PBMFIsMe

iTrader: (5)

I am on the verge of buying an Iron 408 from Texas Speed and am moderately concerned about whether I can run 93 Octane fuel without any issues.


Here is the plan:

Iron 408 from TSP
4.030 Bore
4"Stroke Eagle Forged Crank
Diamond Pistons with -2cc Valve reliefs -.005 out of the hole
6.125" Eagle Rods
.040 Gasket for .035 Quench
12.56:1 Static Compression
Custom 242/248 .612/.615 XFI Lobe cam 114 LSA +0
63cc PRC 5.3L Heads Stage 2.5 w Race Valve job and 2.04/1.575 Valves
8.73:1 Dynamic Compression


Valve Events @ .050 are:

IVC - 55*ABDC
EVO - 58* BBDC
Overlap - 17*

I am running 1 7/8" headers, but I do have cats, so I need to keep the overlap down for that. Otherwise the cam is still up in the air. I just want to make sure I won't be having issues running 93 Octane with a good tune.

I could always go with -10cc dish pistons, but I love the idea of a high compression/high torque motor.

Hopefully some of the more experienced members can chime in about how crazy I am.....or better yet, how good my plan is. Haha.

99mongooSS

iTrader: (4)

the pistons going in my forged 347 are 11.8:1 ill be runnin pump gas

NassyVette

iTrader: (22)

I've seen members on here claiming to run 12.5:1 with pump gas, don't know how they manage to avoid detonation... Good tune, tight quench, good gas??? With my luck I'd be pinging like crazy.

WeberRacingEquipment

iTrader: (4)

We run alot of motors @ 12:1 on pump gas. With a proper tune up it should not be an issue.

Beaflag VonRathburg

iTrader: (176)

The key is you need enough cam to bleed off the compression. Static compression ratio doesn't really matter when you're talking about running pump gas. Getting the right DCR and tight quench distance are the two keys.

Shawn @ VA Speed

iTrader: (3)

We have done a couple with 12.5 compression on pump gas,like said above it comes down to having the right cam and quench,piston and combustion chamber design also come into play,then last but surely not least a great tune.

2001 Pewter WS6

iTrader: (43)

With my old heads, I ran it at 12.5:1 on 93 octane and had no issues. I had a Comp XE-R 244/248 .612/.615 112 +2 my dynamic compression was at 8.9:1. That cam idled and drove perfect.

PBMFIsMe

iTrader: (5)

I'm starting to feel a little better about this.

I've had better luck with smaller cams on my current motor, but bigger cubes means a bigger cam. My most important goal going with this setup was to not need to trade off high RPM power for low RPM torque.

With the higher SCR, I can run a much bigger cam without losing so much torque from a low DCR value. Therefore keeping the torque high and a nice flat power curve instead of the peaky curves you see on low compression/big cam motors.

Do you think the cam I'm looking at would be a good choice? I was considering going with a little higher LSA to lower overlap some more. Maybe a 116+2 instead of a 114+0. That would keep the DCR the same, but lower the overlap to 13*.

The car is and will continue to be a daily driver, so I would like to keep the street manners to a reasonable level as well as having a beast at the track.

DONAIMIAN

iTrader: (18)

Keep in mind that this is an Iron block and it wont deal with heat (something that a lot of people forget about) like the AL block.

b00sted

iTrader: (7)

Why? You'd be on such a fine line between running good and detonation...That's if you can even pull it off.

Ever consider running e-85 or mixing 93 and some 110 or something? I know alot of supra guys that mix toulene into pump gas and it raises the octane a bit...I don't know how much but it's supposed to be a big help.

Or, even meth injection...Have it spray meth whenever you're putting greater than a certain load on the engine.

PBMFIsMe

iTrader: (5)

Why is because I don't ever intend to go FI or Nitrous on the car and want to get every single HP/TQ out of this motor that I can naturally aspirated. I'm OK with running near the edge of detonation. I have the tuning software to adjust as needed for conditions (Hot summer days/ Race gas at the track/etc).

I have considered the E-85, but its almost non-existent here in FL. Meth is a bad idea in my own opinion, because with my luck, the day I need it the most it won't be functioning properly and I've heard it can be difficult to tune. I do have 100 Octane fuel available from my local Sunoco, but at $5.50/gallon its a little expensive to be throwing that in the DD. (Of course, with the way gas prices are going, that may be a good deal here in the next few months LOL).

