Hi all, I thought I would share some of my experiences and findings from dyno testing I have done. All the runs were done with the same car, same gear on the same dyno. The test car for these runs is my 73 914-6 conversion. All the runs were done with the same header and exhaust configuration. The exhaust is a 1-1/2 MSDS header with early two in modified to two out early muffler. Previous testing had show this muffler to be very close in power to megaphones on a stock 3.0SC motor.
I have now had 4 different engines in the car over the last 5 years. I will describe the build on each and the results.
The first motor I built and tested was a 2.4L. It started life as a 72T but was rebuilt with the following. 70.4 stroke crank with 84mm pistons for actual displacement of 2.339L. It had factory 2.4 S pistons listed as 8.5 to 1 compression and E cams. The heads were 2.4T with ports slightly opened to 33mm. The induction was MFI with throttles matched to the heads at 33mm. MFI had stock air cleaner system without warm up valve on inlet. I did quite a bit of tuning with the MFI to get good Air fuel mixtures across the RPM range. Dyno below.
The next engine I built was also a 2.4L 70.4 x 84. On this engine I use 73 CIS 2.4 heads. I removed the CIS bungs and bored and ported the heads to 36MM intake 35mm exhaust to match the dimensions of some 2.7S heads I had. This engine used T cast iron cylinders and JE pistons. I CCed the heads and pistons to achieve 9.5 to 1 compression. Cams were factory E cams as previous engine. Induction was a set of modified Zeniths with bored and ported stock manifolds. Air cleaner was stock Porsche with snorkel cut about 4 inches from cover.The chokes were changed to 30MM and the main jets changed to 145. The dyno below is a comparison of the first engine to this.
The big take away from this engine is that the bigger ports will kill low and mid range power. is not true. This engine with big ports made more power and torque everywhere I am sure the extra compression helped as well. Other take away is that E cams made peak power at just over 6000RPM.
The next engine I tested actual a friends that I helped him build and we tested in my car. This engine was a 2.7 built to RS+ specs. It was 70.4mm x 90mm for 2.685L. Pistons were factory RS with 8.5 to 1 compression. Cams were Elgin MOD S and heads were stock 2.7S with 35mm Intake and 35mm exhaust. Induction was Weber 40IDA with PMO manifolds. It used same factory air cleaner as previous 2.4L. This engine made very good power. Dyno below comparing 2.4 to 2.7, no replacement for displacement and hot cams.
The last engine I built is still in the car. It is a long stroke 2.5L 70.4MM stroke with 86MM bore for 2.452L. This engine uses 86MM nikasil cylinders and JE pistons. After CCing the compression ratio is 9.8 to 1. Heads were like previous 2.4L, they started as 73 CIS heads and were bored and ported to match 2.7S heads with 36MM intakes and 35mm exhaust ports. Cams are Elgin MOD S and induction is Zenith carbs on PMO manifolds. Carbs have 36MM venturi with 145 Mains and 110 Air correction jets. I will detail the difficulty getting these carbs to work with this engine in another thread. This used same factory air cleaner as previous engines. So lessons learned on this engine is that more modern cam grinds really help power everywhere over factory E cams. I also did some testing of the air cleaner system that I will detail later. Dyno comparison of 2.4 E motor to 2.5 MOD S motor.
Take away for me iis the MOD S cams are actually less peaky than factory E cams. The E cams made peak power at 6K and the MOD S cams made peak power at 5700 RPM and more torque everywhere. I know some of the torque is due to larger displacement but not all of it. Modern cam profiles just out perform old factory grinds.
I had read that the factory air cleaner used with Zeniths and MFI systems helped mid range power. I wanted to test this assumption. After getting some base line numbers I removed the Air cleaner cover and did another run. I was very surprised to see that removing the air cleaner cover made a big difference in power. Dyno below is with and with out air cleaner cover.
12HP added with air cleaner removed. So now I wanted to see what was causing the loss so I removed the air filter but left the front cover on. Dyno below with and without filter.
So filter cost a couple HP. Next test was to remove the front of the air cleaner. Dyno comparing with and without front cover but no filter.
Result was more power. So even though I had the inlet snorkel cut off making a larger inlet it was still a restriction.
So last was air cleaner with no front cover or filter compared no air cleaner to see if it helps with low end torque.
There is a little bit of gain with the air cleaner on at 3700 RPM and 5000 RPM but I am not sure it makes up for the loss over 5500. What is clear is that if you run the factory air cleaner make the inlet as large as possible.
Last dyno is comparison of current engine in 914 and 911 SC 3.0 CIS engine in my 911 SC race car. These were also tested on the same dyno. The SC was stock class car with 9.3 to 1 factory pistons, CIS and headers. It made the same power as several other stock class similar engines. The 2.5 made more power and torque than the 3.0SC with stock cams.
2.5 makes more power and torque than 3.0 SC at all points about 3400RPM.
john