## Exhaust Chamber Calculator

I'm looking for feedback and numbers checking on my exhaust chamber calculator. I made this a while ago but I've learned a lot since then and modified some of my formulas. This is based off of Jennings book.

There isn't currently an option for a divergent header, but I may work on that in the future.

Make sure you either download a copy or save a copy to your google drive. I don't want it editable so it won't get messed with and I don't have the experience to make a java or other web-based calculator.

## Re: Exhaust Chamber Calculator

Jake's back baby!

## Re: Exhaust Chamber Calculator

So what're your rpms like with calc'd vs experienced?

## Re: Exhaust Chamber Calculator

I haven't compared this to a bike, it's just theoretical values based off Jennings calculations. Right now I'm just looking for fact checking on my formulas and formatting suggestions.

## Re: Exhaust Chamber Calculator

ohhhh hell yea. this is mopeds

## Re: Exhaust Chamber Calculator

Anyone have time to double check my formulas? Any feedback on the format or improvements? Bueller?

## Re: Exhaust Chamber Calculator

i checked it out and to my highly skilled eye the drawing is definitely an expansion chamber.

## Re: Exhaust Chamber Calculator

The tuned length formula from that book is known for producing pipes that are too long. For most small engines with a limited BDT, the speed of sound is different too. Best to use 1670 ft/sec.

You should eliminate the tuned length in the middle of the baffle cone and change it to the tuned length from the piston face to the to the stinger (or the end of the rear (baffle) cone.

Also, you need approximately 20 degrees of blow down time, for a tuned pipe to actually work. Many small engines have bdt as low as 12 to 14 degrees.

BDT is the distance in degrees from where the exhaust port opens to where the transfer port opens.

If you have less than 19 degrees of BDT, you need a different type of a pipe.

For best performance, if you know where peak torque rpm is, and then use that rpm for your pipes tuned length. Best HP on almost any engine is where peak HP is at the same rpm as peak Torque. When you match those two peaks, a pipe is capable of producing 48 to 50% more HP than an engine with no pipe.

An engine without a pipe will make more low to mid HP than a engine with a pipe. Be careful with your head pipe diameters and front cone angles. Bigger is rarely better.

Bill M.

## Re: Exhaust Chamber Calculator

Jake is this intended for a single speed bike, variator, or shifter? The purpose will influence the design.

## Re: Exhaust Chamber Calculator

Bill and Rob,

This is simply based off Jenning's calculations in his book. The angles of the diffuser and baffle cones can be modified to suit how wide or narrow of a powerband you want and where it will peak.

Blowdown timing is not factored in to these equations, Bill. Most 70cc moped cylinders have 20+ degrees of blowdown though. I see you are located in Riverside and a quick google search shows that you have many, many years of experience in two strokes. I would love to get together sometime and chat about engines. Maybe see your mini dynos if you still have them?

## Re: Exhaust Chamber Calculator

Seems pretty close on some of the older pipes I have

## Re: Exhaust Chamber Calculator

Jake you need to read GP Blair's Basic Design of two stroke engines. It can be found with a search as a pdf. Jenning's used Blair's SAE papers without using them as a reference.

You need to read and reread chapter 2 until you understand it. That said, the basic pipe design in 2 Stroke Tuner's Handbook will still get you close to a great pipe, so long as you change the tuned length to what I suggested. The design is missing tapered head pipes though.

If you are interested in SAE papers, you can find them on microfiche or in books at UC & Cal State university libraries. They are all open to the public, but you can only copy, not check out.

My dyno is all dusty in my old garage. But this site shows me there may still be a market that I can apply it too. It was built for dyno testing giant scale pylon racers with model airplane engines the size of motocross bikes (60cc to 290cc), running 25% nitro.

At the same time I started ROC Racing Products and used the dyno to build 3 great Goped pipes. 2 of my pipes are in the pictures , my Snake Pipe (torque) on the Go-Quad & blue goped, and my Stinger (top end) pipe on the gopeds. The Stinger actually accelerated faster than the torque pipe and made 49% more HP than the stock baseline.

