AGreen

I posted this over in the TPI section, they said come here.

The rotary I just finished blowing up made about 350 wheel horsepower. So let's just get this over with and I'll say now that in no way will I allow my car to hold a bigger engine with less power and more weight. There's potential here and I know I can tap it.

So my current status with this engine is that I have the block and crank at a machine shop getting bored/honed .030 over and it's getting forged aluminum flat top pistons. Based on bets, I have to make this work for under $2k. Sounds crazy, but I can sell my rotary stuff to make up for the losses. So far I am in for $1k. That includes:

Used TPI 350 (and 700r4) from an 88 trans am GTA
Megasquirt 2 version 3.0
SBC to 2nd gen Rx7 built-in cradle kit (allows for a smooth drop in)
700r4 to Rx7 diff driveshaft
Rx7 turbo differential
Sealed Power rebuild kit CMSHP760-300
Intercooler and piping (using the one I ran on the rotary)

So I'm within budget for now. I still need the turbo(s?) and some decent injectors, but I know how to wheel and deal for parts pretty good.

Now, for the BIG concern that I need help with. The flat tops are going to bump the compression ratio up to 9.8:1. That's assuming:
4.030 bore
3.48" stroke
0.019" deck clearance
0.039" compressed head gasket thickness
4.125" head gasket bore
64cc heads (stock)

I need to lower the compression ratio down a bit. I've done my research, but I'm sitting at a fork in the road right now. On one hand, I'm being told that double head gaskets are completely fine. People say they've used then for years and up to 20+ psi and never had a head gasket leak. I'm suspicious, and I feel rightfully so. On the other hand sits the option of larger cc heads. I have found a set of 191 heads for sale for $75 here locally. Some research says they're 76cc heads, some say 64. Some say the valves are 1.94/1.5 and some say 1.84/1.5. I'm still trying to figure out if they're even worth using? Are they one of the more "crack prone" sets of heads? I have also found a set of 993 heads, which are free. I believe they are also 76cc.

Another question: Why is everybody saying that 9:1 (and up) CR is too high? And they're referring to a naturally aspirated build. I built a rotary with 9.4:1 CR at 18 psi that is still running (and just as strong as ever) from many years ago. I have seen 4 cylinders run turbocharged at 9:1 and they're just fine. If higher than 9:1 is bad for NA engines, then how low CR does a small block need to be to not detonate under boost?

TL;DR:
Need to lower CR
191 heads are an option
So are the 993
I don't think the idea of doubling a head gasket is too smart
Are 191 or 993 heads a good, cheap option for my turbocharged application?

-Green

jimw67

Why not use a larger cc piston and use your current 64cc heads? Are the pistons used/already purchased? I ask as you're putting alot of effort into using them.

Are you using the stock cam? If yes, then you definitely need to drop the compression.

191 and 993 heads are pretty poor heads, but they will solve your compression issue. 993 heads for free - you'll probably be taking someone's door stop - lol. Keep in mind that your HP goals are not too lofty, so with that in mind, the 191 or 993 heads will work. And they would be a better choice than using another iron head with 10:1 compression. Hopefully you can find a set with 194/150 valves.

You should get you quench lower - .058" sucks when all you have to do is run a different head gasket. Your idea of doubling the head gaskets is a bad idea that won't last and I'd stop listening to the guy that told you that it will be fine.

Could you list the part number or cc of the pistons you have, the cam or cam specs that you have and your current head part numbers? Better guidance will follow.

Regarding the compression comparison between a Gen 1 SBC and a rotary - apples and oranges. The same comparison applies to a Gen1 and an LS based engine. The more efficient of an intake, exhaust, head then the more compression you can run due to heat transfer and detonation resistance. Your TPI intake is relatively efficient within it's design range of below 5000 rpms. The reason most recommend forged pistons is they will handle detonation better than a cast or hyper piston. The reason for aluminum heads is they transfer heat much better and most aftermarket aluminum heads have a thicker deck surface that helps both detonation resistance and heat transfer.
Regarding 'why is everybody saying that 9:1 CR is too high?' It's not if you have the appropriate parts, but from what you're describing you do not. 8-8.5:1 puts a safety margin into your build to compensate for any tuning errors. At 10:1 with stock iron heads your margin is much thinner.
42lb injectors will hit you goal, but not much more. An e-bay turbo for $250-300 should be adequate - 70mm,76mm GT45, etc.

If you want this to live, then I'd highly recommend a knocksensor like this :
http://www.viatrack.ca/

It is a pretty easy mod to your MS2 board for a simple input. I'm using one and it works better than OK and the MS settings are simple. And it's an inexpensive solution to keeping from blowing your motor to bits.