I guess the bottom line is that I want to make sure I get my money's worth. I'd shoot myself if I spent $4-5K on a bigger cube motor and didn't gain at least 100HP/100TQ over my current setup. My goal is to get this heavy pig of a GTO in the 10's NA, but do it on a budget. I've looked through a lot of the 408 threads in the dyno section and most are around 500HP or so, but most of the ones higher than that are all M6 cars. I want at least 500 in my A4. I figured the extra 4% or so I'd gain from the high compression may help get me there.

I also want to leave myself some room to grow, so next year I may want a new set of heads, or a different cam, or both. I could easily drop the compression with bigger chambered heads if I have too many problems and adjust the cam to maximize the setup to the new castings.

edcmat-l1

iTrader: (10)

We've done several pump gas LS motors in the 12:1 range. And we've learned alot of stuff too.
1: theres a huge difference between 11.9 or 12:1 and 12.5:1
2: sometimes compression doesnt mean all that. Esp when you have to pull timing to keep the spark knock out.
3: we've done a **** ton of BBCs over 12:1 on pump gas, with gigantic cams, but LS motors arent big blocks. They def respond differently.
I dont think that cam, as big as it is is big enough to let you run 12.5 on pump gas.
I would recommend staying under 12:1. not much, but just under it. You'll def have more room to tune. And you wont be giving up much in the way of low end torque, esp if you have to pull out a bunch of timing to keep it from pinging.

racer7088

FWIW and not to step on any toes or say it absolutely can't be done but I have never seen any good running 12.5 to one true pump gas engines down here in Texas. They always seem to detonate quite a bit and have to be tuned rich and have timing taken out etc. so they don't even seem to make any more power usually once you fix them. They're good for a dyno queen deal but not so good for a real long life street deal that really makes power. I have to fix quite a few like this. If you cruise a lot and don't do lots of WOT driving or live in a cooler higher altitude state with good/better gas it might be better but otherwise I can't even imagine doing that really.

The engine masters competition was filled with 700hp 410-434 inch small block engines at 10.0-10.5 compression and yet before with no compresion limits and using pump gas they did not make basically any more power by the time they were tuned for pump gas. Many of these 12.5 to one deals on pump gas actually blew up in only a few dyno pulls as well. Keep in mind that I am talking heat soaked real street engines with good heads and cams really meant to make power down in the normal street <7000 rpm range. Anyway I would get one of TSP's slightly lower compression deals before going with that much compression. For reference even these 9 second pump gas deals down here we have done that are on LS1tech don't have even 12 to 1 compression and they have some pretty big cams.

Also remember that I like compression for any race gas engine and have built some very high compression stuff that runs extremely well but again we aren't stuck running regular pump gas fuels. Your cam would have to be pretty far off to reduce your VE so much you could run that much compression anyway I would think so that wouldn't necessarily be a good cam for making power.

PBMFIsMe

iTrader: (5)

Well. I think that overall my question has been answered. Not really what I wanted to hear, but certainly some good points were made and I'll take them.

I can easily go from the -2 cc flat tops to the -10 cc dish pistons and drop it down to a more reasonable 11.4:1 SCR. At that point, my camshaft choice will need to change, but that isn't a big deal as nothing has been purchased yet.

a 236/242 XFI lobe cam with .615/.612 lift on a 112+2 will yield me an 8.52:1 DCR with a .040 gasket

with these valve events @ .050:

IVO - 8 BTDC
IVC - 48 ABDC
EVO - 55 BBDC
EVC - 7 ATDC
ECL - 114
Overlap - 15 degrees

Do you think that would net me 500 rwhp through a 3600 stall? Or do I need to get a higher duration cam to get there?

racer7088

PBMFIsMe,

Remember that in reality you want the cam that traps the highest amount of air and fuel in your intended rpm range requardless of your compression numbers. Then after you have maximized your VE you add as much compression as you can but not the other way around. All this DCR stuff usually pushes people in the wrong directions.

Anotherwords don't build an engine or spec a cam totally around an arbitrary compression ratio but rather use a compression ratio that will work with the correct engine, heads and cam and then you will have a better engine.

The compression you run is much more related to the fuel you have at your disposal than the cam per se. It is true that having a lot of compression allows you to run a somewhat crazier cam but only within reason and that's why people don't run pump gas in race classes where they can run race gas.

edcmat-l1

iTrader: (10)

Dont get caught up in the dyno numbers with a bigger non-lock up stall. You'll always be disappointed. If you're really concerned about the power numbers, put it on an engine dyno. Otherwise, just take it to the track.
If you're sticking with a lock up style converter (4L60/80) then the stall speed doesnt mean crap if they lock it up on the dyno.

edcmat-l1

iTrader: (10)

Thats the same thing I've seen, dealt with. About 11.9:1 and you're right on the edge of not being able to get enough timing in, giving some up in the tune.