Engine Tech

Breaking in a new Engine

There are a few schools of thought for braking in a new engine, baby it for a 1000 miles change the oil and drive normally. Or Drive it hard sooner than later. The primary task your trying to cover when breaking in a new engine is to get the new piston rings to seat. MotoTune has experience with breaking in engines and says it well.  

CIS Modifications

Wide Band O2 controlled CIS K-Jet

An engine equipped with a Bosch CIS (K-Jetronic) fuel injection system depends on the accuracy of the control pressure for 
starting and drive ability performance.  The WUR can be modified to provide for external adjustments by the addition of a pull-out
screw and nut which permits very accurate movement of the plug. 
DIY Link
At this time we believe the best CIS (K-Jetronic) upgrade is to trick the CIS computer with a after market wideband controller like an innovate
LC-1 or similar.  Here is how we see this working.  Thanks to Wclark vwvortex member for pointing this out.
In closed loop operation the K-Jet will try to keep the O2 sensor input to an average of about .45V by
increasing or decreasing enrichment via the frequency valve assuming averaging .45 results in an engine
running at stochiometric. It does not know what the AFR is only what the O2 sensor input voltage is. So if
we tell the ECU stochiometric is .1V and 13.7:1 AFR is .45V (as we are going to do at WOT) it is perfectly
happy to drive the enrichment to 13.7:1 (Lambda = .85).
LC-1/sensor replaces the stock NB O2 sensor. Analog 1 out is already programmed to look like a stock NB
(Lambda 1.0 in the middle, etc. ), the other analog output needs to be set with a similar range (0-1V) but
centered on your Lambda of choice (e.g. .85 - AFR 13.7:1 on normal gasoline) for WOT. You need a
Windows PC and serial cable (RS-232 to RS-232 or RS-232 to USB) to do this.
WOT switch is disconnected from the K-Jet ECU so it does not go open loop at WOT but continues to use
the O2 sensor input.
The relay connects both LC-1 analog outputs (one at a time) to the O2 sensor input on the K-Jet ECU.
Normal throttle positions has the Analog 1 (stock NB simulation) routed thru and WOT energizes the relay
routing the reprogrammed Analog 2 thru.
In practice any wideband controller could do the same thing as the LC-1/LM-1 so long as it has 2
programmable analog outputs. From what I see there arent too many others that have this. Also any other
wideband controller should be able to calibrate itself against the sensor itself because sensors age and
output versus oxygen content changes, causing errors in the output if not calibrated periodically. The
LM-1/LC-1 have a provision for doing this.

Intake Tuning

Power output of the engine is directly proportional to the amount of air and fuel that it can ingest, the goal is to pack as much mixture as possible into the engine during the intake cycle. Good article on the importance of proper flowing intake system and what affects air flow. The Intake Cycle Part One
Great article that discusses the many factors that affect total intake airflow, and horsepower output. Including intake pressure wave tuning.
The Intake Cycle Part Two

During the intake cycle, the piston is moving down the cylinder, the cylinder volume is increasing, intake charge is rushing in through the intake manifold then through the  open intake valve.  At low rpms the intake charge is rushing in relatively slowly compared to when the engine is at high rpms.  The intake system needs to be designed/engineered such that the intake velocity is not too low, otherwise the fuel and air mixture will start to separate.  If the intake design is such that the velocity is to high at low rpms then the engine will be very restricted at high rpms.  The intake velocity can be used to aid in the cylinder filling, if the intake valve is open long enough  ABDC.   (After Bottom Dead Center - piston has stopped going down and ready to move up).  The intake charge has inertia, more at high rpms.  This inertia will continue to fill the cylinder even when the piston has stopped pulling air in and started the compression cycle.  The two main factors effecting how much how much inertia the intake charge has is the length of the intake runner and the velocity of the air.  Short intake runners with proper high velocity will have less inertia then long runners and proper intake velocity.  One needs to strike a balance between these factors when setting up the induction system.  This duration the intake valve is open after BDC is dependent on the CAM design.  Look for your cams intake valve closing value in degrees ABDC(After Bottom Dead Center).  The later the intake valve closes(higher number of ABDC valve closing) the higher the RPMS needed to make its power.  Engines with cams that have high ABDC may make less low end power due to low intake velocity at low RPMs - in this condition the piston could be pushing some of the intake charge back out the intake valve prior to closing, this is not a good condition.  This condition could also effect the engines ability to idle properly at low rpms, and cause starting/idling issues with CIS based cars.

