2.5 short stroke, GE80 cams how is the idle ?
 

Good day.

I'm in the process of dialing in a 2,5 short stroke that is just broken in. Right now, it pops and farts in the intake a little when idling or even cruising sometimes. Mixture is still quite rich and I need to do some jetting (running PMO 40 carbs). 10.5 compression, twin plug, stock valves, 36 mm intake and exhaust ports.

I may have an ignition problem, and some cross firing, trying to figure it out (HPV1 ignition).

In any event, the question I have for those who are running a similar engine: is it possible to achieve a smooth idle with the combination of the agressive GE 80 cams, the 2.5 short stroke configuration and the high compression ?

TIA

Yes -- with injection. Carbs are more prone to reversion then MFI (which is one reason why Porsche switched to MFI For their race cars starting around 1969). GE80 cams are full race cams with a lot of overlap, I expec that you'll have a real challenge getting them to run well at low revs and part throttle, especially with the "big" ports compared to the engine capacity.

Basically, when you've got a high compression engine with a lot of overlap and the throttles closed, it's possible to have pressure waves develop in the exhausts and intakes which will result in the cylinder pressures being higher then port pressure -- which results in the charge getting sucked or pushed back out the intake during overlap, and before the intake valve closes. The result is the cylinder doesn't get a good charge, and extra mixture collects in the ports. This is reversion. In extreme cases you can get situations where the mixture is pushed all the way out of the ports which results in a cloud of fuel floating over the stacks. None of this is conducive to smooth running in addition to potentially being a fire hazzard.

Michel:

Yessir, you can get a decent, if high idle with a GE-80 cammed, carbureted 2.5.

These things like to idle around 1K RPM but the popping & farting indicates some jetting/mixture issues to be worked out and perhaps fouled plugs.

You may find that the HPV's struggle with keeping the plugs clean when asked to idle for periods of time and I'd recommend a setof hot plug while you get the carbs adjusted.

I hope you have the tall intake manifolds when using such cams,..:)

Thanks.

An idle around 1K is fine with me. I thought a lean condition caused intake pops. Guaranteed my mixture is plenty rich, although I have yet to put my LM-1 on it. I'm running short stacks under my PMO carbs.

BTW I understand PMO 40 carbs are NLA, is that correct ? The reason I ask is that I have a 2.2 E MFI system on my shelf (magnesium stacks and all), and if I could get that re-worked for my engine, and pay for that work at least in part by selling the PMO carbs, I would really think about it.

If not, I guess I need to start tuning my carbs, even if that's not something I'm really familiar with. I'm sure plenty of support is available, though.

Oh, the engine is in a 914-6.

Michel

p.s. Steve, you had given me a quote on the MFI re-adjustment, I'm still thinking about it. I kinda like the MFI system.

Michel:

Intake pops generally reflect leanness, yes.

Short manifolds are not the right ones for such aggressive cams,...you must have the tall ones for the motor to run right.

PMO is behind on shipping carb kits but the 40mm ones are not discontinued. :)

You'll have a tough time getting MFI to run well with GE-80 cams since the pump's space cam is matched to the engine camshaft profile. Talk to Gus @ Pacific FI to see if he can match that combination.

Steve,

Thanks.

I sorta knew about the MFI being hard to make work for my cams. The carbs are on the car now, I'll work on what I have now.

Not sure I understand the reason tall manifolds are needed. A bit of research is needed.

Cheers

I talked to Gus about a similar setup and he said that an adjusted 2.4S/2.7RS space cam should work with some pump tweaks. The way I understood it is that the space cam is tuned for RPM range, of which the S space cam is good to over 8000 RPM, which the GE80s are happy to run in. I believe the adjustments needed would be get the rack set up to properly provide enough fuel.

He does recommend using a twin solenoid 2 liter pump for various reasons, but there is no reason you can't make a 2.2 pump work. It just takes money and with Gus, time...he's backed up about 12 months on pump rebuilds right now.

2.5 stroker
 

Hey Michel,

I've got a similar 2.5 short stroke . . .
89mm p&c, 10.3:1
twin-plugged (thank to steve weiner for the help here)
38/36 intake and exhaust ports
40mm PMO's w/ tall manifolds
dc60 cams
SSI's w/ sport exhaust (2in 2out)

I don't have it in the car yey but I thought I'd share my PMO settings to hopefully help both of us dial it in. Difference in the cams may make this meaningless, but here you go.

Venturi 34
Main jets 135
Air corrector 190
Idle jet 55
Idle air 130
Emulsion tubes F11

How are your PMO's setup?

Regards,

Tom

(That's strange, I thought that I respond to this already, but I can't find it. So here goes again.)

Disclaimer: If Steve disagrees with anything I say below -- he's right.

There are two things that you might be referring to:

1) The Secondary ventury. These are the upside-down T-shaped venturi tubes that sit in the middle of the carb throats. The standard versions are 2" tall while the "Tall" versions are 4+" tall. Visually, if the tubes come up roughly to the top of stacks, you have the "tall" secondaries. These are apparently key to good part-throttle running since the longer venturi tubes allow the air to develop more inertia, and thus make the jetting less sensitive to pulses caused by the cam overlap.

2) Manifold height. These affect performance in race engines in 2 ways.
a) By moving the carbs further "upstream", they move the introduction of the fuel into the mixture further from the valve, which allows the fuel to get better atomized prior to being introduced into the combustion chamber. This is important at high revs when gas speed in the ports increases.

b) By lengthening the intake tract "pipe", you are lowering the frequency of the pipe which has an affect on the accoustical tuning of the intake. This is especially critical in race engines due to the cam overlap, so as you think of the accoustical tuning, you really have to start with the exhausts since it is the high-energy exhaust pulses which really drive the accoustics of the system. I tried to explore this subject a while back in this thread.

