Modified V1 circuit pipe

Trying to get a better powerband out of an old V1 pipe. These pipes rip through the low and mid range, but once you hit about 8500 rpms, it'll just scream at you in agony as you push it to 10k+ trying to over rev a few more mph out of it.

The stock header pipe is 20mm ID all the way from the last U bend, so I cut it back, welded a 30mm ID U bend in, then a 26mm header pipe, and finally, a 21mm ID converging pipe at the flange to match my polini.

And it's a side bleed. (No baffle, because it's being used on a track bike.)

Hopefully this broadens the powerband and doesn't ruin it.

Haven't tested it yet, due to southern California's infamous Santa Ana winds kicking gusts up to 50mph. There's garbage cans blowing down my street faster than my moped, so I'll have to wait a bit.


Re: Modified V1 circuit pipe

Header pipe is also about 3" longer than stock.

And the big patch is where the old mount was. It was cracked there so I grinded off the mount and patched it.

Re: Modified V1 circuit pipe

I hope it works for you. I had a buddy side bleeding circuit pipes a while back. Not really sure how they performed tho. Please report back with details.

Re: Modified V1 circuit pipe

I have an identical stock end bleed circuit, and the side bleed gains 3mph. Not a lot, but every little bit helps when you're racing.

Re: Modified V1 circuit pipe

I ended up doing something similar. Put a 25mm header on a proma then side bleeding it. Hit about 10K on an E50.

Had a weird second hit near 9/9.5K. Pretty good little pipe, had a ton of midrange pull.

I just plugged the end bleed hole, I couldn't notice any changes in performance other than being louder with it open.

I put the side bleed portion near the end of the belly, I wonder if the location matters whole lot or not? I'd imagine it is similar anywhere on the belly of the pipe.


Re: Modified V1 circuit pipe

From everything I've read, the center bleed should be at or behind the center of the belly.

I tried having a tiny bleed hole at the end. I welded a bolt with the same threads as carb jets into the end, and tried 2mm and 2.5mm holes, but it worked best plugged completely.

Re: Modified V1 circuit pipe

Looks awesome! Hope it performs well.

Re: Modified V1 circuit pipe

if the header is 3" longer, you aren't going to really gain any more RPMs unless something else was wrong with the pipe prior.

max power rpm is a function of tuned length and exhaust duration. make the pipe longer, it will rev lower.

who knows tho?

Re: Modified V1 circuit pipe

I'm not looking for more rpms, just a wider powerband.

The stock proma hits hard and fast, I'm trying to mellow it out a bit to keep it hitting in the higher rpms.

Guess I could go test it right now. Be back in a few...

Re: Modified V1 circuit pipe

So, yeah, totally works.

Still has a lot of low end power to blast it off the line, and you can feel it kick in hard at about 7k.

But instead of a short burst, it keeps accelerating, and you can feel another kick just above 9k, where the stock proma punch is over with.

Quick run around the neighborhood hit 52mph at 9875 rpm on my race bike. It's got a little more, but my racer with no suspension is a handful on bumpy streets with cars around, so I didn't push it to it's limit.

The goal was to make a better short track pipe. The Apex kart track has a 650' straight, and the stock proma hits its peak at about 500'. I wanted a pipe specifically for that track, that still rips in and out of the tight corners.

And it worked!

The Grange track (which is now AVS, Apple Valley Speedway,) has a 950' straight, and a lot of races are won in the straight, which kinda sucks, because you can rip all over someone in the corners, and then they can catch up (or pass you) in the straight, and then you have to fight all over again to get ahead.

But I have a pipe for that, too...


Re: Modified V1 circuit pipe

Good job, what did u make the short looking pipe out of?

Re: Modified V1 circuit pipe

Feckin wild! I haven't really messed with promas much just because of the hard shut down, but very cool that you're pushing the limits.

Those are some crazy looking pipes!

Re: Modified V1 circuit pipe

Is the other one a big muff?

Re: Modified V1 circuit pipe

The other pipe is a metragas derby hacker whatever pipe with a header from some unknown pipe welded in. Hits hard in mid-range and carries the powerband up to max rpms. And centerbleeded it, which cut the noise down by half. Weird how that works.

The circuit header parts were the almost 90° section of the above metragas pipe, with a coupla pie cuts to bend it some more, and the last header part is off of some extra pipe I had laying around, maybe from a Sachs? The bends fit almost perfect.

The little converging pipe piece is from a 1" seat post where it tapers smaller to fit a seat clamp. And I cut my own flanges.

The stingers are just some pipe I got from the metal store. I forget the exact diameter.

The circuit is actually upside down, which puts the bleed pipe right under the engine, and exits under my foot. Hopefully it doesn't soak my shoe in oil, cuz I don't want to do a slippy slide getting off my bike. But I think it's pointed down enough... (edited)

Re: Modified V1 circuit pipe

Damn you!! Gonna make me buy a circuit for my trike!!

Re: Modified V1 circuit pipe

I keep telling you too. Probably be the perfect pipe for a heavy bike. And they're only $100 bucks at treats.

Re: Modified V1 circuit pipe

Satan, I have a garelli circuit I could sell ya. I want to get a different pipe for this bike... granted it is another circuit, but a different one.

Re: Modified V1 circuit pipe

I may take you up on that. I need to sell a pipe. I got an mlm people pipe I think?

Re: Modified V1 circuit pipe

Ha yes! The pipe shuffle two step. My favorite dance.

Fun fact, the circuit pipe I have is two piece. The baffle has a tube where the mount is welded and is slid on to the end of the stinger and held on by the frame mount.

Re: Modified V1 circuit pipe

So I sold my mlm pipe and got a circuit. I had a mamba circuit years ago and realy like the low power but the tin sound I wasn't super excited about. I'm sure this will be the same but the trike will surely like it. I got the v1ks running after a bit of fuss and put on a polini I had. I may mod my pipe to side bleed like this but I may just leave it alone as I'm not looking for major performance on the trike, haha

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