Dynamic compression question
 

How many of you guys are running between 7.5 to 1 and 8 to 1 dynamic compression on pump gas (93 octane) and what is the real limit here. After simulating an engine build on my computer, (bore, stroke, cam specs., it calculates 13.4 static compression ratio and 7.6 dynamic compression. Which means, I think, will run marginally on pump gas. Marginally, defined as, drive easy on the street and splash a little of 110 on race day. My parts consist of:
440 cfm tiger heads
535 IA2
264-270 110 lsa roller cam
2- 800 cfm carbs
4000 stall converter
t-400 trans
9" rear with 3.50 gear
This engine is currently running with 30 cc dish pistons and single 1050 carb, and 6.4 dynamic compression and 130 lbs. of cranking compression. Only 760 HP. Gotta do better than this!

Steve

I've never ran any numbers on my engine for "dynamic" compression, but it easily manages 87 octane fuel at 10.99 to 1 static compression ratio.

The build is a 428 block, 455 crank, std 455 bore size, exactly zero deck height, and SD's Old Faithful 236/245/112 HR camshaft. ICL is 109.5 degrees. The OF cam is around 292/308 duration seat to seat, nearly as I can remember.

Cranking pressure is right at 198-200psi in all cylinders.....Cliff

Cliff, that's good news. Now all I gotta do is get flat top pistons and run the cylinder pressure up like your combo. In the end, I'm looking for 900 hp.

Is that 760hp at the wheels or crank?

It also helps when setting up an engine to run on pump fuel, to move peak torque up in the rpm range, and spread it out as much as possible/practical.

Even with big power engines (N/A), they don't fill as well at high rpm's, as there is less time (more cycles per second). If you can get it to manage pump fuel up to and at peak torque, good to go. Beyond peak torque the cylinders don't fill as well, and dynamic compression will start dropping off.

From what I've seen with this topic, small/tight LSA camshafts that pull power down in the rpm range, narrow up power, and increase peak torque, don't work as well for pump fuel as they do for race fuel.

Of course that comment applies more to street engines than full race, but the laws of physics still apply. I've shown with my own engine that we can easily manage currently available pump fuel at a relativley "high" compression ratio.

I wouldn't be afraid for one second to step up an additional 10-15 degrees @ .050" with the camshaft, and up the compression ratio another half point or so. It wouldn't be any more octane sensitive than it is now, with the ICL at the right point and close control of timing/fuel curves....IMHO......Cliff

With 13.4 to 1 static I wouldn't mess around and run 112, regardless of what you calc for dynamic. You don't want to detonate.

What intake are you running? Is everything matched?

I am not sure that cam is working for you either.

There is most likely a few places to bring the power up just in tuning.

WE built a 535 Tiger head combo at 11.2 Static cr and 8.6 DCR thats about our limit for 93 octane.It made 892HP at 6900rpm and 718 torque at 6100rpms.Bill C.

this is a great combo for E85

Bill C. it made 575 ultimately on a chassis dyno. I ran the A/F ratio from 11:1 - 13:5 and it liked 12:9 the best. I ran the cam from straight up to 8 degree advanced. I liked 8 degree advanced best. That is also where the cranking compression finally went to 160 psi.The current intake manifold is a sheetmetal one that I made with a 1050 dominator carb. I know there is a compromise on the low profile intake but I also know it isn't a 150 hp compromise. In its current state I ran the timing up to 42 before the power started to taper off. I backed it off to 39 to be safe. This is why I feel it could use alot more compression. I have ordered 2 new 800 cfm carbs and will situate them over the port runners and hopefully make that area more efficient. The GOOD news is it lost oil pressure while driving down the street and now has a rod knock, so, now I get to buy new flat top pistons to fix the low compression.
Bill C were you running flat top pistons on that engine?

Ponjon not many E85 stations around here

Dished pistons.You should not need 39 degrees timing,Bill C.

Bill, I know I should not need 39 degrees but that's where it ended up. I think that the compression is too low with the 30 cc dish pistons and 72 cc heads. Maybe I should just put a blower on it.

What RPM was peak power?

Any more specifics on your intake?
runner CSA @ head, CSA @ plenum, runner length, plenum volume?

how much lift at the valve?

