barrel spacers

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madforitkev
Posts: 292
Joined: Tue Dec 13, 2016 7:21 pm
Location: Poole

barrel spacers

Post by madforitkev »

My project is moving forwards (will do a big build update with pics soon).

I've basically managed to graft the GT 750 motor into a VFR800 frame with a single sided arm, so I have a suspended engine.

Final step on the major chassis work is deciding if I need a barrel spacer as a step towards the engine tuning/porting. This will drive the front engine mount location weld point on the barrels.

I've seen a few porting options but none that define the size of the ideal barrel spacer.

Does anyone know what drives the spacer size (i've seen up to 3mm referenced) and is there an ideal porting layout/modification?

When my last mounting brackets come back from the shop I will do an update.
Honest baby....this is probably the cheapest bike I have ever bought!!
teazer
Posts: 1035
Joined: Mon Mar 07, 2011 8:10 pm
Location: Chicago

Re: barrel spacers

Post by teazer »

At the risk of stating the obvious, the spacer thickness you need depends on how far you want to raise the ports. The most common is probably 3mm which plus second gasket raises the ports 3.5mm and you then take 4.0mm off the top to bring compression back and to correct the 0.5mm negative deck height. You should lower the port floors, but in practice, there is not a lot of flow when the port is fully open, so not doing it is not a major issue.

You will need to raise the exhaust roof to get sufficient blowdown time/area. That's the time that the cylinder pressure is higher than transfer port pressure and stock is only about 26 degrees IIRC. Aim for 30 degrees. You should really work on time-area rather than just timing, but for the street it will be in the ballpark.

You can also widen all the ports and on JKL blocks the inlet floor needs to be lowered. ON a JKL, leave the floors and raise the roofs until the rings are starting to show at which point the piston skirt will still be hanging down, but if you shorten the skirt you get more intake duration and that hurts bottom end performance with little upside at the big end. Cometic in Ohio will sell you a laser cut spacer, 2 base gaskets and a beautiful head gasket that won;t blow out like OEM and aftermarket do.
madforitkev
Posts: 292
Joined: Tue Dec 13, 2016 7:21 pm
Location: Poole

Re: barrel spacers

Post by madforitkev »

Teazer

Great info. Thanks.

I currently have a set of A barrels.

I am obviously hopeing for near to 100 BHP on the finished item so porting that helps with that is useful.

Thanks again

Pics soon

Kev
Honest baby....this is probably the cheapest bike I have ever bought!!
madforitkev
Posts: 292
Joined: Tue Dec 13, 2016 7:21 pm
Location: Poole

Re: barrel spacers

Post by madforitkev »

Teazer,

Not wishing to be a PIA... ;)

I am sure this must have come up before but can't find it.

I have found several porting diagrams on-line.

If I use the 3mm spacer and machine the 4mm of the top of the barrels to get compression back, my question is which (if any) of the pics is the best approach?
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??
Honest baby....this is probably the cheapest bike I have ever bought!!
teazer
Posts: 1035
Joined: Mon Mar 07, 2011 8:10 pm
Location: Chicago

Re: barrel spacers

Post by teazer »

#1 and #7 are plots of a TR750 probably from the Cycle article. #2 and 3 are from an on line article about porting a race bike. 32mm is pretty high fr a street port... Next one simply refers to making the port divider shorter. #5 is John McCann's TR750 sports car port plot. Next one is best ignored.

Simple answer: None of the above.

What pipes and carbs are you planning on running?

Just as a point of reference I have two sets of barrels ported by well respected tuners. One favors tall ports but not very wide. The other likes very wide ports and not too tall. Both work but are different. The bst answer is probably to see if Stan the man would port them for you or one of the modern two stroke gurus. That way you have the best chance to chat with the tuner to get the powerband you really want.
madforitkev
Posts: 292
Joined: Tue Dec 13, 2016 7:21 pm
Location: Poole

Re: barrel spacers

Post by madforitkev »

32mm lectrons or 35mm mikunis. Haven't decided yet.

Pipes are a bit of an unknown as to a certain extent I need to build them round the new chassis.

I know at least one company who say they can give me a design based on the final porting but if I go with your recommendation I guess the porting guy I use would be best for advice?

I will put some pics up next week when I get the front end in.

Cheers
Honest baby....this is probably the cheapest bike I have ever bought!!
teazer
Posts: 1035
Joined: Mon Mar 07, 2011 8:10 pm
Location: Chicago

Re: barrel spacers

Post by teazer »

If you feel comfortable with a porting tool, you can do it yourself, but you have to work out what you want and then buy the tools. A Gt750 takes a fair amount of time to port because it's a triple and because it is so mild that there's a lot of metal to carve out and it has to be done accurately and consistently.

I prefer to design pipes to match ports - It's the right way to do it, but I am surprised to find that many pipes will work and will give different results. For example a mildly ported GT750 will work with Allspeeds or Basanni or J&R or Jemco and all have different power at different revs. For a 60-80 HP bike that's all fine. To get 100, it's better to match the pipes and ports. A 3 into 1 pipe makes my bike feel much faster at low to mid speeds because it's much lighter than OEM. In reality, it makes less power but that's not how it translates to feel and street performance.

The average moped or lawn mower is a higher state of tune than a GT and that's one aspect of its character that appeals to many owners. There's a lot of scope for performance improvement but they will never be an RGV or TZR. 60HP 250cc bikes equate to say 180HP 750. A stock GT at say 48 rear wheel hp is equivalent to a 16HP 250 single. :?

32 is stock OEM size and is a little small 34-35 is probably better suited to the task, and 38-40 would be better at the top end but maybe not so good for traffic light stop starts.
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manxgtjohn

Re: barrel spacers

Post by manxgtjohn »

into a vfr mmm any photos of this i would love to see that im doing a translpant my self in to a modern chassis
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gixxerman001

Re: barrel spacers

Post by gixxerman001 »

Interesting stuff.
I'm looking to add a little extra to my GT750 too, I've gone for a 2.5mm plate.
Reading the replies and various other info it appears that other than having the compensating machining done on the cylinder block - 3mm when gaskets are taken into account? (2.5mm + 0.5mm....when they're crushed?) that is about it.
I'm planning on a head skim to true the flat surface (which I'm not expecting to be any more than 0.25mm) and a further 1mm off. I'm not sure whether I need to trim 2.5mm off the piston skirt, but that one is easy stuff if it's needed.
I'm looking 75 - 80hp and a good torque & power curve for the street.

Looking forward to hearing how you get on Kev.
swarrans
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Location: Maidstone Kent

Re: barrel spacers

Post by swarrans »

Aren't you going to massively reduce the compression ratio if you only take 1mm off the head? - or did I misunderstand what you meant? (which is probable!)
Simon
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