slowride Posted March 18, 2012 Share Posted March 18, 2012 Hello I have a 78 datsun L28 that has been bored out to a 3.1 stroker. My compression is 220. The car made 210 rwhp on a mustang dyno. Unknown aftermarket Cam, JSK fuel rail, SDS standalone. Due to the high compression the dyno tuner reccomended that I run race gas, because at WOT i risk burning my valves and melting a piston with my current setup. My mechanic is going to be removing head and inspecting motor to see how we can lower compression. Any ideas? We are thinking: -multilayer metal head gasket -shave a bit off the top of each piston -larger cam? Extrudabody 45mm ITBs are waiting to be installed once this high compression gets figured out. Thanks! Quote Link to comment Share on other sites More sharing options...
getoffmyinternet Posted March 18, 2012 Share Posted March 18, 2012 Shaving pistons just sounds like a very bad idea. You could probably find alternate pistons that would work better (variations in distance to the center of the pin without sacrificing strength). Perhaps it's a bad head combo? The head could probably be reworked a little but I don't know how much you could take out to make a significant difference in the chamber size. Head gaskets of varying thicknesses are available, as well as varying rod lengths, which would probably make the greatest difference. Quote Link to comment Share on other sites More sharing options...
BlueStag Posted March 18, 2012 Share Posted March 18, 2012 You don't specify which head you are running. That might be the place to make it right. Quote Link to comment Share on other sites More sharing options...
slowride Posted March 18, 2012 Author Share Posted March 18, 2012 (edited) hey thanks for the quick reply what thickness do you think I should go for? 1 mil? 2 mil? 3 mil? Is there a calculation to bring down my compression ? I am unsure of what head I have on the motor...I purchased it as an unfinished project, got it running last summer and am working all the kinks out. I'll report back with some pics of what head it is next week. Are heads easily identifiable? I think the motor is using the stock head, but its been ported and polished. Apparently compression is 10:1 which seems true to the 220 per cylinder I got when compression testing. Why when I read people l31 stroker builds do they shave the head to gain higher compression? How are they able to run their cars on 94 octane without burning up valves or melting pistons? Am i crazy to try and achieve a 9:1 CR? Edited March 18, 2012 by slowride Quote Link to comment Share on other sites More sharing options...
PMC raceengines Posted March 18, 2012 Share Posted March 18, 2012 (edited) mmm lots of guessing work going on ,, And its not a guessing game. Comp ratio is measured not guessed and you have to have the head off to do it proply Edited March 18, 2012 by PMC raceengines Quote Link to comment Share on other sites More sharing options...
Gollum Posted March 18, 2012 Share Posted March 18, 2012 What he said ^... We have ZERO data here so far. Here's some BASICS that should get us started: What head? What Gasket? What Pisons? What Rods? What Crank? Any detonation detected? What's the ignition setup look like? What's your initial and total advance? The test number you get on a compression test gauge has VERY little to do with static compression and how you want to approach your engine and your goals Knowing the TRUE, MEASURED static compression will give you your start on figuring out a "solution" to this "problem" you're having. I also wouldn't take a dyno operators advice as gospel truth. Their opinion is just that, an opinion. You didn't mention having ANY detonation problems, which makes me wonder why this is even coming up.... Quote Link to comment Share on other sites More sharing options...
PMC raceengines Posted March 18, 2012 Share Posted March 18, 2012 (edited) Just knowing what head its got means nothing ,,, You need real facts how many cc is the chamber . How big is the head gasket in ccs, is the piston positive dec hight. Who can tell over a computer its up to a engine builder to check and do real tests Edited March 18, 2012 by PMC raceengines Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 18, 2012 Share Posted March 18, 2012 Burnt valves and melted pistons are more an effect of lean mixtures which may or may or may not be detonating. I'm with the "any detonation experienced?" crowd. And what "race gas" did you use. What did your AFR do during the test... Matter of fact, where's the dyno printout? Before you go fixing something... Make sure it's broken in the first place!!! Quote Link to comment Share on other sites More sharing options...
madkaw Posted March 18, 2012 Share Posted March 18, 2012 When I read Corky Bells turbo charging book he pretty much condemned the idea of thicker gaskets to alter compression. Basically don't mess with the squish if you don't know what your doing. You might make things worse with a thicker gasket and have worse detonation - If you have any now. Quote Link to comment Share on other sites More sharing options...
