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Date:         Mon, 29 Oct 2001 02:51:30 EST
Reply-To:     FrankGRUN@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Frank Grunthaner <FrankGRUN@AOL.COM>
Subject:      On Engine Efficiency, Comparing I4s, WBs and Subies
Content-Type: text/plain; charset="US-ASCII"

Some time ago as I was surreptitiously lurking on the subaruvanagon list (before being caught and excised by Warren as an unwelcome alien) I perused a minimalist tome penned by pensioner that waxed enthusiastically about the modern design of the Subaru engines and their great efficiency as compared to the VW ilk. Well, as some of you might imagine, this stimulated my seating area, and I'm moved to respond. Numbers are good. Measurements are good. Mechanical (careful here) feelings are often not real. The theory of the nonlinear tail desperately sensing the positive improvement stimulated by the most recent and stunning infusion of cash into the bottomless cash consumer.

So, anyway, I decided to prepare an table that could compare the mechanical efficiency (and therefore engineering prowess) of the various engines that could power these bricks through the ether. The quantity of comparison I have chosen is the brake mean effective pressure developed in the combustion chamber while doing work on the piston. The number is derived from the peak torque developed at the flywheel per unit engine displacement. These numbers are readily calculated from the meager engineering data provided by the reclusive Subaru personnel, and are readily expanded by the addition of your favorite heat pump. I have arranged the values in ascending order of higher pressure (means higher efficiency). Pressure expressed in units of psi. In essence, the torque maximum represents the peak volumetric efficiency of the pump as well as the maximum efficiency of completely consuming the available fuel for air heating. This number is a function of the combustion chamber design, the intake and exhaust manifold flow dynamics, cam and ignition timing, etc.,etc. Bigger is better. (Also often newer.)

Engine Displacement Torque BMEP (liters) (ft.lb.) (psi)

Vanagon 2.0L A/C 101@3000 126.9 VW GX 1.8L I4 96@3000 132.8 VW MZ 1.8L I4 98@3250 135.6 Vanagon 1.9L WB 106@2600 136.6 Subie 2.5 2.5L WB (<96) 144@2800 136.6 Vanagon 2.1L WB 117@3200 136.7 VW 9A 1.8L I4 113@4400 140.8 VW HT 1.8L I4 105@3000 145.3 AUDI 2.3 2.3L I5 140@4500 149.7 VW RV 1.8L I4 109@3800 150.8 Audi 3A 2.0L I4 121@3200 150.8 VW ACC 1.8L I4 107@3500 150.8 VW AAZ 1.9L I4 TD 107@2500 150.8 Subie 2.5 2.5L WB (>97) 162@2800 150.8 Subie SVX 3.3L WB 228@4400 150.8 VW ABA 2.0L I4 122@3200 152.0 VW RD 1.8L I4 110@3200 152.2 VW AAA 2.8L I4 173@4200 152.6 Subie 2.2 2.2L WB 137@4400 153.0 Subie 2.5 2.5L WB P II 166@2800 157.4 VW PF 1.8L I4 114@3800 157.7 SAAB 2.0 2.0L I4 128@3000 159.5 TIICO (SA)2.0L I4 132@3500 164.5 Subie 2.2 2.2L WB P II 149@3600 166.4 VW AHY 1.9L I4 TD i 149@1900 193.7 VW 1.9 TDi1.9L I4 TDi 155@1900 201.5 Subie 2.2T2.2L WB T 181@2800 202.2 VW 1.8T 1.8L I4 T 162@2200 224.1 SAAB 2.0T 2.0L I4 188@3000 234.3

Some comments:

1. The table is in 12 point Monaco Font. 2. The engine longest in the tooth is the old Type 4, followed closely by the old low compression I4 CIS engines, and Vanagon WB engines. 3. Note that the pre 96 Subie 2.5L engine is just as inefficient as the Vanagon WB's. 4. The 1.8L Digifant engines (RV, RD and PF) are much stronger than the VW waterboxers as well as the 1.8L CIS and CIS-E engines. 5. The Subie SVX and the late 2.5 engines are no more efficient than the post 1988 VW engines. 6. The ABA cross-flow head (2.0L) and the VR6 are at the same generation and are very good. 7. The SAAB 2.0L (very similar to the chipped Digifant or Bosch LH-Jetronic driven Audi 3A or RV/PF engines) is a very efficient design. 8. If the TIICo numbers are to be believed, this 8 Valve head with Motronic management outperforms the ABA crossflow head with Motronic. 9. The latest phase II engine design of the Subie 2.2 is a competitive WB, as is the phase II 2.5L Subie. 10. The 1.9L and TDi engines are very efficient in this comparison. 11. The turbocharged gas engines lead the pack, with the highest efficiency. The volumetric efficiency achieved by the pressurized inlet far exceeds pumping losses due to inserting the turbo in the exhaust stream. 12. So, the VW WB's and the old A/C engine are really from an elder time, as are the same vintage Subies. The I4 engines are right there with the strongest. Only the turbos do better. Go to the turbo if you can.

Frank Grunthaner


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