Table of contents

• Screwdriver tip for the Honda CB 750 Seven Fifty Make the exhaust system yourself
• Open-ended experiment
• First registered in February 1994
• Engine characteristics have changed
• The goal slowly comes into view
• Most difficult hurdle: the noise
• Motorcycle now has an individual touch

Ralf Petersen

14th pictures

Ralf Petersen

1/14
The starting point: a Honda CB 750 Sevenfifty for 850 euros, first registration February 1994.

Ralf Petersen

2/14
Unsatisfactory: rear view and sound of the standard exhaust system.

Ralf Petersen

3/14
First stage of renovation. New: short front fender, bellows, mirrors, indicators, bench, partially polished and painted rims.

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4/14
Starting material part 1: standard manifold of the Sevenfifty.

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5/14
Starting material part 2: “Conic Long” silencers from Louis.

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6/14
The centerpiece from a master’s hand – the custom-made (not yet polished) centerpiece.

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7/14
“Fitting” in the exhaust forge.

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8/14
New side view: chic and easy to dismantle for maintenance work.

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9/14
Noise measurement 1: The TuV experts at the “briefing”.

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10/14
Noise measurement 2: measurement in second and third gear.

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11/14
Looks good, the values ​​are in the specified range.

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12/14
Performance measurement: the Sevenfifty with a new exhaust system on the dynamometer.

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13/14
The last hurdle: TuV expert Poggenpohl prepares the report.

Ralf Petersen

14/14
Done: The “Four” cuts a fine figure on the Honda CB 750 Sevenfifty.

counselor

workshop

Screwdriver tip exhaust system – Honda CB 750 Seven Fifty

Screwdriver tip for the Honda CB 750 Seven Fifty
Make the exhaust system yourself

The exhaust system of Honda’s CB 750 Seven Fifty lacks optically the pep, the sound is sober. The stupid thing: there are no accessory systems in the desired four-in-four design on the market. There is only one thing left: do it yourself. We show how to do it.

Ralf Petersen

October 29, 2018

A brief history: When I bought a well-preserved Honda CB 750 Seven Fifty in the summer of 2015, infected by the retro wave, for a ridiculously low price of 850 euros, I wanted to wrap the classic naked bike in a chic retro look. Redesigning the front section with a short mudguard, polishing work, bellows, mirrors and mini-indicators is relatively easy, but changing the side view is even more difficult. Thanks to the seat bench conversion in the 80s style and partially polished rims painted in the vehicle color, I get a satisfactory result here too. Most problematic: the redesign of the stern. The standard rear light, which apparently came from leftover trucks, has been replaced on my machine by an even more terrible LED light. Replacing this with two small taillights is obvious, but there is still a lack of visual flair. With the image of a classic CB 750 Four K6 / K7 in mind, the idea of ​​a four-pipe system matures.

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Open-ended experiment

The performance is given additional appeal due to the weaknesses of the standard exhaust system: On the one hand, the collector sitting under the engine is lovelessly welded together, fissured and therefore prone to corrosion. On the other hand, the sewing machine-like sound of the original system is not very inspiring. However, a corresponding research shows that there is no system in the accessories trade that I like. Inquiries to professional exhaust builders also end in a sobering way. Either there is hardly any interest in this idea, or the asking price of 3,000 to 4,000 euros clearly exceeds my planned budget of a maximum of 1,000 euros. In the end, there is only one thing left for me to do: initiative – the start of an interesting project!

Ralf Petersen

The standard rear end of the Honda didn’t please.

The original idea: just remove the tailpipes of the original four-in-two system, have a kind of Y-piece made and mount four slim and elegant silencers on it. The right tailpipes can be found quickly. Louis has had such things as “Conic Long” in its range for many years at a price of 59 euros per tube. The catch: You do not have an ABE, the converted system must therefore be assessed and approved by the TuV as an individual approval.

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First registered in February 1994

Before the further implementation of the project, I have to contact the TuV, which is the responsibility of DEKRA in East Germany. Involving the experts there as early as possible is an indispensable prerequisite for the success of the project. Competent advice and help can be found with Dipl.-Ing. Gerhard Poggenpohl, the motorcycle expert at TuV-Nord (himself a former owner of a CB 750 Four). If possible, bring all parts, documents, etc. with you to the preliminary meeting. In my case, it turns out that the Seven Fifty I bought was fortunately registered for the first time in February 1994 and that the desired conversion is therefore possible. In the case of an initial registration two months later, I would only have been allowed to use tailpipes with an EC type approval. A performance and noise measurement as well as an assessment by a TuV expert is still required. The costs for this are around 350 euros – not cheap, but feasible.

Ralf Petersen

Dipl.-Ing. Gerhard Poggenpohl from TuV-Nord.

Since I cannot fall back on any experience and find little helpful on the Internet, the success of the campaign is anything but certain. An exhaust system is a very motorcycle-specific component. Changes to this can definitely have a negative effect. Will the sound, volume, power and engine characteristics turn out to be satisfactory in the end? And will I comply with the requirements of the TuV? To be on the safe side, I decide not to destroy the well-preserved exhaust system of the machine for the conversion, but to purchase another used exhaust. In the classifieds market I find a system in poor condition, but with well-preserved manifolds. 180 euros? Recovered! Flexed, cleaned, polished – it already fulfills its purpose. The four Louis Conics arrive as a package a few days later. It’s going well.

