More compression absolutely. 8.87:1 is old school boost compression ratios.

 

The general consensus is that 8.8 is low for a centri setup. It will probably be a little sluggish off boost with stock rear gears.

 

It would also hurt overall power.

 

I have 4:10 gears would that help?

---------- Post added at 07:13 PM ---------- Previous post was at 06:59 PM ----------

What sucks is that I can get a really good deal on it and it is a well built short block.

 

You said it's just a short block? I'm assuming that compression ratio is based on a stock size chamber.

Regardless, it's quite low. I'd only consider that low of compression if it was an old iron head with piss poor valve angles and a shitty chamber with low quality gas. Otherwise, there's no reason to go below 9.5:1 compression. This goes for turbo, centri, and PD. You'd just be throwing away off-boost response and on-boost power.

 

IMO, depends on how good the deal is. At the end of the day, a forged shortblock is still a forged shortblock.

RDY has a point though. Is this 8.8 CR with PI heads? That would be a pretty large piston dish (~23cc or so). A shortblock designed for 8.8 with 4v heads would be more like ~13cc, which would yield about 10-10.1 with PI heads.

 

IMO, depends on how good the deal is. At the end of the day, a forged shortblock is still a forged shortblock.

+1. I grabbed a cobra shortblock because it was a hell of a deal. Think the compression is 8.5:1 with 4v heads and 9.5:1 with 2v heads. Sine I got the Mach I guess I'll be running 8.5:1 in the future or swapping out the pistons for higher compression.

 

The heads are stage III patriots. Here are the specs
Made to Perform. Patriot Performance Stage III 1020 Superflow Bench 3.552 Bore Heads for PI Romeo blocks feature 46cc combustion chambers with 180cc intake runners and 67cc exhaust runners. They come complete with Patriot's Custom Stainless Steel Valves and .600" Lift Springs with titanium retainers. Every last detail of a high performance racing head is taken into account at Patriot including a 5 angle valve job and 5-axis CNC porting.

Stage III Specs. The Stage III heads are designed for forced-induction airflow with their 1.84" intake valves flowing 68 CFM at .100 lift and 226 CFM at the full .600 lift and their 1.453" exhaust valves flowing 53CFM at .100 lift and 201 CFM at .600.

Here is the build sheet for the engine.

 

I am having a compression test with the heads installed Saturday.

---------- Post added at 08:16 PM ---------- Previous post was at 08:07 PM ----------

I was originally going to buy just the short block but decided to go ahead and get it as a longblock. We are having a compression test done Saturday so I can be sure what it actually is. He was running it with the same setup and made 530rwhp. He is supposed to get me the dyno sheet this week.

 

That compression ratio is to low.

evil your fairly local to me which means if you decided to build your own custom block Im here.

 

Thanks David. I may start looking for a block.

 

Sounds good, putting together a SB is possible, the most mistakes are made in the timing of the cams.. Another major issue is the cheeping out on the oil pump. since we have 93 readily available I can see you going 9,5,1 at minimum. I wish I would have went higher but I knew at 20+ I needed to hold back a little.

 

I am going to see what the compression ratio is with the heads installed this weekend. If it's to low I am going to build one.

 

That looks more like a bill than a build sheet. Build sheets I'm familiar with typically lists measurements taken during the machining and assembly, and sometimes include part numbers for the parts used.

 

You are correct t. That was my mistake. That is the invoice on the short block and head work that was done.

 

I know I'm old school on compression ratio but I'll still stand by the rule of thumb that one compression ratio number gets you 2 to 3% HP boost.

Most calculators give numbers in that range, like this one:

http://www.bgsoflex.com/cgi-bin/crchange.cgi?hp=450&oldcr=8.8&newcr=9.8

So lets go with 3%. if the engine is making 450 HP at 8.8:1 compression it will make 464 HP at 9.8 to 1, 14 HP more.

But let see what happens if instead of increasing the compression ratio we run more boost, which the lower compression ratio will allow.