Pastry, the intake was designed to fit under the hood. I matched the head port openings and kept the runners the same dimension all the way. I was recommended to do this by another porter. I do not know the plenum volumn but the runners are 2 7/8 x 1 3/8 to match the port opening. I really DO NOT want to cut the super 400 hood that is on it for a taller intake unless it can be proven to vastly outperform this one.The valve lift is .800. Hopefully I can post the dyno sheet and pics of the intake.

Also bare in mind that the above dyno sheet is a chassis dyno.

The increase in CR will definitely help make more power. But you will have to run race gas.

The look from that intake, imo, is small plenum and short runners. You said no taper. That is a higher rpm intake. With the small plenum, it probably needs more carb than you have.

The cam just doesn't seem to match the combo to me. You said you had to advance it 8 degrees for cranking compression. But did you run it on the dyno to see the power differences?

With the intake being limited, etc and the displacement of the engine I think you need a bigger spread and a fair bit more duration to get good cylinder fill. That and a larger carb and the increase in CR should get you smilin ear to ear.

Ron, I did run it on the dyno and I gained 30 hp to the 8 degree advance from the straight up position. I am also going to remove the top of this intake and refabricate to put 2 -800 cfm edelbrock Thunder series carbs situated over the runners to help the air/fuel mixture from trying to negotiate all the tight turns.

Steve

Cliff, does your engine the 10.99 CR have aluminum heads ? It must because you could not get away with it with iron heads. I would ping itself to death.
Its all about cylinder pressure anyway, not necessarily CR.
The cam in the OPs engine is too small for those heads. its puny.
I run a cam almost that big with iron HOs on pump gas and could use more.

Dragncar, so what size cam would you use. I drive this car on the street more than race. It drives good with current cam but never ran a cam larger that this one and am concerned about over camming.

Steve

The cam you are using is very close to the pump gas 535ci street engine I did that made 813 HP.

The cam was 265/272 @.050 on a 110 with 10.75 to 1 static compression.

http://forums.maxperformanceinc.com/...=687764&page=3

Calvin Hill
Hill Performance
708-250-7420

For reference, my street car has a 272/275 @ 0.050 on a 108 in the 467. 3500 converter. Drives fine.

Calvin, the ultimate goal with this engine is to have over 900 HP on pump gas and drive sensably on the street. I want to be able to drive at least 1 hour to the local track, race, and drive back. If I have to splash a little race gas to race that does not bother me but I want it to run on pump gas on the street. The 4000 converter should keep the load off the engine to help prevent detination. It needs to run some strong 1/4 mi numbers because I would like to get involved in some of this East Coast Pontiac outlaw racing. It certainly won't be the fastest but, If you want to race with these guys you better bring a BIG STICK and dark sunglasses.

Hooter you running mid 11's with a 272 duration cam?

yeah, lack of a roll bar has kept me from trying to tune it to go any faster. Have only ran it down the 1/4 at Norwalk in less than stellar air too. Weighs 3700.

I don't want to sound negative here, but you are going to have to hit it with a big stick. If you are looking to pick up over 100 HP, it's going to take more changes. Compression and cam, ring pack?

Where did the engine make peak power?

Calvin Hill
Hill Performance
708-250-7420

Calvin, gunna raise the compression fron 10.8 to 13.4. Right now it has 30 cc dish pistons and I am going to replace them with flat tops. On the intake side I'm going to 2- 800 cfm carbs from a single 1050. By the way I fabbed the intake with such a low profile I'm thinking that the 2 carbs situated over the runners should make some big gains. Right now the HP peak is 6200, tq @ 4900-5400 kinda flat.
BTW I think I need a roll bar also!
Steve

You can make 900HP with 11to 1CR and the right cam and manifold,WE had a slightly compromised intake on the 892HP combo for hood clearance.1/16.1/16.3/16 std tension ring package,Dyno time was limited as engine had to ship .Bill C.

CFM, was that flat top pistons on the 892 hp engine?

Dished .Bill C.

your motor the one with the old dirty bird heads on it? did you try alot of other timing loops?

