Now it's looking like a top end : )

[tz375]

Arne,

I must have missed the reference to 80 RWHP. That's a big number. What exhausts and carbs are you planning on using?

With all that extra transfer and intake duration, I guess that strong bottom end isn't a priority

Though lane (rngdng) has a thick spacer in and his bike seems to run fine by all accounts. maybe I'm over-thinking the port duration issue

[Arne]

Arne,

If I knew it all, I probably wouldn't bother messing around with this old stuff. I'll just see what it runs at on the dyno, and from what I've read it shouldn't be that far off from 80 - but that's all part of the fun, isn't it.

Carbs are 36mm flatslide mikuni, expansion chambers are period ocelot, ignition is HPI.

I haven't heard any issues with the GT750 bottom end other then the gearing ratios are a bit wide. I'm going to run the stock clutch and see how it does, as its fairly easy to upgrade.

Have you got any info from Bill Tripp and his software (spelling?) on modified GT750 engines and predicted HP?

[Arne]

Nice Arne...looks great! I'm very interested in what you are doing. Do you think Eric might have mapped the ports?

Kevin

I'm not sure, you can PM him on this board - he's Zook-e

[tz375]

Bill and I ran a huge number of different port combos from 60 to 180 hp with all manner of silliness that implies.

HPI is a great ignition. The 36mm flatslides are a great choice, but 38 are probably better. I have a set of 38mm TMs and 34mm TMX's to try. On the Phat Trakka, I'll use the little ones and teh 38s can wait for the reed motor.

Ocelot pipes are old school, but should work. I don't have all their dimensions, so I can't compare them with a modern pipe.

How high are you planning on pushing compression pressure and squish or no squish? The quiescent head may start to be a problem at those power levels. Welding and machining a head is expensive but cheaper than a melt down.

[tz375]

Arne,
When I said "bottom end" I was referring to the powerband, not the crank. I should have been more clear on that one.

I have a "thing" about intake duration because it tends to kill low RPM power - except of course on some road engines that don't fill very effectively....

[water cooled]

Arne, it sounds like you are well on your way to a much improved GT750 engine.

I also performed certain mods to my engine, mostly porting and ignition along with larger carbs and better pipes. I wanted to get my bike tested last fall but it will have to wait until the end of February.

Let me know how the dyno test goes and I will do the same. I never had my barrel mapped after it was ported and skimmed so I will have to do that as well. Good luck with you project....nice job.

Kevin

[Arne]

Bill and I ran a huge number of different port combos from 60 to 180 hp with all manner of silliness that implies.

Very interesting. I'm basing the 80 I quoted mainly off of what Ocelot could get out of their national engine.

I figured they'd go 10% high and then I'd lose 15 from the chain and wheel.

110 - 11 - 15 gives a bit over 80 horse.

http://dsr.racer.net/engines/suzuki/gt750/page1.htm

Ocelot made two basic race engine types:

A 750cc "regional" engine good for 110-115 HP. A 1978 pricing sheet from Ocelot shows the "Introductory" 750cc engine at $2,463

Their killer 850cc 130HP "national" engine. The same 1978 pricing sheet from Ocelot shows the "National" 850cc engine at $3,545.

[Arne]

HPI is a great ignition.

Yah, they're pretty cool.

I wish it had a charging coil on it though so I could pull that huge rotor off the crank.

I also wish I would've waited a few months for the US Dollar to get stronger, I could've saved a bit there, oh well.

[Arne]

The 36mm flatslides are a great choice, but 38 are probably better. I have a set of 38mm TMs and 34mm TMX's to try. On the Phat Trakka, I'll use the little ones and teh 38s can wait for the reed motor.

I really debated myself on whether to go with 36's or 38's.

I decided on the 36's to help the low rpm's a bit, or as much as I could while still getting the 'big' HP number.

I think Lane is running 34's and pulling some pretty good HP out of his blueboy so I thought another two and we'll see what that does. What HP was your bike putting down on the dyno Lane?

This RD racer has some interesting things to say about carb sizes.

http://homes.midmaine.com/~shoeman/inde ... dcarbs.htm

CARBS:I'll admit it, I've never built a race motor for myself that ran stock carbs. Formula RD was not yet here when I started racing, and the classes we ran allowed any size round slide carb you wished to use. For the street, I used re-jetted stockers with K&N's and later a set of SpecII 30's with BIG Uni's. I never ran my street bike on a dyno, but the old seat of the pants test again seemed to register a noticeable gain. The bigger carbs are worth the price just in ease of tuning alone. Main jets are equally simple on both but the needles are so much easier on the 30's it's not funny. F-RD guys must get frustrated wrestling with those slides while the more modded bike tuners can change needle settings in about three minutes. You can still find info out there from the bad old days when in a production class you had to stick with the stock carb size but the usual suspects needed to win at any cost so they bored the stock 28mm bodies out to something approaching 30mm's and found a useful gain. The bore needed to be offset from the centerline to avoid ruining the body, and apparently the tip-off was an idle that could never be brought down as low as a stocker. If you do not have to adhere to some sort of rule like that, then get the bigger carbs, it's a lot less work for the same or maybe more power gain, and you'll get the easier to tune bodies as well. A win-win if there ever was one.

Some people complain of a flat spot at 4k or something like that with the 28's and K&N's, but I never have really noticed it, but then again if you want to ride around at 4000 rpm all day why ride an RD? I always just roll the throttle on at those low revs until the engine gets closer to the powerband. Problem solved in my book. That is how you really want to use the throttle if you want to be fast, even on the track. Bigger carbs can mean bigger power, up to a point!