The rule off thumb on this is one point lower in compression ratio will let you run 2 lbs more boost.

On our motors, each lb of boost will get around at least 16 or 17 more whp so lets say those 2 lbs get us 32 HP.

So going with the 8.8:1 compression ratio with 14 lbs of boost gets us 482 whp while going with a 9.8:1 compression ratio with 12lbs of boost gets us 464 whp.

That's 18 more whp with the lower compression motor and that was using the more aggressive numbers for compression HP gain and more conservative ones for the boost gain.

Granted it will be a little lazier out of boost but its worth it for the extra high end HP.

This low end laziness out of boost can also be compensated for somewhat by adding more spark then, which the lower compression will allow.

So I'd say grab that motor and just pulley it up more.

 

You're leaving out a huge factor with your equation. You're basing that 3% gain on atmospheric (14.7psi) air density. Figure that equation out at 2-3 atmospheres and you'll see the light. That loose 3% figure becomes much higher. That number is increased across the entire rpm range as well, not just at peak power.

Piston design as a lot to do with this also though. You never want a dome piston, flat tops or dished only. Your cam specs will have a big impact as well.

 

Peak numbers don't be everything. Numbers under the curve are important just as much or more. To make a badass boosted engine. Build a kick ass n/a engine. Plain, simple, and PROVEN.

Ps- low compression engines are not kick ass n/a engines.

 

Ok I'm going to make this simple... everyone shut up except Cali and Pate... because you guys are my boys lol

I dont give a damn about static compression ratios. They mean nothing. You need to think in terms of DYNAMIC compression ratio... If you have a great deal on an 8.9:1 shortblock buy the damn thing. Have the heads cut down and run a good cometic gasket. You can get back some of that compression for less than 200 dollars. I get Cometic Gaskets at cost if you're patient. It will take a couple weeks for you to get them but there is no better head gasket.

 

Thanks man, You know what blows my mind is people are so willing to take Engine advice from others who have never themselves built one yet somehow they are ****ing engine experts. That's like watching ER and then just writing out prescriptions because you seen it or heard it on TV. I personally have to go over things twice anymore just to make sure I am doing it right Lol so I essentially build 2 motors in the process of doing one.

OP listen to the few on here that have built these motors and knows what it takes to keep them alive under a lot of boost. You got some really good people helping you like Nitmare, RDY,callboy, who is if I aint mistaken a newby on his first MOD motor however impressing me with his finding on the degreeing process with the trickflow heads and stock cams.

 

Well of course you have to go by dynamic, that's a given. My built engine is 10.8:1 compression and won't have an issue handling 20+psi on 93 octane because of my cam profiles and chamber work. We're working up to that point.

Compression doesn't kill boosted engines, quench does. Fix the chamber instead and you will be rewarded for the compression.

 

I'd pass and build your own longblock but that's me. I don't trust people.

 

I know the guy really well and have bought tons of parts from him with good results so far.

 

Then just get all the documentation.

 

He said he would get me all the documents. I am going to take a chance on it. I am getting a really good deal on it. Worse case scenario I have to swap pistons.

 

If it were me, and it was too good of a deal to pass up, I'd just buy it and not worry much about the compression ratio. The way I see it; A low compression forged engine still has more power potential than the stock shortblock.

 

What centrifugal blower are you running?

 

Vortech V2 S-Trim.

---------- Post added at 07:53 PM ---------- Previous post was at 07:50 PM ----------

I would up buying his longblock instead of just the Short block. The heads are stage 3 patriot and the cams are customer ground comp blower cams. He is getting me the cam specs this weekend.

 

If you end up with the compression that low still in the end, I wouldn't put much cam in it at all. I'd even recommend stock cams just so it doesn't feel like a dog out of boost. You'll want to keep the dynamic pressure up.

 

I swap pistons still. Why go through all the hassle and not get 100% out of it?

 

What's the max boost on that blower?

 

According to Vortech it is good for a max boost of 20psi and 680hp.

 

Less boost more power. Not the other way around.