Originally Posted by c3_dk

Much nicer text now Scott, and it says the same

Originally Posted by 7t9l82

Is it possible the heads don't flow as well as is thought?

Originally Posted by David_at_Triumph

Dear 63Mako,

I am pretty sure I know what is happening to your motor, the same thing happened to me with a nearly identical build. 355 ci, 10:1 compression, 0.015” head gasket, LT-4 hot cam, 1.6 rockers, except I was using a set of ZZ4 heads, cranking pressure was 185-195 psi in all cylinders. Also my engine was port fuel injected single plane holley with a megasquirt ECU controlling both injection and timing using a ZZ4 distributor modified for electronic timing control with a seven pin ignition module.

This “400 HP” engine barely ran better 0-60 and 0-100 mph times than the original 1975 L-48 motor, I was so disgusted. After blaming the injectors, timing, intake manifold, exhaust setup, megasquirt, etc… to no real improvements, I was ready to do the beehive spring change you did. I never got the chance, after running several autocrosses and two track days, the engine had a nervous come apart in the valve train on the track. As it turns out, three of the four lifter on cylinders numbers seven and eight were completely collapsed and stuck down, leaving the rockers very loose - the lifters have about 0.187” of hydraulic travel, so I lost over half the lift of the cam. Ultimately, this also damaged the lower end, rod knock, because with port injection, the cylinder(s) were filling with fuel and partially hydro-locking. Someone following me on the last lap reported clear liquid coming from my left exhaust and the engine was of course rattling, shaking, and sputtering as I came into the pits. (Note of caution to those considering a port injection retrofit, the lower end damage would not have happened with a carb or a throttle body.) That was the last time my beautifully prepared 2,900 lb track shark has left the garage, that was three years ago (so sad).

My engine had the exact same symptoms as your dyno pull is showing - the engine ran really smoothly across the band, sounded like a real killer and kept an optimum 12-13 AFR ratio via the wideband O2 sensor. But, about 4,000 rpm and above, my engine would seem to flatten off just at the point where I thought it should have just been coming on. I could see it in the megasquirt logs, the rpm versus time would start roll off above 4,000 rpm. I could also see it on my trakmate g-level traces on long straights, the acceleration would begin to fall off at the top of every gear.

So, I think the root cause of your problem is that the lifters are collapsing. Here is where the plot thickens a little. You are using a one-piece seal motor intended for the spider lifters and lifters from a truck. My engine was a 1973 L-82 block which I modified to accept spider lifters. Those who know engines are probably thinking, that can’t be done, the spider roller lifters are too tall for the Gen I block. And this is true, the V-8 spider lifters are too tall for a Gen 1 block, but the spider lifters out of a four cylinder GM are exactly the right height. If you know anything about this cheap and cheesy conversion method, it comes back to a guy on ebay that sells you the “conversion” plans for like 20 bucks. Well, I did the conversion, and being a hydraulics engineer, I did not really see any reason why it wouldn’t work. And, like you, I was on a $2,000 budget build for a retro hydraulic roller.

After purchasing retrofit tie bar hydraulic roller lifters, Howards, I have a better idea as to why the GM lifters are collapsing. The GM lifters do not have a return spring inside the lifters like the retrofit units have. So when the GM lifter starts losing oil, they count on oil pressure to extend them back and refill them. For some reason, this is not enough. I believe it does have something to do with the LT-4 hot cam, the spring pressures, and the 1.6 rocker ratio conspiring to put just a little too much force on the lifter such that it doesn’t quite have enough time or pressure to completely refill. This explains why it happens at higher rpm. It may have something to do with the valve lift ramp rate of the LT-4 hot cam causing a little bounce at higher rpms when the valve closes and this may explain the fluid loss and subsequent collapse. Maybe the check valve passage of these lifters is a too small and they can’t refill quickly enough. Anyway, I am pretty sure it is related to one or more of these issues.
As far as the fix goes, that’s a set of $400+ retrofit tie bar lifters and of course the correct length pushrods, and it’s somewhere between the GM roller 7.291” pushrods and the 7.8” flat tappet pushrods, but you already know that I think.

Let me know if you have any questions, best of luck from a fellow C3 corvette nut, Dave

Originally Posted by bluedawg

Thats what I was wondering and posted it in the other mako355 thread.

Originally Posted by Les

Now that you've hit the same wall, I'd take a hard look at this.

Originally Posted by Shark Racer

Unless the heads are complete garbage (IE, worse than a stock 70's GM truck head), I can't see this motor topping out at 270hp. Roller cam, good headers, nigh 10:1 compression I can't see this making under 300hp on an engine dyno.



Makes a lot of sense - way more than anything I've posted. I don't know which lifters came with the cam.... it looks like stock LT1/LT4 lifters are fine, but since the parts were bought used - they may well have been truck lifters.

IDK, but I'm pulling for Mako!

Originally Posted by Calo69

Sucks that your dyno costs are defeating the whole premise of build. Good luck

Originally Posted by cuisinartvette

Was thinking even a stock LT4 cam (not the LT4 hotcam)would peak higher than that wonder what you really got? How does the idle sound?

Originally Posted by Jeff_Keryk

This is a great thread. There is no quit in Mako.