Pros & Cons - 701 61x vs 701 62t vs 760 64u

[ItsRixter]

I've seen a few posts regarding why it's not a great upgrade from a 701 cylinder to a 760. Although it is larger displacement it might have weak bottom end? Some other topics of filling in the cross circuit...what's that?
Is a 701 62t just larger exhaust bolt holes?
Maximum bore for each cylinder?
I'd love to hear your feedback on
701x: pros/ cons
701t: pros/cons
760u: pros/cons

to put this into perspective...let's say these motors are applied into a Round Nose Superjet

 

[SUPERTUNE]

The 760 stock cylinder porting is designed to run the 760 huge volume exhaust pipe and no way can you fit it in the Superjet. The more aggressive 760 cylinder is more like a heavy ported 61X cylinder for closed course racing, a loss in bottom end and big increase in mid to top power.
If you build the 760 like a race setup then the more aggressive porting will make really good power even for freestyle, but most don't want total loss ignitions, race gas compression, expensive custom race carbs and so on...
The port location is a little different and SHOULD be port matched to be correct, bolt pattern is the same, but the 760's use the 10mm bolts instead of 8mm bolts to support the huge stock exhaust from the 760 engine.
No big deal there, as you just screw in 8mm X 1.25 helicoils into the threaded holes as they are the same 1.25 thread pitch.
As far as sleeve's go...yes you have about .080 more in bore size with the stock 760 sleeves.

 

[LBE]

I have 760's in my B1 and my RN. Both are ported with Bpipes, heads, lightened flywheels, w/carb work done. I do like the 760 in the B1, but in the SJ it does not have the low end snap of a 701. I do like the power, but a 701 with similar mods would be more responsive off the bottom.

Depends on what your riding as well. The 701 may be more bottom end, but in the surf I don't miss that. I ride faster and that mid range is great.
To answer the question

61x cyl: Good strong cyl and with a little porting can be very snappy. and is pretty good in stock form.

62t cyl: A good cyl as well but has slightly higher port timing. With some porting can also be a very snappy cyl. May have slightly less bottom than a 61x. I have ridden mod versions of both and can barely tell a difference.

64u cy: higher port timing. Definately a more mid to top end cyl. I think the bore size is noticeable in torque gains (could be in my head). If running it in a SJ, I would run a lower pitch prop so it rides more in its powerband.

It seems that if you want a lot more pop from the 64u (760) cyl, higher compression really brings that cyl alive.

Good luck.

 

[#ZERO]

The 61X cylinder works well for bottom end because the exhaust port roof opens at 90.5 degree crank angle and the exhaust ports are smaller. The stock bore size is 81mm and they can be safely bored out to around 84mm which is limited by the thinner skirt at the sleeve bottom. Also the casting is prone to cracking at the base on the intake side when the compression is increased, if not used with a girdled head.

The 62T 701 cylinder has taller exhaust ports and the exhaust roof is 2mm higher which opens at 87.5 degree crank angle. The later cylinders have reinforced webs on the intake side to keep from cracking at the base. The stock bore size is 81mm and the maximum safe bore size is around 84mm. The bore size is limited because of the groove at the base of the sleeves and they have a rough casting around the transfer port windows.

The 64X 760 cylinder is very similar to the 62T casting with improved sleeves that start at 84mm and can safely be bored up to 85mm which is limited by the thinner skirt at the bottom of the sleeve. Both the 62T & 64X cylinders have a higher blowdown time which is more suited for mid range, top end performance and these sleeves have a smooth machined angle upward around the transfer port windows. The 62T 701 & 64X 760 cylinders not only have the larger 10mm exhaust bolt holes but the actual exhaust port roof casting is 4mm higher.

Here are the port timing and duration specifications for the 61x, 62T 701, and 64X 760 cylinder so you can compare the difference.

90.5° Exhaust Port Opens
121° Transfer Port Opens
179° Exhaust Duration
118° Transfer Duration
30.5° Blowdown Time

Here are the specifications for the 62T 701 & 64X 760 cylinder.

87.5° Exhaust Port Opens
120.5° Transfer Port Opens
185° Exhaust Duration
119° Transfer Duration
33° Blowdown Time

Some other topics of filling in the cross circuit...what's that?

Filling the crossover passage on the intake manifold greatly improves the pulse signal clarity to the fuel pumps and makes the carbs more tunable. The passage is originally intended to offer smoother low-speed response for the stock engines.

 

[WAB]

This should be stickied!
Great posts :burnout:

 

[rossthaboss11]

is it true that for bigger bore motors you need to use race gas with lower compression? so most people would start using race gas in a 701 when you get around 190 psi or higher. would this mean you need to start using race gas in a 760 around 180psi? i have a 760 bored to 780 with about 175 psi

 

[tmc9046]

Great post alot of good info!!

