Rebuild/compression ?

jfran430

New Member
Jan 8, 2007
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Connecticut
Okay everyone, so mid summer last year, i blew my bottem end up in the cobra and am finally getting around to fixing it. I went to a machine/performance shop for the first time today (best of the best around here), after searching everywhere i could think of for forged shortblocks, and talked to the owner about a complete rebuild. So he asks what if i want it for a supercharger or N/A and i tell him it will be supercharged just not for some time (anywhere from 5k - 15kmiles down the road). He tells me that it is not a good idea at all, because of the compression being low, and that it is going to be sluggish and a 'pig' as he put it. Now i could understand losing some power, but is it actually harmful for a N/A engine to run on low compression (prob around 9.0-1)?? He made it very clear that i should not drive the car until it is supercharged and even told me he would rather build something with a higher compression ratio and see me spray it, which im not really that into (although the thought of a stroker kit is intruiging). I have the funds for the build, but not the charger right now, and really only want to have to fix this problem once, so all advice is appriciated. Thanks guys, hope to be back on the roadd soon!!
 
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It's not harmful, but the car will lose some guts in the low end torque department. My Bronco has 8.0:1 compression, talk about Smog Era crap! 13 degrees of base timing took care of the lack of guts, but 400 cubic inches doesn't hurt either.

9.0:1 is not "low" compression. It's the same compression that 2V GTs run. Factory boosted cars often run 8.0:1 - 8.5:1 compression, which is about as low as I'd recommend on any vehicle, boosted or not.
 
It's not harmful, but the car will lose some guts in the low end torque department. My Bronco has 8.0:1 compression, talk about Smog Era crap! 13 degrees of base timing took care of the lack of guts, but 400 cubic inches doesn't hurt either.

9.0:1 is not "low" compression. It's the same compression that 2V GTs run. Factory boosted cars often run 8.0:1 - 8.5:1 compression, which is about as low as I'd recommend on any vehicle, boosted or not.
Only thing near that low is the Terminator at 8.5:1 sompression and that's with the big CC of 4V heads, and for reliability and to deture any detonation on brittle mahle pistons.
9:1 is low compression for our cars, a great area for boost but your car wont feel like a pig with a new bottom end @ 9:1. Stock isn't 9:1 (I had throught this previously as well). It's 9.3 for a NPI and around 9.7 for a PI engine. I'm running 18CC Manley pistons and am only down to 9.6 or so - so 9:1 would be nice and low.
What you need to factor is dynamic compression over static, i've been doing that based on the cams I'm looking to use.

I's perfectly acceptable to use a 9:1 shortblock N/A for as long as you want. I went up to 10.4:1 with my current set-up and i likely only picked up 5-10.
 
I haven't put much thought into the exact parts i will use, but obv everything will be forged. As for the heads and cams i planned on staying with my stock setup (97 cobra) but having a valve job done. The way i see it, even though i could be completly wrong, is it cant hurt because its less stress on the motor anyways.
 
Perfect. Now last question. I know im going tto have to do it again after i charge her, but should i tune it right after the motor goes in? I am planning on it and don't see any reason not too, besides the fact that odv ill have to do it again. But like i said, i only want to do this once so im not really worried about overspending on something like that.
 
Perfect. Now last question. I know im going tto have to do it again after i charge her, but should i tune it right after the motor goes in? I am planning on it and don't see any reason not too, besides the fact that odv ill have to do it again. But like i said, i only want to do this once so im not really worried about overspending on something like that.

is the motor getting poked and stroked? or are you just replacing parts with forged parts?

im currently running a 5.0 DOHC motor with 10.5:1 compression (andf500 heads) and i definatley needs a tune. it still runs pretty well, i just dont want to go racing or really wind it up until then. im going to get mine tuned as soon as i get my tax return check.
 
What you do is buy a tuner from MPH. you'll need a tuner for the charger anyways, so buy one from MPH and tim will put a tune on it for you new set-up that's N/A (doesn't cost extra), load that. Then have your dyno tuner use the tuner to tune your car when it's boosted.
 
is the motor getting poked and stroked? or are you just replacing parts with forged parts?

im currently running a 5.0 DOHC motor with 10.5:1 compression (andf500 heads) and i definatley needs a tune. it still runs pretty well, i just dont want to go racing or really wind it up until then. im going to get mine tuned as soon as i get my tax return check.

That set up sounds pretty nasty, similar to the round I'm going with my rebuild but not really. :rlaugh:
 
What you do is buy a tuner from MPH. you'll need a tuner for the charger anyways, so buy one from MPH and tim will put a tune on it for you new set-up that's N/A (doesn't cost extra), load that. Then have your dyno tuner use the tuner to tune your car when it's boosted.

First time ever seeing/hearing of this site. Hell send me a tuner with a tune for my rebuild that i can just load? I wont have to take it to the dyno? And then after its boosted they can reuse the same tuner, right?