JimV

I have had 8 sets of AFR heads go through my shop. None of them came close to their advertised numbers on the exhaust side. They also post numbers with a "cheater pipe" which makes the numbers look better. They have a problem with the cnc program that gouges the venturi area below the seat insert. The intakes flow very good, within 1.5 percent on my bench. I would recommend anyone have them flow checked before making a cam choice. When the exhaust ports are fixed they will flow 330 cfm w/pipe.

tomcat

Hi Jim: Have you had any Brodix heads go through your hands? How do the BB-2X or the new BB-3X stack up to the heads mentioned above? What I notice on the websites and flow charts from head manufacturers is that flow numbers on the intake do go up with larger models, but exhaust numbers don't seem to move as much. How is that supposed to work, especially with blowers?

JimV

Tom,

I have not seen the -2 X or 3X yet. The advertised numbers look good though. Good point on the exhaust numbers. It's been said that a boosted motor is the same as running a N/A motor one atmosphere below sea level. I can agree with that for the intake side only. There is a ton more cylinder pressure on the exhaust valve with boost. In the big blocks the 1.880 valve seems to flow the best. The cylinder wall will shroud the valve if it gets much bigger. I tried up to a 1.940 valve with little gains. I guess all you can do is add duration.

GPM

Jim, how many more degrees of exhaust duration vs intake would you suggest in a blower application. Intake flowing 400 cfm exhaust at 305 cfm.

NOBODY

Jim, funny you should say that because I also have different views on this subject.

I left Dustin Whipple some (ridiculous questions as Richard Cranium 572 put it) in the TMP 1600 thread to answer since he said he knew everything about supercharged engines, although he didn't answer them. So here goes some of my views.

What do my I/E ratios need to be in supercharged apps? I believe they need to be between 70 to 90 percent.
Why? The engine is still a pump and when you build a high eff. pump you try to build it to have very low frictional losses and extract as much energy you can from the fuel you put to it. The object is to get the maximum VE you can for the CI you picked and have as much USABLE HP you can get in the RPM band it will live in. A lot of times this does not mean bigger ports or cams just the best combination that yields the best results in the REAL WORLD it operates in. Once you have this combination, now It's time to supercharge. You don't build a supercharged engine, you build a eff. N/A engine you supercharge later. Believe it or not the more eff the N/A engine is the more comp. ratio you will get away with with less octane fuel. This is because an eff. engine will burn the air and fuel more eff. and completely. I have my own views on what the best static and dynamic comp ratios need to be, but I'll keep this to myself. So your I/E ratios need to be the same for a high VE NA engine.

Does my exhaust runner need to be bigger? NO
Why? Because you did not change the pump when you supercharged this engine. It's my belief that the exhaust only needs to flow more if you change the CI once you have an eff. combination.

What lobe sep. angle does my cam need to be? It needs to be the same as it was.
Why? Because when I made this good combination (depending on what the app. is for) the best cam that yielded the best usable HP for my app. did not change. Just because I now supercharged it doesn't mean fuel and air will be pushed out the left open valves on a tight lobe sep. angle. This does not happen. The air has just changed around my motor, my motor doesn't know it just got boosted. It thinks I just moved closer to sea level and the adjustments that need to be made when this really does happen need to be made now. It still has to be tuned different.

Does the speed of the intake runner speed up when supercharged? NO
Why? Because my pump didn't change. The air is now denser and still moves the same speed as before. The only thing that will change the speed in the runner now will be more RPM.

As far as exhaust valve sizes, I want to point out the bigger the valves get in the head the less material you have between the valves and when you build more HP you build more heat so the thinner this is on a supercharged endurance engine the more you need to cross you fingers.

Any time you build a supercharged endurance engine you also have to keep in mind all the heavy duty things you must have to make it live and then try to build it to have the most NA HP you can for the app. It's used for.

All the things I've said are my beliefs and are food for thought weather you believe them or not.

JimV

GPM Honestly I would be afraid to give advice for blower motors. I used to do a lot of blower motors a few years back but most of my attention is N/A engines because the majority of my customers have bravo drives and we are making good power with our N/A 540's.

Maybe one of the blower gurus can help out.
The more I learn the dumber I get.

cstraub

Exhaust duartion numbers are determined by the following factors, I/E% ratio, CID, RPM. Importance of each is listed in order with I/E% being 50% of the equation.

The following is a general rule of thumb that I have found over the years of designing cams for NA, Blower, Turbo, and NOS engines.

At 75% I/E ration the cam should be a single pattern.

For every 2 degrees less of I/E ratio below 75% you need to add a degree of exhaust duration to the exhaust.

For every 2 degrees more of I/E ratio above 75% you need to add a degree of intake duration.

Now CID and rpm both play into this with each being around 25% of the equation, but if you use the above you will get pretty damn close.

How do I know this to be true. Over 12 + years of feedback on dyno's and real world testing in circle track, marine, drag, truck pull, road race, and air racing.

Chris

GPM

I'm just curious, thinking about what Tom Cat said about BBC heads. The intakes flow keeps getting better while the exhaust seems to be staying the same. When you start forcing 800 or so cfm into each cylinder what does it take to get it out. How does the 75% IE ratio work in this situation. Would you figure 75% of the 800? How much hp is lost because the cylinder hasn't been evacuated.

tomcat

The best NA engine is the best foundation for centrifugal supercharging, at least that's my experience. So I like NOBODY's take on this. I still wonder about the exhaust flow. Higher cylinder pressure means more pressure differential across the valve and port, and denser exhaust gases, so the exhaust on port any higher HP engine will automatically flow more exhaust mass . What happens when you dyno an engine with one of these heads that makes 400 CFM on intake but only 250 on the exhaust. If you then bring the exhaust up to 300 what is the result?

NOBODY

I heard a QUOTE one time that said----
When you build a turbocharged motor why worry about how much power it makes NA just put the turbo on and keep adding boost till you get the power you want
Answer---
If I have a lazy dog and I put a turbo on him, then I have a lazy dog with a turbo on him

I think the truth behind this is you can have a lazy motor with too little of cam and heads and also be lazy with too big of cam and heads, either way you have a lazy motor. If you then supercharge this motor you WILL have more power, but not what you could of had.