Personally, with your budget limit and what you already have a 4.8 or 5.3 with the appropriate spacer for the trans with an MSD box to run the ignition would have been an easy swap keeping in mind that running 4.8's can be had for virtually nothing - less than $350. And these motors will easily handle high teens-20 psi.

Good luck - Jim

AGreen

Wow thanks a lot!!

Let's see about answering some questions.

The pistons were already bought and are being installed now. I didn't get to thinking that 9.8:1 would be too high until after I had already sent them out to get put on the rods. The pistons are 6.1cc. I forget the number, but they're the ones that came in the listed kit from summit racing.

The cam is a stock L98 unit. All the cams I'm finding are too heavily overlapped for what I want to do, and I don't believe my budget allows for it yet.

My current heads are the stock L98 iron heads. I'll get the numbers when I get back home.

The 993 heads are my neighbor's. He bought a bunch of 350 parts not knowing what he was getting because he wanted to build one for a 3rd gen. He then bought a very nice 400 sbc, and asked if I wanted the heads last night.

I'm all over the megasquirt knock sensor. I ran the same MS on my rx7. Long story short on the rotary, I bought the engine "built". The post-destruction teardown proved that it was, in fact, built with junk parts.

Would you mind elaborating on the quench volume please? I'd love to know a little more on that topic. I've done some research in thicker head gaskets, but it returned very little information, which was confusing at best. Most of the thicker options didn't have the same bore diameter as the ones I am planning on using (4.125").

ZZ3Astro

Cometic makes a .098" thick MLS gasket for 4.125


http://www.summitracing.com/parts/cg...make/Chevrolet

Orr89RocZ

iTrader: (20)

I'd be interested in trying that gasket with piston in the hole no more or less than .005 or so. That will remove quench from system and help reduce detonation and also reduce compression

I may try something like this on my stock 305 to see if it works

AGreen

So far from what I have read up on about quench is that removing quench is bad. So the larger volume heads are a better way to lower CR and maintain quench volume.

This info was pulled from Corky Bell's "maximum boost"

Orr89RocZ

iTrader: (20)

Thats kinda not true to an extent, but application specific

Some modern factory cars dont have any quench area in their head and piston packages, so there is no quench height. Hemi chambers are like that

In a na car, quench is important because you are trying to compress a atmospheric air charge, mix fuel evenly and burn it off as fast and efficiently as possible to extract as much energy as possible. Flame burn rate being faster is better as long as it dont detonate

In boosted motors the air charge is much denser. Flame front travels across the air fuel charge much faster and thus burns off so quick there is a rapid rise in pressure leading to detonation if fuel cant handle it with its octane rating. Thats why we pull timing to an extent to delay the flame travel and control cyl pressures. Here we want to slow burn down and delay travel. Removing quench areas and that "squish zone" quench height will help control flame front travel. This fights detonation and makes the tuning window much wider and safer to work in

Its something alot of big hp cars are doing and getting away from the old mindset of proper quench is .030-.050" in sbc/bbc etc. in na motors you need that, but nitrous and blown motors you dont need it, the density helps make up for it.

Theres also ways to soften the head chamber to remove quench pad area. Thats something you could look into.

I havent done any of this on my car and have .041" quench height and working great. My heads seem to want timing, more than i would have thought. I didnt specifically ask for soften chambers and pretty sure i didnt get them now that i know what to look for. Yet the combo works fine

jimw67

I thought about this today while I was at work (don't tell the boss). Your issues are not going to be the most idealic quench. They are going to be detonation and/or heat resistance.

I'd say put the knock sensor in it and run it right up against it. Get all of the cooling system that will fit in the car and see what gives. The TPI is a good thing for building tons of go below 5000 rpm's. What does that imply - with a decent build you shouldn't have to worry about high rpm issues like oil system, valve train stability, exhaust system flow capacities, windage, etc.. Basically it's an easy bottom end build.

I had great luck with the Knocksense sensor, it probably saved me a motor. It's cheap insurance ($100) and Boris is a great guy.

And everything that Orr said above is correct. For 400-500HP you'll be fine with the big chamber heads. The issue will come when you want more.

Good luck - Jim

AGreen

So what would be the ideal quench?

Another thought passed through my head today. I have been of the mindset that iron heads are more durable and that's why I wanted to use them for forced induction. But the aluminum heads have a better coefficient of heat transfer, so less chance of detonation. So where is the line drawn? Aluminum is good for cooling, but won't take a few knocks? Iron is sturdy and can handle the knocks but more prone to making them?

