High RPM: How to Get It & What It Means

Do you like to go fast? Well get out of that stocker and build a hipo motor for your VW. Come here to talk with others who like to drive fast.

Moderator: Tom Notch

User avatar
FJCamper
Moderator
Posts: 2670
Joined: Wed Nov 14, 2007 5:19 pm
Location: Birmingham AL

High RPM: How to Get It & What It Means

Post by FJCamper » Wed Dec 01, 2010 10:37 pm

Myth Busting.

Image

1. The more RPM the more power.

Untrue. An engine can, and will, stop making power as soon as its ability to "breathe" (pump in more fuel and air and use it) ends, even though it is still gaining RPM and making louder exhaust noise.

2. Air-cooled VW engines cannot reliably sustain high RPM.

Untrue. The opposite true statement should be "Inadequately balanced and oiled VW air-cooled engines made from parts too weak for the stress" cannot reliably sustain high RPM.

RPM numbers get thrown around on forums a lot, and the assumption is the higher the better. But good information and apples-to-apples comparison is rare, leaving most air-cooled VW engine builders using guesswork and making decisions based off hearsay.

You may even have seen where experienced builders disagree (to put it mildly) as to what works and why. That's because builders are experts in different fields and applications, and forget (or never learned) that what works on a drag engine might not work on a road racing engine, or on any degree of street engine between the two. Not to mention that many pontificators have opinions based on theory, not experience.

As the great philosophers say, "first define your terms," so we will.

What is "high" RPM?

VW made its street engines to run at a reliable 4500 RPM before trouble started, so let's take that as a baseline. Porsche took the same basic pushrod engine design and raised it to a reliable 6000 RPM in the Super 90 with a stronger engine case, counterbalanced crank, stronger/lighter valve train parts, and better oiling.

High RPM trouble here (VW or Porsche) means the crankshaft flexing in the middle and pounding out the center main bearing, pushrods bending, valve springs breaking or going into a weird state of oscillating "float," rocker arms breaking, connecting rods stretching, con rod bolts breaking, con rods smashing through the case roof, and valve stem keepers coming loose.

Even in the 1960's the Porsche pushrod guys were lightning their valve train parts to reach 7000 RPM, much less hold it. By the 1980's, SCCA championship pushrod 356's were running 8000 RPM.

The four-cam, four-cylinder Porsche engine used in the Spyders and Carrera models was originally redlined at 8000 for the 1500cc engines and detuned to 7000 for the 2-litre engines. Power ended at about 6200 RPM, however. And the lower 7000 number was on plain bearing cranks, not the more exotic roller bearings of the small engines ... and surviving.

And these four-cam engines could do 7000 all day. But you hear pushrod guys often mention 7000 and even 8000 RPM on this forum.

A "VW type" engine (few or no original parts in it) can turn 7000 with bursts up to 8000 (for upshifts to stay in the powerband) and last a season on the road courses, which is ten to fifteen engine hours. Such an engine consists of:

Aluminum case (for strength) not aluminum-magnesium
Counterweighted crank
H-beam type rods
Lightest possible pistons and wrist pins
Straight-cut cam gear or grooved aluminum cam gear
Lightweight lifters, grooved for better oiling
Tempered aluminum pushrods
Lightest possible rocker arms
Lightest possible valve adjusting screws
Double valve springs and light alloy keepers
26mm oil pump minimum, lightest possible oil weight
Windage tray or dry sump to keep oil spray away from crankshaft

Generally speaking, the shorter the stroke and the lighter the piston (which often means smaller bore) and valve train, the more RPM you can turn, but it also means (cammed right) your power band moves higher as well. A 1600cc and a 2000cc engine might both be able to produce the same power, but the 1600cc will have to do it at 7000 where the 2000cc can do it at 6000.

And you can see the steel pushrods we use for strength with heavy duty springs or double valve springs are way too heavy. This is another plug for Aircooled.Net's tempered aluminum pushrods.

So, the answer to the question "Can a pushrod VW engine hold 7000 RPM for minutes at a time and live to tell about it?" is yes.