My experience with small engines started with Ital-Jet & Vespa scooters and mopeds. But because of California's emission standards the Vespa scooters went away, and all mopeds, 2-stroke motorcycles became 50cc. My friends all had MB5's, some had AR50's, and I have worked on all the Yamaha, Honda 2-stroke scooters. Many of these engines had very low BDTimes. But the majority is in watercraft (Jet Skis & SeaDoo) racing. I specialize in tuning, designing pipes, diagnose and repair of engine failures, but I don't consider myself as a engine builder.

Send me a PM if you want to meet.

Bill M. (edited)

## Re: Exhaust Chamber Calculator

The GoQuad has my torque pipe. I called the Snake Pipe, the owner lived near San Francisco, CA. The blue Goped has a Snake pipe, and lived in Australia. The Goped

in the bottom picture has my top end pipe (Stinger) and lived in Virginia Beach, VA.

Bill M.

## Re: Exhaust Chamber Calculator

how do gopeds rev so high

## Re: Exhaust Chamber Calculator

So what sort of difference in power would be found between two different motors tuned for the same peak rpm, but with different exhaust timings? Like in your example, with 178' timing and 10k rpm peak, the resulting tuned length is 40.5". With 144' timing and the same 10k peak, we get 34.7" pipe length.

Would I be correct in assuming that the 144' timing motor would make more broad power, but less Max power? Making up numbers but, like 4hp from 5k to 11k and and peak at 6hp? While the big timing 178' motor would be much peakier, making 2hp til 8k rpm but then peak power of 10hp at our 10k rpm peak.

I know the blowdown time has a huge impact on this. Like if the blowdown time was matched between the two motors, would the lower timing 144' cylinder be just as peaky? let's say our random blowdown number is 27, we'd only have 90' transfer timing on our low timing cylinder, but on our big timing 178' cylinder we'd have 124' transfer timing. Wheras in more real life practice, that 144' cylinder would probably have a similar transfer timing and much less blowdown as a result. So the broader power is more a function of how much blowdown the cylinder has?

This kinda gets off topic of exhaust design and more into the ports and how they play with exhaust

## Re: Exhaust Chamber Calculator

In my experience (for Reeded cyls)

Pipe has the most to do with where you hit and how hard. I've taken lower times cyls and put high rpm pipes on them and they run great. Put low pipes in higher times cyls and they run great too. I'm so sick of hearing match the pipe to the cyl. Fuck that. Unless you have chopped many many pipes and rebuilt them for different characteristics, y'all just spouting what you've read. 50-70cc is all in the pipe.

Honestly unless you get to crazy blowdown town (28+) timings really don't matter that much. At that point the band is so narrow that you're just trying to find/decide where it hits and have a pipe that doesn't kill all lowend for your application.

What matters more (if you're going for big power) is getting as much mix to the top as you can in the jug, and having the ex port as big as you can, though not so big where it fucks up the rings (65%-70% of bore is fine)

#1 mistake people make is raising the ex port too high making the blowdown shitty. Especially on a one speed. Better to run a high rpm pipe on a lower timed cyl and focus on getting tons of fuel mix to the top.

I've chopped up so many pipes to change their attributes. I do find that a sweet spot for tuned length for 50-70cc is about 36inches to about 27 inches. Much longer or shorter starts to give diminishing returns.

## Re: Exhaust Chamber Calculator

> pat splat Wrote:

> -------------------------------------------------------

> how do gopeds rev so high

There are many reasons, but the Goped transfer ports are large enough to support high rpm.

Bill M.

## Re: Exhaust Chamber Calculator

Lets give our comments some boundaries. Lets say we are talking about Mopeds, Gopeds, and light duty motorcycles. Not MX bikes or pocket bike.

> jordan * Wrote:

> -------------------------------------------------------

> So what sort of difference in power would be found between two different

> motors tuned for the same peak rpm, but with different exhaust timings?

> Like in your example, with 178' timing and 10k rpm peak, the resulting

> tuned length is 40.5". With 144' timing and the same 10k peak, we get

> 34.7" pipe length.

