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Header primary diameter for a small motorcycle engine

General engine tech -- Drag Racing to Circle Track

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ptuomov
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Header primary diameter for a small motorcycle engine

Post by ptuomov »

Rpm limited by rules to 9300rpm. Power band 7000-9300rpm. Four valve head, big stock cams mandated by rules. 600cc total engine capacity, 150cc per cylinder. Don’t know how much engine power it will make, but people talk about 80hp at the crank or 20hp per cylinder. 12.8:1 compression limited by rules, 98 RON octane gasoline. Ignition timing set by rules, would run without knocking on 90 AKI / 94 RON gasoline.

What size (inside diameter) primary header pipe from the exhaust port to the first step?
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Re: Header primary diameter for a small motorcycle engine

Post by Rick! »

For this, right? If you have hired "the Stig" as your pilot, then it looks like you'll do pretty good. Judging by the video, there's only one driver in that race that knows how to wheel his kart.
https://www.youtube.com/watch?v=x-N6rG7_Hdc

The "happy" chassis dyno in your other thread could be due to not running in a 1:1 gear so there is still torque multiplication in the transmission. BTDT.

Pipemax will help you out unless you can bend Calvin's ear. Maybe the latest version has another 600cc bike engine other than the CBR600RR to use as a template. You will be disappointed in the power numbers it estimates when you turn the RPM down to 9300.

Maybe the EngMod4T guys will chime in.

Or, download the Lotus program for those early morning brainstorming sessions. :)
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Re: Header primary diameter for a small motorcycle engine

Post by Momus »

I run several 145 cc Honda singles- 2 and 4 valve- in that per-cylinder HP range or higher and default to 1.125 16 gauge- so 25- 25.4 bore.

I would be testing 1.25" OD 16 guage as well, probably with and without a very short stub at the nominal exhaust port exit diameter leading into the 1.250".
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Re: Header primary diameter for a small motorcycle engine

Post by ptuomov »

Thanks for comments.

Unrestricted, the N738 engine makes

125 hp (93 kW) @ 13,500 rpm
67.7 N⋅m (49.9 lb⋅ft) @ 11,500 rpm

The exhaust valves are 22mm each with 4.5mm stems. The theoretical maximum valve area corresponds to about 30.5mm ID pipe if there's no seat. With 1mm 45-degree seat, I think it's 28.2mm ID pipe equivalent. At the valve, that's the area that exhaust 125 hp worth of exhaust gas... I wouldn't want to go larger than that 28.2mm ID for the 9300 rpm max restricted engine.

Another consideration is that if the power will go down by 29%, just ball parking the effect of bringing down the peak torque to 9300 rpm and computing 1 - (49.9*9300/5250)/125 = 0.71, the correct pipe diameter should be 16% smaller in the restricted engine vs. the unrestricted engine.

The rules don't allow one to modify the head, but nothing says a pipe can't protrude into the exhaust port from the flange. It sounds like one should see how small one can go without making it an awkward transition.
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Re: Header primary diameter for a small motorcycle engine

Post by Momus »

A single cylinder mule would be handy at this stage.

My various engines seem to prefer a pipe ID greater than the exhaust valve diameter, let alone the throat.

I have tried a theoretically better suited 1" 16 guage pipe to a small Honda engine with a single exhaust valve of 20.5 mm and it collapsed- fitting a much larger bore pipe brought it back.

When I finish my countershaft dyno I will be getting to the nitty gritty.

What are the exhaust details of the as delivered exhaust?
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Re: Header primary diameter for a small motorcycle engine

Post by bigmike »

I might be way off here but, I would be taking a look at the Yamaha FZ6R engine and its exhaust system. It was made from a older gen R6 engine and is tuned more for mid range via smaller ports, smaller header primaries, and smaller throttle bodies. Lots of engines on ebay for good prices.
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Re: Header primary diameter for a small motorcycle engine

Post by ptuomov »

The exhaust port is 32mm ID for the 125hp stock unrestricted bike. The header that came off the engine has already been modified to be smaller but the original header was a bit bigger than the port for the first primary section.

If we start with the stock 32mm ID port size as being optimal for unrestricted engine, then we’d likely want to shrink that to 27mm ID for the restricted engine. Right?
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Re: Header primary diameter for a small motorcycle engine

Post by ptuomov »

Momus wrote: Wed Jul 21, 2021 2:16 am My various engines seem to prefer a pipe ID greater than the exhaust valve diameter, let alone the throat.