Engine characteristics have changed

For the missing connector, I turn to the exhaust forge in Viersen, which is known to insiders for its solid work. Here I get further valuable tips and suggestions. Instead of keeping the collector of the original exhaust system, exhaust professional Wulf Peppmoller suggests lengthening the manifold with four stainless steel pipes and leading them directly to the mufflers. As with the original, the individual manifolds are connected with an interference pipe and shaped so perfectly that the main stand remains functional. I liked the result straight away, and excitedly I started my first test drive, which, however, left an ambivalent impression. The engine characteristics have changed, the engine only revs up smoothly and smoothly after 2,000 tours. Even more serious: the sound of the system is reminiscent of the 1970s and the then legendary Marving systems. Clearly too loud for today! But Peppmoller also has advice for this problem and installs four additional stainless steel DB killers in the system.

Ralf Petersen

The center piece was handmade.

The second driving attempt went much better and promptly lifted my spirits. After synchronizing the carburettors, the Seven Fifty not only has a clean draft from 1,500 tours, but also a deep rumbling sound that is neither intrusive nor too loud. Just under 400 euros are due for custom work. This marks the next milestone in the project. A positive side effect: the now possible simple dismantling of the mufflers for maintenance and repair work. Until now, the original system had to be completely dismantled with great effort.

The goal slowly comes into view

The biggest hurdles, however, are undoubtedly still the noise and power measurement as well as the technical acceptance with regard to construction, durability, etc. Of course, the noise measurement is of particular importance. If the permissible limit values ​​are exceeded, acceptance is not possible. In the case of my Seven Fifty, the driving noise limit values ​​for motorcycles with first registration from October 1st, 1990 must be complied with. That’s 82 decibels after all. Before 1990 it was even 86 decibels, after 1995 it was only 80. The problem with an assessment is the subjectivity of the noise. It is not possible to make a reasonable statement about the driving noise based only on the stationary noise. Since the measurement is done at your own risk (the fee of 250 euros has to be paid even if the decision is negative), I buy an inexpensive decibel measuring device from Racefoxx in advance for around 26 euros, which, as it turns out later, delivers surprisingly accurate measurements.

Ralf Petersen

Noise measurement in second and third gear.

I use it to carry out an improvised test by comparing the values ​​of the modified Seven Fifty with those of my Honda NTV of the same year of construction in idle gas and at different speeds. The result is encouraging, and I turn to the Bottrop TuV station responsible for noise measurements in my region. Attention: Only a few TuV stations offer such measurements, as a suitable test track is required. In the case of TuV Bottrop, the test is carried out on the Brabus test track located directly next to the station (mainly in the field of vehicle tuning, especially for Mercedes-Benz models). So ideal conditions.

Most difficult hurdle: the noise

Detlef Gehrmann, the head of the ward, seems very open-minded and not only patiently answered all my questions in advance, but also gives a lot of tips. Nevertheless, he makes it clear that in the end only the measured values ​​will decide. So on a sunny and dry morning at the end of April at eight o’clock, I drive to the appointment in Bottrop with a very queasy feeling, similar to a visit to the dentist. Concentrated TuV competence is already waiting for me there. In addition to expert Gehrmann, his colleague Dietmar Bley, TuV team leader for the Rhine / Ruhr region, also appeared. He drives the Seven Fifty while Gehrmann monitors the measurement. On the Brabus test site, framed by numerous Mercedes and Smart cars, the test cycle takes place in accordance with the specifications for speed and engine speed in second and third gear from both directions, taking into account the condition of the road surface, wind speed and temperature. It remains exciting. The test result lets me breathe a sigh of relief. The measurement results in an average value of 80 decibels and is therefore even two decibels below the limit value. The most difficult hurdle has been overcome!

Ralf Petersen

The performance measurement takes place on the MGM test bench.

I have the performance measurement carried out on a test stand from MGM in Castrop-Rauxel. Master two-wheel mechanic Manuel Galkus, the owner of the company specializing in BMW motorcycles, is known in the scene as a tuning professional and is active in racing. For a very fair 39 euros, in addition to the desired performance diagram, there is also a (positive) assessment of performance and torque curve. The nominal power of the original Seven Fifty is 73 hp. The now measured 75.5 PS are therefore within the tolerance range. For a motorcycle with a mileage of over 60,000 kilometers, this is a very considerable figure.

Motorcycle now has an individual touch

After I have already sent all the relevant documents to my main contact person, Dipl.-Ing. Poggenpohl had sent, now the penultimate step at TuV Herne is pending. Due to the previously clarified situation, the corresponding acceptance takes place as expected without any problems. At the end, you have to go to the road traffic office to enter the change in the vehicle documents. After making an online appointment reservation in advance, this can also be done in less than an hour and the exhaust system legalized.

All’s well that ends well: It’s a long but very interesting path from idea to implementation. And even if I had doubts in between, the considerable effort was ultimately worth it. Sound and optics meet my expectations even better than before. The motorcycle now has an individual touch that makes it unique. With a project like this, it is important not to commit yourself too strongly at the beginning, to remain open to suggestions and to set the schedule fairly generously.

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