep the water out, you'll be fine as long as your ears can take the racket. LOL

#5 crittersf1 OFFLINE  

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Posted May 04, 2016 - 08:16 AM

Straight pipes, no matter what they are on, are nothing but obnoxious.


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#6 shorty ONLINE  

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Posted May 04, 2016 - 09:00 AM

Ok, I wanted stacks that extend something like 24" above hood so that shouldn't be an issue...

You might want to go a bit higher than 24" to get it above ear level. I had a 32" riser with my Kohler. It sure made a nice sound under load.

#7 Greasy6020 OFFLINE  

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Posted May 04, 2016 - 09:38 AM

Ok, thanks for all the opinions!

#8 Greasy6020 OFFLINE  

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Posted May 04, 2016 - 10:24 AM

As long as they don't leak near the port and you keep the water out, you'll be fine as long as your ears can take the racket. LOL


sure I can take the racket... I'll probably put a drain hole near the bottom of the bend of the pipe

#9 LilysDad OFFLINE  

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Posted May 04, 2016 - 10:46 AM


I will be parting out this tractor once something on it breaks, for now it's gonna be for having some fun...

Don't you have any movie shows in your village? The polar bears have enough issues as it is.



#10 Greasy6020 OFFLINE  

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Posted May 04, 2016 - 11:00 AM

Don't you have any movie shows in your village? The polar bears have enough issues as it is.


I'd go but I don't feel like getting stabbed... Theatre is very close to a rough part of town, lots of people with addiction type problems.

I hope the polar bear thing is a joke. I live up north, but not that far north! I don't live in an igloo either, I cut my grass by may long, I can grow corn and eat it too... When we milk the cows we don't get icecream either lol...
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#11 LilysDad OFFLINE  

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Posted May 04, 2016 - 11:03 AM

I was judging by your location info.



#12 Greasy6020 OFFLINE  

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Posted May 04, 2016 - 11:10 AM

I was judging by your location info.


Ok, its all good.

#13 toomanytoys84 OFFLINE  

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Posted May 04, 2016 - 05:09 PM

Straight pipes aren't the problem with burnt valves, might cause exhaust gas velocity issues, but won't hurt the valves if long enough. The problem occurs when straight pipes are too short and the natural reversion caused by overlap, draws cold air back across the hot valve.


Doesn't natural reversion occur when you are on the throttle and off the throttle?

If a tractor is running at a constant RPM wouldn't this make it a non issue?
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#14 toppop52 OFFLINE  

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Posted May 04, 2016 - 07:12 PM

Depends on cam overlap, but in a small stock engine steady throttle shouldn't be an issue, but you have to start and stop it. Either way, With 24"+ pipes it wouldn't matter. I've seen automotive engines damage a valve running with short manifolds and no pipes. The more efficient the exhaust system, the quicker the the cylinders scavenge, and the less problem reversion is.

Edited by toppop52, May 04, 2016 - 07:16 PM.

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