More HP - Suzuki GSX-R Motorcycle Forums Gixxer.com
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post #1 of 17 (permalink) Old 02-16-2017, 02:08 AM Thread Starter
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More HP

I have a GSXR 750 L 1990 and Wonder if I can gain some more hp With a KN filter and do something With the carbs. I am keeping the original air filter Box and the bike have the Stock exhaust.
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post #2 of 17 (permalink) Old 02-16-2017, 02:43 AM
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Re: More HP

The standard silencer is the most restrictive part (and heavy), only when this is swapped will you see any real gains by altering the air filter and jetting. You can get useful gains by doing the silencer/filter/jetting. It's a good idea to keep the airbox , even more so on 750s.
Nice looking 750L Carl.
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post #3 of 17 (permalink) Old 02-16-2017, 03:13 AM Thread Starter
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Re: More HP

Thank you Nickr, only problem is that it is impossible to find a sport silencer for my bike.
Is it possible to fit the Yoshimra exhaust system for 1. gen gsxr 750 or maybe the 2. gen 1100 Yoshimura on my bike?
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post #4 of 17 (permalink) Old 02-16-2017, 12:52 PM
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Re: More HP

Hopefully one of the guys on this board might have a slip-on silencer for you - you've got the red R. In the US they had an extra year of oil cooled 750s, so an even better chance of getting an exhaust . . . hopefully !
You might be able to get a link pipe made for you by an auto pipe company, then you can fit any can on the end.
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post #5 of 17 (permalink) Old 02-17-2017, 03:00 PM
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Re: More HP

As you are in norway , think you will need something with "E" number
Barracuda RS3 Suzuki GSX-R 750/SP L-M 90-91 4-1 Helsystem med EU godk
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post #6 of 17 (permalink) Old 02-18-2017, 02:35 AM Thread Starter
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Re: More HP

Will Barracuda exhaust gain more hp? I wonder if this exhaust will fit my bike and is it as good as Yoshimura? It cant be a direct bolt on for all the Slingshots as they are a little different. Correct me if I am wrong.
NEW 1988-91 suzuki gsxr750 gsxr 750 KERKER EXHAUST 4-1 muffler oem silencer race
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post #7 of 17 (permalink) Old 02-18-2017, 03:01 AM
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Re: More HP

just about any exhaust will be an upgrade, the down tubes on the stock headers are kinda small and restrictive and as previously mentioned the silencer is also restrictive. I have a D&D 4-2-1 setup off an 1100 and it gives good top end power since its freer flowing, 4-1 headers or something a little more restrictive will yield more bottom end power. ignition advancers are yay or nay, some have had good results and others are against it. I think the '90 came with 36mm slingshots? keep those, they are sufficient in performance
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post #8 of 17 (permalink) Old 02-18-2017, 04:17 AM
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Re: More HP

90 had already 38mm
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post #9 of 17 (permalink) Old 02-19-2017, 02:46 AM
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Re: More HP

wow, interesting price on that Kerker exhaust, but it is new.

yes, the 88-89 750's had a different oil-pan, because they originally had dual exhaust, so that Kerker may not fit 88-91.

honestly, if you want any decent increase in HP, then you must fit a proper full race exhaust.
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post #10 of 17 (permalink) Old 02-19-2017, 09:03 AM
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Re: More HP

One thing that I'll mention is that you need to consider just what you want. A race exhaust will potentially allow you to make more power at higher engine speeds but this may also come at the cost of less power and worse carburation at other engine speeds. The area between 4.000 and 6,000 rpm where you spend a lot of time riding on the street is probably the area most affected. This is especially true of the exhaust designs that merge the four header pipes into one, in the collector. A design that merges the four pipes into two pipes, then those two into one (or divides the collector in half, accomplishing a similar thing) can work better. An '88-89 750 seems more prone to this than the long stroke motor bikes.

JR
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post #11 of 17 (permalink) Old 02-19-2017, 09:35 AM Thread Starter
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Re: More HP

What about just a race can with kn filter in the original air filter box and a stage 3 jet kit. Will It affect the power band much between 4000-6000 rpm?
Super tune the GSXR 750
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post #12 of 17 (permalink) Old 02-19-2017, 10:11 AM
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Re: More HP

I'm not a huge fan of K&N filters, as they don't filter the air as well as the stock ones. They are better than no filter...

I would send an email to Factory Pro (Factory Pro: Producers of the EC997 Low Inertia Eddy Current Dynamometer Series and Quality MC performance products) or have a look at the page on their website devoted to the carbs in your bike.

JR
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post #13 of 17 (permalink) Old 02-19-2017, 10:13 AM
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Re: More HP

To save you an hour trying to find the page on their website, here's what they had to say:

Power jet carbs - Mikuni's great addition to a carb used in a high rpm application.

The power jet adjusts high rpm mixture, in the gsxr750 - from 10 to redline, in 1/3rd the step of a main jet change. Changing a main jet, in the 38mm carb, as installed on the gsxr750, adds or subtracts up to 2% CO per main jet change - when the CO% needs to be adjusted in in .2%-.4% for best power attainment.

Changing the power jet allowed much finer increments of change and, just as critically, happened to change the fuel delivery curve to what was optimum for the gsxr750 - something that would have required main air jet changes and other modifications to attain, but would still leave the main jet fuel delivery steps too coarse.

This Powerjet circuit works wonderfully when tuned on the stock airboxed gsxr750 (and it's pretty straightforward to tune on our EC997 Low Inertia Eddy Current dynamometers unlike simple inertia dynos.

The method of operation is as follows.

