Questions on compression, squish clearance and pipes

[ottoman]

Ok so I am new to this forum and fairly new to Honda scooters. I have built many fast Classic type mopeds.. Puch, Tomos, Garrelli etc.and a Zuma based Nitrous drag scooter.

I have been reading the performance and build threads and I keep seeing most people recommending to keep your compression readings in the 135 or so range.

When I build a fast ped I always shoot for between 150 and 175 PSI compression and I found that between .5 and .6 mm squish clearance works best to produce the most power.

The biggest difference I see is fan cooling verses free air.. but I would think that Fan and shroud cooling would be better... am I wrong on this?

The other difference is most ped builds are around 74cc.. slightly smaller than the Honda builds I see on here... another possibility.

Also I never seen the types of pipes such as the V8, PG short and long type I see here. Everything I ran was always more traditional style expansion chambers.

What is the advantage of this type of pipe... if any... over the type of pipe I am use to using?

I always like to learn from guys who have "been there and done that" so I am looking forward to hearing from you guys!

[Bear45-70]

OK, here is my 2 cents on why majority of these guys are burning down their BBK. TOO MUCH CARB! Which makes proper fuel distribution next to impossible to achieve. If they want to run a BIG carb they should be running a CV style carb instead of these slide carbs. *, my Aero 80 has a factory spec of 142 to 200 PSI for compression, but only has a 16 mm carb. The others burning up their builds are doing it because they haven't a clue as to what they should be doing.

[Wheelman-111]

Greetings:

Quoth The Bear:

*, my Aero 80 has a factory spec of 142 to 200 PSI for compression, but only has a 16 mm carb. The others burning up their builds are doing it because they haven't a clue as to what they should be doing.

I represent that remark. Ouch! But learning more each day about the fan-cooled engine's limitations. Big pilot jets protect from off-throttle heat spikes when using bigger carbs, which create a lot of Heat to go along with big Top-End wallop.

Trouble is when throttle closes, you can't just pull in the clutch to let the engine return immediately to idle. Instead momentum drags on for as much as 30 seconds, with reduced fan output over a relatively limited area of cooling fins. Less fuel means less oil is supplied. Even worse with throttle-controlled oil pump if connected.

All this limits the ability to wring maximum output from a hi-po scooter used as a Daily Rider.

Chamber pipes can be phenomenal if tuned to Perfection. A few, like Sanjuro's Burial pipe, are so tuned. The PG Long is attractive because it's relatively cheap and has a pretty wide range of compromise to work at medium to high RPM with a variety of displacements. The chamber I had was On/Off, while the PG powerband is more broad.

[bakaracer]

OK, here is my 2 cents on why majority of these guys are burning down their BBK. TOO MUCH CARB! Which makes proper fuel distribution next to impossible to achieve. If they want to run a BIG carb they should be running a CV style carb instead of these slide carbs. *, my Aero 80 has a factory spec of 142 to 200 PSI for compression, but only has a 16 mm carb. The others burning up their builds are doing it because they haven't a clue as to what they should be doing.

What is the factory spec compression ratio? Yes not 10to1 or higher. The point of lowering the compression on the kitted elite 50s is so that is can run on pump gas for daily riding use without knocking and blowing the engine up. With the aftermarket kits,the compression ratio is above 11.5to1 which is to high for pump gas on these 2 stroke engines

[capturedbyrobots]

OK, here is my 2 cents on why majority of these guys are burning down their BBK. TOO MUCH CARB! Which makes proper fuel distribution next to impossible to achieve. If they want to run a BIG carb they should be running a CV style carb instead of these slide carbs. *, my Aero 80 has a factory spec of 142 to 200 PSI for compression, but only has a 16 mm carb. The others burning up their builds are doing it because they haven't a clue as to what they should be doing.

I keep hearing this from experienced tuners.
So....
for a standard 70cc kit, what size slide carb do you suggest?
14mm, 16mm, 17.5mm, 19mm?
So in theory, a small carb should be fine as long as it's jetted properly....yes?

I did hear the the BWS100 has a 16mm carb stock, I think from Overrev....

[Bear45-70]

I keep hearing this from experienced tuners.
So....
for a standard 70cc kit, what size slide carb do you suggest?
14mm, 16mm, 17.5mm, 19mm?
So in theory, a small carb should be fine as long as it's jetted properly....yes?

I did hear the the BWS100 has a 16mm carb stock, I think from Overrev....