For a race bike (and on my street bikes too) I settled on Mikuni VM34's. Easy to tune, great response, and great rideability. The reed valve RD is forgiving on throttle bore size, so it is possible to use bigger carbs than 34mm and have the bike still run, but I think the powerband and throttle response will start to suffer. Velocity of the airflow through the carb is everything. Too big and the airflow gets real sluggish so throttle response gets poor and it's harder to jet correctly. So avoid the temptation, as strong as it is, to go the He-Man route and jam some '38s into your stock inlet manifolds. You'll be faster with a more conservative approach.

As a little test one spring I dynoed our RD after Daytona, where my rider had insisted on using a set of 38's after speaking with a friend of ours who used to race TZ's and TR's way back when. I must tell you that we were using TR3 cylinders at the time, which are piston-port, not reed valve, so there is no low end power to speak of. I thought the 34's would have been easier to ride and therefore faster, even at Daytona. The rider felt the bigger carbs were faster. So we strapped the bike down and did a few pulls. The bigger carbs helped the engine pull higher revs than it had with the 34's, as had been reported by the rider. It made 58hp @ 10500 rpm. Then the 34's went on. The result was 61hp @ 9500 rpm. The big carbs felt faster than the old setup because they made a bigger "hit" when they finally started to work as compared to when they were running poorly, and the resulting narrower powerband felt more frantic. If you compare the BMEP figures you'll see the 34's made 118psi vs. 101psi for the 38's. Definite de-tune. So we had traded a bunch of driveability for a few more MPH at redline

[Arne]

Ocelot pipes are old school, but should work. I don't have all their dimensions, so I can't compare them with a modern pipe.

I'm pretty excited about the pipes, I've only seen them on two other bikes and two sets go across ebay.

Here are mine:



This bike has them on with newer silencers (does someone have a photo of this bike from the other side?)



I need to fab the internal silencers on them yet (they didn't come with them.)

[Arne]

How high are you planning on pushing compression pressure and squish or no squish? The quiescent head may start to be a problem at those power levels. Welding and machining a head is expensive but cheaper than a melt down.

I haven't checked the volume on the head yet. When I bolt it together if it's lower than 135 psi I'm going to have it planed down.

It doesn't appear to have been planed far enough to raise the compression above 150 psi, but I'm just guessing.

If it ends up at or over 150 psi I'll probably think about cutting some squish into it.

As its a custom top end I'll be starting rich on the jetting and only running the available high octane pump gas in it. I usually stay on the rich side of the jets, so I guess I'll just watch the plugs and see what happens for awhile.

[Arne]

Arne,
When I said "bottom end" I was referring to the powerband,

Ahhh, I see. Yeah, its a modified piston port two stroke so it ain't gonna have alot down at the low rpms - oh well what do you do.

I have a "thing" about intake duration because it tends to kill low RPM power - except of course on some road engines that don't fill very effectively....

Its my understanding that you have to sacrifice bottom end on these old piston port engines to get bigger HP numbers unless you start going the route of reed intakes and such.

A couple years ago I just about forked out the cash to buy a two stroke simulation program but figured I wouldn't be using it enough so I skipped it.

I may end up going that way some day.....

I think Lane has a bit taller spacer than I have as well. How is your bike below 5 grand Lane?

[Arne]

Arne, it sounds like you are well on your way to a much improved GT750 engine.

I also performed certain mods to my engine, mostly porting and ignition along with larger carbs and better pipes. I wanted to get my bike tested last fall but it will have to wait until the end of February.

Let me know how the dyno test goes and I will do the same. I never had my barrel mapped after it was ported and skimmed so I will have to do that as well. Good luck with you project....nice job.

Kevin

I've been following your drag racer since the last board, I really like it. You'll have to post up the numbers when you get them. I don't see too many dyno results from modified GT750's.

Does anybody have any GT750 dyno results available?

[tz375]

Arne,

When you get the bike up and running, we'll have to see if we can't design some better pipes with modern software.

It doesn't always work out that way though. I have run some numbers for an H2 and then ran DENCO numbers and was surprised as to how spot on they were. the software wasn't available back in the day, so it tell us something about the skill and tenacity of the good guys.

All that the latest software has really done is to make it easy for the rest of us to work out what was known by people at the cutting edge back in the day.

I've seen a couple of dyno charts. Stock motors seem to make about 48 at teh rear wheel, and with a set of pipes that goes up to about 52-55. One of the GT750 racers had a chart with 60 some HP on it and a later one with 8oish on his race bike.

i did see a 129hp chart but have no idea if it's real or bogus or how modified that motor was. I hear that Brett in Sydney is approaching 200 on his highly modified 1000cc reed valve race motor, but I have seen no proof of that in the public domain.

Suzukidave had his Yellow Peril tested. That bike I think is basically stock ports but with a set of Bassani pipes and made 55ish I think. His new motor should top 80 with modern pipes and flat slides and hours of port work (mainly by Dave). He has done a heck of a job on that bike (both in fact), and while some of us like to feel we contributed in some small way, the reality is tat Dave did 99% of the work and was responsible for most/all of the design decisions.


Of course some dyno's seem to show much larger numbers than others, so it's not really appropriate to compare one graph with another apart from bench racing that is.

[Arne]

I've seen 'low' HP numbers and 'high' HP numbers posted for the GT750 in few places, but rarely any dyno charts that went with them.

The only way I'll believe a number is if I take my own bike to a dyno with a proven correlation record and run it (which is just what I plan to do, if I can find one : )

I'll probably just stay with the Ocelot chambers on this bike (until I change my mind at least)

Thanks - Arne