At full throttle, as the rpm increases, at 10k, there is enough of a pressure differential between the float bowl and the airbox interior to draw fuel up the black hose on the LH side of the carb and exiting through the hole at the top of the bellmouth of the carb. If you have a broken powerjet hose spigot, an oil pump spigot from an RD400 carb will replace it. They are pressed in, so they will twist out.

The fuel is metered by a jet that is located in the bottom of the float bowl. The jets are sized in increments of 2.5 or .025mm. Usual size for a gsxr750 with a stock airbox and air filter might be between #58 to #67.5.

The power jet circuit, when properly tuned, adds the equivalent of 2-3- main jet sizes "on top" of the main jet, after 10k, so, if you were not using the power jet circuit, i.e. had a "0" or blanked jet installed with a #125 main jet, you would use a #117.5 with a #62.5 power jet installed.

Since this particular circuit works on the pressure difference between the float bowl and the airbox interior, it is absolutely affected by any change in the pressure differential. If the air filter is changed to less restrictive unit or the airbox inlet is modified, creating less restriction - the power jet area (size) should have to be increased above the usual size, though, a BMC or K&N, as installed for stock replacement, may only require 1-2 sizes increase in the power jet (in addition to +2-+3 on the main jet circuit).

I specified "this" application, as there are other "power jet" systems that do not rely on airbox / carb bore pressure differential. They are identified by having a tube that is long enough to reach the center of the carb bore (where air pressure is lowest at high velocities. In the Harley world, it's called a "Thunderjet".

If the airbox is removed on this 750, there is no longer a sufficient pressure differential to pull the fuel up the ~2.5" vertical rise from the float bowl to the outlet in the bellmouth and the circuit is no longer effective.

Why is the Power jet circuit difficult to tune on a simple inertia dyno and easy on our EC997 Low Inertia dynamometer? According to the former owner of Dynojet, the power jet circuit simply doesn't work because there is a lag in fuel delivery at 9.5k rpm - creating a flat spot there. It turns out that the reason that he saw that is that the dynojet dyno has insufficient load to simulate the Real World Loading (tm) that is present on the bike in 4th and higher gears on the road or track. There is a slight delay in the onset of Power jet fuel delivery, but it's only vaguely present in second gear in the real world, and not present in higher gears due to the slower acceleration rate that occurs when you are actually riding. If you were racing, as Yoshimura USA and other non sponsored, large US Suzuki sponsored teams (we lent them carbs for the Finals) verified, the kit outperformed anything dynojet had to offer.

How to tune:
1. Install the main jet that produces the best power at full throttle / 8k-9k. Don't care about 11k-12k+ right now.
2. Install the power jet set that produces the best power at full throttle / 10k to redline.
3. Raise or lower needle for best power at full throttle / 5k-7k.
4. Raise or lower float height to get best power at full throttle / 3k.
4. Recheck main jet and needle height if you needed to lower the fuel appreciably.
5. Adjust fuel screws for best idle.

Powerjet Benefits:
The size of the main jet DOES affect the low and midrange. Excess leanness isn't usually the problem on these carbs. Using a #117.5 vs. a #122.5 main jet (PJ equipped vs. using a #0 PJ ) leans and crispens the low-end and midrange for better off idle and corner exit performance.

There other applications on other motorcycles that use circuits that are called "power jet" circuits that work on different principles - some are electronically controlled and work in the midrange like RGV250, the RS250 for upper topend, where they activate and deactivate through different ranges and still others work for different reasons and by different principles.
"Power Jet" is a catchy sounding name and it gets used every few years or so...

Why did Suzuki specify that US and UK models, for example would have a blank or "0" jet installed, disabling the circuit and other countries, like Canada, got the activated power jet circuit (though with pretty odd settings)?
Emissions? I don't think so. With the basic fuel level and needle settings virtually the same on both applications, using the larger main jet, as required with the circuit blanked, would only increase hydrocarbon emissions under measured conditions.

At any rate, the circuit works extremely well in dealing with the coarse main jet metering steps of the older style gsxr750 carbs - 1st through 5th place at the 1990 WERA Grand National Finals used our Factory Pro #CRB-S06-1.0 Carb Recalibration Kit. Pervasive kit use followed for the next couple of years -until 1992, the last year of the power jet.
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post #14 of 17 (permalink) Old 02-19-2017, 11:03 AM Thread Starter
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Re: More HP

Thank you Javadog, this is very technicaly reading for an accounter. I Just want to replace the stock can and add some more hp without destroy the every day rideability.
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post #15 of 17 (permalink) Old 02-20-2017, 03:53 PM
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Re: More HP

i agree with Java. i'm at a point in my life where i appreciate a quiet/not so loud/stock exhuast, i don't have to prove myself or race others on the street, and i enjoy my oilers for what they are.

your 1990 750, even in stock/original form, is plenty fast, and it has surprisingly high capabilities. if you are looking to lose a bit of weight and enjoy the sounds that these 4 cylinder engines make, then go with a quality slip-on or full exhaust.

i don't know about your stock jetting settings, since they are country/region specific, but if you are willing to spend money, and want the most performance from the stock carbs/airbox, then the installation of Factory-Pro's jet-kit should might make a performance gain, along with having it set-up on a dyno.

good luck and keep us posted.
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post #16 of 17 (permalink) Old 02-21-2017, 07:59 PM
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Re: More HP

I agree with all that has been said so far.
Here is some more reading on tuning,
CV Carb Tuning Procedures
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post #17 of 17 (permalink) Old 02-22-2017, 07:01 AM Thread Starter
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Re: More HP

Ok, thank you guys for informative answers! I have decided to og for the same setup as Wedge. Buy a Yoshimura can and get a custom built midpipe. The bike is fast enough for me as it is. :-) Hope the bike dont loose too much power in the area between 4.000 and 6,000 rpm.
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