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engine will not rev passed 5000 rpm why
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Glenn Premium Member
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PostPosted: Wed Jul 23, 2014 7:11 am    Post subject: Reply with quote

Hyperspace wrote:
Jeff_Birt wrote:
I was watching a program the other day that mentioned that the VW air cooled engine is naturally rev limited by the fan. As the RPM of the fan doubles the power needed to drive it quadruples, i.e. goes up by power of two. At some point your engine can produce the power needed to rotate faster due to the fan. Kind of makes sense now why the drag race guys will not want to run the fan.


The fanbelt starts slipping at 4500 RPM.

Depends on how fast you get up there.
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PostPosted: Wed Jul 23, 2014 9:45 am    Post subject: Reply with quote

Using a carb CFM calculator based on your engine size you need

126.66 CFM @ 5000 RPM
139.33 CFM @ 5500 RPM
152.00 CFM @ 6000 RPM
164.66 CFM @ 6500 RPM
177.33 CFM @ 7000 RPM

Your Kadron 40's should flow 165 CFM each, so x2 = 330 CFM

So air flow should be there .. maybe need to add a wide band air fuel ratio sensor to make sure you're not leaning out/starving for fuel at high rpm, maybe need to get more fuel into the motor (bigger jets, higher volume fuel pump, make sure lines/hoses aren't restricted - may need to try a 3/8" fuel source line directly into a can of gas hooked up to the pump and see what it does).

The fan is an interesting idea .. pop the belt off and see what happens
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mukluk
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PostPosted: Wed Jul 23, 2014 12:03 pm    Post subject: Reply with quote

Still haven't mentioned if you're still running points or not... points have a tendency to float above 5000rpm unless you use a stronger spring... when they no longer follow the cam lobes in the distributor, you'll get intermittent or mistimed spark.
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Glenn Premium Member
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PostPosted: Wed Jul 23, 2014 12:13 pm    Post subject: Reply with quote

mukluk wrote:
Still haven't mentioned if you're still running points or not... points have a tendency to float above 5000rpm unless you use a stronger spring... when they no longer follow the cam lobes in the distributor, you'll get intermittent or mistimed spark.

True, some cheap aftermarket points are crap.

But if you''re using a Bosch 010 you want to use the 01 016 points which are for a early 911. I've run mine deep into the 7000 rpm range with no float.
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advinnie
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PostPosted: Wed Jul 23, 2014 12:42 pm    Post subject: Reply with quote

mukluk wrote:
Still haven't mentioned if you're still running points or not... points have a tendency to float above 5000rpm unless you use a stronger spring... when they no longer follow the cam lobes in the distributor, you'll get intermittent or mistimed spark.


Sorry mate I'm running good old fashioned points.
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PostPosted: Wed Jul 23, 2014 5:05 pm    Post subject: Reply with quote

advinnie wrote:
Sorry mate I'm running good old fashioned points.


Figured you might like to look into some of the cheaper, easier possbile answers before you end up going the route of replacement heads or gearbox.
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Danwvw
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PostPosted: Wed Jul 23, 2014 5:22 pm    Post subject: Reply with quote

You know it's not really about what happens above 5000 rpm in a car you drive on the road. But I built a 74mm using 88mm p&c's, the stock heads, port matched and the DRLA 36mm carbs for my bus and I never rev it past 5000 but I can tell it would go higher easy. It has the Web-cam 163 in it. Did you use the short Rods like I did? They may hold it back a bit combined with your cam!
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PostPosted: Thu Jul 24, 2014 2:14 am    Post subject: Reply with quote

Hi I used stock conrods. I have low compression on cylinders 1&2. Cylinders 1&2 read 115 and 105psi cylinders 3&4 are 125 and 130 psi
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PostPosted: Thu Jul 24, 2014 2:35 am    Post subject: Reply with quote

Did it ever rev to 6000?
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PostPosted: Thu Jul 24, 2014 3:02 am    Post subject: Reply with quote

When it had the stock crank it used to rev to 5500 with all the same cam exhaust and carbs as I have now all I've done is fit a 74mm counter weighted crank and a lightened flywheel and the max rpm have dropped.
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PostPosted: Thu Jul 24, 2014 3:41 am    Post subject: Reply with quote

degree the cam

maintain the geometries are consistent

maybe the keyway is off a few degrees on the crank

do the dirty investigations

most concur it's unnecessary for power and need to attain 5k with the configuration you have.


do you know what happens with long stroke

the entire combustion process changes
piston travel to crank rotation is changed
yea
all different

so change back

you built a truck motor
let it run like one


maybe if you hold it @5k
it'll loosen up that little bit
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advinnie
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PostPosted: Thu Jul 24, 2014 4:20 am    Post subject: Reply with quote

Tell me what happens to long engines
And have i built the engine? What I was after was an engine that would launch me off the line as fast as possible was going looking for 80 mph top end speed. It all bolt to gearbox from a 1200 vw beetle. And the engine is in a bug as well. If I have built the wrong engine what should I of built?
I'm about to start a 1905cc engine build 90.5mm pistons and a 74mm crank what you lot think?
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PostPosted: Thu Jul 24, 2014 5:06 am    Post subject: Reply with quote

This was some interesting information I found about maximum rpm being limited due to rotating mass

Quote:
Maximum Rpm

The HiPerMath equations are:

cubic inches = (3.14)(bore / 2)2(stroke)(number of cylinders)

maximum rpm due to mass = 0.0056(cubic inches)2 - 8.89(cubic inches) + 11527


What you do:

Choose carburetor or throttle body sizes, intake runner diameter, intake valve lift, intake valve duration, intake valve size, bore and stroke to produce the maximum rpm you want.