 

[ATS_Scott]

My guess is, No. Bore size should not affect needed octane. I think that needed octane is more influenced by porting, compression, and timing advance.

is it true that for bigger bore motors you need to use race gas with lower compression? so most people would start using race gas in a 701 when you get around 190 psi or higher. would this mean you need to start using race gas in a 760 around 180psi? i have a 760 bored to 780 with about 175 psi

 

[rossthaboss11]

ok thanks

 

[SUPERTUNE]

is it true that for bigger bore motors you need to use race gas with lower compression? so most people would start using race gas in a 701 when you get around 190 psi or higher. would this mean you need to start using race gas in a 760 around 180psi? i have a 760 bored to 780 with about 175 psi

My guess is, No. Bore size should not affect needed octane. I think that needed octane is more influenced by porting, compression, and timing advance.

I'll agree with Scott...
Also add in combustion chamber design and the MSV factor...
It's more about the setup, for the most part, a ported engine really shouldn't be set up solely by a compression gauge. Learning how to do static measured cc's using a buret to calculate compression ratios will help you learn what limits can be applied to certain engine combinations.
P.S Word to the wise, always keep a note book of notes as you get older you can't remember it all. :thinking:

Chuck

 

[ItsRixter]

Filling the crossover passage on the intake manifold greatly improves the pulse signal clarity to the fuel pumps and makes the carbs more tunable. The passage is originally intended to offer smoother low-speed response for the stock engines.

Can someone post a pic of this?

 

[#ZERO]

Can someone post a pic of this?

Use the search feature and enter the word "crossover". :twak:

 

[ItsRixter]

This thread is so informative. I keep referencing it.

 

[sjetrider]

I have 760's in my B1 and my RN. Both are ported with Bpipes, heads, lightened flywheels, w/carb work done. I do like the 760 in the B1, but in the SJ it does not have the low end snap of a 701. I do like the power, but a 701 with similar mods would be more responsive off the bottom.

Depends on what your riding as well. The 701 may be more bottom end, but in the surf I don't miss that. I ride faster and that mid range is great.
To answer the question

61x cyl: Good strong cyl and with a little porting can be very snappy. and is pretty good in stock form.

62t cyl: A good cyl as well but has slightly higher port timing. With some porting can also be a very snappy cyl. May have slightly less bottom than a 61x. I have ridden mod versions of both and can barely tell a difference.

64u cy: higher port timing. Definately a more mid to top end cyl. I think the bore size is noticeable in torque gains (could be in my head). If running it in a SJ, I would run a lower pitch prop so it rides more in its powerband.

It seems that if you want a lot more pop from the 64u (760) cyl, higher compression really brings that cyl alive.

Good luck.

x2

 

[restosud]

didn't chuck say that you could safely mill the base of the 62t/64u cylinder to lower port timing?something like 0.085in?
i kind of remember him saying that one day but can't find it anymore.

 

[Patrick Star]

Wow, this is GREAT info. This is basically what I have been searching for going on a couple weeks now, I like the sticky idea too. This totally eliminates the 760 for me as a choice/temptaition cause I just want to focus on low to mid range power.
Thanks!

 

[jvann13]

I have the 64u cylinder and love the power. I am running a single 46 with a atp mani and I think the power down low is not bad at all. It has a ton more power than a 701 with a b pipe. Have not riden a 701 with porting or a head. That is my friends next mod and I am looking forward to seeing the difference.

 

[Yamer-Jammer]

My guess is, No. Bore size should not affect needed octane. I think that needed octane is more influenced by porting, compression, and timing advance.

Just FYI, bore size does affect octane.

(domes) - (compression) - (octane required)

701:

29cc - 230lbs - 155 octane
31cc - 215lbs - 110 octane
33cc - 200lbs - 100 octane
34cc - 192lbs - 98 octane
35cc - 185lbs - 91 octane
37cc - 170lbs - 91 octane

760:

29cc - 245lbs - 116 octane
31cc - 230lbs - 115 octane
33cc - 215lbs - 110 octane
35cc - 200lbs - 105 octane
36cc - 193lbs - 105 octane
37cc - 187lbs - 100 octane
38cc - 180lbs - 98 octane
40cc - 165lbs - 91 octane
42cc - 150lbs - 89 octane

Source: These links should be hot (if not just copy an paste)
http://www.adaracing.com/personal-watercraft/head-kit-yamaha-701-super-jet-girdled/ya701mhdkit
http://www.adaracing.com/personal-watercraft/head-kit-yamaha-760-superjet-girdled-billet/ya760mhdkit

 

[stefan]

Have someone here made a big bore 62t cylinder with big bore sleeves and 88 mm piston ?

 

[glennbbautomachine]

didn't chuck say that you could safely mill the base of the 62t/64u cylinder to lower port timing?something like 0.085in?
i kind of remember him saying that one day but can't find it anymore.

So has anybody done this?