Thanks. To be clear, it's not my engine. Just trying to make sense of completely conflicting information I've been told. I think it's pretty clear at this point.
Just so I’m clear, I know guys kill cylinder pressure to reduce the tendency to detonate.
Does it work? Not really for what I said above. If you want to run higher than orthodox compression on pump gas you need to:
Control engine coolant temperature. Anything over 11:1 and the coolant temperature needs to be 160-170 TOPS when rolling down the road. It can creep up 8-10 degrees at long stop lights and such but that’s it.
Control inlet air temperature by running a cold air intake manifold. That means no exhaust heat crossover ever. And an intake that has a gap like a Strip Dominator. Notice I didnt say RPM air gap. I’ll address that later.
Absolutely you must have a timing curve. By that I mean a curve with 1.5-2.0 degrees increase per 1000 rpm. That is non negotiable. You absolutely can NOT have the same ignition timing at peak torque and peak power.
To control engine temperature you need a high flow water pump. A quality high flow thermostat like Stewart Components. That’s the only one I use.
You absolutely MUST overdrive the water pump. At the very worst you need to be driving the pump at crank speed. That is the bare assed minimum. I’m starting my testing with an electric pump. Then I’ll switch to my mechanical pump with my 6% over pulleys. Then I’ll drop on the 20% over pulleys or whatever they are. I’m looking for the power losses. I’ll give up 15 or even 20 if I have to, to keep the temperature at 160. Because if it goes over that then you need to pull timing and that kills power. You must control engine temperature and cold engines make more powa! Mo powa is always best.
Your parts selections are critical. I will not do 11:1 pump gas on a dual plane intake manifold. If you try it you are stepping on your own dick. BTDT. It’s unpleasant to say the least.
Cylinder to cylinder distribution is horrendous at best and you have to tune to the leanest cylinder and set the timing to the fastest burn. You can’t tune high compression pump gas engines doing that. That’s why so many guys end up mixing in race gas. Fuel distribution is paramount and really with a tunnel ram I think I could do 12.75:1 on pump gas if I screwed around with it long enough.
If you are married to a dual plane intake drop your compression ratio.
The other thing that can’t be done is running the engine too far under peak torque and then slamming your foot through the floor and not expect the engine to rattle its teeth out. Driving is a huge factor in how much compression you can use on pump gas. If you think rolling along in high gear at 2000 rpm and then standing the butterflies on end is cool these type of engines is unacceptable.
The last thing (but not the least) is spark plugs. Type and heat range is uber critical.
Opening a plug catalog picking a street plug is a guaranteed parts killer.
For my junk I’m starting with a C61YC Champion on the dyno because my best educated assumption is it will need a C63CX in the car.
Those are racing plug heat ranges. Trying to run a street plug heat range will destroy parts so fast you’ll need race gas mix at the minimum to keep from killing everything.
And you have to learn to read your plugs. If you live in an area like I do where I can run my local track and be anywhere from 2500 feet DA to easily over 5000 feet DA and in 3.5 hours I can be at the track I grew up on and in the spring and fall that place can go MINUS 500-1000 feet DA!!
That’s Disney Land **** right there. But if you can’t read a plug and you can see DA swings like that you’ll be in trouble. As the DA goes down I start dropping the heat range one for every 1k.
If the engine is on the edge for some reason I will pull a degree of timing for every 1k loss of DA. If I have some room I’ll drop 1 degree for every 2k drop in DA.
But the plug is the key. It will tell you exactly what the engine wants. I have tuned engines that needed two different heat ranges to make it happy. It drive guys bonkers but there isn’t a rule that says every cylinder wants the same heat range. You see that with dual plane intakes and some single plane intakes.
You have to be confident in what you see on the plug and then do it.
Of course carb tuning is paramount.
I don’t run these engines with anything but quality annular boosters unless the carb(s) are on the small side.
You have to get the fuel atomized pretty good by the time the fuel is at the opening of the runner so that the greatest amount of fuel is fully vaporized when the plug fires. Liquid fuel will not burn.
So prepping the fuel before it gets to the cylinder is a big deal. You start getting big drops of fuel when the plug goes off the plug will look lean but the exhaust ports will look dead rich. I see that all the time. The fuel burns too late to make power so EGT goes bonkers high and that makes its own mess.
That’s about all I can think of now. Unorthodox compression ratios on pump gas are what I consider pretty easily doable. It takes plenty of forethought but it can be done.
The benefits include a broader, fatter torque and power curves. Far better engine efficiency. The engine will run cleaner.
The engine will drive better. Far better. It’s amazing to drive a car that is done correctly. Even at 11:1. It tames big cams. It’s just hard to explain.
It is more work but IMO it’s beyond worth the extra worth.
Also, when I’m testing my trash I’ll be testing at cruise loading with my fuel pressure regulator. I want to see how far I can drop the fuel pressure with manifold vacuum at a cruise to lean the engine down a bit and get some fuel efficiency back.
And I’m going to test and see if I can tune the distributor well enough to use ported vacuum on the vacuum advance for the above reasons.
I’m looking forward to seeing if I can do it. If I can that will be all the better for fuel efficiency and cruise power and drivability.