Bugfuel
Posts: 3011
Joined: Fri Apr 20, 2001 3:01 am
Location: Front Royal, VA

Re: High RPM: How to Get It & What It Means

Post by Bugfuel » Wed Dec 01, 2010 10:44 pm

Well said :)

User avatar
MinamiKotaro
Posts: 2071
Joined: Sat Aug 06, 2005 8:26 am

Re: High RPM: How to Get It & What It Means

Post by MinamiKotaro » Wed Dec 01, 2010 11:00 pm

Or you could put a turbo on your engine and not need to turn any RPM to make power. :mrgreen:

Sorry, I had to say it. :mrgreen:
'67 Beetle, 2276
MS-1 v3.57

User avatar
yodogg
Posts: 143
Joined: Tue Sep 14, 2010 3:26 pm
Location: Phoenix,AZ

Re: High RPM: How to Get It & What It Means

Post by yodogg » Thu Dec 02, 2010 12:17 am

FJC,

Thank you for yet another informative and practical post.

Good luck with testing this weekend!

Bruce2
Posts: 6771
Joined: Sat Oct 13, 2001 3:01 am

Re: High RPM: How to Get It & What It Means

Post by Bruce2 » Thu Dec 02, 2010 1:13 am

FJCamper wrote: Tempered aluminum pushrods
I bet you meant "Tapered".

FJCamper wrote: Double valve springs and light alloy keepers
Keepers must be hardened high strength steel. You meant alloy retainers, right?
Aluminum retainers, or my preference, Titanium.

User avatar
FJCamper
Moderator
Posts: 2670
Joined: Wed Nov 14, 2007 5:19 pm
Location: Birmingham AL

Re: High RPM: How to Get It & What It Means

Post by FJCamper » Thu Dec 02, 2010 9:29 am

Hi Bruce,

Yes, retainers. Brain fade there.

But Aircooled has referred to their pushrods as tempered. We have a set that was installed on double valve spring heads on a 7k+ engine and the little buggers always endured. They do have dual tapered (a la Porsche 356) but the straight ones look like normal pushrods.

And Aircooled cites up to 9500 RPM for those pushrods!

FJC

PS: To you turbo guys. Yes, the turbo makes all this RPM exotica unnecessary.

User avatar
Kalle
Posts: 279
Joined: Tue Nov 16, 2004 3:10 am

Re: High RPM: How to Get It & What It Means

Post by Kalle » Fri Dec 03, 2010 4:13 pm

There are aircooled engines out there made to perform at high rpms and capable of 10 000 rpms.

1915cc - 240hp. Latest version 255hp. NA - pump fuel

http://www.youtube.com/watch?v=d2frvQNmuS8

1603cc - 196hp. Pump fuel

http://www.youtube.com/watch?v=VPVqYRrQBA0

Best rgs
BB

User avatar
yodogg
Posts: 143
Joined: Tue Sep 14, 2010 3:26 pm
Location: Phoenix,AZ

Re: High RPM: How to Get It & What It Means

Post by yodogg » Fri Dec 03, 2010 6:11 pm

Hej Kalle!

I live here in the states, but was born in Sweden and spent a great deal of my childhood in Stockholm and up in Norrland. Where in Sweden are you? Also, tell us a little more about those two engines!

Johan (Yodogg)

User avatar
doc
Site Admin
Posts: 3398
Joined: Thu Mar 31, 2005 5:38 pm
Location: Bentonville, Arkansas
Contact:

Re: High RPM: How to Get It & What It Means

Post by doc » Fri Dec 03, 2010 6:21 pm

I love reality posts. Thanks FJC.

doc

cal 67
Posts: 123
Joined: Wed Apr 08, 2009 7:03 pm

Re: High RPM: How to Get It & What It Means

Post by cal 67 » Fri Dec 03, 2010 11:19 pm

Those engines are coming from JPM Motorsport. . . Small engine gurus.

User avatar
Kalle
Posts: 279
Joined: Tue Nov 16, 2004 3:10 am

Re: High RPM: How to Get It & What It Means

Post by Kalle » Sat Dec 04, 2010 4:19 am

yodogg wrote:Hej Kalle!

