Reply to "B. Goyaniuk's motor questions"

Bohdan,

Good afternoon!

I am enjoying my Friday off, the weather here in So Cal is warm, with our "Santana" winds blowing air from the desert, the skies are clear & blue, it's the reason So Cal is so wonderful this time of year. I can see smoke from a brush fire rising from the mountains in the distance, that unfortunately is also a usual occurence when the Santana winds blow.

To get to your questions, I'll answer the 2nd question first as it is the easiest to answer. I'll refer you to my recent post to DeTom regarding blueprinting an engine, wherein I mentioned part of blue printing is having the machine shop make the bores perpendicular to the crankshaft, that is what indexing is. Of course its not needed, the motor will operate without this proceedure, but to blue print an engine assembly it is necessary. If a bore is not perpendicular to the crankshaft there will be extra friction & thrust forces on the rotating assembly & cylinder walls.

Consider this, I know you are concerned with the thin cylinder walls of the 351C block, hence your desire to only bore the cylinders 0.010" to 0.015". You want the walls thick enough to survive the thrust forces applied during operation. By indexing the bores you are significantly reducing the thrust forces that concern you.

We can stiffen those cylinder walls by the way by applying a half fill of "hard block" in the water jackets, a subject we should cover at a later time.

One thing I question is your chosen mechanics experience with the 351C motor. To a degree the 351C is a different assembly requiring different handling than other motors. It is best to use somebody completely familiar with this assembly. A Cleveland mechanic knows the cylinder walls can be thin & that you always shoot for a 0.030" over bore, never 0.040". I've never experienced a standard bore block needing more than 0.030" to true up the bores. So his comment about needing a possible 0.040" overbore to index the bores gives me reason to warn you to be cautious. This by no means implies he's not a great mechanic & engine builder, just brings his experience with the 351C into question. Of course, maybe I just got lucky to have never needed to go 0.040" over. But I never heard the engine shops I used quote anything but 0.030" over for a Cleveland.

To answer your question about valve sizes, I can only provide guidance rather than hard numbers. I have no experience with AFD & CHI heads. In general, the smaller port, higher velocity cylinder heads tend to run smaller valve sizes. For instance the C302 heads used 2.15" & 1.65" valves. The Yates heads currently run in NASCAR use 2.10" & 1.60" valves. My best advice would be to consult Dave at AFD, tell him what displacement your motor shall be and ask his advice on what valve sizes to run. And for porting, it is always best to find somebody already experienced with the head, they will know what work to do to achieve the desired results. Somebody new to the head will be experimenting with your cylinder heads, make sense? I believe AFD offers porting of their heads. Camshaft profiles & timing are then selected to compliment the characteristics of the head. Porting a head to compliment a cam would be putting the cart before the horse. Again, AFD is advertising that they will be offering a line of camshafts to compliment their cylinder heads.

One last comment about cylinder heads while I'm on the subject. What your engine builder said is true, modern heads have higher velocity ports, higher turbulence combustion chambers & more centrally located spark plugs, therefore it stands to reason they should make more torque than the older cast iron heads. But the Cleveland heads are not that bad, the combustion chambers are actually quite effective.

As a cylinder head design improves, the amount of ignition advance required decreases, an engine that only requires 30 degrees total advance is assumed to be more thermally efficient than an engine that requires 40 degrees of total ignition advance.

My experience is that a naturally aspirated Cleveland with cast iron heads requires 34 to 42 degrees of total ignition advance, depending upon the specifics of the motor. I have heard that the new alloy heads require 30 degrees or a bit less, so I have assumed they are more thermally efficient. However, earlier this year some Cleveland owners on the Cleveland Forum informed me thay are only running about 32 degrees total ignition advance on their motors with cast iron heads. These settings were arrived at with dyno testing. This is less than I've ever experienced, but I think it is safe to assume that todays fuels & camshaft grinds are different than they were when I was building motors, and this would explain the less total advance required today by Clevelands with iron heads.

So here's my point, if a 351C running 35 year old iron heads only needs 32 degrees timing advance running modern fuel & camshaft grinds, and the alloy heads are requiring 30 degrees running the same fuel & camshaft grinds, just how much more efficient are the new allow heads?

Only back to back testing on a dyno will reveal the differences, which I am sure are subtle. The differences will not be night and day, they will not be large. This brings us back to what I have written over and over, the 351C iron heads are not a week link. For a 35 year old head, they are pretty damn bitchen, which is why Chevy guys have always feared Cleveland equipped autos. Which is also why the current small block Chevy motors running in the new Corvette have canted valve heads very similar to Cleveland heads!

If I have time next week, I thought I would stir the pot a bit & write a post on the DTBB about the Cleveland oiling system. This will be info those who are building motors will find useful. I'll provide the cold hard facts, just like you appreciate my engineer friend! I'm thinking next Friday would be a good day to do it since I'll have the day off.

take care

your friend on the DTBB, George