2.9L /174 ci Large Bore Engine

20141218_152817 20151207_083808[1] 20151211_223940[1]


The 174 cubic Inch Corvair engine is the largest bore that can be safely achieved (in our opinion) without machine work to engine block or heads as with big bore engines.  These engines use a full-fin cylinder and is bored to our specifications by Clark’s Corvair.

We are planning to bore the small rod end to use a .90 pin to use the 2.8 GM V6 Piston.  This piston pin block requires machining for some clearance issues for the crankshaft.

Our original design was to take the Corvair Piston and have enlarged to that bore (3.5439 in) using better alloys and designs that today’s engines use.  The reaction of the Corvair community was mute as they usually wont buy anything not made or endorsed by a major Corvair vendor. We have the design and quotes together for the manufacture of this piston in Forged or cast, but we will not act until we get enough orders to justify such an outlay.

The GM 2.0  and 2.8 piston are not that much different except the 2.8 piston has a slightly larger piston pin block and has to be shaved.  The GM 2.0 is almost perfect for the Corvair except that it has the low compression dish.

V6-173 2.8L 3.5039(IN) 89.0(MM) COMP HT: 1.578 PIN DIA: .9055 Offset, Flat Top

L4-121 2.0L 3.5039(IN) 89.0(MM) COMP HT: 1.578 PIN DIA: .9055 Offset, Dish Top, 2.250” dia. head recess .155” deep.

Nov 2015

We are starting the project with the lowest HP engine possible based on what we had.  This initial engine is to prove the piston/cylinder combination.  This 174 ci engine has -6mm dished pistons and has the same rod pin size as the Corvair (.8002) and using non-machined 110 heads .  This will make the effective CR around 8.4:1 and should be able to use lower grade fuels in higher ambient temps (summer).   We will replace these dished pistons and heads with non-dished (flat pistons) and machined heads (step removed) for a higher horse/CR engine.

24 Nov 15

Engine is built, installed and running in the 61 Loadside and is strong to be a low compression engine, but I didnt get the increase in HP due to the low compression.   The EFI system was installed with the Cubic inches and timing increased 6 degrees but still allows regular fuel without pinging. Very satisfied with low-end torque but missing the peppiness of a higher compression engine and the rule is still correct…. nothing replaces cubic inches.

15 Dec 2015

Low compression piston have been pulled since the modified rods arrived that will accept the GM V6 pistons.  These flat top pistons provide about 9.4:1 CR without any head work.  Timing had to be retarded from the low compression pistons, but a notable increase in HP is present.  The next upgrade will be heads with the step removed to improve burn performance.  Were only at 500 miles into the project and will be a daily driver until it proves itself as dependable or one of those cool rods are hanging outside the case.

1 June 2017

Engine is still running without a hitch.  Has been taken off the road for a new paint job and will be returned to service as our work truck by 1 August 2017.  I had 3 serious inquiries into the 2.9L project, but far from the 20 commits I needed before dropping $7000 for pistons. One piston has a small bit of slap due to tight pin, but our only worry was boring the Corvair connecting rod to .900″ from .8002″ to accept the larger pin of the 2.8 oversized pistons.  Clarks Corvair bored the full-fin set of cylinders and was of excellent quality and are performing well.  At the 25000 mile mark, and nothing has came through the side yet???…. but its a Loadside automatic, so not much drag racing here.