The cams intake valve closing value in degrees ABDC also directly effects the engines dynamic compression ratio.  Please take time to read up on dynamic compression ratio before you purchase that new CAM. 

 AUTOZINE TECHNICAL SCHOOL Tuned Intake and Exhaust Article



Dynamic Compression Ration Explained




air entering the cylinder

Exhaust systems

Exhaust tuning theory


As the exaust exits the cylinder through the open exhaust valve the gases are traveling at a high speed due to the combination of expanding gases and being pushed out of the cylinder by the up moving piston.  This is existing of exhaust gas has inertia also.  The inertia is moving away from the cylinder and can be used to help fill the cylinder with fresh intake charge.  With engines where there is no over lap period(intake and exaust valve open at the same time) then this inertia can create a slight vaccum in the chamber that can help draw air into the chamber once the intake valve starts to open, the gains are very minimal with this setup.  With an engine with a cam with overlap  the inertia from the exiting exhaust gases will start to draw in the intake charge.  With the proper sized exhaust system and proper exhaust port tuning the inertia effect can be more powerful inducing intake charge into the cylider than the pistion moving down on the intake stroke. 

More Discussion Elgin Cams


Pressure Wave Tuning

As the exhaust valve opens a few degrees before BDC(or at BDC for that matter), the exhaust gases exiting the combustion camber create a pressure wave. This pressure wave of exhaust gases also form a sonic pressure wave. A sonic wave travels the speed of sound. The speed of the exhaust gases travels slower and is based on the diameter or the tube and other restrictions. When the pressure high pressure sonic wave reaches a large area change, such as a collector, muffler, or end of pipe, the wave reflects backwards and changes pressure(from positive to negative).

So the high pressure wave exiting the head, entering the header, reaches the first pipe merge reflects back toward the head as a negative wave. Sucking additional gases from the chamber. If the valve timing(during overlap period) and wave timing is the same, then the (sucking) wave pulls air in from the intake system at several times the force than the piston does. Before the piston even has started to pull air in. When this happens the combustion chamber is filled with fresh charge, and the intake process is off to a quick start with high velocity intake charge. this has been called the 5th cycle on racing motors.

Here are some basic calculations I did to see how long the runners on the headers needed to be to give the boost of intake charge.

Sonic wave travels about 1130 ft/sec in hot air
Basic cam timing is 1/2 crank rotation.(keep things simple)
Equation ((1/2 crank rotation/rpm) x wave speed = total distance /2 = runner length
<math>((30/rpm) x 1130)/2</math>

rpm vs. runner length
1000 16.95 ft
2000 8.48 ft
3000 5.65 ft
4000 4.24 ft
5000 3.39 ft
6000 2.83 ft
6500 2.61 ft

It seems that tuning for 6000 to 6500 rpm is reasonable. That would mean that the first collector(point where two pipes merge ie area change) would need to be about 2' long. Since the port length on the 8v motor from valve to gasket surface is about 5 inches.