Factors to consider:

1. Maximum rpm will be determined either by the mass of the moving parts in the engine or the air passages (throttle body, intake runners, intake valves and exhaust valves).

2. The higher the ci (cubic inches or in3) displacement you have, the lower the maximum rpm. Engines with longer strokes and larger pistons have more mass rotating, so there is more stress on the crank. The crank can only handle so much stress, so rpm is limited.

Small bore and stroke allow for a high revving engine. It is possible for the small bore and stroke engine to draw in as much air as a large bore and stroke engine, but the air will be drawn through at much higher rpms.

3. The larger the carburetor or throttle body size, intake runner diameter, intake valve lift, intake valve duration and intake valve size the higher the maximum rpm. Air velocity is limited to the speed of sound, which is 767.58 mph (miles per hour) or 67547.4 fpm (feet per minute). Large carburetor or throttle body size, intake runner diameter, intake valve lift, intake valve duration, or intake valve size all cause the air to move slower and reach the speed of sound at a higher rpm. This is good if you want your power to come in at higher rpms. Large diameters and lifts allow the engine to breath, increasing maximum rpm at the cost of the power coming in later.

Small carburetor or throttle body size, intake runner diameter, intake valve lift, intake valve duration, or intake valve size all cause air to move faster and reach the speed of sound at a lower rpm. This is good if you want your power to come in early and you want big torque. Larger bore and stroke do the same thing, since each stroke of the piston draws more air.

Definitions:

ci = cubic inches or in3
stroke = the distance the piston travels in the cylinder.
bore = the diameter of the cylinder.
rpm = revolutions per minute


Source: http://www.hipermath.com/engines/max_rpm
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PostPosted: Thu Jul 24, 2014 5:21 am    Post subject: Reply with quote

So .. using that formula maximum rpm due to mass = 0.0056(cubic inches)2 - 8.89(cubic inches) + 11527

1699cc = 103.68ci

0.0056(103.6Cool2 - 8.89(103.6Cool + 11527 = 10,665 rpm limited by rotating mass

(If I did my math right .. someone smarter than me please double check)

Hmm .. I don't think I would want to be within 100 feet of an air cooled vw turning 10k

So there goes that theory
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PostPosted: Thu Jul 24, 2014 6:10 am    Post subject: Reply with quote

advinnie wrote:
I think i have built the wrong engine for what I wanted Sad
I was after a engine that would give good acceleration of the line/ street racing. The engine to bolt to a gearbox from a 1200 vw beetle so the engine needed to rev high to get good use out of that gearbox.


Yep.large bore short stroke revs higher and builds torque later

small bore long stroke revs lower and builds torque earlier..


But before you condemn your build run it some ,see if you can master the combination...you wont have to rev as high at the line as a short stoke engine to gain max torque...
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PostPosted: Thu Jul 24, 2014 10:48 am    Post subject: Reply with quote

Well it's all sort of limited stuff, The cam is not that advanced, the heads are just stock, the cylinders are stock, small, there just isn't enough room in the cylinder for it to breath that way. To fix it, use solid rocker shafts and consider the thick wall 88's slip in piston $ cylinders for about $150 or $160. (You may have to trim the skirts on them a little). Is that 278' Duration with 1.4 rockers? Large ID pushrod tubes, so put the CB-Performance 1565 tubes on when you do the cylinders. Either have the heads ported nicely or go with new heads, I am looking at the Steve Tims 37.5mm intake valves for my engine. SuperStock heads, L3 heads, and the Mofoco Super Street 040 heads might be a good choice for a small bore engine like ours, My 88mm thickwall pistons & cylinders use the 92mm bore heads.
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PostPosted: Thu Jul 24, 2014 2:12 pm    Post subject: Reply with quote

I have no idea if the 284 duration is at cam or with the 1.4 rockers. I will look at the cam card I a bit and get back to you.
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advinnie
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PostPosted: Thu Jul 24, 2014 2:14 pm    Post subject: Reply with quote

[quote="johnnypan"]
advinnie wrote:
I think i have built the wrong engine for what I wanted Sad
I was after a engine that would give good acceleration of the line/ street racing. The engine to bolt to a gearbox from a 1200 vw beetle so the engine needed to rev high to get good use out of that gearbox.


Yep.large bore short stroke revs higher and builds torque later

small bore long stroke revs lower and builds torque earlier..


But before you condemn your build run it some ,see if you can master the combination...you wont have to rev as high at the line as a short stoke engine to gain max torque...[/quote

ok how about large bore and large stroke? 78.4mm crank and 90.5mm pistons? What sort of engine will that make? A high rpm motor or a truck engine?
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PostPosted: Thu Jul 24, 2014 2:18 pm    Post subject: Reply with quote

advinnie wrote:
Tell me what happens to long engines
And have i built the engine? What I was after was an engine that would launch me off the line as fast as possible was going looking for 80 mph top end speed. It all bolt to gearbox from a 1200 vw beetle. And the engine is in a bug as well. If I have built the wrong engine what should I of built?
I'm about to start a 1905cc engine build 90.5mm pistons and a 74mm crank what you lot think?


Torque is good. Did you drive it?
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advinnie
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PostPosted: Thu Jul 24, 2014 2:28 pm    Post subject: Reply with quote

Yes mate i built it and have put just over 500 mile on it now. It is a massive improvement over the stock 1600 engine. It pulls hard in every gear untill 5000 rpm where it just stops accelerating.
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