Just talking about VWs

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

blane.c

Well-Known Member
HBA Supporter
Joined
Jun 27, 2015
Messages
3,879
Location
capital district NY
Rocker ratios? If you wanted to build up a big cube engine and take this threads advise to use a stock cam and single port heads for low end torque, would there be any advantage or disadvantage to using different ratio rockers vs. stock (considering stock 1.1 to 1)?
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
4,825
Location
US
Rocker ratios? If you wanted to build up a big cube engine and take this threads advise to use a stock cam and single port heads for low end torque, would there be any advantage or disadvantage to using different ratio rockers vs. stock (considering stock 1.1 to 1)?
I'm assuming you are talking about a direct drive engine, how many RPMs will you be turning? A large VW engine that will be turning at 3400-3600 RPMs or so for max HP would probably benefit from dual-port heads (see a related post from Pops, here). I don't think you'd benefit from a modified ratio on the rockers if you are using a racing/street performance head with the larger valves. They provide plenty of breathing for a large VW running at "only" 3600 RPM.
 

blane.c

Well-Known Member
HBA Supporter
Joined
Jun 27, 2015
Messages
3,879
Location
capital district NY
Cruise 2700 RPM, maybe 3000 RPM + or - max because large diameter propeller. I like large diameter propellers and low RPM engines so why build a screamer I won't like. I am wondering if using a stock cam, whether low RPM torque would benefit from say a 1.25 to 1 lifter compared to a 1.1 to 1 lifter or if it would be adverse to your low RPM torque goals?
 

Pops

Well-Known Member
Log Member
Joined
Jan 1, 2013
Messages
8,000
Location
USA.
Cruise 2700 RPM, maybe 3000 RPM + or - max because large diameter propeller. I like large diameter propellers and low RPM engines so why build a screamer I won't like. I am wondering if using a stock cam, whether low RPM torque would benefit from say a 1.25 to 1 lifter compared to a 1.1 to 1 lifter or if it would be adverse to your low RPM torque goals?
The ratio rockers are for better breathing at a lot higher rpm than you want to run, I would stay with the stock cam and rockers.
 

blane.c

Well-Known Member
HBA Supporter
Joined
Jun 27, 2015
Messages
3,879
Location
capital district NY
The pushrod tubes? The stock pushrod tubes will be to short on a stroker engine? Some of the after-market tubes are two piece with a spring and a additional seal (another place to spring a leak) are these the solution or is there a better method?
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
4,825
Location
US
Single-port heads vs dual port

We've had some discussion on single-port heads vs dual port. I think there's consensus that the single-port heads work well at improving torque at low RPMs (as I understand it, the proposed explanation is that the restricted/smaller pipe flows at a higher speed and "packs" the cylinder a bit due to dynamic pressure). But, at somewhat higher RPMs and larger engine displacements, the more restrictive single port heads don't flow well enough to get an optimum charge into the cylinder during the intake stroke.

I've never seen a comparative dyno chart, so I lack a true fact-based comparison, but here's some reasoning we can work from:

If I've got this right, VW engineers used single port heads in their 1500 cc engines. Dual parts were adopted for the 1600cc engines (there were some 16000cc engines with single ports, some with dual ports). So, apparently, somewhere around 1500cc and 1600CC is where dual ports were found to be useful.
It looks like the stock VW 1600cc engine puts out max HP at about 4250 RPM (see below).

If we assume this was a deliberate design decision (that VW engineers wanted their peak HP to be at 4250 RPM), then we can figure out the place where they found that they needed to go to dual ports to achieve that.
4250 RPM x 1/4 (since one intake stroke per RPM) x 1500cc (engine displacement)= 1,594 liters per minute. A single port was apparently okay for this average flow rate (i.e. single ports worked for 1500cc engines).
4250 RPM x 1/4 x 1600cc = 1,700 liters per minute. Somewhere at or below this level is where dual ports were found to be needed to achieve max HP at 4250 RPM.

So, let's pick 1,650 liter/min as an estimated average flow rate where the single port heads were found to be too restrictive at 4250 RPM.

Here's what RPM produces that average flow rate for some VW displacements we see in airplane engines:
1600cc: 4125 RPM (since this higher RPMs than a plane would use, single ports would always be OK/better for an aero 1600cc engine)

1835cc: 3596 RPM
1916cc: 3445 RPM
2180cc: 3027 RPM (so, for planes that will be turning 3100 to 3600 RPM, a single port wouldn't be enough)
2387cc: 2765 RPM

This is just some back-of-the-envelope figuring and, for simplicity, it ignores a lot of things (e.g. it assumes that average flow rate is a good stand-in for max flow rate, when we know max flow rate is more important for intake sizing. It also ignores any benefits to be gained from any higher flow velocity of a single port). Still, it at least is a set of numbers, and it agrees well with what Pops has mentioned previously as far as the likely spot (RPMs/displacement) where dual heads might be preferable.
 
Last edited:

BBerson

Well-Known Member
HBA Supporter
Joined
Dec 16, 2007
Messages
13,082
Location
Port Townsend WA
Re: Single-port heads vs dual port

Wouldn't dual ports of the correct smaller size for 3200 rpm be ideal?
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
4,825
Location
US
Re: Single-port heads vs dual port

Wouldn't dual ports of the correct smaller size for 3200 rpm be ideal?
Many folks use higher than 3200 RPM. Also, I don't know the mechanics of optimizing the intake diameter for power--I gotta think the geometry of the flow around the valves is at least as important as the diameter of the runner inside the head. Finally, if we are using automotive parts, we're stuck with whatever they make. A dedicated VW head for airplane use would be a great thing--better fins/flow at the exhaust port, smaller valves to preserve more space between valves and between valves and spark plug holes, a better spot for a second plug, etc.

