Max Dynamic Compression Ratio on 91 pump gas
 

I am in upgrading my 525 sc 177 blower engine and i want to know if he have anyone know what is the max dynamic comp ratio on 91 pump gas?

I will put the Edelbrock performer 454 rpm aluminum marine head.
I want to puting up my static comp ratio at 8 or 8.2 to 1
I want also puting the 2.85 blower pulley to rase up the boost loss due to arond 40 - 50 more flow with the new head.
I dont know how much boost i can lost with 40 - 50 more flow head ?? But i want to bring it in the 6lb range.

I want a reliable engine so i want to be on the safe side

Does the dynamic comp ratio can be no more than 9:1 on marine blower engine??

The stock 525sc dynamic comp are at 8.77:1 at 5.5 boost.

Anyone can help me???

With stock boost and stock 7.5:1 pistons, the effective compression ratio is about 10.3:1 and is already close to max for 91 octane.
http://www.holley.com/data/TechServi...ech%20Info.pdf

I did run almost 8# of boost on my 525SC for an effective CR of about 11.5:1. I always ran 92 or 93 octane.

Increasing the static compression ratio has the same effect as increasing boost. I think if you do both, you may have issues.

I'm not sure exactly what you are referring to as far as "dynamic comp" and if you are referring to the effective CR.
The Holley chart I linked to is a pretty good rule of thumb for CR and boost anyway though.

you already there if not past with a stock 525...water/alcohol injection will help lower the needs...

The dynamic comp ratio is calculate with the closing valve cam deg so i know that the DCR is always lower than the static comp ratio.
Depending on witch cam you chosse you can get a lower DCR without lowering your static como ratio.

Thats why i need to know what is the max dynamic comp ratio on pump gas...

I would raise the compression to 8.5
Run aluminum heads
Change over to a hydraulic roller
Run about 5 lbs of boost ( spinning the little blower too fast just makes more heat with little return in power).
Run the motor with no thermostat.

i pretty much agree,i like 8.0 to 8.5 compression,the small blowers stop making boost when you spin them to fast,they just make more heat in the intake charge.innercooler really helps when you start to run any supercharger over 7 lbs boost.every supercharger has its limitations.when you try to make too much boost with a small blower,you will be flirting with disaster,been there,done that.i hope this helps.

Another question about comp ratio

Does a supercharge engine can go more higher effective comp ratio (under 12:1 like weiand technical info) than a naturaly aspirated engine??

I have notice that the merc 500hp engine run with 8.75:1 and the stock 525sc engine at 7.5:1 have an effective ratio of near 10.31:1.


Thanks all for your input

I run the 525sc blowers with 3.25" pulleys and make 6.2psi. Edelbrock rpm performer heads. 9.5:1 compression on 93 octane. Much care is put into my engines though to ensure the setup is right (quench, a/f, timing, valves, mls, ect. so that may be a little much for you. Certainly 9:1 on 91oct, 30* all in, 11.8 wot a/f and a cool running alum headed engine would be doable (12.7:1 dynamic). To be aggressive, last season I ran 9.6:1 compression and 6.2psi for a 13.4:1 dynamic ratio. Rule of thumb is supposed to be 12:1 total but I have debunked that, at least with my junk. This year I am going to add a bit of margin and run 9.4:1 with 6.2psi on 93oct.

How much boost do you have lost when you have putting the rpm performer head?

What is your cam??

Your comp ratio number is the effective ratio with the blower.

The dynamic comp ratio is calculating with the cam intake valve closing degree at the (ABDC point) @.050 . ( make a big effect on the dynamic comp ratio).
I think this ratio is very important to know and have big effect in mod engine if i want to run on 91 pump gas but i dont know at wich number is the limite

Here is the link to calculate:http://www.rbracing-rsr.com/comprAdvHD.htm

Interesting, I am then referring to effective compression ratio in my dialogue above. My cam is crane 139011, essentially thier old 731 cam. ABDC on intake .050" would be 40* with cam straight up. I installed them retarded 2* so call it 42*abdc So what is my dynamic compression and what would it be with the cam installed as recommended? I check out your link later and see if I can figure it out too.