Yes, the exhaust length would be the same, but the load each engine could carry would be different The port dimensions of most 144' engines (that I have experience with) will prevent a moderate BDT for a pipe, in favor of a longer power stroke and transfer ports that support a low rpm peak torque.

Depending on the engine size and load, the 178' engine has the potential of being a poor performer because too much of the power stroke was cut out of it. In this case increased BDT doesn't imply more performance.

A compromise for a moped sized engine would be 166 ex dur, 83' EO, 60' TO, 23' BDT.

High HP% improvements come from stacking peak HP on top of peak Torque at the highest rpm possible. This statement implies that there is only a very narrow range of TL that is capable of HP increases in the 45 to 50% range. One pipe can do it all!!

> Would I be correct in assuming that the 144' timing motor would make

> more broad power, but less Max power?

Yes. Less peak hp because of transfer port and rpm restrictions. The type of power depends the design of the pipe and cylinder. But it doesn't really have anything to do with front and rear cone angles. Unless it is just a poor design.

Broad power for a 144' engine with a low BDT can be achieved by good pipe design. Good examples are both 50cc Honda & Yamaha scooter mufflers. Good scavenging mufflers will produce 25 to 30% more HP than an engine with no pipe.

> I know the blowdown time has a huge impact on this. Like if the blowdown

> time was matched between the two motors, would the lower timing 144'

> cylinder be just as peaky? let's say our random blowdown number is 27,

> we'd only have 90' transfer timing on our low timing cylinder, but on

> our big timing 178' cylinder we'd have 124' transfer timing. Wheras in

> more real life practice, that 144' cylinder would probably have a

> similar transfer timing and much less blowdown as a result.

>So the broader power is more a function of how much blowdown the cylinder has?

No.

Keep in mind that the idea of a peaky powerband is different when you compare a dyno graph to a test ride. A dyno graph may show peak HP to only last 500 rpm (compared to peak HP 2500 rpm wide). It is still possible for the dyno graph (with a narrow 500 rpm peak HP) to still have great acceleration, a broad powerband and great peak HP and good power after the peak.

But when we ride a goped/moped, we may make a change to the pipe or engine and discover it has a hard hitting powerband and conclude that we made a good change. So we may also consider a hard hitting powerband to have peaky HP, and that is a good description. The problem is hard hitting powerbands do not indicate an increase in peak HP. (I wish I had bold print for the previous sentence).

Instead it is the tell-tale sign of horsepower that was lost before peak HP occurs. The more HP lost in the low to mid range, the harder the engine hits the pipe.

Bill M. (edited)

## Re: Exhaust Chamber Calculator

I used to think that raising the BDT on an engine always resulted in a peaky or light-switch powerband until a few years ago. For a long time I directly saw that correlation in Kawasaki 650, 750, 800 twin jetski engines. Recently (4 or 5 yrs ago) we received a ported 800 from a top engine builder (34' BDT), and I noticed they raised the transfer ports. But they didn't do a good job, they just increased the angles of T1 and T2 greater than 25 degrees.

So we flattened out the transfer ports, and ran stock dual 40mm carbs instead of the more common dual 44, 46, 48mm carbs. This engine has a great, broad powerband, and more top end speed than other race boats.

It turns out Group K ports their cylinders to the same specs and 34' BDT as the DASA cylinder before we re-ported the transfers. The Group K JetSki engine had the same port specs, carburetion, and exhaust pipe. But it had a horrible hard hitting (Peaky) power band (with nearly the exact same set-up), and less peak HP.

The difference was the transfer port angles that we changed, gave the first engine's transfer ports an increased effective time area over the Group K porting.

On other smaller displacement engines, when you raise the exhaust port, you expect to make more peak HP. Because torque times rpm = HP, so if you increase the rpm, you naturally expect more HP. But in most cases the transfer ports are already maxed out, and when when you raise just the exhaust port and extend the rpm range, you do not gain any significant peak HP. The transfer ports were already maxed out, so when you extend the rpm, the torque curve is falling off fast. So with

increased rpm and less torque = poor results.