Does your Limbach (or the Sauer engines) have a head specifically made for aircraft engines? At the price they charged, and given the need to have the audit trail needed for certification, I would think so. Does it have one port or two?
 

Pops

Well-Known Member
Log Member
Joined
Jan 1, 2013
Messages
8,000
Location
USA.
VW engineers design the VW engine for a highway cruise of 3000-3100 rpm @ 60 mph . Transaxle gear ratio/ 27.5" tall tires, etc. A stock factory built VW engine also has a smooth spot at those rpm's. But different years transaxles with different gear ratios can be swaped.
The 1200 cc, 40 hp engine develops max torque at 3600-3900 rpm ( can't remember which but I think its 3600 rpm). You can take a 1200, 40 hp engine out of a bug and install a 1600 cc engine and see a lot less torque at street rpm than you expect. Not a lot of different . You can take a single port 1600 cc engine out of a bug and install a 1835cc engine with dual ports heads and not see any driveable torque increase at street rpm's. The only increase will be at higher interstate speeds and rpm's.

BTW --- My two sons and I are going to the big VW car show and flea market at Columbus, OH. tomorrow. Think it the biggest VW get together in the U.S. Its east of town just off interstate 70 close to the drag strip. The Trailways drag strip always have their Mopar weekend at the same time. Will be a busy place. Couple friends of mine will have their Big GM V-8 powered VW's there.
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
4,825
Location
US
1650 liters min x 35.32 = 58,278 cfm? Trying to make sense out of this chart.

View attachment 73458
1 CFM = 28.317 cubic liters per minute. So, 1650 cubic liters per minute = 58.27 CFM

Remember,that's our WAG at something between the average flow required by the 1500CC engine and the 1600cc engine at 4250 RPM. The heads should be sized to accomodate peak flow, which might be >many, many< times the average flow rate (since intake flow only occurs 1/4 of the time, and it's not steady over that small fraction of the stroke).
I don't know how to interpret the chart. My questions would be:
1) What does the ".100", ".200", ".300" row represent? If it is valve openings, you'd need to know what size valves are used (some of the heads are sold with several sizes)
2) Is the CFM for one cylinder, one head, or one engine?

Any head that gives good performance in an 1835cc engine at 4000 RPM should provide enough capacity to feed a 2180cc or larger engine at 3600 RPMs.
 
Last edited:

blane.c

Well-Known Member
HBA Supporter
Joined
Jun 27, 2015
Messages
3,879
Location
capital district NY
That makes more sense. It had to be off by some factor of zero's.

A port and polish job on a single port head should improve it's flow rate quite a bit even with stock valves. The work is simple if you are just going to clean things up, matching flow is were it is at.

Does anyone know how to build an economical flow bench suitable for VW size heads?
 

Vigilant1

Well-Known Member
Lifetime Supporter
Joined
Jan 24, 2011
Messages
4,825
Location
US
A port and polish job on a single port head should improve it's flow rate quite a bit even with stock valves. The work is simple if you are just going to clean things up, matching flow is were it is at.
I guess doing something to improve flow would be most valuable if induction is a limiting factor, but is that the case for a VW engine in aircraft use? At least for the higher HPs (at any RPM), getting more output isn't especially useful because the limiting factor is the ability to get rid of the heat. We could pick up some HP pretty easily with smoother intakes, higher CR, etc, but it can't be used because the heat generated by burning the fuel for the additional HP will greatly reduce the life/reliability of the exhaust valves/head.

Porting and smoothing do reduce pumping losses slightly, but that wouldn't be noticeable.

Something that would benefit VW aero engine fliers: A test bench to measure how well heads shed heat. Build a standardized baffling setup, "cool tins" etc to go around the head and use a standardized flow around the outside (find out a typical difference in static pressure at the "in" and "out" of a typical airplane plenum and use that to establish the cooling airflow rate across the head). Insulate the head from other objects, fill the combustion chamber with very hot oil, keep everything heated until the head and oil are at the same high temp, then cover the top with an insulating layer, start the blower, see how long it takes to cool the boil by 100 degrees. Test a lot of heads and the comparison would do a pretty good job of showing which ones shed heat well. Weld on some fat fins and try more tests. Do some specific tests of the exhaust valve seat temps with the cooling flow on and a big heat source inside the head.
Maybe if something like that got published and popular, somebody that manufactures heads might take notice . . .
 

blane.c

Well-Known Member
HBA Supporter
Joined
Jun 27, 2015
Messages
3,879
Location
capital district NY
Carbon fiber (with the proper matrix) sheds heat well. It is susceptible to corroding aluminum (less of a problem with proper matrix). If you used add-on carbon fiber fins with a sacrificial aluminum (time life throw away) adapters/clips that hand press fit (clicked) onto the existing cylinder fins you potentially could pull additional heat from the cylinder for less weight. It would require experimentation.
 

pictsidhe

Well-Known Member
Joined
Jul 15, 2014
Messages
7,419
Location
North Carolina
That makes more sense. It had to be off by some factor of zero's.

A port and polish job on a single port head should improve it's flow rate quite a bit even with stock valves. The work is simple if you are just going to clean things up, matching flow is were it is at.

Does anyone know how to build an economical flow bench suitable for VW size heads?
Porting that yields an improvement is harder to do than most people think.
You can build a budget flow bench from a vacuum cleaner and water manometers. You don't have to keep pressure drop constant to see whether you are improvinf things, or not. Flow is proportional ro rhe square of pressure drop. It's not quite perfect, but thousands of dollars cheaper than a proper bench.
With a goodwill vacuum, you should be able to build a bench for under $100.
 
2
Top