how many cubes can my heads handle? 1

[stout1]

I am building a motor starting with the heads. I have a set of Brodix BB2-plus heads and would like to run 540 cubes or more. Brodix says that these heads are good up to 522 cubes if ported and flow about 370 CFM @ 28". The BB3 heads flow the same but have a larger intake runner volume so they say it can handle more cubes. My question is that if the heads flow the same why can't they handle the same size engine? What do others think of the heads I have, can I run a 540 or more through them and how well will this combo do? Looking for others opinions/experiences.
Thank You.

Michael

 

[dcasto]

The compression ration will increase with a larger displacement engine. At the higher ration, the pressures may exceed the heads ability to seal and maintain that pressure. You will also have a detonation problem too.

To test this theory, you'll need the head volume.

 

[patprimmer]

200 CFM heads will run on a 600 CI motor, but power might peak at say 3000 rpm. 400 CFM heads will run on a 300 CI motor, but the usable RPM range will be very different, say around 8000 to 9000 rpm.

The CFM of your heads and the CI displacement of your engine need to be matched to the power output and engine speed range you require.

Compression ratio will be dependant on a number of factors, being combustion chamber volume, deck clearance, head gasket thickness, piston crown shape and the cylinders displacement.

The phrasing of the OP indicates maybe you are in over your head, and therefore not seeing through sales hype and very broad generalisations. Do you know a good reputable engine builder who has built similar engines for similar applications. His advice might be valuable.

If you want more advice from here, you need to give more details of the other engine components and your application and aspirations.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[thundair]

Just to add a point that may interest you, is that the compression ratio you need to look at is the volume from the time the inlet valve closes, delta to the volume at TDC.

Unless you are going to build engines professionally I would not recommend using all those expensive parts as a learning curve...

Cheers

I don't know anything but the people that do.

 

[stout1]

In the past when building an engine I have always had the cubes determined by sanctions such as IHRA, NHRA, NTPA, etc. For this engine cubes are unlimited and are being restricted by the heads that I have (I don't want to spend the money for bigger heads since I have these around). I want to design an engine around the heads that I have. The best way to this as I see it is to let the CFM of the intake side determine the amount of energy available and from this there should be a way to determine the max cubic inch to run efficiently. I'm planning to run around 7000-7500 RPM. I'm also a long way off from calculating the compression ratio needed so we don't need to discuss this yet. I'm only currently looking at the displacement that these heads will be able to handle. Thank You.

Michael

 

[Fabrico]

Port, or intake runner, volume plays a big role in charging the cylinder. Small port volume is for smaller displacement -OR- lower power range. An example would be a 300cc port on a 500 cu in BB. It would be a stump puller but also would be falling short at perhaps somewhere around 5,000 to 6,000 RPM.

 

[dragula]

Dont pay attn to the CFM #, thats all advertising. You need to measure the average CSA and minimum CSA throughout the port and calculate from there.

For max power... CSA x 184,136/Stroke x Bore2=Max RPM

 

[stout1]

Thanks for the replys so far. Dragula, I have the formulas set up in an excel spread sheet for HP and Torque using the CSA as you have stated. My thought was that using the minimum CSA would give me max potential numbers. You may not always be able to achieve this depending on the design of the runner and such. I was wondering if there was a way to use the CFM at a given constant (the inches it was flowed at) since this is what the runner is actually doing. I guess to help explain what I mean is to say it like this - CSA calculation = potential ; CFM flow = actual. Is there a way to use the actual in the calculation of cubic inch instead of potential. Thanks for the help.

Michael

 

[Marquis]

"Dont pay attn to the CFM #, thats all advertising. You need to measure the average CSA and minimum CSA throughout the port and calculate from there."

Problem with using cross sectional area is it's only considering a 2 dimentional cross sectional area cut in a plane and not looking at 3 D effects of the port geometry such as flow shear, flow break away, etc etc.

A steady state Flow Rig (which is where the CFM numbers come from)does. Granted, it only allows deductive reasoning from obeservation and may not allow you to really understand why (you need CFD for that) but I think the CFM numbers are useful. I once worked with Ford but now have gone all metric I'm afraid, so am using AlphaKs (relative to bores)and metres cubed per second, and then use a 1 D code to evaluate. So I can't help the original poster right now I'm afraid.

 

[patprimmer]

I must thank Marquis for reintroducing logic back to this thread.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Fabrico]

Have you taken a look around the Airflow Research (AFR) website? There is some good info there. They have done a tremendous amount of development for BBC heads and are about the best in the business.

http://www.airflowresearch.com/index.php

 

[stout1]

Marquis is heading in the direction I was looking for (too many engineering classes and not enough english make it difficult for me to communicate my ideas). I understand stand that the CFM is advertised but I also verified it with my flow bench. Since the numbers are correct is there a way to use them? It's monday and I just got back from vacation so I'll leave it there for now. Thank You for all the responses.

Michael

 

[larrycoyle]

Michael,

Whenever you set out to build a particular engine combination there are several things to consider. One thing I am amazed that no one has asked you yet is the type of car you are going to run, how much it weighs and several other variables. The first thing you need to do is determine what you want to accomplish and design everything toward that goal. Now, if you already have the -2 heads as it appears from your initial post then why don't you consider building a smaller engine. If you have no parts and want to build the bigger engine then you need to consider the requirements to build that combo successfully. Airflow is certainly important but there are so many other variables to consider. The intake runner size/volume will go a long way to determine where in the RPM range you are going to make power but also you need to remember that it takes a certain amount of volume to feed certain size motors efficiently.

It sounds as though you need to consult a reputable engine builder in your area or a good cylinder head shop to help you make the right decision. These resources are available to you so why try to make such an important decision on your own? One last thing is that you need to be realistic with your goals for this combination a take everything into consideration. Especially your financial situation.

I am not trying to muddy the waters here but if you were going to have to buy a new cylinder head you should probably consider the fairly new Brodix -3XTRA series which has been rolled to 24° instead of the standard -3.


Good Luck

Larry

Larry Coyle
Cylinder Head Engineering, LLC

http://www.cylinderheadengineering.com