I like that you used "when" and not "if". It's like you already know me so well

I think I may be making a mountain out of a mole hill about quench. If only my block would get back from the machine shop I'd go out and build the damn thing instead of armchair engineering it to death (and armchair engineering my budget out the window)

Orr89RocZ

iTrader: (20)

No one who builds a turbo motor ever makes 1 boost level power number and leaves it there. Its tooo easy to adjust boost and make more. You will get used to the power. It happens to everyone, so you will turn it up.

It comes down to if you can afford to wait and build it right, with proper parts, you'll be much more happy down the road

jimw67

I'm on my first boosted gas engine. Our (my son's an I) plan was to run 7-8 psi and leave it there. Well, after two waste gate spring changes to 10 and then 14 we are now ready to make 500+ whp. I'll bet that by this time next year we'll be talking about 18-20.

'Boost is addictive' is not just a saying, it's the truth. Melt something and replace it with something that won't melt. 'More fuel' is like like telling a fat guy that there is another pizza is ready after the first one is almost gone.

FWIW, just pulled the car in the garage to put in a new cam and add another fuel pump.

AGreen

I know all too well the dangers. I bought a turbo rotary drivetrain with the intention of only running it stock. It was only an increase in power from 146 to 190, so not much. The very first "modification" I did to it was a 3" exhaust. I didn't have an exhaust that matched up to the turbo downpipe, so I made my own exhaust for about $50. Once I got it going, it had horrible boost creep from the 11mm (7/16") diameter internal wastegate. When I felt that power, I knew I wanted it, but I had to do it safely. So I ported the wastegate to 35mm to keep it under control. Then I built the MS ECU. Then came the injectors. Then I built a large hybrid turbo. Then I built a v-mount intercooler. It goes on and on until I was somewhat happy with 350 whp at 18 psi.

400 horses in an Rx7 is a lot. It'll keep me satisfied until I raise the budget again, which I will do. I assume I'll eventually turbocharge an LS engine and stick it in there, but the L98 was cheap and just looks cool. And I'm sure it can hold 400 for a while at least.

Thanks for all the insights. I'll most definitely post some build pictures as I go along. The transmission is already in the car, I'm anxious to get this going. And it's got to be done within a year because I'm transferring at the end of '16, and it has to be drivable.

For the "right now" aspect of this build I already know I will have to get it moving on NA power. That's to make sure the engine is broken in, the tune is right, and everything works properly before adding the turbo(s). I definitely don't want to go all out all at once and miss something important. For instance, what if it smokes? It'll be that much more difficult figuring out if it's coming from the engine or turbo, and I'll have to tear it halfway apart to figure out where and why.

Orr89RocZ

iTrader: (20)

When you set it up, depending on what your final quench height and head choice ends up being, go relatively conservative on timing. Make it pull almost 2 deg per lb boost for starters if comp is higher than you like. Keep it safe

ZZ3Astro

Are you guys running talking about KnocksenseMS (for megasquirt)? These things are really not meant for full throttle knock tuning. You have to crank down the sensitivity as rpm increases and generally it is useless about 5000 rpm. I didn't know this at the time but after reading the instructions and numerous forums along with my own testing, the product is on my car but isn't used at all now. Have any of you experienced otherwise?

Orr89RocZ

iTrader: (20)

I use stock stuff. Even then i am not so sure its accurate. Ideally track and dyno testing to see what makes max power and when 1 deg change no longer produces significant gains, i stop

jimw67

They are more sensitive than the stock stuff - IMO. They will pickup roller rocker noise. I have mine mounted on the side of the block a couple of inches below the head. I will agree that having it on the intake is a waste of time because it shows knock at whatever setting. After moving mine around on the engine, I believe alot of what you're reading is based on the location of the sensor. I read/heard alot of the same info that you've read before I put it on, but figured it was worth a shot for the money.

Regarding picking up noise above 5000 rpm's, if you're using it on a TPI setup, then running much above 5000 isn't much of a concern. But mine doesn't seem to have that issue upto 5700 (my motor hasn't run 6000 ever - no point). And as Justin pointed out, detonation can be seen on a dyno graph; comparing a dyno graph to the knock input is a great way to get it dialed in - that's what I did. I initially dialed it in while driving as the instructions say and it worked pretty well. After hitting the dyno and setting it, I put a lot of faith in it.

I sound like a salesman for Knocksense, but I always appreciate a part that works. And for a $100 it's cheap insurance against a weak fuel pump, plugged fuel filter, etc.