I always ran the edelbrocks so I cannot speak to any boost change due to improved flow.

This is what i get from the calculator from your engine info:
4.250 bore, 4 stroke 6.135 rod with 6.2 boost ant 9.6:1 comp ratio at 1000 feet altitude.



Camshaft, Rod Length, Boost and Altitude Correction to Compression


Your engine summary is as follows: Bore 4.25 inches, stroke 4 inches, rod c-c length 6.135 inches, with a static compression ratio of 9.6 :1. Your camshaft specifications call for an inlet valve closing of 42 degrees ABDC (after bottom dead center).

Your chamber volume is 108.13 cc's. With this camshaft your dynamic, or effective stroke is 3.63 inches. Your dynamic compression ratio is 8.60 :1 corrected for cam timing, altitude, and rod length. Your dynamic cranking pressure, corrected for cam timing, rod length and altitude is 174.55 PSI. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and 6.2 PSI is 12.23 :1.

Knowledge is power.




you are at 12.23:1 dynamic and the stock merc 525sc is arond 9.18:1 at 1000 altitude (feet).

So i dont know at wich ratio we are to high with this calculation......

While all the calculations are cool, I would be more interested in what others have done, and done successfully. Your 525SC is pretty common engine, that lots of guys on here have hopped up in various ways.

For starters, going with Aluminum Heads is a big help in fighting detonation. Polishing the combustion chambers can also help prevent any hot spots.

Boost psi can be tricky. For example, like you said, your boost will drop based on better flowing heads, and cam profile. So lets say stock maybe you were driving the blower at a 2:1 Ratio and making 5psi. Now, with the mods, you may have to drive the blower at 2.3, or maybe even 2.45 to make your goal of 6psi. Now you are getting into not only heat, but you will also really be "beating up" the air in the blower. Lets say you put a decent duration cam, and prop for 5500. Your blower will be turning either 12650 rpm or 13500RPM at WOT. Where stock at 5200RPM engine speed and 2:1 Ratio, you were turning the blower at 10,400RPM.

If your gonna keep the 177, i'd keep the drive ratio around 2:1. Like Kvogt said, keep it at 4-5psi, and run it. Can you make more power by upping the boost, yes, but not with a 177 on a stout 454ci. With good heads, properly selected hyd roller cam, modest static compression, 600HP is about where you'll be at 5psi. If 650-700 is what your looking for, more boost will get you there, but your gonna need a bigger compressor to do it.

Very interesting and 12.23 sound much more realistic to me and how I think of compression ratios. 13.4 was way high. I'm not sure what chamber volume is though and where you got that? My heads are 116cc.

My real numbers entered I get 12.45 dynamic at 42*abdc. Loading the cam straight up would have increased the dynamic to 12.58 (40*). That is a great calculator, thanks for sharing. It would be nice to get some correlation though. I'm going to be redegreeing both cams in the next couple weeks so I may go back to the straight up 107ICL recommended.

Also, for reference. On my 460CI engines, with Dart Aluminum heads, aggressive hyd roller cam, compression around 8.4:1, with a 177 weiand driven at 2:15:1, i got about 4.5psi. Swapping to a 250 B&M setup, 1.6:1 ratio gave me 5psi. 1.8:1 gave me 6.5-7psi.

I went with a 40% larger blower, although to get equal boost psi, I only slowed the blower 25%. Why is that, I dont know. Is it because the air charge is cooler, hence lower boost psi in the intake, or was the weiand a more effecient design than the B&M, I dont know. I do know that the best I got out of the 177 setup at 5psi was about 78mph on a cool day. The 250 setup at 7psi, got me 84mph on a cool day, and that was with fat jets and conservative timing. Probably a couple more mph in it with tweaking.

