Bike Carbs on pinto and cvh

[Rik178m]

Got 2 engines which will eventually go in my roadster a 1600 CVH and a 205 pinto.

Just got a set of ZX9R carbs as they were fairly cheap. Are they suitable for either engine?

I notice on another forum alot of people are using 600cc carbs. Are these better? I would have thought bigger the cc the better for car engines?

Whats should I be looking for ?

[Bonzo]

Although new to all of this malarkey,here are my thoughts for what they are worth.

I would imagine that the carbs you have bought will be fine once suitably jetted to deliver the correct amount of fuel.

From my research. I believe that the 600cc carbs are just about the ideal size to provide the correct fuel to air ratio needed on the 1800-2000cc car engine. Only minimal carb jet changes needed.

I assume this is the reason that 600cc carbs fetch so much more money than the larger ones.

[davidimurray]

Hello Guys

I've thought about this for quite some time back in my uni days when invloved with Formula Student. There we have the situation that all the air must pass through a single throttle and then a 20mm restrictor before the plenum.

Now, without any forced induction, and ignoring potential pressure waves due to organ pipe/helmholtz theory, then the most air a cylinde can draw in, is the capacity of a that single cylinder. However, you don't get that due to frictional losses in the intake system, i.e. the volumetric efficiency, and the reason we spend lots of money on porting heads etc. A typical engine will only draw in about 75-80% of its actual capacity, a tuned engine might have a VE upto 85%

So if there is only a certain amount of air you can draw in, once you reach this limit fitting bigger carbs will have no affect on the top end power, BUT where it will have an affect is on the throttle response. With the 20mm restrictor example, I've used 50,40 and 30mm throttles and the difference was amazing. The 50mm was like an on/off switch, the 30 on the other hand was a dream of driveability.

So my thoughts on this is that you may be better going for the smallest throttles possible for the equivalent airflow. We can quite easily work out the volume of air required for a 2l pinto at the red line, we can then also do the same for various bikes - the ultimate air being to try and match the redline air flow requirements of the bike carbs to the redline air flow requirements of the pinto. I must say that this is only a theory adn I haven't proved it (yet )

If I get a quiet 5 min at work tomorrow I will try and knock out a spread sheet to work this out.

Does the theory make sense?

Cheers

Dave

[Cyberbeej]

i was just getting into that,
please continue..........

BJ

[spud69]

Originally Posted by Rik178m

Got 2 engines which will eventually go in my roadster a 1600 CVH and a 205 pinto.

Just got a set of ZX9R carbs as they were fairly cheap. Are they suitable for either engine?

I notice on another forum alot of people are using 600cc carbs. Are these better? I would have thought bigger the cc the better for car engines?

Whats should I be looking for ?

As a response to your question Rik....

I've just upgraded my car with a 2.0l zetec blacktop with zx6r bike carbs and a megajolt ignition system. These carbs and cheap and relatively easy to fit and match the 2l a treat, if you get a later model they come with a tps for 3d mapping, transforms the engine with excellent throttle response and great torque all the way from 1500rpm. Unless you have the specialist gear you will need to get the carbs setup for the engine on a rolling road, Bogg Brothers in North Yorkshire did an excellent job on mine for only £90. Basically all they needed to do was setup the tickover and high rev mixtures and drilled out the main jets to 1.55mm. There's a lot of people using zx9r carbs too, i don't think there's much in it.

160bhp at the wheels....

Very chuffed with mine....>Andy

[davidimurray]

Ok, just been playing in Excel and got some interesting answers! I will upload the spreadsheet this evening, but in the meantime have a look at the figures below, based around a VE of 0.8

1.8l engine 6000 RPM 18l/s
2l engine 6000 RPM 20l/s
600cc bike 16000RPM 16l/s
750cc bike 13000RPM 16.3l/s
1000cc bike 12000RPM 20l/s

So by my guess - the carb from a 1000cc bike should be ideal, but loads of people use 600cc ones without problems. I guess it probably gives them a very sharp throttle response as opposed to pure top end. The 600 carb should still be able to flow 20l/s but at higher velocities and with a little top end loss.

Cheers

Dave

[Bonzo]

Wow, now that is a lot of science for an old brain to soak up

I assume this is why the Supercharger is the ulimate upgrade.

Nice work guys.

[davidimurray]

uh, oh - now we are gonna get into the age old supercharger v turbocharger debate! The thing with either of these methods is that you normally get a VE value greater than 1, i.e. forcing more air into the engine than it can suck in.

Remember though, that I am just using this as an approximate set of numbers to keep the carbs around their original operating range. You can flow as much air as the engine will pull, but with increased velocities and losses. To really show off I got some pretty graphs for later

[Bonzo]

Originally Posted by davidimurray

uh, oh - now we are gonna get into the age old supercharger v turbocharger debate! The thing with either of these methods is that you normally get a VE value greater than 1, i.e. forcing more air into the engine than it can suck in.

Noooo, we don't even want to go there !!??

It's just the science of all things engine, blows my mind. ( Well what little of it that is left )

After almost 2 years of boring other forum members over engine choice. I have only recently decided to go the Zetec route.

Lots & lots of reading since then.

I can honestly say that as far as modern engines go, I know hell of a lot more now than I did a couple of months ago.

I can still fit it on a piece of A5 paper though.

[davidimurray]

The big thing I find with engine related work is that it is all about care and attention. You need to consider the whole system, no point fitting a ported head with a stuffy air filter! Yes there is a lot of skill and experienced invovled, but if you can build a whole car then engine tweaking should be childsplay.

If you haven't come across it yet look for organ pipe or helmholtz tuning for runner lengths - that confused the hell out of me for a while until I got the chance to actually do some tuning myself.

On that note has anyone played with tuning inltake/exhaust tuned lengths?

Cheers

Dave