Carb vs cylinder pressure

[phantomfab]

Does a larger carburetor have a higher cylinder pressure on the same engine as a smaller carb if they are both at WOT.

If so does this effect take place everywhere in the throttle range?

[kimosabi]

If the smaller carb is so small it restricts airflow so much it affects the cam, heads and valvetrain flow capabilities it will let less air in through the intake tract. Yes.

There's a reason why you find vacuum under the carb.

[mag2555]

When you apply too small of a Carb to a motor in its peak hp rpm range that the motor is built for you make the motor have to work and suck in air instead of Atmospheric pressure driving air into the motor!
This is the best untechical way that I know how to describe it, maybe others here can do better?

[phantomfab]

I was just asking because i was reading about cylinder pressures being lower at park throttle cruising This made me wonder if the carb size made a difference in cylinder pressure on the engine. Such as could a 350 hp 8.5:1 engine with a 750 cfm carb at 6000 rpm have a higher cylinder pressure then a 420ish hp engine with 9.5:1 with a 500 cfm carb at 6800? Will the power show a difference in response to these pressures?

[phantomfab]

Wouldnt cylinder pressure being higher mean more power and throttle response? If this is the case why does a smaller carb usually seem to have more throttle response? Or is this throttle response all due to the amount of velocity And/Or difference of pressure at the throttle blades?

[MadBill]

As above, the short answer to your question ("yes") doesn't tell you much because the relationship between carb size and cylinder pressure is complex. Additionally, cylinder pressure is highly variable throughout the four strokes and at various engine speeds and throttle openings and is affected by many other factors.

Ignoring the other factors that affect the engine's "breathing" ability and simplifying a bit, at WOT and low RPM anything bigger than a really small carburetor will result in almost full atmospheric pressure in the manifold, and thus relatively high cylinder pressure during the power stroke. As the RPM increases, the carb's restriction will result in a rising pressure drop ("manifold vacuum") that limits the mass of air/fuel mix and thus lowers cylinder pressure. A larger carburetor with its lesser restriction will flow more mixture than a small one, resulting in more pressure and increased power.

Such increases are subject to diminishing returns though, and furthermore a carburetor relies on airflow to draw in fuel and mix it with air, so a too large one results in low air speed and poor mixing at low RPM.

[phantomfab]

Thank you for the explanation. The larger size working poorly at lower RPM was what i suspected to be the difference. But was curious about when sanctioning bodies in racing such as the Bmods i run. I was wondering if the difference was enough between the 500 and the open rule. And was the open carb rule on a 350 hp engine better than a 500 cfm ruled open motor other than open chamber heads and 360 max cubic inch.

[MadBill]

Usually such rules are designed to produce performance parity and it takes a lot of knowledge and/or experience to determine which if either combination holds an advantage.

[mag2555]

As in any motor and it performing it best for its intended purpose it's the average power numbers that mean the most, be it the need for a power band that's 2500 rpm wide, or like in pro stock motors now where some are set up / tuned to max out numbers in a 900 rpm wide power band.

With any motor HP numbers will continue to climb as rpm goes up until the frictional HP loss of the motor over takes the auctual HP being made.

In roundly round racing dialing in on what you need can be more difficult then straight line racing since how the driver reacts to track conditions takes place thru out the whole race, at the strip for the most part the driver effects on the race are 1/2 second long!


Throttle response is in near direct proportion to the air velocity in the complete intake track , as in small throttle opening passing a small amount of high speed air, or large throttle opening pass a large amount of high speed air.

The funny thing is when you drive on a motor that has too much air speed for the rpm it's at the throttle feels the same as too large a Carb being used at low speed!

[David Redszus]

Perhaps we can persuade Nitro to post a comparison using PV curves to explain the pressure curves.

[jmarkaudio]

Steady state part throttle at any given RPM the restrictions will be the same, until the larger carb is open past the restrictive point of the smaller carb, which will be close to WOT. How the fuel metering works will be different, the small carb has a larger throttle opening to get the same RPM at the same loads the tune will be different. A larger carb will likely be operating on the idle/transition metering until a higher RPM. And obviously the closer in size the less the difference, going from a 750 to a 1250 would be more dramatic.