Carbs or FI ?

[jnp914]

In my track-car-of-the-future, since I am not concerned with maintaining street legality, is there any real advantage in keeping the stock FI on the stock 1.7, or is converting to a pair of Solex's the move to make?

I don't see this being full race. More PCA Driver's Education and some time trials is more like it. A play car, just like the Harley 250 I am building up as a play bike.

Thanks for the responses and info.

[pbanders]

If it's a track-only car, I'd suggest carbs, as they allow you to use a performance cam that will give you top end HP, as well as permit you to do head and exhaust work to free up breathing. See my web page for more info on carbs vs. FI.

[jnp914]

Seems to be a reasonable, articulate position you put forward on your site. Probably leave it FI until I do some motor work.

Thanks

[skline]

This sounds like an argument looking for a place to happen. I think I will stay out of it.

[Ron Meier]

Driveability = fuel infection

Carbs w/FI Cam = image + poorer fuel mileage + less driveability + more noise

As they said - change the motor characteristics - go carbs or megasquirt - stock motor for seat time - stay with the FI (if it's working)

Pappy is not afraid to start an argument. Do a search on this forum for your answers.

[M. Hendrix]

Originally posted by skline
This sounds like an argument looking for a place to happen. I think I will stay out of it.

No, it doesn't, and no, you won't.




M

[skline]

Yes it does, and yes I will!!

[sammyg2]

Carbs make real cool noises at WOT that you can't get from the stock F.I.

that's about the only real benefit you can get from carbs unless you change the cam.

[Stuart Lehr]

One thing not mentioned- the carbs just look so much cleaner.

[bash555]

Hey Ron,

Could you explain to me again the difference in an early 1970's FI cam vs a carbureted cam?

Thanks,
D

[Dave at Pelican Parts]

I'm not Ron, but I can tell you that the FI cam is very very very very mild. When you switch over to carbs (or some other very adjustable fuel system) you can deal with a much more aggressive cam. You'll get more power at the top end. You'll usually give away some in the lower RPMs, but you can make up for that by raising the compression...

--DD

[bash555]

right,

but as far as losing perf by using a carb on a old FI cam, what, specifically is the problem?

is it:

a) cam timing problem?
b) duration problem ?
c) lobe seperation problem ?

it can't be lift.

would increasing the lift by ratio rockers help a carb on the stock FI cam?

I can understand modern FI cams, that have been optimised for the FI, having issues with a carb, but not a 30+ year old cam in an engine that uses the first pratical FI system.

[Brant]

bash555,

do a search or two..
you will find exactly the same question asked many many times,
and there will be enough responses to keep you busy until monday.....


But one more thing...
DECIDE WHERE (sanctioning body) YOU INTEND TO DRIVE IT FIRST.


many stock classes do not allow carbs.
you could be bumped into a modified class where the car is not competetive......

This from a person who now has his first FI motor after 17 years!
brant

[Dave at Pelican Parts]

Originally posted by bash555
but as far as losing perf by using a carb on a old FI cam, what, specifically is the problem?

The problem isn't that the carb'ed performance is less than the FI'd performance (when everything is "right", I don't think the differences are large enough to notice). The problem is that there is so much more that can be done if you use a more radical cam grind.

The timing of the opening and closing events, and most especially the overlap where both exhaust and intake valves are open at the same time. Those are the biggest problems that stock FI has with aggressive cams. But those are the kinds of things that help an engine breathe better, particularly at higher RPMs and flow rates.

--DD

[pbanders]

The issue here with carbs vs. FI is the application. For general street use, the FI cam and FI setup will produce far better results than any cam/carb setup - better starting, better low-end torque, better fuel economy, lower emissions, better general drivability. But for track use, you are primarily concerned with maximum power output, as you will be using your engine in a narrow RPM range near redline. Under these conditions, an optimized carb setup with a carefully selected cam will produce significantly higher peak power output than a stock FI setup and cam. The key here is selecting the cam so that it is the best balance of peak power and torque band to match up with the tracks you're planning on running.