The factory uses a 16 mm carb on their 80 cc scooter motors. They us 24 mm carbs on their 80 cc motocross hot rod motors. Nowhere do they use 28 or 30 mm carbs on less than 100 cc engines and rarely then.

I wish the Japanese used CFM (Cubic Feet per Minute or some variation of that system) ratings rather than venturi size for rating their carbs, since there is a simple formula for determining how much carb you need for a specific sized engine at a specific redline RPM.

A small carb is much more forgiving of minor jetting errors than a really big carb. Also the small carb's metering systems start flowing sooner, that is at a lower RPM that a big bore carb does. I had a Holley tech at the Tulsa Nationals back in the early 70's tell me that "A lot of little holes will always work better than a couple of big holes." He was referring to using 3X2 barrels or 2X4 barrels rather than a single really big 4 barrel on an engine.

For street use a smaller carb enhances low end power and throttle response, whereas a big carb hurts both those with only a minor improvement in top end power and speed. I still think a CV (Constant Velocity) carb would be a better way to go. It helps retain the low end throttle response while still allowing the big bore carb for that little extra top end.

For the 72 and up cc crowd a 24 or a 26 at the most. I am considering a 20 for my 80 cc scooter, but then I'm only looking for a few mph increase and not the max I can pull from the engine. Oh I have been told (I have no proof yet that the Aero 125 uses the same 16 mm carb the Aero 80 does. I find this hard to believe but this would explain the rather disappointing top end that they show.

[ottoman]

In my experience with 70-75cc mopeds.. if the porting was on the performance side the bigger carb always gave better acceleration and top end. I played with 15, 16, 18,19,20,21 and 24mm carbs. Most of these motors were one speed non variated.
The smaller carbs might have been crisper in the first 20 feet but after that the bigger carb was always better... when tuned right.
The smaller carbs always gave better fuel mileage... but how many people really care how many miles per gallon their scooter gets?

[ottoman]

Chamber pipes can be phenomenal if tuned to Perfection. A few, like Sanjuro's Burial pipe, are so tuned. The PG Long is attractive because it's relatively cheap and has a pretty wide range of compromise to work at medium to high RPM with a variety of displacements. The chamber I had was On/Off, while the PG powerband is more broad.

[/quote]

so a PG long is a more forgiving pipe with a broader power band than a Chamber pipe (as you call them) but I suspect makes less power than a chamber pipe when the chamber pipe is "on the pipe"... Interesting.

I know when I was building my Zuma based dragger the only race pipes I could find were the chamber style... and there were I bet a hundred to choose from... I mainly see the PG long type pipes on Hondas.. or maybe I am looking in the wrong places

[Bear45-70]

In my experience with 70-75cc mopeds.. if the porting was on the performance side the bigger carb always gave better acceleration and top end. I played with 15, 16, 18,19,20,21 and 24mm carbs. Most of these motors were one speed non variated.
The smaller carbs might have been crisper in the first 20 feet but after that the bigger carb was always better... when tuned right.
The smaller carbs always gave better fuel mileage... but how many people really care how many miles per gallon their scooter gets?

Ah, a seat of the pants performance recording device I see. Have done the racing thing for many years, that's in several different venues with 2 and 4 strokes and I know without a doubt you are wrong. Your are the guys I beat for the several national championships I won. Bigger is better only works with CID and then only if you can make it live.

[ottoman]

In my experience with 70-75cc mopeds.. if the porting was on the performance side the bigger carb always gave better acceleration and top end. I played with 15, 16, 18,19,20,21 and 24mm carbs. Most of these motors were one speed non variated.
The smaller carbs might have been crisper in the first 20 feet but after that the bigger carb was always better... when tuned right.
The smaller carbs always gave better fuel mileage... but how many people really care how many miles per gallon their scooter gets?

Ah, a seat of the pants performance recording device I see. Have done the racing thing for many years, that's in several different venues with 2 and 4 strokes and I know without a doubt you are wrong. Your are the guys I beat for the several national championships I won. Bigger is better only works with CID and then only if you can make it live.

Well I will put my experience up against yours.... who knows you may have more.

