Fordboy- in one of your excellent posts here you mentioned the effect of rod ratio on flow demand and position of that max. Along with this was the comment that a 4 valve head with better low lift flow than the average 2 valve would, generally speaking, work better with a shorter rod (lower rod ratio) to take advantage of the low lift flow since location of peak demand was somewhat sooner in the cycle. Since you and Midget will need to destroke the K motor to fit the class how are you planning to help mitigate this issue or take advantage of it. You can't just shorten the rods or you get a seriously top heavy piston (longer compression height) which has it's own disadvantages. I am particularly interested as I find myself with a short block with similar issues and am going from a very good 2 valve head to a very good 4 valve head that does indeed have better low lift flow, but similar high lift flow to the 2 valve. I'm not sure there is anything I can do but am curious what possibilities there may be. The 4 valve head was designed for the displacement I am running but the bore and stroke are a bit different than what it was designed for. Less of a mismatch than the 2 valve head was. Thoughts?
Jack
Jack,
I'm working on flow bench adaptors and some simulations for the 'K', whilst juggling some other projects, AND, most importantly, some home improvements regally decreed by the "Mighty Queen". If you catch my drift . . . .
One of the issues I have not rambled about is the "match" or "balance" between "flow demand" and "flow capacity".
"Flow demand" is the minimum the engine needs. "Flow capacity" is the induction tract's ability to fulfill this "need". Current ideas about the definition of "flow capacity" revolve around the inlet tract flow numbers combined with the valve train's "ability" to provide the "flow" required Vs crankshaft position. I'm not sure how clear that is conceptually, so I'm going to post up the graphs I generate for this project as we go along, so readers can gain a better understanding of what is going on.
Also of importance to note is that for normally aspirated engines, the ability to "fulfill" "flow demand" early enough in the cycle, might be unlikely with a given "build geometry" and "valve train". That puts the engine in "catch up mode". And it is also why the "intake ramming" portion (BDC to intake valve close) of the intake cycle becomes so critically important. This is where comparatively "small" changes to either the "build geometry" and/or the "valve train" can pay off. It is also important to note that when I say "valve train", I mean
everything from the cam to the valve, and, probably including the cam drive method as well.
The point being is that if you have the opportunity to choose some of these dimensions/parts/etc, it is important to choose "wisely". I don't think you can know any of this without spending some time doing the number crunching and analysis. My own personal opinion is that I would rather know the numbers, (even if they are "poor") than just rely on chance/dumb luck. I want every opportunity to influence/improve the results of my efforts.
You are correct though, when you say that there are situations where you can't do very much. But I am of the opinion that it is always a "compromise" anyway, so I want to pick the "best" compromise based on several factors, say planned rpm range, "realistic" part loadings, airflow capability, etc, etc. Somebody said,
"It's complicated", I agree. I think you are going to have to make some hard choices with your build and you should run the numbers so you can make the best choices.
The 'K' is going to have some of the same problems you are encountering. Namely:
1) We have to destroke. Reducing bore isn't really a good choice for this build. You might be in a different situation.
2) F1 type rod/stroke ratio. We are stuck with the block deck height, for various reasons.
3) Flow capacity perhaps "too large". This remains to be verified by flow testing.
My initial thoughts are:
1) That since the stock compression height is very short, we can shorten the rod a "bit" without making the piston too heavy.
2) Flow capacity can be altered appropriately with high intensity camshafts.
My thinking on this is subject to change without notice, based on the measurements & analysis . . . . . .
One final comment: If you utilize a low duration cam/valve train to reduce "flow capacity" and/or increase velocity, you probably can't fulfill "flow demand" on a timely basis. In other words: You screwed yourself . . . . . . This is why it's important to do the math and pick "the best compromise".
Fordboy