I'll try to spare you most of my frustrations with my engine rebuild, but as of right now, I'm staring at the build sheet and the CR is @ 10.2:1. I told the builder it came with 10.5:1 stock and I would like to see it closer to 11:1. Is there a ratio or percentage of power gained or lost by changes in compression. I know the LT4 came with 10.8:1 and it made 30 more hp.

If .3=30 hp, then it would seem .8=80hp. I know there was a little more done to an LT4 than just a change in compression. I am mostly thinking out loud right now...

 

hmmmmm no. there are more variables to include to predict a power increase with a high CR. the lt4 made more power than the lt1 due to the larger lt4 heads and a cam. the 10.8 CR had little to do with the power increase. to get a power increase with Cr you need to push 13:1 and higher.

 

I wouldn't worry about .3, its minimal.

Also what numbers are you using to figure? Did he actually cc your heads? 3cc could = .3 compression.

 

For max power with no other changes. You can see 2-3% power increase per point of change this is for race type engines with comp over 12:1. In the more streetable ranges the increase in power will be less. That is not to say you wont see gains here, but most of them will come in the form of part throttle cruising. You will make more low end tq during part throttle. I would not worry about chasing compression ratio around too much, but putting large cam in with lower compression isnt giong to yield the best cruiser but the diff is going to be rather small.

 

I'd be pretty pissed if I specified 11.0 and got 10.2. And wondering what other things were screwed up.

But the OP makes it sound like it may have been more like general ramblings than a specification.

Regardless, if it's supposed to be a performance build then I'd say the builder doesn't know what he's doing.

If it's supposed to be a stock rebuild then you weren't concerned with HP in the first place so why worry about 1-2%.

P.S. - https://www.camaroz28.com/calcs/crchange.html

 

My message to the builder was to reuse as many stock components while maximizing the HP. We debated for a few weeks before he finally ASKED me what cam I was going to throw in (because I didn't want the cam he was selling) and then he'd match the engine to the cam. I was also tossing out the idea of putting in new heads, but we decided to rebuild the ones I had. So, what I got were some ported stock heads with 53.5 cc chambers (new stainless 1.94 valves), -14 cc dished pistons, 6.0 connecting rods, .30 overbore, xfi280 comp cam with the beehive springs, 1.6 promag RR. Looking at the prices listed it seems like we spared no expense on anything replaced, so everything should be good. I'm just a little frustrated with the lack of CR (and his lack of note taking). He said it would be easier to tune with the lower compression.

 

honing or boreing on the cylinders, getting a larger combustion chamber in the head, or even goin from say (for example reasons) domed topped pistons to flat tops will all lower compression some.
some changers will be more than others boring the cylinders being the largest factor usually.

 

i remember hearing a general guideline on a typical street engine, for each full point of compression something like 10.something horsepower is gained

 

According to the calculator Steve threw out, it would have been 8-10 hp. With there being no forseen difference in cost, I would want that extra hp.

 

http://www.sunyabem.org/images/chart.bmp

The chart is if nothing changes but the CR. If you take advantage of the higher CR with a bigger cam, the gains are larger but come higher in the rpm range.

Rich

 

Why wouldnt lower compression decrease power? Unless you planning on doing some f/i I would be kinda pissed. Then again it just 10 hp, throw an electric waterpump on and be done with it.

 

I think the dished pistons were the biggest mistake, personally. Some good Mahle flat tops would have been a better choice.

 

More then likely the dished pistons did not help at all attempting to get higher then stock compression. You may look into a small domed piston, or just buy flat tops and use a thinner head gasket maybe. Im no expert just throwing some ideas out.

 

Is this a stock cube motor?

 

he said .30 over. So im assuming its a 355, have you thought about getting the heads milled for more compression?