I have tried a theoretically better suited 1" 16 guage pipe to a small Honda engine with a single exhaust valve of 20.5 mm and it collapsed- fitting a much larger bore pipe brought it back.
The exhaust valves are sized for about 41% more power than the restricted engine will be making. My thinking was that given that exhaust valves are “too large”, there’s no point in going even bigger with the first pipe section. But that’s just intuition, no real theory or data backing that up.

It makes sense to me that going too big will have a modest lower rpm penalty at 7000rpm but going too small will have a large 9300 redline rpm penalty. Getting it wrong likely has an asymmetric penalty.
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Re: Header primary diameter for a small motorcycle engine

Post by Rick! »

What is an unrestricted gixxer 600?
A 2021 gixxer 600 is around 104hp with 44ft-lbs of torque.
These guys show the same engine at around 106hp stock for the years given.
You've told us several times that aftermarket anything is out of bounds. Does this include a Dynojet Powercommander?
How are you limiting the rpm to 9300? A jailbroke ECM?

Your barometer of performance is your buddy at the dyno shop. The pic of the dyno graph shows 57.5ish, maybe 58 hp going down to maybe just over 56ish hp with your intake and exhaust mods. Filling in the hole was a good start but I don't know if I would cherry pick that and state a huge gain was made as the peak hp went down a tad. You're stating "everyone" is hitting 90hp at 9300rpm on his dyno? Did I miss anything? I'm not sure how you can believe that is feasible. (the highly optimistic data point number 2350 appears to be an anomaly as gaining 15hp in 13rpms smells funny.)

How does your "Stig" do with the kart you have now? Is he within a second of the top dogs? Is he faster?

How many engine parts and combos have the top dogs burned up to create their perfect power recipe? In drag racing, the SS guys usually go through several sets of available stuff to come up with their performance. I would assume it would be the same in any racing venue. A lot of minuscule gains add up to a significant gain as home runs don't exist.

Maybe you could put out all the pertinent data in one post so some of the folks here can play some simulation games and respond to your first question on primary pipe diameter. it's easy to find bore and stroke, you gave an exh valve dia so the intake valve dia is needed, cam info is needed, rod length is a good thing, and any flow numbers you might have dug up could be useful. Then some simulation time can be burned for your quest.

When we were trying to develop an 8000rpm 4S for a sled, it was really hard to make any serious hp to make up for the weight penalty. It's one of the reasons why the RX-1 used a gear reducer on a 10,000rpm engine to keep the clutch at 8000rpms. Power density is where it's at so maybe another way to look at your quest: Is there gains to be made on the lower end by making the top end stronger? Or is it a game of stepping on the balloon and trying to coax hp below 9300 and removing hp to the right of 9300?

nitro 2 has already stated you need a proper 4 into 1 pipe design so a phone call and a few franklins will give you big insights into this engine and exhaust tuning.

I do like your enthusiasm so keep on it and let us know what happens.
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Re: Header primary diameter for a small motorcycle engine

Post by ptuomov »

Since nobody including the manufacturer knows (or at least anybody reliable refuses to say) what power these engines make, I'm just going by the relative to baseline both in power and sizing. You'll get all sorts of numbers between 100 and 125 hp for the stock engine in the bike.
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Re: Header primary diameter for a small motorcycle engine

Post by ptuomov »

bigmike wrote: Wed Jul 21, 2021 4:54 am I might be way off here but, I would be taking a look at the Yamaha FZ6R engine and its exhaust system. It was made from a older gen R6 engine and is tuned more for mid range via smaller ports, smaller header primaries, and smaller throttle bodies. Lots of engines on ebay for good prices.
The Yamaha 600cc competitor is useful in the sense that in a bike it's in the ball park in the same power level as the Suzuki and there's more data about the Yamaha.
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Re: Header primary diameter for a small motorcycle engine

Post by ptuomov »

I punched in both the unrestricted engine and restricted engine into pipemax. Im assuming 71% of the unrestricted power for the restricted engine, and jerryrigged a 6900-9400rpm power band for the restricted engine header. With 16G pipes and 40mm exhaust port length, pipemax recommends -16% smaller pipe ODs and +50% longer pipe lengths across the board for the restricted engine.
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Re: Header primary diameter for a small motorcycle engine

Post by ptuomov »

What does common sense and past experience say about header primary pipe length in a motorcycle 4-1 system (or primary plus secondary length in 4-2-1 system)? The power band that is used the most is 7000-9300 rpm.
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