Lotus Super Seven, Engine Rebuild, Part I
March 19, 1996
As some of you may remember, I removed the Seven engine last Thanksgiving and delivered it to the engine builder soon after. I knew it had poor compression in the 3 and 4 cylinders and about 18,000 miles on the engine. I thought it was a stock 2270 Ford Range engine, currently available in England, new.
My engine builder is Mike Stimson in Redwood City, California and does Formula Fords and Porsches. (The latter engines are all over his shop and made me a bit nervous...) I have his permission to discuss this on the Internet, but he does not have access.
He has torn the engine down and looked at all the parts. The crank has been magnafluxed and is ok. Many of the parts that experience wear, like the chain tensioner, show little wear in fact.
The piston rings in 3 and 4 did not bed in, and there are black carbon deposits past the compression rings on those pistons. Mike mentioned the back carb may have been running rich, allowing gas to wipe the walls of lubrication. But this would cause the rings to wear prematurely, would it not? Nevertheless, now I know why the compression was bad. Pistons, bearings, cam, rockers, block, and all are in nice shape and reusable.
The engine is not stock. It has been ported, at least on the intake side, and he mentioned it looked like it was done in England, whatever that means. The cam appears to be a Cosworth A2 (3000-7000 rpm, other specs uncertain...).
So, what does he suggest? The target is about 120 hp, free reving engine, with a broad power range, lots of torque. I do not want a road race engine with a narrow range and lots of power high up. Autocrossing needs the former, and I am not so competitive to require the latter for our open track days.
Convert the head to big valves, using Honda Accord intakes and Nissan 300 ZX exhaust valves. These are modern technology, hardened, and modern materials. He will help shape them. Special bronze guides and dual springs with an interferance fit to damp out harmonics.
Surface the head, and block. The latter will reduce the squish as the combustion chamber is in the pistons and the head is flat. Compression will go from around 8.7 to 9.3 or so just by doing these. Some power here.
Deglaze the cylinders. Regarding the current discussion on using a torque plate, he says it is not necessary here because all we are doing is cleaning the bore and getting it ready to seat new rings.
File rings to fit. He will get .005 over rings and file to fit. Some magic here I suspect.
Lighten the fly wheel. I thought perhaps getting an aluminium flywheel might be nice to improve response, but he thought the price was $300! JAE?
A racing balance. There are specifications here for how many grams an inch out and he mentioned something like a racing spec is 3 gm +/- ? but his guy does it within a gram.
He originally suggested matching the intake manifold, but later thought the match is close enough, although a bit off center.
FelPro gasket set, TwinCam rod bolts, a steel cam block, and possible oil pump rebuild finish off the known stuff.
He talked about some people using DCOE 45s in place of the 40s, and he can enlarge the intake manifold to accomodate them, just rounding the entry and radiusing it. I have not decided but I have this extra set of 45s from my new 41C project car that I could use until it needs to be returned.
I was fascinated with a couple of other concepts. He talked about how the bottom end needs to breath as well as the top end. I understood good breathing getting the good air in and the bad air out, but the bottom end? It turns out the air needs to rush around under the pistons as well and you can radius the webbing in the crankcase, around were the cam passes to greatly facilitate the flow. Good for 5 hp but expensive, like five hours! I passed.
No modifications suggested for the distributor, but he can reduce the lash.
No modifications suggested on the cam, which surprised me because this is where I thought much of the power could be found. He thinks the power will be great and does not suggest a higher lift or longer duration. The former will cause wear and tear on the valve train for little return, and the latter will affect the smog. I still have hopes of being able to easily smog this car.
Things we are not doing, because they are expensive or because the hp gain will be minimal include:
port the exhaust port intake more (he will check these two after cleaning the head) grind rockers for more lift check valve to piston clearance (he says we are not modifying this so much as to worry about it) fab a scraper rail lighten rods, pistons, pins shim distributor end play grind the cam hardened valve seats (He suggests a 1:4 racing gas mix if I am going full throttle at events for any length of time) and something about intake and exhaust seals...
Comments, suggestions, thoughts are welcome! What did I miss? What am I doing that I should not?
And he thinks the goal of 120hp is reachable with this setup. I can hardly wait!