t-tom Posted March 18, 2012 Share Posted March 18, 2012 I'd worry more about putting a proper tune with the itb's on your motor. It's amazing what a proper tune will do. I'd be Leary of anyone that told me they could tell how high my compression was without knowing what pistons, head and chamber mods have been done. I'm with the don't fix it of its not broken or breaking camp Quote Link to comment Share on other sites More sharing options...
slowride Posted March 18, 2012 Author Share Posted March 18, 2012 (edited) i am having mechanic dig into the motor this week.... I never did run race gas, I just used 94 and didnt full throttle it, although I barely drove it last summer due to this. I wanted to get it running properly before enjoying her. I know the car hesitates when at WOT and didnt seem to pull all that hard. I dont have the dyno sheet, its in a box somewhere in my storage. I do remember the AFRs being awfully high at WOT, I'll get you guys some more info like what head I have on it, pistons, rods, length of stroke. I know head was ported and polished.(PO told me) Do I need to send pistons and head off to machine shop to determine CC's of each cylinder? Or is that something any mechanic can do? Can i identify the crank by just pulling the oil pan? The piston positive deck height is the amount that the piston sits above the block when the crack is at TDC? If the piston is flush with block or it is up 1MM this will help determine what piston I have correct? Is 10 to 1 compression too high for a street car? My thoughts are to figure out what I have and get the car running good, then install ITBs and back to dyno to get it tuned properly. Thanks for all the advice and direction, I appreciate it. Yea I know the wiring looks a lil rough it was all taped up and we are going to be cleaning it up after addressing all these issues. Edited March 18, 2012 by slowride Quote Link to comment Share on other sites More sharing options...
Chemicalblue Posted March 18, 2012 Share Posted March 18, 2012 How are you tuning it, what EMS? Quote Link to comment Share on other sites More sharing options...
slowride Posted March 18, 2012 Author Share Posted March 18, 2012 sds Quote Link to comment Share on other sites More sharing options...
Xnke Posted March 18, 2012 Share Posted March 18, 2012 Ok, this is getting a little silly. First off, you don't need 91 octane to run an L28 at 9.5:1, let alone stock compression. If you are pinging, then find out what timing and temperature you're running at, and adjust to compensate. Chances are you have some mismatched parts. Second, the only way you can *know* what you have, is to measure it. No one has mentioned head casting numbers. If the head was ported by the PO, and not by a shop...chances are good that you have a stock combustion chamber. Most people's idea of "port and polish" revolves around sanding out the casting flash and smoothing the surface to a mirror finish, ignoring the shape and consistency port-to-port. It is a minority of home porters who actually go through each step, because most do not have the patience to finish. It's a long job. What head casting number do you have? It's on the forward passenger side of the cylinder head, near the gasket surface. If you really have a 10:1 compression ratio motor, and you're worried about detonation resistance, then check to see if you are running a stock cam. If you are, get a bigger cam. It'll lower dynamic compression, and add some power. The higher compression ratio makes this a good option for a 230-240 degree cam, but I would not go larger than that without more info. What injectors are you running? What fuel rail pressure? What is your total ignition timing, spark plug heat range, are the plugs indexed correctly? Your hesitation at WOT could be a LOT of things... Quote Link to comment Share on other sites More sharing options...
steve260z Posted March 18, 2012 Share Posted March 18, 2012 All your concern is over the "dyno tuner's" comments? Who could actually no little more than operating the dyno. btw 10.1 on a stroker is not too high. Certainly with a cam. What head do you have and does it ping and what is the timing? Quote Link to comment Share on other sites More sharing options...
t-tom Posted March 18, 2012 Share Posted March 18, 2012 Unless you just want too spend money just too spend it I wouldn't have it torn down until you know there is a problem. 10-1 is not to high for a street motor, but all your parts need to work together. I'd find out what you have head casting number would be a big help then let the experts here help you out Quote Link to comment Share on other sites More sharing options...
slowride Posted March 19, 2012 Author Share Posted March 19, 2012 (edited) hey guys ill get you the head info tomorrow, the car is at a shop right now the motor has a very big cam,,idles at 950rpm very loupy the hesitation and the comments of "i cant give you a proper tune with your current setup without you running race gas " has started me on this quest to determine what I actually have in the motor and to get it running properly. my car has a gauge that measures fuel pressure, I believe it was at 40 or 50psi when the car is running. I know I have an external holley pump and a JSK fuel rail. I'll find the block casting number, what kinda injectors I have and timing tomorrow. I want to ensure my car is going to run right and make as much power as possible with my setup. Id like to see 300hp, but I wont even get close until I have it running properly. What would most of you do? Just retard the timing till proper AFR and no pinging call it a day? Edited March 19, 2012 by slowride Quote Link to comment Share on other sites More sharing options...