Mystic, I am perplexed on this engine. It seems to have all the goods but just don't deliver. Before I advanced the cam 8 degrees, it only had 130 lbs. (hot) cranking compression. When I simulated the engine on my software it was correct within 5 lbs. On the software program I kept advancing the cam til I could get the most cranking compression practical. In the end it ended up at 8 degrees. I advanced the cam 8 degrees and it picked up power across the whole board and even on the top end. This engine is not gas sensitive at all. I seems to me to be too low on compression with the 30 cc dish pistons.
I did buy the old dirty bird heads but could not use them from all the pinholes. I spent about a day trying to repair them and Dave tried also. I ended up just buying a new set of tigers and Dave Wilcox ported them. I will post a picture of a roughed port.

We made 900hp with 11:1 compression as well with a 496ci. Compression isnt your issue.

Slowbird, what cam were you running? Duration?

276-286
You need at least another 500rpm to make your 900hp. Your cam timing & engine timing I believe are pointing to issues.

x2!

Slowbird and Mike, so in your experience if I left the compression alone and went with more cam, I should make the power? Are there any specific "issues" or general "issues"?

Well here goes best as i can. There could be many things that are hurting you. 1st indicator that something is really wrong is the hp per cfm on the heads. At 440 cfm with the area of the tiger head making 2 hp per cfm should not be an extraordinary feat. 880 is what i would be looking for as an acceptable number. The fact that it requires 39 deg timing is also an indicator. I've helped a couple guys sort out situations like this and heres how it goes...one of 2 directions productive or basically not. The guys who never find it are the same ones that will not actually check everything, they say things like "i know thats not it" or have too much pride to have a pro look at something. Not sayin this is you....but this is were most people are. They will live with a wrong converter or cam because they picked it or its too much work to change in the racing season. Not tooting my horn at all but i sent my converter back 5 times to get it right just this last year. So the moral is its gonna take some work to find it.
Heres what i recommend for a productive plan to find the issue...
1. verify actual static compression ratio. If you are at 13.2/1 you should most likely be beating the bearings out of that thing on pump gas. This is also where i find the most deviation from the specs i'm given at the start. really look at the deck hight and pour the combustion chamber and piston top. You need to KNOW what you have.
2. I would contact a reputable cam guy and have them design a custom cam around your combination. Keep in mind that any cam manufacturer or arm chair quater back is going to have an opinion and try to sell you something. They all think they are experts...most are salesmen trying to sell you something. I use scott at http://www.buyracingparts.com/ Most times he does what i think i would have ordered....sometimes not but the results have been consistently really good. The cam you have is not what i would have picked for your engine and goals btw.
3. Take your heads to a REALLY reputable head guy with a reputation for getting results and have him look at them. there more to it than just the flow bench number.
4. See if you can borrow a tiger or bop intake for testing from someone. Your sheet looks good but there is a lot to consider when designing one. I personally posses the fab skills to make one but would never attempt it because i care about results more than i want to prove i could do it. That said yours may be close....or it just might be 100hp off. It wouldn't be the first time.
5.ensure that all the machine work was done correctly. A bad hone/ valve job etc can kill hp.
6. after all this is done schedule a dyno session with a good operator. If the dyno is stingy even better. But you really want a good operator with a lot of experience that can shed some light if you are having issues while there. Chassis dynos are notoriously off for a lot fo reasons. A bad converter can show a miserable performance loss. They are really good at helping you find things in the car though. If it improves it was an actual improvement in other words.

I hope this helps...

I've been reading this thread and thinking about all of the variables.

I first questioned the dyno numbers, have you been to the track? MPH/Weight/RPM is all you need to know.

Using the dyno information, the engine peaks way to early for a large port 535ci engine. As Slowbird has indicated, 6900 RPM would be a good target to get the piston speed up to over 5000 fpm. That cylinder head with the correct valve train and cam should have a nice long flat torque curve.

Another fact is the low cylinder pressure the engine is building and the need to advance the cam that much.

Again, I don't want to make this sound negative. Big ports generally have poor low to mid lift flow numbers. By design, these types of heads require very specific timing events on the cam and increased RPM to make power.

I would check leakdown, have the cam run on a cam reader and then evaluate my cylinder head flow data before I made any other changes.

Calvin Hill
Hill Performance
708-250-7420

3&4!

