Boost Question

[itSSlow98]

Long story short i popped my 8.5 compression motor back in october and it made 700/760 on 20psi. The new motor that Im currently breaking in is 10:1. How much boost would I need to run to make roughly the same power as before? I was thinking 16psi should do it but Im not real familiar with the difference in compression and just how much extra you gain per lb.

 

[MGC]

Long story short i popped my 8.5 compression motor back in october and it made 700/760 on 20psi. The new motor that Im currently breaking in is 10:1. How much boost would I need to run to make roughly the same power as before? I was thinking 16psi should do it but Im not real familiar with the difference in compression and just how much extra you gain per lb.

IMO 15/16 psi will about the same #'s, IIRC mihovetz said with the higher compression you take about 3 degree's out of it for every 1 bump in compression, so as a baseline take 5 degree's out of your original tune, a car that doesn't need as much timing is more efficient.

 

[itSSlow98]

Good info thanks. Current tune is 20 psi/17 timing so I figured 16/14 would be good. Ill start a thread with how the dyno tune goes next Saturday.

 

[MGC]

No problem, I did a little searching and found the quote I was referring to:

Hi to all. Here's the short edited version of how this works. I have proven over and over that compression just makes the engine perform better. I don't think I have seen any combos similar to mine that make anywhere near the same power using less compression and more boost. The pump gas combo is the same. I guarantee a similarly equipped low compression engine will make less power than the high compression version with less boost.

There are ways to work around the higher compression and get the best of both worlds. A lot of it is in the cam choices you make and where they are installed at. Every test I have ever done on these engines yields far more than the conventional thinking 4% per point of compression. My results are actually around 7% or 8%. Doesn't matter turbo, screw blower or centrifugal blower. Also the compression makes the turbos work better thus enabling someone to effectively use a pretty large turbo and still spool up quickly.

I see a lot of discussion regarding timing and usually lots of it. I can guarantee you that an engine that uses a lot of timing is not efficient. Simple example is an 8.5 to 1 motor might run well at 16 degrees while a 10 to 1 motor usually will only want to be around 12 at the same boost. The differences being that it is easier for the piston to overcome TDC and exert more accelerative force on the crankshaft with the higher compression motor especially from the 1/8 mile mark to the finish line. Even engines of the same power level will show higher MPH with less timing. Usually 3-4 mph is pretty normal.

On the tests I have done that were back to back the fuel maps were identical on either motor as long as the boost was the same. The difference was simply efficiency. That means the same fuel was used across the board just the high compression engine yielded more power across the board. We usually see a brake specific number around .38 on the blower motors The same holds true on street driven cars. To maintain or accelerate the vehicle similarly the low compression car needs more throttle to do the same thing. I could safely say the high compression version will yield 10% better fuel economy in all situations.

Where's the negatives? The negatives come from not thinking outside of the box. Conventional thinking will have you all running 7.5 compression like back in the pushrod days. Most of what you hear comes from that type of thinking and an unwillingness to try new things. For sure the engine can be more sensitive to detonation. You don't need a genius to tune it. Just someone with 2 ears and some simple common sense. THE FUEL IS USUALLY THE SAME JUST USE LESS TIMING. By that I mean -3 degrees for every point of compression is a good start.

Compression is free and boost costs money. If you have a set horsepower figure in mind these factors play very well into what you are doing. If you can make the same power with 6psi less boost there are many benefits. Less air charge temps. Being able to run the blower where it is efficient. Less parasitic drag. Less underhood temps. Less possibility of belt slip.

Another thought behind the lower compression was head gaskets. Back in the day (I was there also) the pushrod engines couldn't keep a head gasket. We just don't have that problem. The problem with every blown head gasket modular engine I have ever seen is detonation. Now the "tuners" may disagree but the parts don't lie. I have seen the insides of hundreds of these engines particularly ones that were "for sure it didn't detonate". In every case you will find that the wrist pin in that cylinder is also bent. 100% of the time. Most aftermarket piston companies usually sell a pin that is too thin to save weight. Typically .120 to .150 max. Guarantee it will bend if detonated. No other force can bend the pin.

An example is we use a custom made M-2 tool steel casidiam coated pin that is .250 thick. Pretty heavy pin for sure. But we just changed from the old version that was H-13 tool steel and .220 thick because we were bending those. We aren't bending them because of detonation, we are bending them because of our 2 step routine to build boost. We fire the cylinder appx 10-14 degrees AFTER TDC. The pin is compressing liquid in the cylinder and its just is too much fuel at that time. We usually run 11-12 degrees at 50-54 psi down track and take timing away after 800 ft.

Please continue your debate.

John Mihovetz

 

[itSSlow98]

Well I can't ask for much better info then that.

 

[Yaadman_GT]

Mihovetz DOES know WTF he is talking about!!