Wow. Great info guys. Curious though, I thought that an I/E ratio of 75% was where you wanted it to be on a stock or ported set of LT1 heads. I thought that 80% was too much exhaust flow, if there is such a thing. If 75% isn't where you'd want it, where would the optimal point be and why?

 

I'm a little pooped after a day of racing. Take a look for revearse split cams in the search feature on advance tech. There is a good thread on that. I've never found a problem with 80% I/E ratio heads, good exhaust flow seems to help, but not as much as intake flow.

If you don't find what you need, I'll probably come back and put soemthing else down later.

Bret

 

My example is different because it's designed to work best with a supercharger. Cars with either a SC or nitrous need more than a 75% E/I ratio.

I am not sure of how to calculate the optimum, but I know knowledgable builders of serious nitrous race motors will increase exhaust valve size at the expense of the size of the intake to a surprising extent. Nitrous, even more than a blower, is demanding of exhaust flow as compared to intake. In terms of getting the nitrous into the cylinders, a port nitrous system is fairly insenstive to the whole intake side. But the amount of combustion gasses produced will be proportional to the size of the nitrous shot. For example, let's say you had a 500hp/400ci small block and were using a 300hp nitrous shot shot. The amount of gas you need to exhaust when injecting nitrous would increase by approximately 60%, similar to having an 800hp/640ci big block instead of a 500hp/400ci small block.

Setups using a centrifugal SC are more sensitive to intake flow, but still much less so than an NA setup. When under boost, the intake manifold is under constant positive pressure, a very different situation than NA. I am not sure and would like to know how to extrapolate the relationship between intake flow as it's usually measured and what the flow is under boost. But the exhaust analogy with nitrous must still hold and the need for exhaust flow also has to account for the power used to overcome the parasitic drag of the blower. So, I suspect that the ideal E/I for a blower might even be higher than for nitrous. If we had the same 500hp/400ci SB with a blower that added 300hp, we would need to add another 50+hp taken to drive the blower into the equation and would be producing an equivalent amount of exhaust gas to an 850hp/680ci BB, more or less.

The amount of hp to be gained by optimizing exhaust flow on a FI combo are not trivial. David Vizard, that indefatiguable dyno tester, gave some examples in one of his books. I can't put my hands on it at the moment, but he was testing a nitrous small block with heads having different size exhaust/intake valves. going from stock sizes to fitting the biggest exhaust valve feasible made a difference of more than 10% in the power output!

As I think about this, I really wish I had an engine dyno and some time (and money) to experiment for myself. Interesting stuff! But in any case, if I go further next year with my wierd street/strip blower+N2O combo I may get a set of bizzaro heads with huge exhaust valves. The theory is compelling and if you look at heads for serious nitrous setups, obviously other people have had the same thoughts. Probably I'd need to use a BB valve with an 11/32" stem, but I imagine that isn't too hard to fit 11/32" valve guides in small block heads. Bret probably knows about that? To bad I wasn't thinking of this last night, I was at track talking to an engine builder/machinist who would have known the in and outs of that.

Wish I knew more about heads. I agree with Bret and the Old Guy that "good" heads and a "bad" cam are far better than the reverse though, at least I know that much.

Rich Krause

 

"As I think about this, I really wish I had an engine dyno and some time (and money) to experiment for myself. Interesting stuff!

Me too, kind of the point of trying to win a contest put on by Superflow! $30K and I could do that all day long.


But in any case, if I go further next year with my wierd street/strip blower+N2O combo I may get a set of bizzaro heads with huge exhaust valves. The theory is compelling and if you look at heads for serious nitrous setups, obviously other people have had the same thoughts. Probably I'd need to use a BB valve with an 11/32" stem, but I imagine that isn't too hard to fit 11/32" valve guides in small block heads. Bret probably knows about that? To bad I wasn't thinking of this last night, I was at track talking to an engine builder/machinist who would have known the in and outs of that."

Two things with that.

11/32, no need for that. Pro Stock engines use a 8mm stem with unreal spring pressure, rpm and lift. So 5/16 isin't out of the question there. Probably smaller. What you really need to look for is a head that has a wider valve spacing from stock. Like a Brodix T1X, 10X or 11X. I saw a Dragster yesterday at the track running a set of these on a 418 SBC with Hilborn FI on it. I've have two heads in mind for something like that Rich. Both of which you could run a stud mount rocker and HR cam with.


One thing to remeber about a optimized N2O setup is that off the N2O it's not going to nbe close to optimal, but 10% on the bottle more is worth it.

Bret

 

Bret: we will be talking if I try this idea, for sure! I like wierd things that work. It's fun to be different as long as it's different/better and not just different.

Any idea how you can calculate optimum exhaust flow for a given amount of nitrous? Is it as simple as I was thinking in my post?

Rich Krause

 

The best educated guess would be to simulate it. The high cylinder pressure is going to be a big part of that. 800hp in a 383 is different from 800hp in a 454, especially when one is on juice and one NA. One reason why the larger curtain area that a big exhaust valve give you helps so much is because it allows so much more by at a higher pressure.

Bret

 

Define “too much.” The issue is that people are looking to make a stock headed LT1 do something that it really doesn’t have the port flow to do, so something like the 306 is picked that has an extremely long duration (for the intended use) to make up for the crappy flow of the heads and relatively low lift to avoid having to go with a more $$$ valvetrain. The end combination will run faster (with what a lot of us would consider awful driveability), but not as fast as a shorter, higher lift cam with a better valvetrain or a much smaller cam with better heads.

You’re somewhat asking the wrong question… There’s power in airflow, and in choosing a duration/lift that takes the best advantage of the airflow characteristics that you have.

My feeling here is that in a perfect world if you were shooting for the best combination, you’d be shooting for exhaust port flow that does not require a split duration cam for it to keep up with the intake (You might still want to run a split duration cam for an assortment of reasons, but in that case you’re more likely to be looking for a ‘reverse split’ cam).

Real world pressures keep this from happening in most cases. The fact is that in most cases, intake flow and duration have a bigger effect on both the power output and the characteristics of the engine so the intake side gets most of the focus and $. Crutching a bad exhaust port with a big exhaust lobe doesn’t have nearly the same consequences as doing the same on an intake.

I think that I’d be safe in saying that we all wish we knew more and that good heads are almost always THE biggest concern in building power.

While I’m a firm believer that in a dynamic situation there is something to be gained from exhaust ports that are smaller then most people would shoot for (you won’t see it in flow #’s, but you will see it in HP numbers as the result of better scavenging), I’m either missing something here or I think that you made too strong a statement. I don’t really see how an exhaust port optimized for flow while spraying will pose a significant loss when NA. Yes, you’ll loose a little bit WRT to scavenging, but I have a hard time believing that “it’s not going to be close to optimal”

 

Mark: I was referring to an extreme situation. Building an "ideal" nitrous motor in this context would require a very large exhaust valve. As you know, there is only so much room in a head for valves. At some point, enlarging the exhaust valve requires shrinking the intake valve. A smaller intake valve will substantially impact performance off nitrous, all else being equal. That's were I saw the problem, not with changes to the port.

I think the next set of heads I try may test this idea (of a large exhaust valve at the expense of the intake) if I continue with the boost+nitrous concept.

Rich Krause