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. Works great, but pin is way too large and rods has to be bored razor thin.
L4-121 2.0L 3.5039(IN) 89.0(MM) COMP HT: 1.578 PIN DIA: .8002 Offset, Dish Top, 2.250” dia. head recess .155” deep. – Perfect fit except its low compression.
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 didn’t 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. I did get a pin tight after install and is making a clicking sound….. I should have removed piston and done it right, but I took the quick way out.
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 over sized 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.
Engine is still going strong with the new injection system (2nd Generation) and may be the last update as nothing really has changed. There is interest in having the pistons made, but not enough the warrant a large cash outlay by me, Clarks should have picked up on this project as it the only way to increase the cubic inches of the Corvair without the outlay of thousands in machine work. Remember, this is half way to a 3.1 L engine, so for the “naysayers” to say its not enough to warrant the cost makes so since as the parts are actually cheaper than currently available Corvair Pistons. The GM V6 Pistons, 1 mm over seem fine, but boring the rod might be dangerous on a high performance engine and don’t want to be responsible for destroying an engine. Might be a better way forward to make new rods that support the .900 rod end, but I see rod modifications (new) costing more than the pistons. I am happy with the engine, so that is where it stops unless someone else picks it up. A big thanks to Ed Kelly for his help and advice on this project.
Still running strong. Upgraded recently to Gen2 EFI with distributor option. Love driving this thing as its a auto with plenty of kick. Just finished looking at aftermarket forged rods that would fit (.9055 on the small end). I have to order $3600, but that is cheaper than the $7200 for pistons and rings.
This engine has become the test bed for new ideas with the EFI system. Its easy to access and I drive it as part of my job… so it has to run. I have installed GM Smart DIS and runs about as the other systems, but is super simple.