JFM-jr

For the life of me I cant comprehend how a lash cap increases plunger rod throw on the high pressure pump. I liken it to a longer pushrod adding valve lift wich is not possible. What am I missing?

FYREANT

To understand the lash cap, you must first understand the fuel lobe on the cam as well as the HPFP. The fuel lobe on the cam cannot be -larger- than stock. Why? Because the stock cam lobe is as large as it can be already. If we made the lobe larger, we wouldn’t be able to get the cam into the block as the lobe would be larger than the bearing journals.

So how do these companies sell cams with 32% fuel lobes? What they change is the base of circle. Base of circle is the part of the lobe that is off the excentric, (where there is zero lift). So, the aftermarket companies make the base of circle smaller so that when the high pressure fuel pump is fully extended, it can go closer to the middle of the cam shaft, while still having the same height on the peak of the lobe thus giving you an overdriven fuel lobe/HPFP setup. The problem here is that when the pump is extended too far, the opening in the HPFP for the fuel to enter before it gets pushed out gets closed before it can “preload” more fuel. So while this works, you don’t truly get the full benefit of the 32% lobe. You get something less, likely closer to about 20-25% more fuel, not 32%

Follow so far? Ok so where does the lash cap come in to play? There is a lifter that is used between the HPFP and the lobe on the cam. The top of this lifter is where the lash cap goes. But why? Well, we know we can overdrive the pump by changing the cam’s fuel lobe base of circle right? But doing so we over stroke the pump in only one direction so much we don’t get any preload to allow the fuel to basically “pre-fill” at the end of the stroke in the pump. That time is then wasted and we are left with a half great solution.

This “preload” is essential to ensuring you get the right amount of fuel in the pump before it pushes it out! Think of this like an air pump that you use to fill up pool toys. If you use a pump that only works on the down thrust, you don’t get nearly as much as one that prefills the air back in to pump on the up thrust. Make sense?

So Enter the lash cap. The lash cap allows us to adjust where the stroke of the pump is taking place in the bore of the pump by “shimming” the lifter. This allows that much needed preload to happen.

If you aren’t careful though and don’t measure the proper amount of preload you could bottom out the pump on the upstroke so you have to measure this!

I wrote a guide on all of this for the forum. It can be found here:

https://www.corvetteforum.com/forums...learances.html

Hope it helps!

Ant

JFM-jr

I have a good idea on how all this works I guess I should have been more specific thats my fault. Im talking about adding the lash cap on a stock cam and stock fuel system. Some believe it can increase fuel supply i dont see how. Sidenote the % increase referenced on aftermarket cam lobes I believe refers to the additional stroke added to the pump not fuel volume increase any confirmation on that?

FYREANT

A lash cap will indeed increase fuel supply. Its overdriving the pump still. When the stock cam is in play, the lash cap will overdrive it the opposite way basically. You’ll get more lift out of the pump on the upstroke. It will likely only be about 15% more. 20% if you’re lucky.

Regarding the percentage on the cam advertisement, it has to be fuel volume increase. Theres just not an additional 32% stroke distance in the pump for that to be possible. 32% more fuel on the other hand is possible because of it being able to “fit more” into the bore when pumping. Maybe I am wrong, but thats my understanding of it.

JFM-jr

So the additional fuel has to come soley from where the new stroke takes place? This cap will not increase the actual stroke distance (like an aftermarket cam) but only relocate where the pumps stroke now takes place?

FYREANT

No, the bit about where the stroke takes place needs to be seen as “this is where it optimally performs to get the most out of it”. The lash cap will increase the fuel by increasing the stroke on the pump directly. It will have a shorter distance effect on the stroke on the base of circle area of the lobe since you can’t change “ground zero” on the stock cam, but then when it gets to the top of the lobe, it will push the pluger higher into the HPFP bore increasing the stroke distance on the upstroke. Make sense?

JFM-jr

So it adds to the bottom (on the base circle of cam increasing pump preload) and relocates this to the top (the nose of the cam lobe). This area of the pump its reloacted to is the area of more optimized pump output?

