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  #31  
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Keithybloodygeorge Keithybloodygeorge is offline
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Quote:
Originally Posted by chevelle View Post
Some of that info is from my dyno threads.
My 4&5/8" 65" stroker runs 3500 pistons advertised at 9:1.
I also have run P cams w/ this combo.
Runs great, but is a challenge to kickstart w/ a mag.
I believe the minus minus exh cams would ease the kickstarting quite a bit.
I'm not a fan of the minus minus int. cams but if kickstarting
a magneto stroker is a challenge for you than they might come in handy for that purpose.
That thread is such a great read Chevelle. I've been back many times to digest all the info but I know there's still plenty I haven't got my head around yet.

I know your thoughts on the minus minus intakes but would the soft bottom end pick up with the increased compression? Did you get to run any tests with different CR pistons by any chance?
Quote:
Originally Posted by Ferrous Head View Post
I think we all go looking for that holy grail of a flat torque curve all across the rev range that start's out very high and just stays there. Sigh.
In the end we all have to make compromises. What we gain on the round-a-bouts we lose on the swings. That's just how it is with engine tuning.
If your hoping to kick this thing into life on a regular basis I would suggest using something like the minus minus cams.
Yes, you will lose some bottom end torque. But you have to remember that your GAINING a lot of bottom end torque by building a bigger engine to start with. So the loss of the bottom end torque is only RELATIVE to what the engine COULD make.

I also suggest that using P cams will result in a cranking pressure that won't be suitable for pump gas.

The down side to using something close to minus minus is that the engine will now make more HP. Up at 6 grand which is fine for a race engine but scary fast on the street. What your building is a bike that's capable of 150MPH. (With a nod to Ralph Nader I was going to add "on a bike that's not safe at any speed" but thought better of it.)

If your heads are set up for the higher lift cams then later on if you feel the bike needs more bottom end you can change out the cams easy enough.
Yep I hear ya Ferrous there's always some give and take

So you know what CR can I reasonably run on 95/98 octane without huge risk?

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  #32  
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Personally, I think you're way too concerned with the compression ratio. Unless you want to build an all out motor; 9 or 10:1 is really good for a streetable motor. The further you push the CR the more heat you build and the more vital everything else becomes.

Instead, I would go for the biggest displacement change you can afford and can afford to kick.

You gain your power in a stroker from the displacement and the resulting torque rush. A higher compression ratio WILL create more "push" on the piston -- hence more torque. But if you aren't modifying everything else or riding in the type of setting where you NEED and can USE that extra torque -- then you may just be creating a hassle to gain very little in the rev range in which you ride.


When I built strokers for guys . . . I rarely went over 10:1 and most were 9:1. I went for max displacement and mid-range (2500-5000rpm) power. That is the recipe I found most guys were really looking for.

My normal formula for my own bikes when getting hot rodded is:
1) 50-60 rear wheel horsepower and 50-70 foot pounds of torque
2) Must run on 89 octane without severe detonation up a hill in HOT weather
3) No more than 500 pounds of bike -- wet.
4) No more than 180 pounds of rider -- sorry tubby folks.
5) Sticky, sticky tires
6) Really good suspension and chassis
7) Must not require any special work for at least 10-20K miles.

I found this formula to result in the most grins per $ spent over the longest period of time.

Your results may vary.

Conversely, I couldn't stroke the sprint or severly overbore it. So, I worked with a guy in NC to have a 10.5:1 wiseco piston fit into an aluminum cylinder (my year sprint still had cast iron) and a trick head. That woke the motor up; even though I did nothing else to the bike. Stock carb, stock exhaust. I instead focused on the timing and took a long time to dial it in. I've now got a reliable 25-28 rear wheel horsepower and some really nice torque in a bike that weighs about 275 pounds. It does get HOT when I get on it -- but with 90-91 octane I haven't had it ping.

What sucks is kicking it. It starts easily -- that's the easy part. What isn't up to snuff is the kicker mechanism. I upgraded to a later version for more strength, but I am routinely stripping kickers once a season. The splines just disappear and then I have to bump start it. My NOS stack is down to one set . . . and I will likely have to machine a new set out of stronger material. I'm dreading buying a splining broach . . .

Oh, that and I've cracked every fender strut at least once :-) But it does rev cleanly to 9500ish . . . and it is addictive.
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I dont think the high comp. ratio is worth the trouble for the minimal gains in a small stroker.
I also like being able to fill up at the gas station.
I wouldn't recommend running any piston that is not easily replaceable, but any 3" stroker pistons are fairly rare so....
I believe there was a seller on ebay selling 3" stroker pistons, but a member here said he could not make them work, he thought the above pin height was incorrect I believe.
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  #34  
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Chuck's on the money where he's telling you to build what I call a "happy" engine.
And he's right about the heat. Heat is always the enemy.
A lot of people build drag race engines and then want to ride them a lot longer than 11 seconds. The heat builds and you get problems.
Because of all the variables involved I prefer to work on cranking pressures. Measured correctly this number will tell you what is really happening inside the engine as opposed to what the numbers on the bits of paper say.
What I found was that an engine stressed to the max (hauling a serious load) for 20 minutes or more really needs 100 octane gas with 165 PSI.
Now 165 is not a lot. That's about what you get with a lightly modified 900 on a "good" carb and exhaust system. Slightly here just means bigger valves, ported heads and Andrews H style cams. And that would be with 9 to 1 pistons.

Most of us never run WOT hauling 900 lbs of rider and bike around a track for 20 minutes. So you can get away with the above setup with 92-95 octane gas.

Your engine will be happy with a lot less compression than you think. I regularly see engines with 120 PSI and the owners aren't complaining about the lack of torque - they whinge about the smoke.