For your chamber volume, this calculator does not take care on your actually deck height, head gasket thickness and your actually squish.

Probably calculating with standard spec for that.

I know that i have a limitation with this blower. I am also looking for a 250 or 256 blower if i can find one. But for now i will start with the 177 in prediction of a bigger one.

What is your cam??? just curious

Another interresting DCR link....

http://members.uia.net/pkelley2/DynamicCR.html

Realy interresting...

That's cool, I've never seen that calculator with the boost built in. This is the one I've always used for NA stuff.

http://kb-silvolite.com/calc.php?action=comp

I can't give you an exact number for the dynamic comp. ratio, but I can tell you this. I have found that I as long as I stay under 190 psi on the dynamic cranking compression I'm OK on 93 octane with a good tune. For 91 octane, I would assume that number would be around 175-180 psi. Good luck.
Eddie

Is that for a NA or blower motor?

:evilb::evilb: Um, Yeah. I thought we were discussing BLOWER motors :evilb::evilb:
FOR SALE: (8) nice 4.630 dia ashtrays, only used ONCE!!

That's interesting because mine with 3.25" pulleys (2.15:1) I get 6psi. Must be my dome pistons reducing volume in the chamber vs your 8.4:1 pistons. Why you didn't have to slow the b&m down as much I'd say the 177 rpm was at peak efficiency and now the b&m rpm is under its most efficient speed. Just a guess though.

My cranking pressure is 175lbs so that might explain why I stay out of detonation so well with the 9.5:1 and forced induction on 93 oct.

This is one of the best threads I've seen in a long while.

BT

Have you verified that psi with another gauge? It seems high for a blower motor, that's why I was asking Eddie if his numbers were for a blower deal too. I could be way on the conservative side but mine are usually around 150ish or a little lower depending on the application. Clearly yours is working well which is awesome and you can't argue with real world results!

Another cool thing is how underated the mini blowers are. Everyone discounts them right away but they work great for what they are. We built a 557 out of spare parts last year with bb2x's, a crane 651 cam, and a 250 blower with a pair of 750's. Before I ran it I called the blower shop to ask if there was anything I could do to get some boost out of it bcause I was sure the blower was too small. They told me it probably wouldn't make any boost on that size engine but we tried it anyway. It made 6lbs and almost 800hp on 89 octane! I thought it was pretty cool for a spare parts motor with a mini blower.

It doesn't actually translate into pressure on a gauge. Just because it's 180 or 190 with using the formula doesn't mean it does it on a gauge.
I just did a comp test on a particular engine before we pulled it down to be freshened. It's a 598 with a 5 liter. The formula says it's almost 180 psi. Actually, it shows 160 on a comp gauge.
I use that number for both NA and sc. I don't neccessarily get all the way up to it for either engine. I just make sure that it's under that. It depends on the what I'm building and what it's going in as to how close to that number I want to get. However, I have been right there with an SC engine. It has to have a very good tune and there isn't much room for error. I generally try to stay in the 175 area or lower. The 190 number is an absolute max for me.
Eddie

Same deal with a buddy of mine. He has some 598's, dart heads, weiand 256 blower, no chiller, 3psi of boost. Engines made over 800hp. For 3psi, he is driving his blower the same ratio as I am, but I get 6.5-7 with my little motors. His 598s with that setup have been extremely reliable with that setup for 4 seasons.

I have a similar set up, 540 CID, alum heads, 250 SC (2) 750's, No intercooler, 4 lbs boost @ 5,000 rpm. I have jetted for 12.0 - 13.0 A/F . Timing is 30 degrees. Dyo sheets says 650 Hp, I have Aluminum Gil type manifolds with EGT probes just above the aluminum in the SS pipe. @ 3,000 rpm EGT is 1250 degrees, 5,000 rpm 1,380 degrees. Boat is V bottom 12,000 lbs . Is this too hot or any ideas how to lower EGT

Maybe a little more timing and fuel?