[M. Hendrix]

Originally posted by skline
Yes it does, and yes I will!!

You will be back.. and no, it doesn't.



M

[bash555]

Thanks again to Dave and Brad.

But what I am really looking for, (and I must have poor search ability), is the specific difference between a FI cam and a Carb cam and why a FI cam does not send the right "signal" to the Carb..

I did find while searching that supposedly the FI cam closes the valve sooner than a normal carb cam causing some of the air/fuel charge to get left behind in the port.

I would guess that the reasoning behind closing the valve sooner was to increase the low to mid torque numbers by increasing the dynamic compression down low.

If that's the case, that definately hurts carb performance down low (ie. Rich) and limits it on the top end (low dynamic compression).

I am curious if it could be helped a bit by using a higher ratio rocker arm causing more lift which might alleviate the early closing.

Also, who else besides Pauter makes ratio rockers for the Type 4?
I am assuming that Raby just uses the stock ones.
Anybody know??

Thanks again for all of your opinions; that's what it's all about!!

[sammyg2]

The stock fuel injection cam has a reasonable amount of lift but has very conservative numbers when it comes to valve timing, diuration, and overlap. basically an RV cam grind. The reason for that is to make it driveable on the street with more torque down low and a wider torque powerband at the expense of higher rpm horsepower. The stock fuel injection systems on 914s need a mild cam like this in order to meter the fuel correctly and to idle at anything under 1000 rpm. Carbs will work fine with this cam if they are dialed in correctly.
If you put too wild of a cam in a type 4 with stock D-jet it probably won't run worth a crap.

The only advantage of carbs is that they allow you to run a more agressive cam grind and get more top end power.

I can't give you exact lift, calve timing, duration etc numbers for the stock cam if that's what you are looking for.

[pbanders]

Sammy covered it, I'll try another way.

The stock cam (IIRC, that is) has -2 degrees of overlap (i.e. when the intake and exhaust valves are open). So - there is NO time at when the intake and exhaust valves are open. This was selected for several reasons, but the primary reason was to reduce CO and HC emissions. With static valve timing, overlap results in some unburned fuel going straight out the exhaust, causing high CO and HC.

The second reason is that low or negative overlap improves low-end torque. Overlap is desirable because at high engine speeds, there is a scavenging effect on the exhaust side that actually pulls intake charge into the cylinder, leading to a higher volumetric efficiency (VE - pumping efficiency). By careful tuning of the intake and exhaust system design, with a proper cam, VE's of near (and over) 100% can be achieved in narrow engine speed ranges. But you don't get something for nothing. Overlap means also that at lower engine speeds, you get poor pumping efficiency, as there is no scavenging effect. You have low engine vacuum at idle, requiring a very rich mixture for good burning (poor fuel economy and HC and CO emissions). Additionally, the vacuum fluctuations are much more pronounced, leading to poor idle quality and smoothness (regardless of if you're using carbs or any form of FI). Modern engines get around this problem by using systems that vary the intake and/or exhaust valve timing so that at low engine speeds, there is minimal or zero overlap, and at high engine speeds, there is appropriate overlap. More advanced systems can vary valve lift and ramps, as well as either switched or continuous control of intake runner length, to better optimize VE over the engine speed range.

As for lift and ramp, the stock cam was again chosen for good street performance. High lift usually means high ramp, which causes high engine friction (poor fuel economy), lost low-end torque, and rapid cam and follower wear.

There's a lot more to it than what I say here. Go to the Crane cams web site for plenty of more information, or simply search on Google.

[Jake Raby]

I stay away from the aftermarket rockers..They are not self oilers, and do not correctly match mnany cams off the shelf. I only have 2 engine combos that use them- Nothing is as good as stock.
As for carbs Vs. FI....Well once upon a time the FI was a great system,now its old and hurting, and expensive to fix.....and a nightmare to troubleshoot. I Like carbs because they are simple and if you break in the middle of no where you can patch it up and get home----not true with FI.
Ironically you will find that the guys that love the stock FI and preach about it , share something...They all sell parts to make it work again......"Them parts ain't cheap"