Optimum EGT for Merc Drag

**CI STV** · 11-01-2021, 04:16 PM

I know that a lot of you drag race guys don’t like to divulge trade secrets, but I’m trying to dial in my STV and I’m convinced that there’s more left in it, since I’m getting max EGTs of around 1060-1100 at 9,000+ RPM. Lower in the RPM band the EGTs are significantly lower.
The setup is a Diamond built modded Drag 2.5 with 21cc PPE heads and Diamond slider intake, with a solid fuel pressure at 56psi. Running Sunoco 112 (Supreme) with Merc red oil @ 32:1.
Am I right that this is a little low on EGT?
Also, I’m hoping to swap in some 17cc chambers soon. To what extent will that affect the air/fuel mixture?

**Stoker boy** · 11-01-2021, 04:30 PM

Are the probes in bolts 16 and 17 of the exhaust divider plate or on the adapter plate. I personally have noticed about 100° difference between the two locations. There’s a whole lot of other factors involved, I found the best way is to read the plugs and the piston tops to know for sure.

**Jay Smith** · 11-01-2021, 05:13 PM

I agree Stroker boy when I was racing I’d make a hard pass early in the day kill it clean roll #5 to BDC put bend o light in plug hole observe the intake vapor wash coming across intake side of piston , if the wash is wet the distance of a silver dollar motor is fat, if it’s distance is size of a quarter , pretty close , size of a dime better richen up a bit for that particular day , also I find for my guys that are not held to speck gas rules you can make more power blending down the octane, remember unless you have lots of compression and require octane to support the high octane it WILL hurt your power if you OVER octane your 2 cycle . My guys that race use a Kestrel weather station to make adjustments in box fuel curve to compensate for humidity , temperature, and altitude…
Just old school ways from old fart …
Cheers ,
Jay @ JSRE

**Stoker boy** · 11-01-2021, 06:19 PM

If you’re running 21cc heads I believe you should be cranking about 215 psi. If I remember correctly when I ran 21 cc I was about 103 octane. 110 race fuel and pump premium I had no detonation problems. Like Jay said you can actually lose performance with too much octane. If you do run 17 cc chambers I would definitely run straight 112 octane, keep an eye on the Pyros, spark plugs and pistons.

**CI STV** · 11-01-2021, 10:14 PM

Originally Posted by Stoker boy

Are the probes in bolts 16 and 17 of the exhaust divider plate or on the adapter plate. I personally have noticed about 100° difference between the two locations. There’s a whole lot of other factors involved, I found the best way is to read the plugs and the piston tops to know for sure.

The pyros are in the two bottom bolt holes of the exhaust plate, so they’re reading exhaust temps at the bottom of the chest cavity. I’m assuming that the further away they are from the exhaust ports, the cooler they should be?
I will try to get a reading on the plugs and piston tops. The only problem is that I would have to kill the power up top and get someone to tow me back in to get an accurate reading.

**CI STV** · 11-01-2021, 10:24 PM

Originally Posted by Jay Smith

I agree Stroker boy when I was racing I’d make a hard pass early in the day kill it clean roll #5 to BDC put bend o light in plug hole observe the intake vapor wash coming across intake side of piston , if the wash is wet the distance of a silver dollar motor is fat, if it’s distance is size of a quarter , pretty close , size of a dime better richen up a bit for that particular day , also I find for my guys that are not held to speck gas rules you can make more power blending down the octane, remember unless you have lots of compression and require octane to support the high octane it WILL hurt your power if you OVER octane your 2 cycle . My guys that race use a Kestrel weather station to make adjustments in box fuel curve to compensate for humidity , temperature, and altitude…
Just old school ways from old fart …
Cheers ,
Jay @ JSRE

Thank you sir. Old school works for me. I’ve seen your tuning advice on the bigger hammer website and that makes perfect sense to me. I’ll see if I can figure out how to get a good reading on the piston tops.
Thanks for the tip on the octane. The only reason I’m running the 112 now is because that’s the only option available to me that doesn’t have MTBE or ethanol in it. I didn’t want to mess with Sunoco GT260 or MS109 (reg) for that reason (and their low stoich number) and the only other leaded race fuels I could get were VP C16 (way too much octane) or Sunoco Maximal (same thing). I suppose I could try blending in a bit of 93 octane pump gas though…

**CI STV** · 11-01-2021, 10:26 PM

Originally Posted by Stoker boy

If you’re running 21cc heads I believe you should be cranking about 215 psi. If I remember correctly when I ran 21 cc I was about 103 octane. 110 race fuel and pump premium I had no detonation problems. Like Jay said you can actually lose performance with too much octane. If you do run 17 cc chambers I would definitely run straight 112 octane, keep an eye on the Pyros, spark plugs and pistons.