Slow, Mike and Calvin, I bought this engine already built and am asking these questions because I have never built an engine of this caliber. I have been building my own engine for 25 years but this whole new combo I felt was out of my league and wanted a professinal to do it. So I did have a professional build it and here I am trying to figure it out $22,000.00 later. I will soon pull the engine and go through it myself. I know Curtis Boggs from RFD and may have him look the heads over. (My sheet metal shop used to be next to his shop in Lorton,Va.) When I first fired this engine up it smoked continuously and blew oil all over the place. The oil pan, valve covers, timing cover all blew oil out. The valley pan would literally fill up with in 10 miles of driving. I removed and sealed the leaks the best I could and bought a vacuum pump which fixed that problem but the smoking problem still existed although it was starting to smoke less the more I drove it. The last time I took it out it lost oil pressure about a 1/2 mi from the shop and now has a knock. Yes, it has plenty of oil, the vacuum pump did not suck it all out, I do know to monitor that. This motor is supposed to have 11to1 comp. but I did not measure it but will. I was wondering if the compression was really low to produce weak cranking comp., 130 lbs., and weak HP numbers. That's why I increased the cam timing, to get more cylinder pressure! My original question is how much dynamic compression you guys have that run on the hairy edge with pump gas. That's where this car needs to be, on the hairy edge. This is a very occasional street car that I want to run good at the track. I have my own dynojet chassis dyno so getting dyno #'s arn't a problem. I also tune other peoples cars and have not had negative feedback most are happy with the results gained. Its a really fun passtime/side job and generate a little cash for the habit. Almost all are stund that their 700 hp small block made 340 rwhp. Mike, I am certainly not too proud to ask as here I am askin and just trying to learn and hopefully will teach someone else down the road.

Steve

When I first fired this engine up it smoked continuously and blew oil all over the place. The oil pan, valve covers, timing cover all blew oil out. The valley pan would literally fill up with in 10 miles of driving. I removed and sealed the leaks the best I could and bought a vacuum pump which fixed that problem but the smoking problem still existed although it was starting to smoke less the more I drove it. The last time I took it out it lost oil pressure about a 1/2 mi from the shop and now has a knock.

Sounds like this engine had issues from the getgo. Your whole problem could be right here. Eating bearings, and lots of crank case pressure. Sounds like bad hone/bore/rings, my:2cents:

That sounds like a very good plan!! Tear it down find where the oil is coming from and who knows what else you might find (sounds like there are quite a few issues!). Also letting Curtis Boggs check you heads is a great idea, you might also consider letting him look at your intake.
My cranking compression was mid 150 if I remember correctly. You have a smaller cam than I did with about the same compression so I am curious as to why your cranking compression is so much lower than mine (I know there are many factors that play into it but I wouldn't think there would be a 20psi difference, I could be wrong though).

FYI I ran you numbers through the program I use and it showed 165psi with your cam straight up and 176 with it 8 degrees advanced. Something is wrong and I wouldn't be surprised if your rings aren't seated at all based on the oiling issues you are having. Or you might find that your compression is way low.

I can't thank you guys enough for all the willing help. This has been a 2 1/2 year ride and I'm ready to get off of it.

Ditto,Plus dyno with wideband o2 in each cylinder.Bill C.

Bill, that is also a good idea to have individual exhaust readings. Would really help out on manifold efficiency data. Dynojet sells the add on stack for that feature but I bet its somewhere aroud $1000.00- $2000.00 for the hardware. Right now all I have is the single wideband sensor that sniffs the tail pipe.

WE have the individual cylinder wideband o2 on our dyno,helps a bunch with manifold development and firing orders.WE have built the 535 Tiger head pump gas engine at 892HP,alsc a 572 BBC at 10.8cr made 878HP,and a 565 BBC at 10.3 cr made 895HP All with single carb cast manifold.Also use a blowby meter on your chassis dyno ,good tool.Bill C.

Bill, never heard of a blowby meter! How does that work?

Steve

Bill, also as I add up all the cc's I think I am closer to 9.8 cr. as its built right now. I will change that soon enough.

Your combustion chamber looks pretty large do you know what it cc's at?

theres 50 hp right there with that cam you had...maybe more