1500 7.5:1comp or 9:1comp piston which one?

[recordsj]

If I went to the flat top pistons I would get a 9.6:1 compression ratio based on my calculations of the volumes from measurements I made (the higher compression possibly that the head has been skimmed by a previous owner).

Is going this much over 9:1 going to be too much for this engine?

 

[Hap Waldrop]

Thats always a issue, when someone builds a high compression engine with low compression pistons, you will still be streetable at 9.6 to 1 though. 1500 heads have huge combustion chamber and thick decks, racers cut these heads .100"+ with no issues, I done a few SCCA limited prep 1500 race engines, in which I had to deck the heads .100". I found something to be true with 1500 engines, if you deck the head a good bit, you need custom pushrods that relfect the amount you take off the head, for example deck .100", you need .100" shorter pushords, but companies like Smith Brothers are glad to make you custom length pushords and they are not that expensive, probably not a issue with lesser cuts, but worth checking valve lift and paying close attention to rocker arm geomentry when doing this.

 

[Guest]

I did some reading a week or so ago that stated ethanol has a higher octane rating than regular fuel. E10 (10% ethanol) has about 95-96 and E85 is supposedly around 104. I don't know if this is true or not but I don't like the stuff anyway. Maybe it's just gov'ment propaganda to get us guys to like it.

I'll swap you a good low comp head for yours if you don't want to go as high as 9.6.

 

[recordsj]

my car is not for racing, just street use. Would 9.6:1 be to high for the longitivity of the engine in comparison to a 9:1?

I figure at least if it was for racing it wouldn't be that big of deal since on a race engine you are used to rebuilding the engine more often... but since mine is not for racing what are your thoughts?

 

[Guest]

Thats always a issue, when someone builds a high compression engine with low compression pistons,

Hap, did you mean w/ low compression heads, or did you mean cutting the low comp head and using dish piston to get to 9.6?

 

[recordsj]

I was also wondering if I go to a higher compression engine, would I need to alter the tuning of the carb? (currently have a weber DGV).

 

[hooey]

here something to think about that is a 3 main bearing motor upping the compression will give you more problems down there
work on getting the head to flow better will get you more in the long run
I have tryed all the way up to 15:1 on that motor and found it can't handle more than 12:1 the power is in the heads getting it in and out! cut the head down and work on the flow of the head if the pistion are good use what you got and put your money in to the heads

 

[recordsj]

Hap,

For a street car (not a race car in which you expect to rebuild on a regular basis), if I went to with the high compression pistons and the resulting compression is 9.6:1 would you think that would be too much for the engine and force rebuilds more often than a 8:1 which is where my engine is now? Or even a 9:1?

Also if I go to 9.6:1 I assume there would be some tuning on the carb (I have a weber dgv 32/36)?

 

[Hap Waldrop]

No, I think you will be fine, I build street engine to those CR numbers everyday, on race engine we go much, much, much higher

Yeah you may need to richen the carbs some, but it will be worth it, for the performance gains.

Oh and here's my opinion on raising the CR in these engine, anything under 10.0 to 1 is fine for the street, and raising the CR is the best bang for the buck in HP increase you can get, and I see no reason honestly in porting head without raising the CR, it not going to gain you much of anything, stock hads flow decntly on the flowbench, once you gotten to the porting stage with a street engine it's normally part of a overall perfoamnce build , meaning head work, aised CR, cam and so on, they all work together and the horse needs to be before the wagon so to speak.

In short for these motors, 10 to 1 is not that radical, and does not make the bottom end wear out quickly, but it is preety close to limit we can run on pump gas, if we had better pump gas I go higher on CR on street engines and not worry about it. If we can't blow the crank out of the block on these motors at 15-16 to 1 in the race engines turning 8000-9000 rpms, you not going to hurt your motor if built properly at 10 to 1 on pump gas and turning 6000 rpm on occasion.

 

[Hap Waldrop]

Thats always a issue, when someone builds a high compression engine with low compression pistons,

Hap, did you mean w/ low compression heads, or did you mean cutting the low comp head and using dish piston to get to 9.6?

I like to start off with the highest compression off the shelf piston I can get, that makes life alot simplier, then from there head or block decking to get to desired CR. FWIW you can take a ton off the 1500 head without worry, I personally skimmed the 1500 heads .100" + for racing with no issue, and that way more than street engine should have to do.

I currenly building a TR6 engine, the 1500's cousin w/ two more cylinders , these engines are very simular in design. It has a deck hieght of ..015' w. flat top pistons and the head has 49cc chamber, and it is currently at 8.5 to 1, our target for this engine is 9.0 to 1, so I'll need to deck the head aprox. about .030" to get to the 9.0 to 1. Botton line you gotta do the math when raising the CR, never assume your block or head has not been decked, who knows what the PO has done.

 

[Guest]

Thats always a issue, when someone builds a high compression engine with low compression pistons,

Hap, did you mean w/ low compression heads, or did you mean cutting the low comp head and using dish piston to get to 9.6?

. Botton line you gotta do the math when raising the CR, never assume your block or head has not been decked, who knows what the PO has done.

I know mine's been cut but not by how much. There is a core motor near me I have to go look at but I haven't had the time.

 

[hooey]

If we can't blow the crank out of the block on these motors at 15-16 to 1 in the race engines turning 8000-9000 rpms, you not going to hurt your motor if built properly at 10 to 1 on pump gas and turning 6000 rpm on occasion.

15--16 you will have a motor that last about 1.5 hr at 8000 -9000 rpm (good for .5 hr race)the point I was trying to make is that if the pistion are good you can mill the head and or the deck of the block cheeper than pistion and put your money in to the heads ( don't go cheep on the head ) new valves new guids and do what seat work you need and maybe a cam, one other thing, valve spring use just what you need the harder thy are the move wear you will have on the valve train

 

[recordsj]

Hap,

thank you very much for all this info and discussion. it has been very useful

 

[Guest]

Gotta love Hap. I might just give him a hamburger one day. :thumbsup: