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FAQ: JBS Spec CNC Performance Heads.



What are the benefits of a gas flowed cylinder head?

A gas flowed cylinder head improves the flow of air and fuel in to the engine making it more efficient and making more power and torque. All modifications such as turbos, supercharging, exhaust etc will all be enhanced by a gas flowed head. If you plan a series of modifications you are well advised to do the head first to reap the full benefit from other modifications.

Can a JBS Spec gas flowed cylinder head be used on a road car?

Yes. We aim at top race standard because racers demand the best possible head but there is no reason why you cannot use our head on a road car. Many of our customers are road car enthusiasts. Whatever the state of tune of your car, a gas flowed cylinder head will improve performance.

Why are some heads measured at 10" of pressure and some at 25"?

10" was a European standard which we used and is suitable for Smaller European engines (as opposed to big V8's). The flow bench is capable of 25" or 28" pressure. 25" is the pressure achieved by a normally aspirated engine and will therefore simulate the swirl and tumble effects you can expect which can affect flow. Our power predictions for both pressures are accurate as mathematical predictions (from our Port Flow Analyzer software) and most heads have been verified by an actual engine build and test on our in house Dyno.

Do you add material to the port (such as resin1840.00s)?

No, we don't because we are aiming at many race series which do not allow this technique. We recognise this legitimate technique for many engines (especially gas flowed cylinder heads for motorcycle engines) but we have not (yet) adopted this technique for our heads. As such they remain eligible for many race series.

How do I know you haven't just made these flow figures up?

We don't run the data through a spreadsheet or anything else which would allow us to manipulate the figures you see. The flow figures come directly from the software used with the flow bench. The Graphs and other data we show are paper print outs direct from the Port Flow Analyzer software. We simply scan the printout and insert in to the website to show we don't mess with the figures we see on the flow bench. It's easy for us to do and it shows we hide nothing.

How do I know the head you sell has the flow you claim?

We compare our finished heads to the original final prototype on the same flowbench. We do this as part of our quality control process. We see less than 1% difference. Our flow results are consistent between heads and between cylinders in the head. This is only possible with CNC engineering. We are totally sure about the flow of the heads we sell which is why we publish the figures and why many gas flow firms don't.

How do I know the horsepower figures you claim are achievable?

Once flow figures are measured on the flow bench and the data is captured via the Port Flow Analyzer software, it is possible to calculate horsepower by adding known parameters such as number of cylinders, camshaft profile, lift details, valve size etc. It is also possible to specify a custom camshaft profile and we can use this data to manufacture a custom camshaft if required. The claimed horsepower is derived by an algorithm using the parameters to derive a predicted horsepower and torque at a known RPM. Obviously we know we need to go further than this.

We have verified the BHP predictions many times through full engine builds and testing on our dyno. We have full faith in the predictions. It is also something which you won't see on any other gas flow engineering firm. But we stand by it because we can prove it.

Horsepower using a turbo or Supercharger is more difficult to calculate as boost pressure is variable electronically and by the client.

All of the heads we supply have been either tested in our dyno cell or via our rolling road when fitted to a car. We are 100% confident about our power claims for engines of a known state of tune and condition.

If you put our head on a knackered old engine though, it's unlikely to match the results we see on one of our engine builds.

Do you guarantee the power you claim?

Yes - but only on engines we build and tune. This is a fair condition. Our claims are verified but we cannot predict BHP on engines we haven't seen. Engines must be of known condition and state of tune. The only way we can be sure of this is by having full control over the engine build.

I can get the same power increase from a re-chip for less money.

You can achieve a lot with electronic tuning especially with Turbo and Turbo diesel engines. (We are a Custom-Code dealer so we know what we are saying here)

But the performance improvements are always limited by the standard head. Electronic tuning might give you 20% with a standard head. A similar state of tune with one of our heads might give you 40% or more. If you have a normally aspirated engine, it may be quite difficult to get really good improvements without a gas flowed cylinder head. If you anticipate doing a series of changes (say, new camshaft, electronics, exhaust  or even turbo or supercharging), it would be wise to look at the head first as it will improve the benefits gained from all the other modifications you are planning.

You may also find you can run lower boost, rpm limits, cam lift and still get the power you are looking for. This will improve reliability. Also, the power will have a better profile across the rev range with better low and mid range power. Maximise the potential from the head and all other changes will be maximised also.

I read a book on gas flowed cylinder heads and I reckon I can gas flow my cylinder head myself with a die grinder, some carborundum grinding paste and a polishing tool.

Good luck to you. Many people try this and some get a modest result and we can totally understand the pleasure gained from having a go and achieving something for your engine by something like this.


  • You won't be able to match years of experience on the job with specialist tools using home tools and tips from a book. Gas flowing takes a lot of experience, specialist techniques and tooling.
  • You will not be able to measure flow and verify the result without a flow bench. It is quite easy to get worse flow (we have seen lots of gas flowed heads that do flow less than standard... sad but true)
  • -You will not be able to replicate flow across the cylinders by manual means without a flow bench, a CNC tool or a lot of time.
  • A JBS Spec CNC heads are far more than just a port polish job. It involves substantial re-profiling of the port with specialist tools and careful incremental measurement on the flow bench.
  • Finally, as we know well, you could quite easily completely ruin your precious head by breaking in to the water circuit or making some other error, and then you might be in the market for a new head anyway.


I have a good head on my engine can you copy it and replicate it?

Yes. We can also check the flow and see how good it is - call us.

Can you repair my cylinder head?

If it is possible to repair, yes. Replication/cnc work may also be a viable repair option. We may also be able to deal with rare or special heads. Call us to discuss.

Can you fit valves to the head for me?

Yes, no problem. Call to discuss costs.

Can you fit the cylinder head for me?

Yes. Drive in - drive out service available by appointment - although it's not a while you wait job! The job may take several days. Cost will depend on the engine.

I don't see my engine on your list - what can you do ?

We have many heads in development. If we don't, we may be able to gas flow your cylinder head. Give us a call.

Can you gas flow my cylinder head for me?

If we have done this head before, yes, we can CNC engineer your head to our specification or exchange it.