I want to start off by stating I am not a experienced/knowledgable engine designer and also want to thank Jake for the email alterting me to the discrepency .
I have a 94mm x 78mm stroker crank with a Web 73 cam. P and P heads but it was suggested by the engine builder that I go with 48mm intake and 38mm exhaust. The builders are fairly known race engine builders for VW and others, but predominately in the short track circuit (outlaws, circuit, etc.)
I have been told that the 48mm intakes limit the rpm of the engine despite the builder stating that the engine is an 8,000 rpm engine build.
My questions are:
A larger intake valve would increase the volume of air flow or less resistance, so how does that decrease the potential rpms.
If as it seems to be a fact and I want/need a higher rpm engine, can it be adjusted for with a programable ECU like a megasquirt?
Or do I just need to rebuild the heads with smaller intake valves and what size is optimal?
Thanks,
Larry

Get Jake to match your engine components for you. That is his business and he has done the dyno work to confirm his designs.

What I've learned during the process of having my engine built is that valve size isn't the "final" solution to making hp with our engines.

However cam design is and Jake can match a cam to match your heads.

And if you want to really go crazy Jake can get you heads that have huge velocity and flow.

Either you're misunderstanding the builder, or the builder isn't really what he's cracked up to be.

Valve sizes (and the ports behind them) are typically a key factor in limited the RPM potential of an engine. In a non-turbo engine, the main limit to airflow through a valve is the speed of sound. Once the airflow nears supersonic speeds trying to flow past a valve, flow basically stops increasing. As the engine speed rises, the airflow speed also has to rise to continue to fill the cylinder during the intake stroke, so there's your limit. The bottom end of the engne may be able to tolerate 8000rpm, but if the head won't flow enough air at 8000rpm, you're not going to reach 8000rpm with the engine under it's own power, as the airflow will go supersonic at the valve first, so the cylinder won't fill adequate, and the power will drop off.

If you look at the power curve on any engine, it will rise to a plateau, then fall off as the valves choke off the flow at higher revs.

You can solve this with bigger valves, or with a cam that opens the valve farther (lift). Both come with their own limitations. Bigger valves take up more space in the head, so there's a physical limit there from sheer space in the head. Bigger valves also weigh more, so they can't be opened as far as quickly before their inertia overcomes the valve springs and they float. Lift has limits, as at higher revs the valves need to be accelerated up to speed faster and faster to open the same distance, and eventually you exceed the physical strength of the part (or you hit inertia effects again).

Four valve heads also solve this problem, as two smaller valves use the space in the head more efficiently, allowing more valve area in a given head. Two smaller valves are also each lighter, so it takes longer to run into inertia effects.

Valve area, in short, is pretty much the key factor in determining how high an engine can usefully rev, which is the key factor in determining how much power an engine of a given displacement will make. Engines with many cylinders have more total valve area than engines with fewer cylinders (given the same engine displacement), which is why you tended to see 12 cylinder engines in F1 (before they were banned), or even 16 cylinder engines. Alas, more cylinders also means more ring and bearing area, so more friction, which cuts into your gains from the additional airflow.

Thanks all.
Yes, I respect Jake's knowledge and experience. I would now wish to mistaken as to being disrespectfully of him. I also, do not wish to be disrespectfully of the builders of my engine.
lapuwali, I appreciate and I think I understand the physics you are using. The larger the unobstructed valve area, the lower the reynolds turbulance and the higher the flow rates. That was the principle used to choose a 48mm intake valve. We also used dual valve springes and 911 rocker adjusters.
I had an engine program where you plugged in the compoents, lift, duration, angle, valve size, head flows, piston size, stroke to choose the optimal combination of compents. Output of that program predicted a 180-190BHP with160FLB torque at about 7000 rpm normally aspirated. The Web 73 cam roughly approximated that combination (ie. I used the 73 cam lift, duration, angle as input data).
Thus, it would seem that the larger intake valve would able to handle higher rpms. I'm sure, using the MPS alters the final product in that I have created a different plenum vacum per rpm/load, thus my question if this could be managed by a programable system like Megasquirt.
What else am I missing?
Larry

What you may be missing is what happens at lower rpms. If this is a street car that is where you will spend more time. With the large valve you will give up velocity at lower rpms which might make for a difficult car to drive. Also, if you can get the air in can you get it out? What are your plans for exhaust?

To answer your question about PEFI. It can provide the fuel but it can't overcome design flaws in the engine. Don't get me wrong I'm not saying what you're building is wrong I don't have the experience to judge it. I just want it clear that FI won't eliminate problems with the engine design.

Dave

Thanks for the information Dave.
It seems to drive fine, except for the rough idle, using the stock D-jet.
I need to order the WB O2 sensor kit to monitor the A/F. The cylinder heads run about 200-250F degrees with a Oil temp about 120-150C degrees. I have adjusted the A/F with my CO sensor in the garage at Idle, 2000 and 3500 rpm and A/F are about 13.3 - 13.7.
Larry

To be slightly more specific: with carbs, the flow velocity through the ports makes a big difference to drivability simply due to the flow velocity through the carb itself. Carbs depend on flow velocity to meter the fuel, and too little velocity disrupts this. With EFI, there's no such dependency. However, flow velocity has other effects, like determining how much turbulence there is in the chamber past the valve. At low engine speeds, high turbulence in the chamber improves charge mixing and the flame path after the plug fires, so using ports/valves that are too big tends to disrupt this at lower engine speeds.