FYREANT

Not to the cam lobe directly. It adds to lifter. But basically can be understood the way you explain in your first sentence. It does not relocate the to a different area of more optimized pump output though. The scenario we are talking about is with a stock cam and therefore stock base of circle. No optimation relocation happens there. You are simply stroking the shaft longer. Its very simple with just a lash cap. Not as simple to explain when there is a lash cap and a 32% cam lobe involved.

JFM-jr

I think I get it . You will end up with more preload on the pump with the cap vs without correct?

FYREANT

In theory, yes, you should end up with more preload in that situation.

JFM-jr

Lastly has anybody confirmed this with data to back it up? Stock cam stock pump with vs without cap.

FYREANT

I have not confirmed it personally. I have only confirmed a 32% cam with and without the lobe.

JFM-jr

Thanks for your time and input here

FYREANT

Anytime!

StarFox

I still don't follow how a lash cap alone increases fuel. With a changed fuel lobe, I get it because the base circle has been reduced, which means your preload will probably go to 0 (or actually less than 0). Then you add the lash cap to get preload back to a positive number, and viola - your stroke has been increased because the fuel lobe has a longer stroke, and you have preload at the bottom which means the pump is using the entirety of the larger stroke length.

But with a stock cam, I don't get it. First, I am assuming that the stock cam has *some* preload. Then adding a lash cap is exactly like increasing pushrod length. You end up with a valve that is cracked open on the base circle of the cam - or in this case a fuel pump with a ton of preload.
Here is an example I am thinking of:
Let's say that with the pump out of the car, fully extended, the plunger is sitting at height 0.
Let's suppose that at the base circle of the fuel lobe, with no lash cap, and the pump is tightened down, the pump plunger is sitting at height .020. This is the preload on the installed pump.
And with stock cam, at max lift the pump is sitting at 1.020 (making these numbers up). That means my total stroke is 1. Max height minus preload height.
Now we add a lash cap of 0.060 to the lifter. This will increase the installed preload plunger height to 0.080 because we added 0.060 to the existing 0.020 preload.
Additionally, the max lift has now increased by 0.060 to become 1.080.

So the result is that at every point, the stroke is still the same as before. The lash cap has added more "pushrod" length but that hasn't changed the stroke. I know the pushrod sits in a spring, but when measuring we remove the outer spring and the inner spring is directly connected to the plunger, like a valve. And then under the lash cap is a lifter which I know has a little bit of play like any other lifter. but it should pump up and act mostly as a rigid device. So what am I missing? How do we get more stroke distance from changing the length of the lifter?

Update: I just wanted to say I'm not contradicting anyone here, just genuinely trying to understand the mechanics of what is happening.

JHEBERT

I would only run a lash cap if I had a smaller base circle cam. I would be concerned with bottoming the HPFP rod and breaking the pumps mounting flange. Not like it hasn’t happened before...

That said, who wants to run a stock AFM cam? Only people I know that want a stock cam wear panties

John

FYREANT

Consider this - in the scenario you describe, the lifter as the same over length. If you consider that the fuel is pumped into the lifter shaft in-line with the lifter, then it may not matter to change the stroke. However, the fue is not pumped into the lift shaft in line. Its pumped in perpendicular to the shaft (hence why the high pressure lines come out sideways from the pump). Why does this matter? Well, it means thet depending where on the shaft the fuel enters, it has a direcr correlation to the amount of time and space the lifter shaft can fill with fuel. Does that make sense?

That is my understanding at least.

StarFox

Yes, that makes sense. So by changing the min and max position of the shaft, you are giving the fuel entry location into the compression area of the pump more time or a clearer path into the chamber to fill up. Hence pumping more fuel. So in theory if I added that 0.060 lash cap to my lt4 pump I would have even more fuel as long as I didn't exceed the max stroke and destroy the pump.

FYREANT

Exactly. I’ve not personally disassembled one of these pumps, but based on everything I know that is how it works on the inside.

robert miller

^^^ Look at what this guy is telling you he is one of the masters on the problems of not getting the lash cap correct in a aftermarket cam. Robert