I owned and operated a Racing engine machine shop in Wisconsin for 15 years before I sold out to my partner. I was in the business for many years before I went out on my own.
Built everything...every type of oval motors from Sprint cars to 4cyls, drag engines, tractor pullers (gas and diesel) road race engines, motor cycle engines (drag road race and flat track) did head work (porting etc) for the factory Harley flat track team. Did R&D work for Kohler company' engine division... heck cant remember all the stuff we were into.
Had all the toys.. flow bench, 1600hp land and sea dyno.. did everything in house... crank grinding, balancing everything... been there done that until I just burnt out.
Now I build mopeds and scooters for fun and I dont come on here saying I know everything.. in fact I was asking questions because I respect those who have built fast stuff and I know I can always learn from others.
I am willing to share my experiences and things I know to work but I mainly like to read about what others have found to work or not work.
Yea I do a lot of seat of the pants dynoing... its th oldest form of dyno you know
But I also run my stuff against my other peds to see if it really is faster and I rely on my GPS also.
And yes my 24mm Mikuni flatslide accelerates the fastest and posts the highest top speed on my Puch 74cc Giladoni E50 and I tried everything from 16mm up to the 24mm... I was going to try a 26 but I never got around to it. Once you slow down and you cant tune it out, then you went too big... screw the theories the race track is the only test that doesnt lie.

[Bear45-70]

Well I will put my experience up against yours.... who knows you may have more.

I owned and operated a Racing engine machine shop in Wisconsin for 15 years before I sold out to my partner. I was in the business for many years before I went out on my own.
Built everything...every type of oval motors from Sprint cars to 4cyls, drag engines, tractor pullers (gas and diesel) road race engines, motor cycle engines (drag road race and flat track) did head work (porting etc) for the factory Harley flat track team. Did R&D work for Kohler company' engine division... heck cant remember all the stuff we were into.
Had all the toys.. flow bench, 1600hp land and sea dyno.. did everything in house... crank grinding, balancing everything... been there done that until I just burnt out.
Now I build mopeds and scooters for fun and I dont come on here saying I know everything.. in fact I was asking questions because I respect those who have built fast stuff and I know I can always learn from others.
I am willing to share my experiences and things I know to work but I mainly like to read about what others have found to work or not work.
Yea I do a lot of seat of the pants dynoing... its th oldest form of dyno you know
But I also run my stuff against my other peds to see if it really is faster and I rely on my GPS also.
And yes my 24mm Mikuni flatslide accelerates the fastest and posts the highest top speed on my Puch 74cc Giladoni E50 and I tried everything from 16mm up to the 24mm... I was going to try a 26 but I never got around to it. Once you slow down and you cant tune it out, then you went too big... screw the theories the race track is the only test that doesnt lie.

OK , How about 2 National High Point Championships along with 2 second in National High points and 7 Divisional Championships and 13 Regional Championships and 3 National speed records and all this with only minor sponsorship. But then I was also professional factory certified mechanical and also was a mechanic for an offshore race team. I raced drag raced cars and did all my own mechanical work plus tuning and won locally but back then I was still in the service and had little money, but I won anyway. I also ran my own shop until I almost died in a motorcycle accident. But hey, I actually did win on my own without having to have a complete race/machine shop to support me.

Oh and this makes no sense what so ever; "Once you slow down and you cant tune it out, then you went too big..."

You can't teach me much about winning that I haven't already learned but hey you can try.

Oh and I know an engineer who will dispute you statement "...screw the theories the race track is the only test that doesn't lie." His credentials makes you and me look like we have done and know nothing.

[capturedbyrobots]

Uh oh...
* match...

[sanjuro]

I see where otto and bear are both coming from. Wish there was a reset button so there wouldn't be any acrimony.

[ottoman]

Oh and this makes no sense what so ever; "Once you slow down and you cant tune it out, then you went too big..."

Oh and I know an engineer who will dispute your statement "...screw the theories the race track is the only test that doesn't lie."

Actually it makes perfect sense.... you keep going bigger until you slow down....you will know on the track when you are over carbed.... you lose measurable performance.

So if your engineer friend says a 16mm carb is the optimum for my my motor and I turn a faster time with a higher speed in the 1/8th mile (for instance) with a 24mm... that makes me wrong? Heck then I want to be wrong!

I have worked with many engineers over the years... numbers are an exact science but how you apply them to a problem isnt.
If you put 10 engineers in a room and ask them a technical question you will get a minimum of 5 different answers.

I doubt these guys want to listen to a couple old has beens having a * contest.... lets just have a drag race.

[ottoman]

Hers is my scoot... lets rock!

PS... its over carburated