I recommend that you test your modifications on a 50 yard time trial. Measure your elapsed time, calculate, mph, and with a little physics you may even calculate HP like Drag Racers can. More than 100 yards I think would be pointless.

After a while you could build an Arduino weather station and apply the weather conditions to estimate how much HP you are loosing that day.

Remember HP equals speed, not increased rpm.

Bill M.

## Re: Exhaust Chamber Calculator

Apparently ran out of time while I was editing my previous post. This is the finished post.

I used to think that raising the BDT on an engine always resulted in a peaky or light-switch powerband until a few years ago. For a long time I directly saw a correlation in Kawasaki 650, 750, 800 twin jetski engines. Recently (4 or 5 yrs ago) we received a ported 800 from (DASA) a top engine builder (34' BDT), and I noticed they raised the transfer ports. But they didn't do a good job, they just increased the angles of T1 and T2 greater than 25 degrees.

So we flattened out the transfer ports, and ran stock dual 40mm carbs instead of the more common dual 44, 46, 48mm carbs. This engine has a great, broad powerband, and more top end speed than other race boats.

It turns out Group K ports their cylinders to the same specs and 34' BDT as the DASA cylinder before we re-ported the transfers. The Group K JetSki engine had the same port specs, carburetion, and exhaust pipe. But it had a horrible hard hitting (Peaky) power band (with nearly the exact same set-up), and less peak HP.

The difference was the transfer port angles that we changed, it gave the first engine's transfer ports an increased effective time area over the Group K, and DASA porting.

===================================================================

On other smaller displacement engines, when you raise the exhaust port, you expect to make more peak HP. Because torque times rpm = HP, so if you increase the rpm, you naturally expect more HP. But in most cases the transfer ports are already maxed out because peak torque occurred earlier in the rpm range, and when when you raise just the exhaust port and extend the rpm range, you do not gain any significant peak HP. The transfer ports were already maxed out, so when you extend the rpm, the torque curve is falling off fast. So with increased rpm and less torque = poor results.

The exception is engines with low BDT of 10 to 18 degrees. On these engines raising the exhaust to only 21 to 23 deg BDT, is a good mod, because the transfer ports are large enough to support increased rpm. Increasing BDT to higher #'s than 23' is not recommended because you are just reducing the power stroke.

====================================================================

I recommend that you test your modifications on a 50 yard time trial. Measure your elapsed time, calculate, mph, and with a little physics you may even calculate HP like Drag Racers can. More than 100 yards I think would be pointless.

After a while you could build an Arduino weather station and apply the weather conditions to estimate how much HP you are loosing that day.

Remember HP equals speed, not increased rpm.

Bill M. (edited)

## Re: Exhaust Chamber Calculator

Why are you using the foot ( ' ) designation in place of degree ( ° ) ?

To use the correct designation , press Alt and 0176 .

If that's too laborious for you , set up a hot key for the degree sign .

You put so much into these posts and then short yourself and everyone else .

## Re: Exhaust Chamber Calculator

> P D Wrote:

> -------------------------------------------------------

> Why are you using the foot ( ' ) designation in place of degree ( ° ) ?

> To use the correct designation , press Alt and 0176 .

> If that's too laborious for you , set up a hot key for the degree sign .

> You put so much into these posts and then short yourself and everyone

> else .

Thank You P D

## Re: Exhaust Chamber Calculator

Sorry if I seem to be too picky . Perfectionist part of myself ...

I can appreciate the time and effort you put into your posts .

## Re: Exhaust Chamber Calculator

i modified the stock 50v bazooka

i increased the stinger stub w/1&1/4'' pipe 1&7/8th out of the end plug cap.

cut down the inside to rebel moby's suggestion. carb work was just as important as pipe work.

also choked up the rear spring as he suggested.

did the ramp mod to 1mm more. used a kevlar drive belt ( the stock one would last a little over a month)

it worked better for low end, off the line, gets through stop lights quicker than cars.

i'm satisfied w/labor intensive result's, it's a little louder but that's the trade off...it was cheap...

after all it's still a moped.

« Go to Topics — end of thread

Want to post in this forum? We'd love to have you join the discussion, but first: