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tuning for pump gas

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gas pump tuning
3K views 16 replies 6 participants last post by  71hallett 
#1 ·
do any of you guys running pump gas have a hard time keeping the egt below 1500*. I always find that I need to jet way rich with pump 91 compared to race gas and today was no different , was tuning a new 516 bbc 10:1 compression afr 305 heads ,victor intake with 850 demon , comp xr292r , dry headers . to get egt below 1600* square jetted at 97's with no power valves ,timing set at 36*. it's in a 19.5 tunnel with B impeller @ 6200 and ran out of time with egt 1400* @ 4-4500 cruise 1550* @ wot. more jet , smaller air bleeds ? oh and no a/f meter but would like to get one . any suggestions thanks Rick .
 
#2 ·
When I ran 850 holleys I ran # 80 jets with power valves.
If you dump too much fuel in it will still be burning coming out the pipes and your egts will be high. Is the motor sluggish with that much fuel ?

I would not tune by the EGTs.
I removed my EGT gauge and tune 100% from the O2 sensor.

36 degrees of timing might be high for pump gas.
 
#5 ·
the 850 Demon has annular boosters and was 80 pri. with a p.v. and 88 sec. out of the box ,with 30* timing the first time out saw over 1700* yikes. pulled the p.v. and jetted square with 92's and 32* timing better but still saw 1650* at wot.then it got 94's with 34* better but still around 1600*finally the 97's with the timing at 36*. runs nice but started to feel a little fat the last time on the bottom coming out of the hole if you just were rolling into it. what do you guys that use pyro get your egt at ? btw water temps steady at 160 * and the plugs show no signs of meltdown on them :D thanks Rick
 
#6 ·
what should max egt be

going to try this again on the weekend . engine builder says timing should be good at 36*. going to jet up to a couple sizes and try timing up and down a couple * and try to get pyro's to stay under 1500* anybody got egt # 's to share ? what are the bad ass 800hp pump gas guys running at ? this is just a 600 hp 516 :)sphss some leaded gas with some octane would do it but the owner wants to be able to run 91 unleaded:)st
 
#7 ·
On my deal we never measured EGT's on the dyno just AFR and re-jetted to get that correct. We ran 100 oct unleaded on our first hard pulls to be safe and found that the motor didn't like any more or any less than 33* of timing. We then tried the 91 oct pump gas and made the final tuning on that. If I remember correctly we did jet up a untill the HP started to fall off and then went back. Ended up with 907 on the final pull. However I don't trust the consistency of the 91 from pump to pump. My motor is right at the edge at 11.5 to 1 comp so I mix 1 part 111 race fuel with 4 parts pump gas to get 95 oct. Insurance!!! I wish we did measure EGT's but one day on the dyno is just not enough time.
 
#8 ·
Most AFR BBC's between 8.75:1-10:1 I've seen in boat application end up around 35º-36º, however these have less overlap cams for I/O wet exhausts.

Too much fuel can raise EGT's.
Less compression can have higher EGT's than more compression. Are we sure 10:1 ? Is this measured or guessed ?
Not enough timing can raise EGT's. Worth to try another few degrees for a run but be careful.

Also, do you have another 850+ carb to swap on and see how that works ?

EGT's can be nice but don't always tell you what to try next. I suggest to keep reading plugs especially with a light and magnifying glass to make sure there are no tiny and shiny aluminum specs (from your pistons!) showing up on it.

If anyone's interested in the cam specs to give an idea of this motor:
 

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#14 ·
71hallett , no i didn't take you serious so it's all good . were actually happy with the power , on a dyno with a bigger carb it might pull 675 ? but i've found from tuning in a boat and looking at the impeller charts there not as generous , teaches you to build for torque . the guy has a history of flat footing everything to death so he bought the 850 thinking it would limit the top end while still making around 600 hp. btw advance is locked out . anymore ideas i'm all ears sounds like your doing a lot of similar stuff to what we are , how's your 8-71 coming along ?did the impeller and helped with the tune-up on one couple years back 433bbc A-B Aggressor impeller @ 6000 with 8lb's boost. be good to hear from more 500 to 700 hp guys must be a ton out there .
 
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