Re: 335 vs GTR vs Supra Engine Talk
PerverTT;309667 said:
Shaun - Is this the reason for the use of an iron block in relatively modern engines like the ones used in the E46 M3 and in the VAG 1.8T? The former is designed to run at high revs while the latter is often subject to insane boost pressures. Had always thought of iron as a low cost, low tech material for use in an engine block.
The use of cast iron is just to keep costs low I believe. It's cheap and strong, but heavy.
To have an aluminium block as strong as, or stronger than, a cast iron one requires either lots of time spent designing it .. lots of FEA and tests...iterative evolution, etc. and good alloys, giving the result of high strength and rigidity per pound (F1 and other big money manufacturers' route), or a block with fairly large external displacement from being beefy everywhere, overbuilt, and not as light as it could be but still somewhat lighter than cast iron (route just about everyone else takes).
Even then there are very few apple to apple comparisons of cast iron to aluminium blocks. In the american V8 markets there are some extremely high quality aluminium versions of cast iron stuff, but they're never make as much power as cast iron. Typically 10-25 hp down on a 700-900hp engine. The thing is the aluminium stuff isn't built specifically to be as strong though since the block itself is 50-65% of cast iron weight, leaving you with lighter, better handling / accelerating vehicle.. which goes further in the direction of the goal which is winning races, not making the most power.
To some degree I don't think you can actually build an aluminium block using traditional alloys used so far, to do what iron does, because of aluminium properties.. is just ends having to be real thick everywhere assuming you absolutely match strength and rigidity. The thickness is not really a concern in most places except cylinder walls where you would end up with larger temp gradients covering a larger distance in the thicker walls. The greater expansion and contraction can't be easy to deal with in a long service life engine. There have been some solid alu billet blocks just about devoid of water jackets, but those are ultra short duration drag race only and still rarely used. Memory on F1 tech regs is fuzzy, but I think they have some degree of freedom to use non typical alu alloys up till a point. Their engines are extremely light for what they do, but also so finely engineered and replaced relatively often. Even if they were forced to use traditional alloys, I'm sure they live with the tradeoffs in strength and stiffness because light weight is such a big factor in laptimes. Of course they try to absolutely minimize tradeoff by perfecting it within their given engine weight limit.
In production cars, move away from cast iron is mainly due to wanting to keep weight noses light so all else equal, car doesn't handle like a pig. They're never chasing outright power from a given external displacement, so alu is perfect. Also to save overall weight so they can put more gadgets in the car.
Re: BMEP.. there are currently a few racing classes where NA gasoline engines that make close to and IIRC even exceed F1 BMEPs at peak power. Of course F1 is running those BMEPs at stratospheric engine speeds so it is a feat in itself. There also have been engines in the past used in similar applications (F3000 , high speed formula cars) that ran 17 bar BMEPs at peak power.. which is some sort of NA gasoline record. It was a result of rev restriction rule and the forcing of making more power through charging efficiency.
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Faster.. wow rod ratio.. that argument can cover pages and pages.. so given that I can't even cover enough on that topic to really do justice to a reply.. I will provide short summary.. forgiveness please. In this realm, especially in beefy blocks, it really does not matter despite what you've read in just about all the mags, hear the "gurus" mention. Ratios of 1.5 have been run very successfully in 24 hour races. Further when you take apart a a high mileage high stress engines of supposedly bad rod ratios, you still wind the bores clean up in a few thou, all except near the top where ring friction at breakover digs into the walls over time. You just never really want to give up displacement, where not regulated, to reach a rod ratio - unless your starting rod ratio is plain ridiculous.. like 1.2 and lower (not fixed) if that's even possible before running into other issues.. because once you factor the displacement and normalize for flow, your peak loads everywhere except on the thrust axis are lower, and the fact that is it higher still doesn't factor in the average block, much less cast iron stuff. The range where you actually have to start really calculating tradeoff is just never seen in the real world. When you gain displacement at the expense of rod ratio in the usual ranges, you see BSFC improve. Which shows overall efficiency gain despite greater thrust forces. And if those thrust forces aren't damaging anything over a long period, what else is there to worry about?
F1 has traditionally been at 2.6-2.9, but that's due to packaging limits of the airbox. As you cut down the decks on the V to reach lower more sane rod ratios, you reduce space available for an efficient inlet tract. The engineers have mentioned they would like to cut down the decks, save on block weight and rod weight, get rod ratios to much lower values, if it didn't end up with overall compromise on power because of the stacks on opposite banks getting too close together and the shape of the airbox with no space between the V. Again all tradeoffs.
Re: liners.... they are interference fit with a minimum of what will hold after block is up to operating temp (usually .002-.003 on a ~4" bore at room temp, depending on alloy). Liners helps somewhat with more evenly loading the aluminium behind it, but in the end if the base material is moving, nothing will help, so the base structure being strong and stiff is always better. I believe there have been some aluminium blocks run linerless with only a coating directly applied to the bores. Expensive stuff with specific design and very homogeneous castings to work.
Hot honing deals with the issue of cylindricity at operating temps. At room temps the bores are all distorted which is fine. F1 engines can't turn over without preheat, for this reason. Neither can any pure race engine, safely, without preheat. Clearances in general, including mains are also tiny, about 1/3-1/5 vs even other very high performance stuff.
