xblazinlv

Well I've been reading up on normal compression numbers, and came up with the following:

115+ is like new
100-115 is healthy
90-100 is getting weak
* < 90 could blow at any moment


I'm sure this is standard information, but is this 100% correct??



My motor is a fresh rebuild, less than 2000 miles on it with 2mm apex seals and a mild street port. I just piston compression checked it and all 3 faces are seeing around 30psi with the tester opened, and closed, it reads 90-95psi on the front and rear housing.

I had the car on the dyno last night, and it made **** for power. I couldn't believe how low the HP numbers were. I know I was on an AWD Mustang Dyno in 2WD mode, and it is a low reading dyno from what all the local people can tell, but I barely put down 220 at the wheels with 11psi and I have just about every bolt on you can possibly have.


Mods:
Intake
Downpipe
Catback
Midpipe
Non-Sequential Stock Twins
Bigger SMIC
1200cc Secondaries
Supra Pump


Am I suffering from low compression, or is the tuning done on my car so far ****? The a/f ratio across the board at 11psi was 11.6-11.7 to redline

WTF??

And do rebuilt motors typically not see higher compression readings? I mean this thing was JUST rebuilt about 1500 miles ago. I don't get why it would be so low already, when the car has never seen over 9psi until last night, and it was pig rich with no knock most of the night on the dyno.

level7

iTrader: (3)

I dont think compression is your problem. Even if you were making low compression, motors can still make good power up top. 220 seems pretty low. Are you running an EMS? I didnt notice one in your mod list. Wish i could be more help but I dont think its the motor unless your feeling the other symptoms (bad hot start, idle jumping, shut off....)

xblazinlv

Oops sorry, I'm running an Apexi Power FC and we were using a DATALOGIT to tune with.

xblazinlv

Going to bump this, I'm pretty bummed / upset about all of this. So much time and effort has gone into this build so far, this really sucks

xblazinlv

Up top again

imluvinit

iTrader: (1)

i dont but i know my 7 has low compression and it turned 320hp on a junk ebay turbo+ most of the usual bolt ons

imluvinit

iTrader: (1)

id have to guess that theres something else to look at.

mdpalmer

iTrader: (24)

You didn't mention ambient air temps. Were they running a fairly massive fan on the front end of your car during the dyno session? I've seen people lose 20-30 rwhp just from heat soaking the intercooler (high intake temps). You might want to get a "real" rotary compression check to be sure, and also make sure your boost is consistent, especially at higher revs/load... you didn't mention any boost controls. Make sure your 11psi is steady when you load that motor up I had problems with that when I was having my car tuned without an electronic boost controller. But I haven't had my car on a dyno, so what do I know

xblazinlv

I have a Blitz SBC ID III boost controller, and boost is rock solid.

As for heat soak and ambient air temps, it was hot outside. Even at night in Vegas air temps are in the high 90's / low 100's

We had a HUGE fan in front of the car (water temps stayed cool on the dyno) and another fan made to shoot air into a FMIC right up into the ducting that goes to the intercooler. In between runs the dyno guy also sprayed down the intercooler with water.


Either way, I'm missing a ton of HP even for a lower reading Mustang Dyno. On this dyno, to be consistent with what most people on the forum see HP wise, I would have had to put down atleast 260 corrected, because on a dynojet out here that would be around 300. I know I have a lot of room to tune, and I'm starting to feel a little better about the car's compression after talking with several people.

mdpalmer

iTrader: (24)

This may be a stuipd question, but: what gas were you using in the car? 91 octane? Also, it would be nice to see a dyno graph.... I think a lot of people here could help you out a bit better if they see power curves. It would be easy to see any ignition breakup and people can scrutinize the A/F mix too...

Two other things to check are fuel and spark... I remember you having some issues with your injectors flowing more than what they were rated at... you sure your plugs are good? etc. etc... just trying to throw stuff out man. I can understand how frustrating a underpowered FD can be. I mean, people look at the car and think it's some kind of spaceship or something. They expect it to be fast

xblazinlv

I'll get a print out of the 220 HP dyno graph tonight since I'm heading back down there. We did so many pulls, the only ones I have a graph of are the ones at 13psi where the a/f ratio was in the 9's lol


As for the injectors, I have 1200cc injectors now that flow what they should. So that's not the issue, and same goes for the spark plugs. I put in brand new BUR9EQ's all the way around the day of the dyno, and tested them out on the freeway on the way there, the car felt fine.


The only thing that makes me feel a little bit better about putting down 220 is that an F-body with an LS1 on the very same dyno, only put down 238 with an intake and exhaust. On a dynojet, an LS1 with intake and exhaust out here usually puts down around 290 at the wheels, so I'm really not that far away from where I should be, I think tuning is going to play a big part in bringing out the numbers, that and waiting for it to cool down. Vegas has no humidity, and it's 110 degrees of dry heat everyday. The night of the dyno is was like 101 outside, and we are also somewhere around 2200ft above sea level, atleast our track is I'm not sure about in the local area, I think we are higher.


Anyway, all my coming dyno pulls will be on a dynojet that I know reads a lot more accurate (we've put our C6 Vette on it before) so I'll report back.

Hopefully someone with some Rotary tuning knowledge can help me out by then and I can get everything sorted out