I live here in the states, but was born in Sweden and spent a great deal of my childhood in Stockholm and up in Norrland. Where in Sweden are you? Also, tell us a little more about those two engines!

Johan (Yodogg)
Tjena Johan,

I`m from Norway and the two vids I posted are from JPM (Johannes Persson Motorsport) in Helsingborg, Sweden. JPM has been making some very powerful NA engines for a while now and it all started with a mouse engine thread on The Lounge. JPM has his own range of very high quality products like the MS230 heads, throttle bodies, pushrods and camshafts. He is still developing the 1915cc engine in the video above here and he hope he will have time to take it racing the coming season. Last I talked to him he believed the engine should produce 258 - 260hp @ 9700 rpms when finished and that he would start thinking about a big engine afterwards.

If you want big NA numbers with a high effect per litre there is no other way to reach those numbers other than high rpms. The challenge is to build a engine capable of taking the high rpms time after time and still keep it driveable at lower rpms. Short stroke, narrow engine, light valve train, good heads and mathematics are basic tools when you want to step up from internet builds and average results.

Best rgs
BB

Breaker
Posts: 75
Joined: Mon Apr 19, 2010 1:54 pm
Location: deepest, darkest Switzerland.

Re: High RPM: How to Get It & What It Means

Post by Breaker » Sat Dec 04, 2010 5:01 pm

Hello, I mean this in the kindest way,....but,

I dissagree with your first statement. The torque curve of certain engines, say stock vw, has good breathing, till, say, 2500 rpm...That is where breathing is best, the torque peak. And yet, as rpm increases, power goes up, till, say a max of 4100 rpm. Why? Because hp is a function of torque and rpm, and in the case of the vw, rpm is still going up, while torque is slowly dropping off, through the rpm, as breathing becomes more restricted.

I hope this makes sense, and is, of course, very different for other engines... The G.T.I.Golf, f'rinstance, has a quite high torque peak, and only a thousand rpm or two separates max torque from max bhp., meaning max power is quite close to max breathing, but still above it...

Again, all in the interests of better vw's...for all.

User avatar
joosep84
Posts: 113
Joined: Sun Sep 20, 2009 1:55 am
Location: Estonia
Contact:

Re: High RPM: How to Get It & What It Means

Post by joosep84 » Sun Dec 05, 2010 7:47 am

My first aircooled engine made 6500RPM with all stock and not balanced. I used it for a 1 season and cranck didn´t flex eaven with 2 pistons pointing to wrong way :D
Do I need better valve springs when I don´t go any higher but keep using the same shiftpoint?
I think I can´t go any higher with stock crank :roll:

User avatar
FJCamper
Moderator
Posts: 2670
Joined: Wed Nov 14, 2007 5:19 pm
Location: Birmingham AL

Re: High RPM: How to Get It & What It Means

Post by FJCamper » Tue Dec 07, 2010 7:54 am

Hi Breaker,

You are of course technically correct.

I was giving the short non-tech explanation, using the general term "power" rather than horsepower or torque, (and being a bit flippant about it) and if you'll reread my statement, you might see that we're both saying the same thing.

FJC

User avatar
yodogg
Posts: 143
Joined: Tue Sep 14, 2010 3:26 pm
Location: Phoenix,AZ

Re: High RPM: How to Get It & What It Means

Post by yodogg » Tue Dec 07, 2010 8:19 am

Kalle,
I apologize for assuming you were from "the flat land". ;) My father-in-law's family comes from Alesund (Valderoy) and Orsta in the western coast area.

Back to the subject at hand though. I understand that the crank, pistons (wt), valves, and valve springs all determine how high one can go in terms of rpm. However isn't it ultimately up to the cam characteristics and the air flow into and out of the engine (valve size, carb size and exhaust size) to determine what the engine does with those rpms? I would describe my own understanding of this relationship as basic at best and would be happy to learn a bit more about cam selection and aspiration.

Post Reply