getoffmyinternet Posted March 19, 2012 Share Posted March 19, 2012 What would most of you do? Just retard the timing till proper AFR and no pinging call it a day? No. Get the details. If the work was done properly at all there should at least be some documentation on which parts were used. You can calculate the proper head gasket thickness etc. but you need to know what you're starting with, no guessing on any of it. I wouldn't use the cylinder pressures to guestimate compression either, it should be the other way around. You calculate your compression and use that to get a target pressure. The actual pressure will always be less than the theoretical compression, how much less depends on the quality of the tune and how worn out the engine. Take measurements and calculate your compression before even thinking about what to do next. Quote Link to comment Share on other sites More sharing options...
Gollum Posted March 19, 2012 Share Posted March 19, 2012 I'm not going to repeat what all has been said, as I feel hybridZ as a community is actually giving much more advice than would normally be given in a thread like this, but I do want to touch on something. You mentioned that the tuner "wouldn't touch your engine without running race gas". While I totally understand why the tuner said this, I also feel it's not right to let you just be ignorant as to why. I've only tuned a small handful of engines and can tell you right off the bat the true-er reality. Race gas doesn't burn the same as pump gas, no matter what people might say. Some like to think that race gas is just "less prone to ping" which is only a half truth. In fact, it's WHY and HOW it burns differently that makes it less prone to detonation. This is a GOOD thing. BUT, what can tend to happen in most shops is that they use race gas to dial in ideal timing and find the rich best torque and lean best torque ranges for that motor at various load points and then extrapolate from there. Then they'll spend a justifiable amount of time fine tuning some values but they'll NEVER get the same amount of tuning done that you can get done when you take an "ongoing" approach to tuning and take the responsibility of a good tune into your OWN hands. They'll never reach a good 40% of the load tables unless they're using a dynamic brake dyno and running it at various loads, AND at various throttle levels (very rare). The exception is when tuning on an engine dyno, as you spend a lot more time and money getting it rigged up, so you better bet they're going to take a lot more time getting the cells dialed in. The real issue that's on topic here though, is that if the tuner uses race gas and doesn't switch back to pump to basically re-do a good quarter of the tests he did before, then he isn't really verifying the engine did what was expected. And in many cases, if the tuner just puts pump gas back in, and then just does some runs to make sure it's running right, as expected, without adjusting cells, then he just left power on the table. Why leave power on the table? Because it's "safe". Safe tunes can tend to be better for the shop as they get less cars coming back in a 6 month period with broken parts. It's easy to tune on race gas as it's forgiving and you can find that balance between rich and lean best torque and figure out what could be called "safe" for that motor, dial it in, then switch gases. I don't say all this to say go find another tuner. I say all this to make sure you understand that educations reigns supreme in any subject, and being an ignorant consumer is the more sure bet way to make sure you're not satisfied with a product. Quote Link to comment Share on other sites More sharing options...
Tony D Posted March 20, 2012 Share Posted March 20, 2012 On a Mustang Dyno you can go from nothing to fully hitting all the load cells of a given EMS system in about 45 minutes if you have a good cooling system and fan on the radiator. Full Throttle Curve should be optimized in about 15 minutes, and the rest of the time is holding at load points. Where the throttle is puts you at the load points so TPS (unless you are using TPS based EFI for some reason like huge cam no MAP below 4000rpms) is pretty much irrelevant. MAP/RPM will set you up. The original 8X8 MS tables were QUICK. In about 20 minutes you were optimized and had better throttle response than stock EMS. The time is in warmup, accel enrichment, decel enrichment, hot restart enrichment, cold to warm curve, decel fuel cut, etc... You can get the car done in an hour on the dyno, but all the tweaks for cold and off-operating temperature smooth operation can take a few hours over the next couple of months of starts from cold... After that, it's SET. Leave it alone! But this thread is just going wild with conjecture. Quantify what you have and stop with the nebulous comments "Big Cam" idling at 950 and lopey? Not even close! Hell, that could be a vacuum leak. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.