Jets vs Exhaust

[tkent02]

So where are the dyno results on the 1981 650G in question?
Simple to prove one way or the other.

[Graham]

So where are the dyno results on the 650G in question?
Simple question.

If your asking about renegadmonk's GS650G? Never in question.
If your asking about the 77 KZ650 that I posted we never question that eather. That was my old street racer, twist the throttle and lift the wheel in the first 3 gears, upper 11s in the quarter, and just under 140 on the top. God I loved that bike. 19 traffic (+ 4 out of state) tickets before I was 20, in jail 3 times, a judge that knew my face. Boy it was a fun bike. I still can't drive in Indiana. LOL, my only question was is 78hp all it put out. Sorry took a walk down memory lane.

[tkent02]

If your asking about renegadmonk's GS650G? Never in question.

So, 6 or 8% probably?

[Graham]

So, 6 or 8% probably?

Ok now I understand what you’re asking. Never saw any dyno results.
I really think it would be up in the double-digit %hp increase because the stock exhaust on an 81 GS650G would be very restrictive. I think the GS650G had like 62 or 64hp, I could see like 69 to 75hp out of one. A friend had a 650g back in 94 and it was pretty snappy, he said it would keep up with my old KZ650 but I never rode it so I can say.

[tkent02]

Ok now I understand what you’re asking. Never saw any dyno results.
I really think it up in the double-digit %hp increase because the stock exhaust on an 81 GS650G would be very restrictive. I think the GS650G had like 62 or 64hp, I could see like 69 to 75hp out of one. A friend had a 650g back in 94 and it was pretty snappy, he said it would keep up with my old KZ650 but I never rode it so I can say.

Suzuki GS650GT GS650GGS 650 GT 1981
Overall Length: 2 170 mm (85.4 in)
Overall Width: 830 mm (32.7 in)
Overall Height: 1 170 mm (46.1 in)
Seat Height: 780 mm (30.7 in)
Wheelbase: 1 440 mm (56.7 in)
Ground Clearance: 160 mm (6.3 in)
Dry Weight: 213 kg (468 lbs)
Engine type: Air-cooled 673 cc inline-4, SOHC, 8 valves. 73 hp (54 kW)/ 9.400 rpm, 57,2 Nm (5,88 kg-m)/ 8.000 rpm. Shaft drive.

OK, I could see 75hp too. From a bike that had 73 from the factory.
So, only a 3% increase from a pipe, I would have thought it would do more good.

[Graham]

Suzuki GS650GT GS650GGS 650 GT 1981
Overall Length: 2 170 mm (85.4 in)
Overall Width: 830 mm (32.7 in)
Overall Height: 1 170 mm (46.1 in)
Seat Height: 780 mm (30.7 in)
Wheelbase: 1 440 mm (56.7 in)
Ground Clearance: 160 mm (6.3 in)
Dry Weight: 213 kg (468 lbs)
Engine type: Air-cooled 673 cc inline-4, SOHC, 8 valves. 73 hp (54 kW)/ 9.400 rpm, 57,2 Nm (5,88 kg-m)/ 8.000 rpm. Shaft drive.

OK, I could see 75hp too. From a bike that had 73 from the factory.
So, only a 3% increase from a pipe, I would have thought it would do more good.

No I didn’t check the stats on the bike. But your right it does have 73hp stock, wow a lot of hp for a 81 650. So a good header should push it up to 80hp or more. It has got to have a beefy stock cam to make 73hp stock. I’ll have to find out if the cam is different then the 550 cam, hmm.
8%hp increase is to low for an 81 GS650G with a good header system. If you only get 8% then you need to retune or rebuild the 81 GS650G. Not every bike will get a big hp increase from a header system. Mostly 4 cylinder motorcycles from 73 to 86.
BTW where did you get 6 to 8%hp increase? Your work? Off the net? I got mine from dyno test we did in the 80s to check the truth in the header ads.

[tkent02]

No I didn’t check the stats on the bike. But your right it does have 73hp stock, wow a lot of hp for a 81 650. So a good header should push it up to 80hp or more. It has got to have a beefy stock cam to make 73hp stock. I’ll have to find out if the cam is different then the 550 cam, hmm.
8%hp increase is to low for an 81 GS650G with a good header system. If you only get 8% then you need to retune or rebuild the 81 GS650G. Not every bike will get a big hp increase from a header system. Mostly 4 cylinder motorcycles from 73 to 86.
BTW where did you get 6 to 8%hp increase? Your work? Off the net? I got mine from dyno test we did in the 80s to check the truth in the header ads.

A gain of 6 to 8 per cent is what I have read the last thirty years or so, it has always seemed about correct to me. Pipes make it somewhat faster on top, not a huge difference, probably not worth the loss of midrange and low grunt except for a bike to be used for racing. The increase in sound, the sudden difference in feel as it comes out of the less powerful midrange into the range where the scavenging works, and the overblown advertising claims always made it seem like a lot more improvement that it really was.

The 650 cam is milder than the 550 cam, for a broader powerband. It gets it power from better airflow, head and intake design, etc. Maybe it's the exhaust, I dunno. Not a 650 expert, but they definitely seem better than the others GSes for the engine size. They got something right.

[mlinder]

The 650 cam has more lift and less duration. It's not milder, it's different, and the combustion chamber design is completely different.

[MisterCinders]

awww man...i was dreaming of the big 25% gain.....

That's what SHE said.

(obligatory)

[tkent02]

The 650 cam has more lift and less duration. It's not milder, it's different, and the combustion chamber design is completely different.

For the 550/650 conversion, it sounds like the 650 cam is the one to use, then? I was thinking the 550 cam would give it better high RPM performance, maybe this is wrong?

[mlinder]

Also, claimed HP on these is at the crank, not the wheel.

Let's take a look at the 550 and 650.

The 550 cams have a bit more overlap than the 650 cams. In order to meet EPA standards, the intake had to be restricted at the airbox, and leaned out. Stock is well below a happy AF ratio.

With that said, the 550 made a claimed 49 crank HP. That's about .09hp per CC.
The 650 makes a claimed 73hp from 673ccs. That's about .108hp per CC.

That's about a 17% increase in HP per CC.

Where did it come from? Head design is certainly different between the two, but we just don't know how much the combustion design increased the horsepower.

However, I can tell you what reducing the overlap but increasing the lift on the cams did...

A fair bit of your 'dirty' exhaust, the unburnt fuel, is a lot of what causes machines not to meet EPA standards.

The more overlap in the cam profiles, the more unburnt fuel gets exhausted.

So, by reducing overlap, Suzuki created a situation where they could introduce a richer mix to the cylinders, increasing power output, and using the extra CC's more efficiently while meeting EPA standards.

Now, it stands to reason then, that creating a closer-to-ideal mix on the 550's will also give you more power, at the cost of 'being green.'

Again, at some point in the next few months, I have to get the 350's to the dyno.

I'll bring the 550's to the dyno at the same time.
Mine should be a 650 by then, but a friend has a 550 which we've opened up a bit. I will get it on the dyno and we can see what it makes. My guess? 58 or 59 RWHP.

[tkent02]

Can't wait to see the results.

[mlinder]

For the 550/650 conversion, it sounds like the 650 cam is the one to use, then? I was thinking the 550 cam would give it better high RPM performance, maybe this is wrong?

I'm going 650 intake cam, 550 exhaust cam, degreed to give a fair bit of overlap.

(edit: by the way, we don't often use pods on CV carbs. They usually don't work well. The airboxes have nice little velocity stacks in them. We just deristrict the box pre-filter. At least on the GS 550's.)