Here's what I have:

Venturi 36
Main jets 140
Air corrector 190
Idle jet 55
Idle air : not sure
Emulsion tubes F11

I just took the top of my carbs apart to confirm what I have, but I dit not note what my idle air is.

The intake manifold is approximately 75mm high.

I now need to install my exhaust analyzer in and go get some hard data before I start jetting.

I will update the thread as progress is made.

How significant is the idle air ?

To be honest, the data out there regarding selecting Emulsion tubes is garbage at best, and in some cases contradictoray. If my memory is correct, you need to pick your Emulsion Tubes based on the cam since they "tweek" the jetting curves based on the size of the holes and where they are located on the tube. I believe that F3's are generally used for "moderate" cams like E's. If you're using GE80's, I think that you should be looking for F24s based on my conversations with Richard Parr in the past and the 2.3 race engine that I'm building using similar cams. Cegerer's car is a 2.5 with Solex cams (somewhat milder then GE60s) and F26 Emulsion tubes. The last time that I checked, he reported that it was running slightly rich, but he was happy with it.

In all cases I'm referring to 40 mm carbs. 46's are something different again.

Well, I can report tha the engine now runs smoother with the adjustment to the fuel level, even if it's still not quite as precise as it should be. I'll do a little more tweeking of that. Then, it's a matter of putting the LM-1 on it and getting some data to try and figure what is happening.

I'll post more information later. This group is very knowledgeable ! I'll also ask Richard Parr, I have bought carbs and jets directly from him and I understand he's happy to provide after-sales service.

Cheers

If you still have problems with low RPM tuning...you might try Anti-Reversion cones in the exhaust ports.
These are cones with the big end at the port and the small end out into the pipe, and they reduce the diameter of the opening by about 10% only.
It sounds wierd, but they will change the tuning of the intake drasticaly.
We used them in the Formula Fords years ago with very good results (had the competition scratching their heads for 4 years).
The idea is to prevent the pulse wave from coming back to the exhaust port and creating back-flow into the cylinder.
The side effect is a broader tuning band for the headers.
Good luck.
Bob

Bob

Bob
if you use reversion tubes does that mean you need larger headers

Walko:
The idea with the cones is only to reduce the over-all area by about 10 - 15%.
In a 1 5/8" header (for example) the small opening of the cone would be about 1 9/16".
I have not run into a problem with header size using this method.
And...as an aside....the inspectors and other competitors in Formula Ford did not see the cones for almost 4 years of racing.
Then someone finally noticed the thicker flange (actually a double flange) on our headers.
What I had done was fab a second flange with the cones to fit inside the headers, for testing purposes.
It worked so well, I never got around to putting the cones into the main header assy.
Bob

Is there any special sauce to location of the cones in terms of how far downstream to place them? It sounds like you are locating them directly after the exhaust ports.

You are correct sir....the cones go as close as possible to the port in the head.
The reason given (not deduced by me) by a very knowledgable mechanic, was, that you are trying to prevent the back pressure wave from entering the port and upsetting the outward flow.
If they are set up correctly, you will see a wider torque curve in the middle area.
We saw a change on the formula ford in the second, third and fourth gears starting at 3500rpm (about).
This was noticed by the driver as being able to pull ahead of the others steadily coming out of a second gear corner, and proceeding through 3rd and 4th all the way to the end of the straight.
Without the cones, all was even with the other cars.
We figured that he had maybe 10 -15HP more available.
I realize that streamlining/drag has a lot to do with it, but not in 2nd gear and continuous through the whole range.
The side benefit is fuel usage/economy, just a little bit better because of cleaner burning in the chambers (better fill...less old gases).
Bob

Bob,

Thanks for the tip, but I've got a bunch of stuff to try first. I put the LM-1 on the car. Once warmed up, the idle is super rich, in the mid teens, cruise is rich in mid twelves and wide open is actually a little lean.

Also, when it's first starting up in the morning and still cold, and running lean, the idle is much smoother. So I strongly suspect that a big part of my problem is plugs fouling when warm and when I have to idle in traffic.

Spoke to Richard Parr, he suggests hotter plugs to start, then maybe smaller idle jets and smaller air-correction jets.

Making progress !

Bob,

thanks for revealing this little secret

Michael

I am glad to let out some of the wierd and wonderful things about engines that I have gathered in over 45 years of building them.
Some of the tricks no longer apply to modern engines, but some do.
I learned by watching and asking stupid questions, probably annoying the heck out of the mechanic doing the job!
I started with Flathead Ford V8's, and found out how to set one up and make it put out lots of power.
The thing is, most of the old tricks still work, and in getting the engine to run more efficiently, you get more HP per gallon.
So the old wives tale is mostly true, the best engine for power and economy is a highly tuned clean running one.
The only reason that we have all these computer controls on our engines today is because we got lazy!!
If we kept our machines in the highest state of tune every Saturday morning, we could throw most of the ECU stuff in the garbage.
My $.02....LOL
Have fun
Bob

would a small port size coupled with a larger diameter primary tube accomplish the same thing? I realize its probably not as efficient but would still serve to break up the backpressure wave.

JP:

Not really; all that does is kill the velocity and we see that on the dyno as a major loss of low & mid-range torque.