Links for reference
Exhaust Cycle Part One


Picking the Right Muffler

"The trend is that as flow is added to an initially flow-restricted engine, power increases rapidly at first then gains tail off. Once the available flow exceeds about 2.2 cfm per hp, the gains possible by increasing muffler capacity drop to less than 1 percent."
"Knowing that 2.2 cfm per open-pipe hp means zero loss from back pressure allows us to determine how much muffler flow your engine needs."
For instance, a VW 90 horsepower on open exhaust will require 90x 2.2 = 198CFM

Exhaust Science Demystified


The stock JH 8 valve motor found in the Rabbit GTI and some Scirocco's has a static compression of 8.5:1. There are various techniques to up the compression to up the horsepower and torque. Replacing the pistons with flat top pistons or decking the head. The pistons found in the JH motor have dished pistons.

Compression Ratio increase

% increase of horsepower(normally with equal torque increase)

8.0:1 to 9.0:1

1.8% increase

9.0:1 to 10.0:1

1.3 % increase

10.0:1 to 11.0:1


11.0 to 12.0:1


12.0:1 to 13.0:1

.6 %

13.0:1 to 14.0:1

.5 %

Source "Volkswagen Performance Handbook" by- Greg Raven 1987
Compression Ratio = (CD + PV + HV + HGV)/(PV +HV +HGV)

CD = Cylinder Displacement
PV = Piston Volume
HV = Head Volume
HGV = Head Gasket Volume

Cylinder Displacement = (bore x bore x stroke x .0031416) /4
Plugging in JH values in mm:
445.22 CC = (81 X 81 x 86.40 x .0031416 ) /4

Values for stock JH motor:
PV = 21 cc
HV = 28 cc (this varies between heads, and combustion chamber on the same head 28 cc -30 cc )
HGV = 9 cc

Compression Ratio plugging in JH values
(445.22 + 21 + 28 + 9)/21 + 28 + 9)
(503.22)/58 = 8.67  
Expressed as 8.67:1

Increasing Compression

Decking the head - removing material from the surface to decrease the volume of the combustion chamber. Removing 0.040" of an inch reduced the volume to about 26 cc.

Rerun calculation with revised HV values
(445.22 + 21 + 26 + 9)/21 + 26 + 9)
(501.22)/56 = 8.95  
Expressed as 8.95:1

Now that you see the math behind the calculations, there is a very nice online calculator to do this for you.
Online calculator: link



CC check March 25/2007 New Piston Volume = 10cc

Wheel Horsepower WHP & Crank/Block Horsepower BHP

WHP is the amount of power available at the wheels - including the drive train losses.
BHP is the amount of power available at the end of the crankshaft.
Obviously BHP numbers appear higher since they don't include the amount of power loss through the transmission etc..
Here is how to estimate BHP from WHP Assuming a conservative 15% or 18% drive train loss:
WHP + (WHP x .15) = BHP
or WHP + (WHP x .18) = BHP
FYI If you Dyno results show 100 WHP than you can estimate that the BHP is 115.
NOTE: Not all Dyno's are created equally, and produce varying results. Dynapack and Mustang Dyno's seem to give favorable results for the A1 platform. Dynojet results seem to be a bit low. Your mileage may vary. When comparing dyno results keep the dyno brand and correction factors in mind.


Increasing air flow into and out of the engine is how it makes horsepower. The majority of the improvement potential is in the head.

There are various techniques to increase the air flow. Here you'll find links to the best resources to describe these porting techniques and technologies.
Whats Extrude hone?

How to port and polish your cylinder head. With careful attention to port shape and smooth-ness you can port your own Mk1/ Rabbit/ Caddy/ Cabby and Scirocco 8v VW head
Porting article

How to port and polish your intake manifold.
Porting a 8V intake, got tips?

Tests results for various VW mk1 intake manifolds. Provides the best performance on the VW 8v motor?
A1 Intake Manifold Testing


List of terms and their meaning.
CC - Cubic Centimeters, a measure of volume
bore - diameter usable portion of a cylinder chamber
stroke - length usable portion of a cylinder chamber
static compression - compression not taking into effect cam timing, just compressing volume x into smaller portion of x
CE2 - second generation of central electronics,VW standardized wiring system based on the same fuse block and harness connectors from 1990.52 - 1992