That's awesome, whether it's a spare parts engine or not. Have you done any of the 250/256 blowers with a single Dom.? Do you like the 2-4150's better than the single Dom? I have always done a single Dom on that blower with good results. Never tried 2 smaller carbs.
Eddie

What kind of egt gauge? I had gaffrigs and they read 1550+ at wot all the time, I poured fuel to it and could only get them to 1500 and boat was pig rich, plugs were almost BLACK, not even brown, put it back where it was and ignored them, Smitty

Eddie, something I noticed when I recently tore my engines down. The front cylinders appeared to be running quite a bit leaner than the rears. I set my dominators up with squared jetting, and even powervalves. Made me scratch my head about that, and do some research. Apparently, from what I read, that was a common problem on the b&m/holley 250 blower, when running the single carb. I found out that they came out with a new style single carb adapter, that put the carb a bit further towards the front of the blower. You can still get these adapters, sold under the weiand name, but they are for the old B&M 250's.

Something else I noticed, and didn't like. When mounting the dominator on the carb plate, you can clearly see how the plate was designed for a 4150 style carb. While someone did some grinding on my adapter plate, its still shoddy looking. When you open the carb to wot, the outer sides of the barrels pretty much hit a wall, and its not a smother transition at all. What I thought about there, is using a 4150 to 4500 carb adapter. More of a funnel shape spacer. Whether or not its worth anything, I can't see that hurting? I do like the simplicity of a single dominator. Engines idled, cruised, and went balls out without a sputter or hiccup. Also would idle all day long in gear at 700rpm. :drink:

I'm using Autometer # 6545 Digital pyrometer, I have a gauge on each engine and swap from bank to bank with the
o2 sensor to be sure all cyl are firing. When I lean out the engine to 14.1 the EGT climbs to 1550 F . Does anyone else have experience with EGT in a SC configuration ?

Why are you leaning it out to 14-1!? :angry-smiley-044:

I see temps similar to Smitty's. Unless you can crank some more timing into it along with the fuel, the fuel alone won't make a big difference and will even start to go the wrong way if too rich and not enough advance. Running that lean at any engine load with a blown marine engine is gonna make some heat for sure.. I find even the idle stabilizes better at around 13.5-13.8

I only ran for few minutes one engine @ 14.1 A/F, I was testing the PV & jetting to understand how many jet sizes made a difference. With 2 750 carbs & 8 jets & 2 PV I was thinking one jet size would make a big differnece, it did not. One jet size was giving me about .3-.5 AF change on the FAST digital o2 sensor. I have been careful to test my octane with an electonic test instrument. I have been running 91.5 - 94.5 Octane through the sample period. I understand that Octane has nothing to do with EGT or A/F but the details remain important. When the pistons melt off the rods I will not have to wonder why

Anybody know what the A/F map is for merc SC 700 ?

Next time you've got some good gas in there try 34 degrees and 11.8-12.0 afr and see what it does. What's your static comp?

Returning to blower question...

I want to know what you think guys about the 250 blower size or a 420 blower size on a 454 ??? Is the 420 are too big?

A 420 is not too big for a 454. It just depends on how much power you are looking to make.
A 250 is plenty adequate to make 650hp or so and a decent improvement over the 177.
It won't make nearly as much heat running the same amount of boost and has more potential to increase flow.

My egt's were always sky high BUT my afr's are in the high 11's at low boost and mid 11's at high boost, never melted it down yet. I never based my motor around cranking compression BUT at 8.4-1 with the cam I am running I see between 150-155 on a gauge. If I cranked it up and saw 190 I would say the base compression is too high or cam is too small unless you were running race gas or VERY low boost.
Eddie, your post is very confusing!:eek:

FWIW too, I hear it's not unusual for a blower motor to have high egts because so much raw fuel is blown out the exhaust that it creates a secondary burn or something, I would be more concerned about seeing safe afr's on both banks, Smitty