This is my documented engine rebuild of my steel GTB engine. The engine had 83k miles and still ran very strong, but was burning a little more oil then I was comfortable with which was about a quart every couple hundred miles depending on how hard I drove. The engine dyno'd last before the rebuild at 199.7bhp at the wheels which I definitely classify as a very strong 308. Have not completed enough tuning to know the final outcome of this engine, but I suspect it will be around 215 at the wheels. Entire process took about two months, but I am still not anywhere near completed tuning the thing.
New Ferrea SS valves
New SSi guides
10.5:1 cpr pistons
new everything else
The guides had a good .010-.012 clearance in them and the stem seals were of course shot. Piston rings had about a .027-.029 gap. The main and rod bearings all measured well within spec and had about .001 clearance with all of them. The main especially looked really good, but all were of course replaced.
There were all kinds of issues with the machine shop who had the heads. In short, they sucked. Make sure you use a very good machine shop that stands by their work. If possible, use a shop that knows Ferrari or Porsche or the like.
The pistons are an old Borgo forged piston Ferrari made in what they called a 'sprint pack' which was essentially a factory kit which included these pistons, factory P6 cams, different carb jetting etc. For some reason, the factory quoted these pistons as being 9.7:1 compression, but the dome is a giant 15cc compared to the stock 6cc dome. With the 38cc combustion chamber my chamber cc'd to be, this puts compression well over 10.5:1 by my math. I do not know the original cost of this kit, but I know it was a very pricey procedure as the engine had to come out and be rebuilt from the ground up to do it. The factory quoted around 300bhp with this kit, but 275-280bhp is the realistic figure. I have P6 cams coming to me, but for now I am using the stock early US/Euro cams which I will set up and am expecting 260bhp once done tuning. The original piston rings with this kit were a hulking piece of kit and were very very tight. I substituded those rings for a much more easy going set of NPR piston rings which just happen to fit a '85 Toyota Corolla engine. The giant 5mm oil ring was the only odd-ball which I located with Total Seal rings.
The cams were timed to 105 degree lobe centers as opposed to the factory 96 degree intake
center and 91 degree exhaust center. Power is up in the rpm band a few hundred rpm.
Jetting in the new engine started out with 145 mains 190 a/c with F24 tubes but I am still really lean.
The first dyno session pegs me at 202bhp at the wheels, but the spark plugs had detonation over 6500rpm AND the mixture is 15:1. Current spark plugs are BP9ES which are pretty cool, but I am going for the non-projected insulator setup which will keep things really cool. I have a very strong set of Mallory ignition boxes firing things but I will still keep the gaps set to a reasonable .025 until I have things figured out a bit more.
Prior ignition timing was about 39 degree total. Needless to say I can't get anywhere near that at the moment. 35 degrees is the only setting where I don't get any detonation due to the hot plugs. I suspect once I fatten up the mixture and cool the plugs a shade more I will be able to get some more timing in there. I will have this figured out soon.
I remember when I first started tuning this engine a few years ago how awful my first dyno session was. I had just serviced the engine and set the timing spot on, but was still lean and came up with something like 177bhp at the wheels, which is actually pretty typical for one of these engines.
Just goes to show you how much bhp is left on the table for proper tuning.
The engine already driving much better due to a stronger torque curve.
Will keep everyone posted and if there are any tuning suggestions, please comment and let me know!