We all like torque and the easiest, cheapest way to get torque is to increase capacity. Higher compression's also work AGAINST the engine as you probably know. Try kicking over a really high comp IH and you find out in a hurry what the engine has to overcome while running.
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Oh. JE makes 3" bore stroker pistons. I have a JE part number somewhere but I believe there is still at least one business selling them on EPray. They are nice pistons and running in a couple of my race engines. Maybe a little heavy (I think 413 grams from memory) but they certainly seem to do the job.
Having ready access to stroker pistons in stock to +79 over with 0.010 steps is really cool. And they run out at something under $350 ? a set.
They do probably have a little too much dome to run with P or PB cams. But better cam selection or two thick base gaskets would fix that.
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Quote:
Originally Posted by Keithybloodygeorge View Post
I’m also toying with the idea of measuring the combustion chambers properly to get some real accuracy on the figures but I don’t know if that’s overkill on a build like this
I don't think taking measurements of this stuff is overkill at all. It's all good information to have. For example there is an accurate 3" hemisphere cut in the head and that's what you're using for your calculations. But in that hemisphere there is a depression caused by the sparkplug hole and you've got two. There are 2 big valves that can easily increase the volume of the chamber a few cc's if they are recessed from a valve job. The counterbored area that the fire ring goes into should be added to the hemisphere part to get the total head volume. This total volume will be closer to 8 cu. in. than the 7.068 you used.

This head volume will be decreased by the fire ring and the piston deck which will be above the the head gasket surface. That distance is something you'll need to figure out. And then there's the piston dome. Another volume you'll want to know.

If the piston can be put in the cylinder the most accurate way to find the clearance volume is to assemble the engine, tilt the engine and fill the cylinder at TDC through the spark plug hole with fluid from a burette.

Your calculations are based on a claimed piston C.R. of 10.5 to 1.
That is a number that would apply to a standard bore piston. So if no changes are made to the dome and the cylinder is at .090 overbore the extra cylinder volume will increase the CR a half point. So 10.5 becomes 11 to 1. You can't take anything for granted. Measurements are good, not overkill.

If the piston has a thick enough top a great way to decrease compression is to lower the height of the dome. If you can get the compression you want with a lower dome that's a plus. And the thickness of both gaskets are part of the game too.
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  #37  
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Quote:
Originally Posted by Chuckthebeatertruck View Post
Personally, I think you're way too concerned with the compression ratio. Unless you want to build an all out motor; 9 or 10:1 is really good for a streetable motor. The further you push the CR the more heat you build and the more vital everything else becomes.

Instead, I would go for the biggest displacement change you can afford and can afford to kick.

You gain your power in a stroker from the displacement and the resulting torque rush. A higher compression ratio WILL create more "push" on the piston -- hence more torque. But if you aren't modifying everything else or riding in the type of setting where you NEED and can USE that extra torque -- then you may just be creating a hassle to gain very little in the rev range in which you ride.

When I built strokers for guys . . . I rarely went over 10:1 and most were 9:1. I went for max displacement and mid-range (2500-5000rpm) power. That is the recipe I found most guys were really looking for.
I’m actually most concerned with keeping the CR to a safe limit more than anything. The fact that these pistons are quoted at 10.5:1 is the bit that worries me but of course if I’m aiming for a number I might as well make it the right one I guess.

Your description above sounds pretty ideal to be honest chuck

Quote:
Originally Posted by Ferrous Head View Post
Chuck's on the money where he's telling you to build what I call a "happy" engine.

And he's right about the heat. Heat is always the enemy.

A lot of people build drag race engines and then want to ride them a lot longer than 11 seconds. The heat builds and you get problems...............



We all like torque and the easiest, cheapest way to get torque is to increase capacity. Higher compression's also work AGAINST the engine as you probably know. Try kicking over a really high comp IH and you find out in a hurry what the engine has to overcome while running.

I hear ya Ferrous, this ain’t no drag engine and I definitely need to be able to kick the bugger!

Quote:
Originally Posted by Ferrous Head View Post
Oh. JE makes 3" bore stroker pistons. I have a JE part number somewhere but I believe there is still at least one business selling them on EPray. They are nice pistons and running in a couple of my race engines. Maybe a little heavy (I think 413 grams from memory) but they certainly seem to do the job.

This is good to know

Quote:
Originally Posted by chevelle View Post
I dont think the high comp. ratio is worth the trouble for the minimal gains in a small stroker.
I also like being able to fill up at the gas station.
I wouldn't recommend running any piston that is not easily replaceable, but any 3" stroker pistons are fairly rare so....
I believe there was a seller on ebay selling 3" stroker pistons, but a member here said he could not make them work, he thought the above pin height was incorrect I believe.

No I really don’t want it too high either chevelle. I was mostly asking out of curiosity regarding the different cam options. I’ve read about the need to adjust your static CR for different cams and wondered whether you’d ever played about in that space

Quote:
Originally Posted by needspeed View Post
I don't think taking measurements of this stuff is overkill at all. It's all good information to have. For example there is an accurate 3" hemisphere cut in the head and that's what you're using for your calculations. But in that hemisphere there is a depression caused by the sparkplug hole and you've got two. There are 2 big valves that can easily increase the volume of the chamber a few cc's if they are recessed from a valve job. The counterbored area that the fire ring goes into should be added to the hemisphere part to get the total head volume. This total volume will be closer to 8 cu. in. than the 7.068 you used.

Yeah there’s a lot of variables there NS. I’d love to do the full blown fluid measurement but I guess we’ll see down the track. Either way I’ll play with these numbers as best I can before settling on the stroke. At this stage the 4 9/16 is looking good though if I just run the full set of siftons

Thanks for the input guys, it’s much appreciated


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