Thanks.

**CI STV** · 11-02-2021, 07:47 AM

So, if I increase the compression by going from 21cc to 17cc heads, would that typically require me to add fuel, or just octane?

**RBT** · 11-02-2021, 07:55 AM

The type of probes also change the temp as does the depth at which they are installed. So just handing out a target temp is foolish. All EGT is good for is for issues. Once you have reference it is good.
I suspect the motor off the dyno is set a bit fat, and your temps are a bit low regardless of which probes. But going to 17cc you might be close.

**CI STV** · 11-02-2021, 08:11 AM

Originally Posted by RBT

The type of probes also change the temp as does the depth at which they are installed. So just handing out a target temp is foolish. All EGT is good for is for issues. Once you have reference it is good.
I suspect the motor off the dyno is set a bit fat, and your temps are a bit low regardless of which probes. But going to 17cc you might be close.

Thanks. I’m using the probes that I got from PPE with my AiM dash. Not sure which brand they are. I wasn’t looking for an exact number, but just some guidance on the range I should be seeing.
From my experience, anything under 1100 typically is rich, and I’m seeing around 1060* with 56psi of fuel pressure.
Although I haven’t cut the power on a high speed run and checked then, from what I can see of the plugs and piston tops, it certainly looks a bit rich.

**RBT** · 11-02-2021, 08:27 AM

His probes are consistent 1150

**CI STV** · 11-02-2021, 08:57 AM

Thanks RBT. I was thinking it should be around 1150, and I have my AiM Dash set up to alarm (flash red) if either bank hits 1200* at any RPM. I also have an alarm for low fuel pressure <53psi and low water pressure <10 psi @ 5,000+ rpm and >50kph.
I love being able to datalog every run. Makes a world of a difference.
I’ll continue read plugs (and piston tops) regardless.

**Stoker boy** · 11-02-2021, 09:27 AM

Originally Posted by CI STV

So, if I increase the compression by going from 21cc to 17cc heads, would that typically require me to add fuel, or just octane?

The octane is so you don’t have detonation. Since I don’t run mercury‘s drag box I have to make fuel curve adjustments at various RPM ranges anytime I change the prop or change heads. Sometimes I make adjustments to timing. It’s always best to start out a little bit on the rich side. If you have a good baseline with your Pyros and can adjust the fuel curve from the dash that will help you on your tuning. 849849A6 is the mercury box my friend uses I thought one time he told me he didn’t need to make adjustments when he changed heads and prop now he tells me something different so I’m not sure.

It may be inconvenient but you still need to read the plugs and the piston tops.

**PanRonnie** · 11-02-2021, 11:40 AM

You have the diverters installed!?
In order to go on the limit you would want to equalize as much as possible
Cooling, injectors, ignition
This way when reading piston tops everything should look as equal as possible
Then decide if you want the bottom cylinders to run richer

**CI STV** · 11-02-2021, 12:58 PM

Originally Posted by Stoker boy

The octane is so you don’t have detonation. Since I don’t run mercury‘s drag box I have to make fuel curve adjustments at various RPM ranges anytime I change the prop or change heads. Sometimes I make adjustments to timing. It’s always best to start out a little bit on the rich side. If you have a good baseline with your Pyros and can adjust the fuel curve from the dash that will help you on your tuning. 849849A6 is the mercury box my friend uses I thought one time he told me he didn’t need to make adjustments when he changed heads and prop now he tells me something different so I’m not sure.

It may be inconvenient but you still need to read the plugs and the piston tops.

Thanks. I’m using a Brucato ACU and the motor was dyno tuned at Diamond, with factory 21cc heads. I changed those out to PPE heads so that I could increase the compression.
I’m thinking of getting the Steamwheel, so that I can make simple small adjustments, without opening the ECU.
Rather than messing with that, when I change the heads, I’ll probably bump the fuel pressure up a pound or two to fatten up fuel delivery and then dial it in from there.