Cruise Engine RPM and MAP Values

[CBDZ]

Hello All. It has been a long time since I have visited here. I am still playing with my MS1 set-up. I know, its outdated, but it does what I need it to do. Nothing fancy, just batch fire and wasted spark.

I have what I think is a dilemma. I am trying to work out a better AFR right at 2400-2600 RPM, which is where I want my cruise speed to be. This gets me about 60-65 MPH. I'd actually like to bring that up to about 70 MPH, which is about 2800 RPM. The issue is that at these values, the MAP is right about 80-85 KPA. These values are also where the engines passes through on a typical acceleration pattern. (not a WOT, balls to the wall pull).

If I lean-out the AFR/add timing at these cells, then acceleration will suffer. If I tune for acceleration, then cruising MPG suffers. Is there a way to lower the MAP at this RPM range during cruise? Is this just the nature of the engine? Am I asking too much to cruise at this speed? If I were to change the intake tract would this change? (currently running stock German EFI Plenum, with CB ends, and Ford Escort TB.

Thanks!

[Chip Birks]

Seems like a lot of throttle input is required to hit that speed. 80-85 assuming sea level is pretty high for a low load cruise. What cam is in the engine?

[CBDZ]

If I remember correctly, it has a Engle 110 cam. The static compression ratio is 8.1:1. I was planning on adding a turbo, so I kept the CR low.

Valves are 42x37.5 (I think) It has been a while since I put this together.

[Clonebug]

It's also geared pretty high....

My buggy cruises at 3100 rpm and 70 mph.

A buggy is a brick going through the air...it has a lot of resistance the faster you go. With your lower rpm it is probably putting a pretty good load on it.
If I cruise at 75-80 my kpa is pretty high also.
You could try adjusting your timing at cruise.....Sometimes if you have someone change timing while driving you can feel it in the pedal when you gain torque and you can let off the pedal and maintain speed. Mess with timing and AFR to get that sweet spot.

[CBDZ]

Clonebug,

31" tires really affect the gear ratio. I should see about a 4.375 ring and pinion. That would drop me down quite a bit. I have a late IRS box with the following gear ratios.
R/P = 3.875
1st = 3.78
2nd = 2.06
3rd = 1.26
4th = 0.93

Are you in boost at 3100 rpm / 70 mph? Is 70 mph a comfortable cruising speed, or do you feel like the engine is screaming?

As I look at my tuning map, I feel that I have a very narrow MAP range of operation. It is always above 75 kpa +. I am hoping to bring the MAP range down and give myself a broader usable MAP range.

[Chip Birks]

Man, you guys and your tall tires. I think I'm cruising at 3800 at 70. I could be wrong though. It's not 3100. 26 inch tall tires though. I fell like my engine has about the least usable map range on the planet. Ambient is 83, idles in the mid to high 70s, but cruises on the freeway in the mid 50s I think. Been a while since I've paid much attention. I do however tune fuel based on throttle position rather than map for all of my non boosted driving. It improved drivability by huge leaps. Timing is still map based I think though.
I went that route because I found that I was using a very very narrow window on my map. Throttle based let's me use way more of my table. Not sure what ms1 is capable. I've never really messed with it.

[Max Welton]

I never thought about it before, but MAP is a clear indication of engine load. And yours tells me that you are pulling a pretty tall effective gear which is really causing extra load on the engine. And the buggy body isn't helping either.

I cruise (65-67 MPH) at ~54 kPa at 6000'. That is about 3400 RPM. My altitude helps reduce aero drag, but still. This was in my type-3 squareback years ago:



Max

[CBDZ]

So, the solutions is to cruise in 3rd gear.

[Max Welton]

Easy enough to try.

I'm sure the tall tires look cool, but the loss of mechanical advantage is a real problem both for MPG and for acceleration. You'd do much better if you could get the revs back up.

Max

[CBDZ]

Max,
Just out of curiosity, what is your idle MAP?

Mine is higher than I thought it would be, making me think I've got a bigger issue.

[Max Welton]

I idle around 30 kPa.

Max

[Chip Birks]

I idle around 30 kPa.

Max

What cam?

[CBDZ]

Wow, my idles is at 60. Maybe I do have some bigger issues to deal with.

[Max Welton]

I idle around 30 kPa.

Max

What cam?

EXV264HYD on 106º lobe centers from Steve Long Racing.

The engine is a 1776 type-3 using the stock type-3 FI intake manifolds and TB.

Max

[Clonebug]

Mine idles between 50-60 depending on engine temp and idle rpm. I like an idle between 900 and 1100 rpm but that is also ambient temp dependent.