Large ports also exacerbate the "harmonic" effects of flow. With wilder cams (much wilder than those 73s you're using), mostly with lots of overlap, there are effects coupling the intake and exhaust port, valve, and tract lengths. You're basically creating a tuned tube like a brass musical instrument, which uses the bizarre dynamics of airflow to "amplify" the airflow at certain velocities, and "de-amplify" it at others. The torque curve of an engine with big cams and large ports tends to be a series of ridges and valleys, where the hills are the places where all of the resonances all add up, and the valleys are where they all cancel. Small ports/valves tend to damp these effects, so they're not as pronounced.

Thus, on an engine destined for mostly street use, you won't see valve sizes and cam timings that allow really big power, as that harms drivability. Building a Type IV that will make good power at 8000rpm will very likely produce an engine that will make great power from 7000-8000rpm, and will barely run at 4000rpm.

There are a LOT of tradeoffs to make when designing an engine, which is why people like Jake go though so much experimentation to find the right combinations. A lot of this can't be exactly calculated even with a room full of supercomputers.

That's way too large an intake valve for that exhaust valve size. It does you absolutely no good to get more mixture into the cylinder if you don't also get it out. Those valve sizes are what you used to see in dune buggies, etc, but they also ran really hot and not all that well. They sure weren't making the power that those huge sizes would lead you to think. Stick to standard ratios for intake/exhaust valve sizes unless you have some really REALLY different in the head or cam to justify it.

Thanks everyone,
I think I am begining to understand.
THus, If I find this a poor combination, I can pull the heads and resize the intake valve to a smaller dia. What size seems to be best in this displacement?
Larry

I think I would not put so much faith in your engine-designing software. A mild grind such as a Web 73 should not be making power at 7000 RPMs. Stock peak power is at just shy of 5000, and the Web 73 isn't wild enough to move that peak up by more than 2000 RPM. Not unless there's a whole lot more going on that you haven't told us or that I don't understand.

And I'm completely ignoring the fact that it'll be a stone bitch to control the movement of larger-than-stock valves at 8000 RPM...

--DD

Just want to slightly hijack for 2 seconds:

James, thanks for taking the time to put your 2 replies together. Extremely well thought out and easy to understand. They took many bits of info I sort of understood in the past, and put it all into the proper context. I really learned a few things.
Thanks man.


End of hijack. Return to your seats.

Thanks, Chris.

I wrote this up a couple of years ago:

Wonders of Science

Peak power can also be roughly constrained by looking at hp/liter. Most two-valve engines with street-quality drivability will do, at best, 60-70hp/liter. Above that, you're compromising streetability badly for a top-weighted power curve. 180hp from 2165cc would be 83hp/liter, which is wilder than the '73 911S (190 from 2.4L, 79hp/liter), which most people feel was on the borderline of streetable, and had very wild cams and MFI. Getting 83hp/liter with D-Jet would be well-nigh impossible.

Motorcycle engines commonly top 150hp/liter these days, and I can tell you from personal experience they have a VERY top-heavy power curve. Snap open the throttle on the typical 100hp 600cc bike at 4000rpm, and a Yugo will out-accelerate you. Snap the throttle open at 8000rpm and you'll have the front wheel pointing at the sky.

Modern car engines are also doing 100-120hp/liter now, but they're cheating. Using variable valve timing, they effectively have both mild and wild cams in the same engine, so they can produce the good bottom-end power of a mildly cammed engine, yet also still breathe at high revs. Other tricks like variable intake and exhaust geometries can extend this window even more, allowing good drivability with very high peak power. F1 engines have all of these things (well, they did before the rules makers started to ruin things), and they could make good power in the top half of their rev range (10-20K rpm), rather than only the top 10-15% of their rev range, as they'd do without such neat toys. The most extreme examples were 50cc four-stroke racing motorcycle engines of the 1960s, which peaked at 14,000rpm, and made zero power below 13,000rpm, and blew up at 14,500rpm. These little wonders used up to 17-speed gearboxes to handle their tiny rev bands.

Thanks again James, (now I know you first name).
I looked at Jake's heads on the other thread, RCR. and really they look like the ones we made. Of courese I cannot say the geometrical diffferences and the flows looked good on the bench. This is a project in learning. If I cannot get what I want out of the 48mm then I guess I could replace the intakes. From my reading, it seems that a 44mm intake is optimal for lower and mid range performance.
Larry

I wanted to make some points clear here, but somehow I got this thread confused with another one about cylinder heads that is currently going on as well.... I received an email from a member asking for my input on the subject of this thread, but somehow I got the two posts confused and even combined them in my head and also in the text I wrote!!!

So the other thread that I responded to is here

Other thread..

The Web 73 will never make power to 7or 8000 RPM.

120C-150C converts to roughly 250F-300F, which if I read correctly, is a bit high for the oil temps, isn't it?

Now it's my turn to threadjack. Just for my own curiousity, what lenght rods are you using?
Mark D.
P.S. Threadjack over